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T Dissertation U l V l l Information Service
University Microfilms International A Bell & Howell Information Company 300 N. Zeeb Road, Ann Arbor, Michigan 48106 8618760
Chang, Yin
EVALUATION OF THE MORELAND COLOR MATCH AS AN INDICATOR OF RETINAL PATHOLOGY
The Ohio State University Ph.D. 1986
University Microfilms
International 300 N. Zeeb Road, Ann Arbor, Ml 48106 Evaluation of the Moreland Color Match as an
Indicator of Retinal Pathology
DISSERTATION
Presented in Partial Fulfillment of the Requirements for
the Degree Doctor of Philosophy in the Graduate
School of The Ohio State University
By
Yin Chang, B.A., Ph.D.
*****
The Ohio State University
1986
Dissertation Committee: Approved by
Richard M. Campbell Ph.D.
Carl R. Ingling, Jr. Ph.D.
Ronald Jones Ph.D. tichard M. Campbell,Advise Biomedical Engineering DEDICATION
To Mom
• • - 11 - ACKNOWLEDGEMENTS
I wish to thank the following people for their assistance in preparing this dissertation:
Professor Richard M. Campbell, for his assistance in soloving the noise problem of the power circuit, and pro viding useful comments and criticism.
Professor Ronald Jones, a member of my committee, for his interest in my study.
Doctor Paul Weber, for his helping me collect the data at the Ohio State University Hospital.
Professor Carl R. Ingling,Jr., and Dr. Brian Tsou, for suggesting the topic, and for working so closely with me during the last four years. Their examples, and the count less hours they spent with me have greatly influenced near ly every aspect of my study. VITA
August 28, 1950 Born, Taipei, Taiwan, R. 0. C.
1973 ------B.S., Chung-Yuan University, Chung Li, Taiwan, R. 0. C.
1973-1975 ------2nd Lt., Army, R. 0. C.
1975-1976 ------Electronic Engineer, Philips Co. Taiwan, R. 0. C.
1976-1980 ------Research Asistant, Chung-Shan Institute of Science and Technology
1980-1982 ------Research Associate, Department of Electrical Engineering, The Ohio State University Columbus, OHIO
1982-1986 ------Research Associate, Department of Biomedical Engineering, The Ohio State University, Columbus, OHIO
Major Field Biomedical Engineering TABLE OF CONTENTS
Page
DEDICATION ...... ii
ACKNOWLEDGEMENTS ...... iii
VITA ...... iv
LIST OF T ABLES ...... viii
LIST OF FIGURES ...... ix
CHAPTER
I. INTRODUCTION ...... 1
Statement of Problem ...... 1
II. METHODS ...... 4
Approach ...... 4 Apparatus ...... 6 Optical Apparatus ...... 7 Electronic Devices ...... 12 Key Circuit Description: 15 Calibrations of the Apparatus ...... 19 Procedures & Conditions ...... 20 Midmatching Point and Matching Range . . 21 Testing Procedure ...... 21 The Subjects ...... 24
- v - III. R E S U L T S ...... 25
Data and Analysis ...... 25
IV. THEORY AND CONCLUSION ...... 33
Color Deficiency and Confusion Line .... 33 C o n c l u s i o n ...... 35 S u m m a r y ...... 36
APPENDICES
A ...... 38
B a c k g r o u n d ...... 38 Some Basic Structures of Eye Related to G l a u c o m a ...... 41 Aqueous Humor Dynamics and Intraocular Pressure ...... 41 What is normal intraocular pressure? . . 50 How Does Aqueous Humor Dynamics Influence Intraocular Pressure? . . 52 The Optic Nerve Head And Visual Fields...... 53 Color Vision Defect and Glaucoma .... 59 Clinical Color Vision Testing Methods . . 61 A Possible Color Vision Testing Method Applied to Glaucoma Detection at Early Stage ...... 65
B ...... 67
Standaralizing the Color Match ...... 67
C ...... 70
The Control Program ...... 70
D ...... 77
Trichromatic Color Theory ...... 77 Opponent-Color Theory ...... 79 Color Matching and The Color Equation . . . 83 The CIE System ...... 90
- vi - The Chromaticity Diagram ...... 93
REFERENCE ...... 95
- vii - LIST OF TABLES
TABLE PAGE
1. Primary Data of Color Vision Test & IOP Measurement ...... 26
2. Matching Range, Frequency, Cumulated Percentage of the Normal Subjects ...... 28
3. The Distribution of I O P ...... 29
4. Reported IOP Distributions General Populations . 52
5. Differences Between Congenital and Acquired D e f e c t s ...... 60
- viii - LIST OF FIGURES
FIGURE PAGE
1. The intersection of two dash lines (580 nm - 480 nm and 500 nm - 440 nm) represents the match point of the four lights...... 5
2. The interactions between the computer set, electronic control box, power control box and optical integration box...... 6
3. (a)Front view of the apparatus (b)Top view of the optical assembly part; HRM: heat- reflecting mirror; PB: plate beamsplitter; F: interference filter; M: mirror; C: chopper; L: l a m p ...... 9
4. (a) heat-reflecting flat mirror and (b) normalized log scale transmittance curves of bandpass interference filters...... 9
5. A Bipartite Field ...... 10
6. A block diagram of feedback control circuit . . 13
7. Block Diagram of Addressing Circuit ...... 15
8. A photodiode and an operational amplifier form a feedback element...... 16
9. (a)The temperature characteristic of S1337.66 BR (b)The linearity of S1337.66 BR . . 17
10. Feedback Control System ...... 18
11. Color match on the confusion line ...... 23
12. Two critical points of matching range ...... 23
13. Histogram of Matching Range ...... 27
14. A Histogram of I O P ...... 30
- ix - 15. A Distribution of Matching Range V.S. I O P ( m m H g ) ...... 32
16. Chromaticity confusion made by (a), protanopes (b), deuteranopes (c), tritanopes . 34
17. A Flow Path of Aqueous H u m o r ...... 42
18. Cut-away view of Schiotz-type indentation tonometer ...... 44
19. Goldmann Applanation Tonometry ...... 47
20. (a) The prism assembly, schematic side view; (b) view through biprism (1) reveals circular meniscus, (2) which is converted into semicircles, (3) by prism...... 49
21. The pattern is seen through the microscope. . 50
22. A distribution of IOP in non-glaucoma(N) & glaucoma(G) population ...... 51
23. Divisions of the optic nerve head. A: Surface nerve fiber layer B: Prelaminar region C: Lamina cribrosa region D: Retrolaminar region ...... 53
24. "Island of vision surrounded by a sea of blindness" (A) the profile of normal visual field; (B) (f) corresponds to the fovea and the blind spot(bs), of the optic nerve head...... 55
25. Normal isopters measured with Goldmann p e r i m e t e r ...... 57
26. Section of a subject's profile illustrating how error scores are plotted...... 64
27. Graded electrical response of retinal cell to various wavelengths of light ...... 81
28. Schematic diagram indicating how a three- pigment system might be connected to produce an opponent-process neural response ...... 83
29. Triangular classification of colors by Mayer using red, yellow and blue primary colors. Color graduations take place in steps of one part on a scale of twelve parts...... 84 - x - 30. Helmholtz's hypothetical curve of the locus of spectral c o l o r s ...... 87
31. Chromaticity diagram of CIE 1931 standard colorimetric observer in the system of real primary stimuli R.G.B...... 92
32. Chromaticity diagram of CIE 1931 standard colorimetric observer in the transformed system of imaginary primary stimuli X,Y,Z. . . 94
- x i - ABSTRACT
Glaucoma patients and normal observers were tested on a specially designed 4-channel anomaloscope incorporating
Moreland's equation (Moreland & Keer ,1978) . This anoma loscope is totally electronic and computer-controlled. A
Commodore 64 microcomputer controls the light intensity to
0.5% accuracy. The matching point, color matching range and eye pressure(IOP) were collected from a screening test on patients and normal observers at the Ohio State Univer sity Hospital. The results indicate that the glaucomatous subjects have a wider color matching range than the normal subjects. The color matching range is also highly corre lated with IOP if a general population is tested. However there is no correlation between color matching range and
IOP in the "normal" range of the distribution. CHAPTER I
INTRODUCTION
STATEMENT OF PROBLEM
Several studies have shown a relation between the degree of color vision defect and the amount of visual field loss in chronic simple glaucoma. The disability in hue discrim ination is in proportion to the visual field loss. (Aus tin, 1974; Fishman, Krill and Fishman, 1974)
Fishman, Krill and Fishman (1974) reported their stud ies on 30 ocular hypertensive and 23 glaucomatous patients using the FM 100-hue test. In the ocular hypertension group, all subjects had normal optic disks, normal visual fields , visual acuity of 20/30 or better, and absence of significant lens opacities or macular pathology. They were normal, except for their intraocular pressures which were
24 mm Hg or greater (measured by Applanation tonometer).
In the group of glaucomatous subjects approximately the same conditions were present except that they had glaucoma tous optic disks or glaucoma compatible visual fields.
These results showed that in the ocular hypertension group,
- 1 - 60% of the eyes had normal color vision, 20% had tritan defects, while the other 20% of the subjects had what is defined as low discrimination. Meanwhile, within the glau coma group, 83% of the eyes with damaged optic disc And visual field had tritan defects, 63% of the eyes with either damaged optic disc or abnormal visual field had tritan defects.
The conclusion of the above results may indicate that color vision defects may be apparent before visual field defects happen(Fishman, Krill & Fishman, 1974; Lakowski,
Bryett & Drance, .1972). There also are several reports
(Cox, 1960; Krill & Fishman,1971) which confirm that type
III violet-yellow defect is the most common disturbance of color vision found in glaucoma. Moreover, at the late stage of glaucoma, a reduction in red-green discriminations
(type II color defect) is also reported (Koliopoulos,1978).
Therefore, this evidence suggests that type III color defect might be a warning signal of the onset of glaucoma with an elevated intraocular pressure. Moreover, after the type III color defect is found,the visual field will gradu ally decrease from the peripheral area into the central area.
Since glaucoma is a non-reversible disease, a very sen sitive and accurate color vision test should be introduced to the glaucoma diagnostic procedure or to the general eye examination. Although there are not many studies reported about this in the past, yet it is worthy to begin. CHAPTER II
METHODS
APPROACH
Available evidence indicates that the B-cones may be most
susceptible to damage by intraocular pressure. Therefore, a
test of B-cone function is required. If B-cone function is
impaired,it may be reflected in an impaired ability to make
color-matches which involve the B-cones. The conventional
Rayleigh match used to screen for Red-Green defects does
not involve the B-cones. Matches which do involve the
B-cones are the Engelking-Trendelenburg match and the More
land match. In 1978, Moreland and Kerr (p.162-166) devel oped a four-primary color-match equation for tritan defect
that allows a perfect match and minimizes the effect of macular pigment.1 The equation is
a579 nm ♦ b480 nm = c439 nm + d499 nm (2.1)
1 Macular pigment is a yellow substance of the macula lutea which is a part of the retina. Its function seems to be to prevent short-wave light from reaching the central area of the retina, and it completely covers the fovea--- the region that has the highest acuity. - 4 - For convenience, the equation (2.1) is modified as (2.2) in this study.
a580 nm ♦ b480 nm = c440 nm ♦ d500 nm (2.2)
Fig. 1 shows the four lights used to make the match defined by equation (2.2). The ratio of 580 nm to 480 nm is fixed. However the ratio of the 500 nm to 400 nm is var ied. The intensities on both sides of the equation are always kept constant.
520 0.8 540
560 0.6
500 ,580
y ,600 ^Equal ,620 energy ,650 700 nm
0.2
480'
0.2 0.4 0.6 0.8 x
Fioure It The intersection of two dash lines (580 nm - 480 nm and 500 nm - 440 nm) represents the match point of the four lights. APPARATUS
The instrument is mainly based on the use of one optical stimulus box, one electronic control box, one power control box and one microcomputer set which includes a
Commondore-64 microcomputer, a Commodore-1541 disk drive and a monitor. The interactions between the boxes and microcomputer set are shown in the following block diagram
(Fig.2).
Computer Electronic Power Set Box Box
Optical Integration Box
Figure 2: The interactions between the computer set, elec tronic control box, power control box and opti cal integration box. 7 Qplic.al_-Appar.atus
The outside dimensions of the apparatus are about 16(W) x 11(H) x 9(D) inches. It contains two parts: The upper
(Fig. 3) part of the apparatus is an assembly of lamps, mirrors, filters, and a motor driven chopper.
The lover part of the apparatus is an air compressing buff er in which a fan provides flowing air from this buffer
into four lamp chambers for carrying out heat.
In Fig. 3a, each lamp is surrounded by a black-painted can. The can has baffled holes on the top, about 3 inches
in diameter and 8.5 inches in height. Each can is very easily moveable from its mounted floor by a slight swing and pull. Inside the can, there is nothing on the floor but a hole which allows the can to make a direct connection with the air compressing buffer. A light bulb socket is fixed on a metal bar which is mounted on the floor across the hole. The can has a window approximately 1.5 inches in diameter on its wall and 5 inches above the floor. Outside the window, a u-shaped holder is fixed on the can. A heat- reflecting mirror* and an interference filter3 are set in
2 Heat-reflecting mirror is a multilayer dielectric mirror. It can reflect or divert heat to a location in the system where dissipation is more convenient and also allows the visible light to pass through.
3 Interference filter is simply a light bandpass filter. In general, a narrowband interference filter permits iso lation of wavelength intervals down to a few nanometers or less in width without the use of dispersion element 8
optical assembly part
air compressing buffer part
(a)
c\Z
HRrt F
efi 3
HRH cA 1
(b)
Figure 3; (a)Front view of the apparatus (b)Top view of the optical assembly part; HRM: heat-reflecting mirror; PB: plate beamsplitter; F: interference filter; Mi mirror; C: chopper; L: lamp
this holder. Therefore the light emitted from the lamp, through the window,to the heat reflecting mirror and interference filter becomes a monochromatic light. The heat radiated from the lamp is also reflected into the can by the heat reflecting mirror. Thus the heat is carried away from the can and won't hurt the interference filter.
The transmittance characteristics of heat-reflecting mirror and narrow band interference filter are shown below(Fig.
4a, 4b).
too
FWHM
Z 100 e Hot Mirror Coating, s 1 ctvity Normal and 45° Incidence u) UUJ z z K2 2 0 2 (/) 1 z ac < o He z *- z cS UJ £ co UJ CL
300 400 500 600 700 800 900 1000 1100 1200 DEVIATION FROM CENTER WAVELENGTH IN FWHM UNITS
WAVELENGTH IN NANOMETERS I FWHM I
(a) (b)
Figure 4i (a) heat-reflecting flat mirror and (b) normal ized log scale transmittance curves of bandpass interference filters.
such as prims or gratings. 10
As shown in Fig. 3b, the monochromatic light beams com ing from channel 3 and channel 4 are combined by a beam splitter* at 45 degrees. Thereafter, 50% transmitted light of channel 4 and 50% deflected light of channel 3 mix together and move forward to a mirror, from where an observer can see this mixed light on the right half field of a bipartite field(Fig.5) through an aperture on the front panel of this apparatus. The angular diameter of this aperture is about 2 degree of visual angle to the observer.
500nm + 440nm 580nm * 480nm
Figure 5: A Bipartite Field
To calculate the visual angle by the aperture size and viewing distance, the following formula(Pickford & Lakow- ski;1960) is used.
tan(B/2) = e/(2R) (2.3)
* The function of plate beamsplitter can partly reflect a.nd partly transimit the incident light. The reflection and transmission ratio of light is a constant value. The general nominal value of R/T ratios are 30/70, 50/50, and 70/30. Where B= Visual Angle
e* aperture size (diameter)
R* viewing distance from observer's cornea to
the aperture
For example, if ve need a 2 degree visual angle and the viewing distance is about 30cm, then aperture size is about
10.5 mm, which is calculated by equation (2.3). The reason for choosing a 2 degree visual angle is that "--- the use of a larger field, although more in keeping with usual methods of viewing, was avoided because of possible non additivity due to rods being present in fields larger than
2 degree. --- These large field color matches therefore relate explicitly to the extra foveal retina rather than to the whole field which includes the foveal rod-free area".(Trezona, 1970)
At the other side of the apparatus, a plastic mirror chopper is located at 45 degree between the channel 1 and channel 2 light sources. The chopper is driven by a series a.c. motor whose speed can be manually and computer con trolled. The two functions of the chopper are:
1. It chops the two channels of light into light pulses.
The beam through the chopper from channel 2 is direct
ly projected onto the left side of the bipartite
field. The beam from the other channel (channel 1) is 12
reflected by the chopper and projects on the same
field as channel 2 . The observer sees a mixed color
light if the motor speed is high enough. Thus if the
luminance of the two light beams is varied, the
observer would see color changing in the range of the
fixed wavelengths of channel 1 and channel 2.
2. The chopper, combined with the two channels of light
sources, can perform as a flicker photometer. The
functions of the flicker photometer will be described
in detail in a later section.
The four photo-sensitive detectors(photodiodes)
which are mounted on the U-shaped holder simultaneous
ly monitor the intensity changes of the four channels.
These signals are fed into four individual amplifiers
in the electronic box. All the signal processing will
be discussed in the latter section.
Electronic Devices
The basic technique for achieving a precise linear brightness control on each channel is based on a simple
feedback theory (note: each channel has the same feedback control circuit except for the feedback resistor) as shown
in the following diagram(Fig. 6).
The equations describing this system in terms of the trans
form variable are: 13
R(S) G(s) * C(S)
H(s)
Figure 6; A block diagram of feedback control circuit
C(s) = G(s) E(s) (2.4)
B(s) = H ( s ) C(s) (2.5)
E(s) = R(s)-B(s) (2.6)
Combining these equations produces the overall transfer
function,
C(s)/R(s) = G(s)/(1 ♦ G(s)H(s)) (2.7)
In this specific control system, the feedback elements
include a photodiode and a chopper stabilized operational
amplifier. The control elements and control system include
an electrical charge holding circuit and a power control
system. Meanwhile, an 8-bit computer used in the whole
system here serves as a commander. It reads the two con
trol potentiometers(pots) voltages through two A/D con verters and then sends a reference input R to each channel 14 by means of a D/A converter. The information received and transmitted is through a so called I/O port in the comput er.
In order to communicate with so many devices(4 D/A con verters for 4 channels of reference inputs, 2 A/D convert ers for two pots, and 2 more A/D converters for another two optional pots for calibration use) and only limited I/O buses available, an addressing circuit board is needed for the computer to talk to each device one at a time. A simplified block diagram(Fig.7) is shown on the following page. A control program (Appendix c) in the computer sequentially does the routine until a push-button is pushed on the back of the control pot box. PA
Figure 7: Block Diagram of Addressing Circuit
Key Circuit Description;
!• Feedback Elements of Fig. 7 16
Figure 8; A photodiode and an operational amplifier form a feedback element.
As shown in Fig. 8, a photodiode (Hamamatsu,
S1337.66 BR) in this circuit functions as a light
brightness sensor. Because of its nearly perfect lin
earity of output current v.s. input light intensity
(Fig. 9(a)) and very stable temperature coeffi-
cient(Fig. 9(b)) in the whole visible wavelength, it
is an excellent device to monitor the changing light
when a change occurs.
A chopper stabilized operational amplifi
e d Intersil, ICL7650) here provides a very low input
bias current (lOpA max), low offset voltage (1 uV over
the temperature range), low long-term and temperature
drifts of input offset voltage and also high gain
(CMRR and PSRR, min 120dB) and high slew rate(2.5
v/us). These features of the operational amplifier
ensure that the amplifier varies linearly with lamp
output. . oto Eeet i Fg 7 Fig. in Elements Control 2. iue : 9 Figure
TEMPERATURE COEFFICIENT (K/*C) .5 1 — - - I.S + + value of the capacitor, the output voltage becomes voltage output the capacitor, the of value pure resistor feedback. However, due to a very small very a to due However, feedback. resistor pure eoe I (- x(tR) isedo I, s usual as IR, of instead exp(-t/RC)) (1- x IR becomes prxmtl IR. approximately voltage output the Then system. the from noise to current no almost deliver to required is photodiode 0.5 ut ie lwps fle t fle u unnecessary out filter to filter low-pass a like just h apiir Teeoe h hl cret flows performs current here circuit RC whole The the Therefore circuit. RC the through amplifier. the 0.5 nu fo te optr Te ice i a feedback a is B circled The computer. the from input 1.0 150 n i. 0 te ice i atal reference a actually is R circled the 10, Fig. In eas f h ey o ipt is urn, the current, bias input low very the of Because 400 bTe iert f 13.6 BR S1337.66 of linearity (b)The aTe eprtr caatrsi o S376 BR S1337.66 of characteristic temperature (a)The AEEGH n ) (nm WAVELENGTH (a) £00 100 1000 < 10 • 10 ' GT NEST ( ) (W INTENSITY UGHT O E VAL JES L A V » NE TO (b) (Typ. at SG0nm.2rC) at (Typ. 100 17
18
+13*
r~. ...i
Figure 10: Feedback Control System
voltage and the circled E is the actuating signal in
Fig. 6. If E=0 volt, there is a change on the capaci
tor voltage because of the symmetrical circuit. But
whenever E varies, both outputs of A1 and A2 are
simultaneously altered. Thereafter the currents in
transistors T1 and T2 become unbalanced which causes
the capacitor to charge or discharge. Then the volt
age on the capacitor is compared with a 120 Hz 5 volt
ramp whose zero voltage corresponds to the time.of
the zero-crossing voltage of the 110V a.c.. The 19
advantage of using the zero-crossing as a timing ref
erence is that the triac can be smoothly fired at any
time in the period of an a.c. cycle. The output of
the comparator is a duration varied pulse which is
dependent on the variation of the feedback voltage.
Furthermore, the duration varied pulses from the out
put of the comparator activate a photo-isolating
device. The changing rate and the firing time of the
SCR in the photo-isolate device are also a function of
the pulse rate and pulse width. Moreover, the current
through the SCR triggers a triac whose conduction
results in the load(lamp) lightening. Thus, the whole
system is a servo system. When the reference input
voltage increases the feedback voltage increases until
B=R or E=0, and vice versa. Therefore, the computer
controls the light linearly for each channel.
CALIBRATIONS OF THE APPARATUS
1. Electronic Circuit:
As is shown in Fig. 8 the feedback resistor is
adjustable. Since the 4 channels have different wav
elengths, the values of the currents in the 4 photo
diodes are also different. Thus the outputs from each
amplifier have to be adjusted to full output scale 20
(5.00 V D.C.) by means of the feedback resistors.
This step must be ensured because the maximum refer
ence input voltage from the computer controlled D/A
converter is 5.00 V D.C.. Otherwise, when the gain of
each amplifier is too high or too low, resulting in an
output voltage greater than 5.00 V or less than 5.00
V then the whole system will never reach balance.
In the mean time, a reference voltage (not refer
ence input voltage as is mentioned above) and a ref
erence current to the A/D and D/A converters are also
very important. Hence, very stable, low temperature
drift voltage and current sources are needed to meet
the above requirements. Otherwise, the drifting "ref
erence" voltage or current would cause inaccurate
results at the outputs of the A/D or D/A converters.
2. The yellow(580 nm) field was calibrated and set to a
luminance of 50 cd/(meter square) with a Pritchard
telephotometer referred to a standard luminance
source.
PROCEDURES & CONDITIONS 21
Midmatchin? Point and Matching Range
The anomalous trichromat's match reflects his/her visual photopigments which differ from those of normal trichro mats. Thus the match point has theoretical significance.
On the other hand, the matching range is an indication of the observer's chromatic discriminative ability. Measure ment of the matching range is practically important. On the average,the matching range of the simple anomalous trichromat is wider than that of the normal trichromat.
As has been mentioned above this instrument is a
4-primary anomaloscope. It is difficult and time-consuming to reach the color match by using 4 primaries. That is why
Moreland suggests fixing the yellow/blue ratio in order to make matching easier. The yellow/blue ratio is not arbi trarily chosen. This ratio should be chosen at the inter section point as shown in Fig. 1. There is no simple way of obtaining this point but by performing a real match experimentally. To find the correct yellow-blue ratio, matches were made by three normal observers and averaged
(appendix B).
Testing Procedure
1. In the testing procedure, the examination starts with
a 5-minute preadaptation to the bipartite field light. 22
The observer is presented with a normal match prepared
by the examiner in advance.
2. The examiner starts by asking the observer to comment
on the "brightness match" of the two fields. If the
answer is negative, the examiner should adjust the
brightness-control pot of the yellow/ blue (right
half) field for the observer until a brightness match
is identified.
3. Then, the observer is asked to comment on the color
appearance of the left half field, comparing it with
the right half field, by answering "same", "too vio
let" or "too green".
4. Under the answering "same" condition, the examiner
slowly turns the color-control pot and gradually
changes the green/violet ratio in either direction on
the green-violet line (Fig. 11) until the observer can
indicate that the left half field is just a little
"more violet" or a little "more green" than the right
half field.
5. At this point, the observer may have a psychological
response problem in making the decision to say "same",
"too green" or "too violet" about this critical
matching point. Therefore, the examiner should vary
slightly the green/violet ratio back and forth through
this "critical point"8 until the answer "same" is
8 the "critical point" should be called "same" by the 23
violet (440nm) ------— M ■ ------green(500nm)
match point
Figure lit Color match on the confusion line
obtained (Fig. 12). Then a push button is pressed by
the examiner following the above answer.
violet(440nm)------» . — > ------green(500nm) / *\ critical point match point critical point
Figure 12: Two critical points of matching range
6. Step (5) is repeated several times until consistent
responses are reported.
7. Then, the examiner changes the green/violet ratio by
turning the color- control pot in another direction.
Another "critical point" should be found by following
(5) and (6). The "distance" between the two "critical
points" is called the matching rangg«
observer. 24
8. For the comment of "too violet" or "too green" in step
(3), the examiner will give the left half field more
green or more violet by slowly adjusting the color-
control pot until a "same" comment is obtained. This
is one of the two "critical points" that the examiner
is trying to find. Then the examiner gives more
green or more violet depending on the original answer
ing "too violet" or "too greener" and performs the
same processes as in (5) and (6) until another " crit
ical point" is found.
THE SUBJECTS
The subjects come from two different sources normal and patients . The patient who has glaucoma or who is a glaucoma suspect is asked whether he/she is willing to do this color vision test when he/she comes back to the hospi tal for follow-up checks. For those normal subjects, the intraocular pressure (IOP) measurement is also provided in addition to the color vision test. If the subject has potential eye problems indicated by the two tests, a more detailed eye examination will be given in the hospital.
There is no restriction on the subject's age, sex, or race in this study. CHAPTER III
RESULTS
DATA. ANP-ANALYSIS
A color vision test using Moreland's equation, and an
intraocular pressure (IOP) measurement with the applanation tonometer were performed in this study. The color vision test measures (1) matching range and (2) mid-matching point which is defined as the center point of the matching range. The correlation between these two variables and IOP are calculated .
Totally twenty one subjects (42 eyes) are examined with the two tests. Four of them are known with glaucoma on one eye or both. The patients were undergoing treatment with eye drop or laser. Table 1 shows the primary data from both color vision test and IOP measurement. Subject No. with "*” indicates glaucoma patients. R and L mean right eye and left eye.
In order to get the "normal" subject's baseline, the four glaucomatous subjects are excluded in the first calcu lation. Table 2 shows the matching range, the frequency
- 25 - 26
Table 1
Primary_Data of Color Vision Test & IOP Measure ment
Subject violet color matching range IOP (mm Hg) No. R L R L
1* 22-78 23-47 28 28
2 22-23 19-27 15 13
3 22-23 21-22 18 16
4 22-23 23-24 13 15
5 20-21 21-23 18 16
6 23-24 27-28 16 15
7* 23-29 23-93 25 24
8* 23-37 23-65 22 22
9 26-30 24-34 17 14
10 25-28 23-25 12 12
11 22-23 21-22 19 23
12 22-25 25-26 14 14
13 23-29 26-30 12 12
14 25-32 28-34 16 18
15 21-21 21-22 10 10
16 22-24 20-22 17 13
17 23-25 22-23 16 16 18 29-34 30-33 7 7
19 21-24 20-25 16 17
20 19-22 22-25 16 16
21* 27-32 30-51 24 23 27 and the cumulated percentage of the normal subjects. ‘ Fig ure 13 displays the histogram of the width of matching range.
Frequence 13 H
12
n
10
s ■
7 •
6 -
5 •
1 ■
2 -
1 -
0 1 2 3 >1 5 6
RANGE
Figure 13: Histogram of Matching Range Table 2
Matching Range. Frequency. Cumulated Percentage of the Normal Subjects
Matching Range Frequency Cumulated Percentage
0 1 2.941
1 13 41.176
2 5 55.882
3 6 73.529
4 2 79.412
5 2 85.294
6 2 91.176
7 1 94.118
8 1 97.059
10 1 100.000
If 95% is chosen as confidence interval, then the width of matching range should be less than 8 according to the 34 normal eyes'(17 subjects) distribution. The mean is 2.824 and the standard deviation is 2.355. The cause of high standard deviation is because of the higher weight on the tail of the match-range and low population. The statisti cal correlation between the match range and IOP of the nor mal subjects is not significant (correlation coefficient®
-0.01550, p=0.9307). 29
The distribution of IOP is also shown on Table 3 and the Fig.14 shows the histogram.
Table 3
The_Di-S.tr ibut ion oi IOP.
IOP Frequency Cumulated Percentage
7 2 5.882
10 2 11.765
12 4 23.529
13 3 32.353
14 3 41.176
15 3 50.000
16 9 76.471
17 3 85.294
18 3 94.118
19 1 97.059
23 1 100.000
(The mean= 14.68, S.D.= 3.27) Frequence 9 h
2 -
t -
OQ DCKXXI DOCOq 10 12 13 14 15 16 IOP(mmHg)
Figure 14: A Histogram of IOP 31
On the other hand, if the four glaucomatous subjects are included in this statistical caculation, the statistical correlation coefficient between matching range and IOP is - significant ( R= 0.62, p* 0.0001). Fig. 15 shows a plot of matching range v.s. IOP If high IOP indicates glaucoma, then the points scattered on the right half side of the diagram are meaningful. Some points located on the bottom in this right half side need interpretations. The point at
(1, 23) on range-IOP plane is a reqular subject with high violet-green color discrimination but also with high IOP value. He is arranged to do a complete eye examination at the University Hospital Clinic. If the results show no glaucomatous indications, then he probably has a pressure- resistant optic nerve. Therefore, he is an exception in the high IOP area without glaucoma. If his test results positively indicate that he has glaucoma, then he may have lost a very small area of visual field and uses the remain ing normal area to see. The other two points with coordi nates at (5, 24) and (6, 25) are interpreted as localized visual field (very small area) loss according to the medi cal records of these glaucoma patients.
The points scattered in the area that IOP> 20 and range>
10 are recorded from the glaucomatous subjects. These sub jects have diffused visual field loss in the peripheral area according to their medical records. 32
Range
70 -
85 -
BO -
55 -
50
45 •
40 -
35 -
30 -
25
20
15
10
B a "7" ■rrrT " 11 13 IS 17 19 21 23 2 5 27
IOP(mmHg)
Figure 15: A Distribution of Matching Range V.S. IOP(mmHg) CHAPTER IV
THEORY AND CONCLUSION
COLOR DEFICIENCY AND CONFUSION LINE
The CIE chromaticity diagram is sometimes used to char acterize the major color-vision deficiencies. The way it is used for this purpose is to plot on the CIE chart the chromaticities of stimuli that are confused by an observer with a deficiency of a given type and to link these chro- maticities by connecting them with a straight line. These lines are called confusion lines or confusion loci.
Different patterns of confusion lines emerge for the different types of color deficiency. Fig 16(a) shows the protanopic confusion loci, Fig. 16(b) the deuteranopic confusion loci and Fig. 16(c) the tritanopic confusion loci. These plots of confusion chromaticities accord com pletely with what we know about the different kinds of dichromats.(Hurvich, 1981)
- 33 - 34
0 h O h
560 06
580 y 600
02
0 02 0.4 0 6 0 8 0.2 0.4 0.6 0.8 X
(a) (b)
Oi.
y 0 4
.650 0.2
4 8 tl
0 02 0.4 0.6 0 8
(c)
Figure 1 6 : Chromaticity confusion made by (a), protanopes (b), deuteranopes (c), tritanopes 35
Moreland's color-matching equation, which this study is based on, is off (but close to) the tritanopic confusion loci. (Notice that the 500 nm-440 nm line does not emerge to the 400 nm point) The reason one chooses this equation which has been mentioned in Chapter One is that the violet color-vision deficiency occurs in the early stage of glau coma. As a result, the patient with violet-color deficien cy has a wider color matching range than the normal observ ers. The evidences is this study also support this point of view.
CONCLUSION
The purpose of designing this apparatus is to detect the diseases which result in color vision defect (especial ly in violet- green region) in the early stage. A large population of normal people is needed for obtaining a rea sonable baseline of this color vision test. Thirty is a reasonable number, statistically, to do a test. Therefore,
34 eyes from 17 regular people should fit the above requirement. However, depending on the patients' willing ness to run the test, a sufficiently large population of patients is somewhat difficult to obtain for the study. ,
Only four patients' data could be collected in about three months. Howerver, the statistical results indicate that by 36 using this color vision test glaucoma patients can be iden tified. Since all the glaucomatous patients in this study have high IOP, if some low tension glaucomatous patients could be found, the results should be interesting. No sub ject with digoxin toxicity or with diabetes is involved in this study. However, color vision deficiencies are also reported to accompany these two kinds of diseases (Chuman,
Lesage, 1985; Kinnear,Aspinall,Lakowski, 1972).
SUMMARY
More subjects will be required to show that the approaches and test described here can reliably identify glaucoma patients.
However, the present results indicate that matching range correlates highly with the presence of glaucoma. The advantages of the test are: (1) The apparatus is very flexible to control because the testing parameters are con trolled by software.. (2) The light intensity in each channel can be precisely controlled to 0.5% accuracy. (3)
The whole test on both eyes could be done within 20 min utes. (4) A standard color displayed on the right half field of the viewing aperture improves discrimination and increases sensitivity. The disadvantages of it are: (1) it contains a computer equipment, an electronic circuit box, an optic integration box, a power control box and some accessary parts such as a small power lamp for viewing background, a chin supporter and holder, a board standing on the table for keeping the eye from interference by. side light, and two small boxes with potentiometers for color,
intensity and motor speed control. Therefore, it is not portable. (2) The observer needs about 3-5 minutes to adapt to the light. APPENDIX A
BACKGROUND
At the beginning, several questions are posed to help us to understand how serious eye problems are in this country.
These questions and answers are given according to "Vision
Problems in the U.S." reported by National Society to Pre vent Blindness (1980).
Q: How many people are blind in the U.S.?
A: Nearly 500,000 people are legally blind. They have a central visual acuity6 for distance of 20/200 or poorer in the better eye with correction, or a field of vision7 no greater than 20 degree in its widest diameter.
Q: How many become blind each year?
6 Visual acuity: Measurement of the ability of the eye to perceive the shape of objects in the direct line of vision and to distinquish detail;generally determined by finding the smallest Snellen symbol that can be recogniz ed. A visual acuity of 20/200 means that a person can see at a distance no greater than 20 feet what one with "normal" sight can see at 200 feet.
7 Field of vision: Entire area that can be seen without shifting the gaze, i.e., without moving the head or eyes.
- 38 - 39
As An estimated 47,000--- one person every 11 minutes.
Q: What are the leading causes of blindness?
A: In.order of frequency: glaucoma, macular degenera tion, senile cataract, optic nerve atrophy, diabetic reti nopathy and retinitis pigmentosa. These causes account for
51% of the blind people.
Nov based on the above statistical results, ve have a general idea that glaucoma is the number one eye disease which can cause blindness. Therefore, this study focuses on it and tries to find an approach to detect this disease in a very early stage.
Glaucoma, a progressive eye disease is usually charac terized by elevated intraocular pressure which is causally related to its common pathological outcome. This elevated pressure not only reduces the blood supply to the retina, but also causes irreversible damage to the optic nerve.
Once the blindness of glaucoma has occurred, treatment often can relieve the elevated introcular pressure and pre vent or slow the progression of optic nerve damage. Never- lethess the existing nerve damage can not be reversed nor vision restored.
Nearly 2 million Americans age 35 and over have glauco ma, more than 1 million people are undetected, and an esti mated 62,000 or 12.5% of the blind people, because of glau coma, are blind. Glaucoma is not a fatal disease, but it 40 may occur any time in life. Aging is an important factor
of developing glaucoma. An estimated 3% of the population
age 65 and over have this disease. Family heredity is also
a factor. The chance of getting the disease among persons
whose blood relatives have glaucoma is about four times
higher than those who do not have relatives with glaucoma.
Since glaucoma is not curable, those persons who have glau
coma family heredity should have an annual eye examination.
Khat-.ar.e the signs or the..symp.t Q ms...pf glaucoma? —
Frequent changes of glasses, none of which is satisfactory;
inability to adjust the eyes to darkened rooms, such as
theatres; loss of side vision, or rainbow-colored rings
around lights. Difficulty in focusing on close work are
the general signals of developing glaucoma. The best
defense against glaucoma is having an eye examination at
least once every two years, especially after the age of 35.
What are.the causes of glaucoma?. Four different mecha
nisms about pathogenesis of glaucomatous optic atrophy were
introduced after the mid-19th century:
1. Mechanical theory: In 1858, Muller proposed that the
elevated intraocular pressure led to direct compres
sion and death of the neurons.
2. Vascular theory: In the same year, Von Jaeger suggest
ed that a vascular abnormality was the underlying
cause of the optic atrophy. 41
3. Schnabel's cavernous atrophy theory: In 1892, Schnabel
-proposed that atrophy of neural elements created empty
spaces, which pulled the nerve head posteriorly.
4. Axoplasmic flow theory:In 1968, a concept of axoplasm-
ic flow was introduced, which revived support for the
mechanical theory but did not exclude the possible
influence of ischemia.
The investigation about the pathogenesis of glaucomatous optic atrophy is still continuing. However,the present evi dence suggests that obstruction to axoplasmic flow may be involved in the pathoenesis of glaucomatous atrophy.
Therefore, there is no clear idea whether mechanical or vascular factors are primarily responsible for this obstruction. Maybe there is more than one mechanism of optic atrophy involved in an eye with glaucoma as Spaeth
(1975,1977) suggested.
SOME BASIC STRUCTURES OF EYE RELATED TO GLAUCOMA
Aqueous Humor Dynamics and Intraocular Pressure
1. The Production and Flow of Aqueous Humor
In the eye, a fluid called aqueous humor circulates
between the cornea and the lens, bathing and providing
nourishment to these tissues. As is shown in Figure
17, aqueous humor is derived from plasma within the
capillary network of the ciliary processes and first 42
Tnnsparent cornea
Corneal epitnelium
’ Descemet's Canal o f Schlemm membrane
Aqueous humour circulating in direction of arrows
Sphincter Lens Choroidal Pectinate ligunent pupillae- epithelium layer of iris Opaque sclera and trabecular Dilator pupulae mesh-work
s'- / . 1 . ^ , T < i * ^ / ' Y. ■■■« T i "wmid------Double layer of pigment epithelium ■ /..* ’-. I'*jf.
Lens fibres
Ciliary Circular fibres of muscle ciliary muscle Suspensory ligament Lens capsule to which zonular or zonula of Zinn Margin of 'Nucleus' fibres are attached Basement membrane of lens to which fibres of Secretory cells humour the zonula are attached overlying pigment epithelium
Figure 17t A Flow Path of Aqueous Humor
enters the posterior chamber. It then passes forward,
through the pupil to the anterior chamber, where it
leaves the eye by way of the trabecular meshwork,
Schlemm's Canal, Intrascleral Channels and finally
enters the Episcleral Vein.
Aqueous humor plays a major role in creating and
maintaining the intraocular pressure. It not only
provides necessary nutrition to the cornea and lens,
but also washes out the metabolic waste from these
tissues. The rate of aqueous humor production is aproximately 2.5 ul/min (Shields, 1982) in the undis turbed human eye. An elevation of intraocular pres sure is associated with an increase in aqueous produc tion. A decrease in intraocular pressure may be asso ciated with a transient decline in aqueous production.
Intraocular Pressure Measurement
A device called a tonometer is designed to measure the intraocular pressure. In the following para graphs, two major types of tonometers will be intro duced .
a. Indentation Tonometer and Tonometry
i. Tonometer(Fig. 18) The Schiotz tonometer
is the prototype of this group. It contains
a scale, an indicator needle, a holder, a
weight which can be incremented( except a
5.5 gram weight is permanently fixed to the
plunger), a plunger and a footplate. The
footplate rests on the cornea when it is in
use. The plunger with weight moves freely
within a shaft in the footplate. Meanwhile
the needle is dragged by the plunger. When
the artifical force which the weight creates
is balanced by intraocular pressure, the 44
needle stops moving . The balanced force
created at the ocular site is not just only
intraocular pressure, but the resistance of
the cornea is also involved. Therefore a
conversion table is needed to relate the
nonlinear relationship among scale reading,
weight and intraocular pressure.
Scale
Needle
Plunger
Footplate
Figure 18; Cut-away view of Schiotz-type indentation tonometer Schiotz Tonometry
1) The subject should be in a supine posi
tion and fixating on a target just
overhead. The examiner separates the
eyelid and gently rests the tonometer
footplate on the anesthetized cornea,
at the position where the plunger can
move vertically and freely.
2) When the tonometer is properly posi
tioned, the examiner will observe a
fine movement of the indicator needle
on the scale. However the scale read
ing is averaged between the extremes
and recorded. Thus, if the scale read
ing is 4 or less, the fixed 5.5g
weight, additional weight should be
added to the plunger.
A conversion table is then used to
convert the intraocular pressure in
millimeters of Mercury(mmHg) from the
scale reading and plunger weight.
Applanation Tonometer and Tonometry(Shields,
56-57,1982) Based on the Imbert-Fick law, Goldmann intro duced a new version of the applanation Tonometer
in 1954. The basic theory of the applanation
tonometer is simply that when a flat surface is pressed against a sphere,the force on the surface equals the pressure in the sphere times the area
flattened.
Pressure (in the sphere)=Force (outside of the surface)/A (flattened area)
With the Goldmann applanation tonometer one measures the force required to establish a stan dard area of contact. If the area of contact is kept to a small value (diameter=3.06 mm.), the volume of fluid displaced by the flattening pro cess will be small, and the pressure in the eye will be elevated only slightly; that is , the final measurement of intraocular pressure will be very close to initial intraocular pressure,
i. Tonometer(Moses, 1958) As shown in Fig.19 47
Biprism „ Direction of Observor’s View
— Rod
Housing
Adjustment Knob
Figure 19: Goldmann Applanation Tonometry
it contains:
1) a plane surface which is a transparent
piece of plastic, 7 mm. in diameter,
mounted on the end of a lever. The
lever is hinged and can be swung out of
the way when the tonometer is not in
use.
2) a device for estimating the correct
contact diameter is adjacent to the
plastic plane and consists of a pair of
prisms one next to the other with their
bases in opposite directions (Fig. 20(a)). Thus, the field seen through
the prism pair is displaced, upper half
to the left and lover half to the right
by a fixed amount (Fig. 20(b))
3) The force comes from a coil spring
through a series of levers which is
finally applied to the prism. The
force is varied by varying the length
of the spring. This is accomplished by
manually rotating a dial calibrated
directly in centimeters of mercury.
4) The entire tonometer is mounted on the
Haag-Streit slitlamp microscope. The
contact area is illuminated by the
slitlamp and is viewed through the
right ocular of the microscope at mag
nification x20. A violet filter in a
hinged mount is rotated in front of the
slit.
Tonometry
1) The edge of the contact area is made
visible by instilling a small amount of
sodium fluorescein in the tear. The
meniscus of the tear fluid now appears
light green. The cornea and biprism (a) (b)
Figure 20: (a) The prism assembly, schematic side view; (b) view through biprism (1) reveals circular meniscus, (2) which is converted into semicir cles, (3) by prism.
are illuminated with a cobalt violet
light from the slitlamp and a violet
filter which is swung in front of the
slit.
2) A drop of anesthetic is instilled into
the patient’s eye and a set fluorescein
paper is touched to the inside of the
lower eyelid. The patient is then
positioned as for microscopy,
3) The fluorescent semicircles are viewed
through the biprism or microscope. The
force against the cornea is adjusted
until the inner edges overlap and two
semicircles are equal in size. (fig.
21) The intraocular pressure is then
read directly from a scale on the
tonometer housing. Too small Too large standard
Figure 21; The pattern is seen through the microscope.
What is normal intraocular pressure?
According to Webster's Ninth New Collegiate Dictionary,
"normal" is explained as "relating to, involving, or being a normal curve or normal distribution." Therefore we can define "normal" intraocular pressure as a distribution in the general population relative to groups of individuals with glaucomatous damage of different pressure ranges.
In the past thirty years, several studies related to intraocular pressure distributions in general populations were reported . Table 4 shows the test results as measured with a Schiotz tonometer and applanation tonometer by two groups of investigators. Fig. 22 shows a distribution of intraocular pressure in non-glaucoma (N) and glaucoma (G) populations. Note the overlap between the two groups. Prevalence
10 15 20
IOP(mmHg)
Figure 22: A distribution of IOP in non-glaucoma(N) glaucoma(G) population 52
Table 4
Reported IQP Distributions General Populations
Investigators Number of Ages IOP Individuals (yr) Mean SD
A. Tested with Schiotz Tonometers
Leydhecker 10,000 10-69 15.5 2.57 et al.('58)
Johnson('66) 7,577 >41 15.4 2.65
Segal et al.(’67) 15,695 >30 15.3-15. 9 (women)
15.0-15. 2(men)
B. Tested with Applanation Tonometers
Armaly('65) 2,316 20-79 15.91 3.14
Perkins('65) 2,000 >40 15.2 2.5(OD)
14.9 2.5(OS)
Loewen et. al .('76) 4,661 9-89 17.18 3.78
Ruprecht et. al.('78) 8,899 5-94 16.25 3.45
Computed from data reported according to sex and age groups.
Hov Does Aqueous Humor Dynamics Influence Intraocular
Pressure?
The aqueous humor is constantly produced by the ciliary
body, and it normally flows from the eye through a tiny drainage system into the blood stream. It also creates the
intraocular pressure to prevent the cornea and iris from 53 collapsing. But, if this normal outflow becomes obstructed from any cause, the intraocular pressure elevates. In association with the elevated pressure, the optic nerve tends to become deformed at the site where it emerges from the eye. Gradually, some or all of its fibers may die.
Vision is lost in proportion to the nerve fiber loss, resulting in complete blindness in the worst cases.
The Optic Nerve Head And Visual Fields.
1. A brief overview of optic nerve head structure.
Optic Nerve Head
Retina
Choroid
S clera <
I
Figure 23; ' Divisions of the optic nerve head. A: Surface nerve fiber layer B: Prelaminar region C: Lamina cribrosa region D: Retrolaminar region The optic nerve head (Fig. 23) contains between 1.1 and 1.3 million fibers. The axons originate in the ganglion cell layer of the retina and converge upon the nerve head from all points in the fundus. Most of these axons are concerned with vision: some provide a pathway for the pupillary light reflex, others relay general responses to light. Within the nerve head, the axons are grouped into approximately 1,000 bundles and are supported by astrocytes. The optic nerve head is also the site of entry and exit of the retinal ves sels. This vascular system supplies some branches to the optic nerve head, although the predominant blood supply for the nerve head comes from the ciliary cir culation. Three neurons form the visual path way from the retina to the occipital visual cortex. The first of these neurons is the bipolar cell that is complete ly within the retina. The second neuron is the gan glion cell that lies in the inner retina. Its axons extend through the optic nerve to the chiasm and end in the lateral geniculate body. The third neuron extends from the geniculate body to the occipital vis ual cortex. The rods and cones are the sensory recep tors in the pathway, their outer segments contain pho topigments which initiate the excitatory visual responses to light. Visual Field 55
bs
> . _,<< N?<. .* ‘
Figure 24: "Island of vision surrounded by a sea of blind ness" (A) the profile of normal visual field; (B) (f) corresponds to the fovea and the blind spot(bs), of the optic nerve head.
The visual field is the portion of space in which
objects are simultaneously visible to the steadily
fixating eye. Scott(1957) used the term "visual
field" described by Traquair who compared the visual
field to " an island hill of vision surrounded by a
sea of blindness"(Fig. 24). It(visual field or the
island hill) follows that small objects are seen with distinctness within the visual field only when small objects are near the visual axis and that larger and brighter stimuli are required if they are to be per ceived in the peripheral field.
The surface of the retina receives an image of objects in the visual field in much the same manner as a photographic film records a landscape. The nerve fibers of the retina are arranged in a pattern that is repeated in the visual pathway from the optic nerve through the chiasmal decussation and onto the visual cortex in the occipital lobes. The visual field cor responds to an inverse cross section of the visual pathway at any point from the receptor organ in the retina to the terminal center in the occipital cortex.
Thus any defect or abnormality of the visual field may reflect desease or damage to a specific portion of the visual pathway.
Normal Visual Field
The normal monocular visual field is a slightly irregular oval centered on the point of fixation. It is approximately 60 degrees upward and 60 degrees inward, 70 to 75 degrees downward and 100 to 110 degrees outward (Fig. 25) The whole binocular field forms a rough oval extending to about 200 degrees lat erally and 130 degrees vertically.
Examination of The Visual Field Figure 25: Normal isopters measured with Goldmann perime ter
The examination of the visual field is geared to
the patient's responses. A device which is used to
examine the visual field is called a perimeter. The
Goldmann perimeter has become the standard instrument
throughout the Western world,since it embodies so
many desirable features that the others do not have.
An example of an excellent set of specifications for a
perimeter is found in the descriptive brochure written
by Goldmann and supplied with the instrument by the
manufacturer:(Harrington, 1981)
It is a spherical projection perimeter with a recording device. A Nitra lamp illumi nates a circumscribed peripheral area above and inside the bowl which is of 300mm radius and painted matte white. The lamp is shaded from the rest of the hemisphere by a hood. A portion of the light is sent by a condens ing lens through a hollow lever arm con taining the projection system for the perim eter target. By this means a slight variation in the brightness of the lamp effects background and target luminosity equally. The movement of the projection arm is produced by a pantograph, controlled by handle which slides on a vertical plate of opal glass illuminated from behind on the back of the perimeter. This plate carries the recording chart. Each position of the handle corresponds exactly with the position of the spot of light on the hemisphere. By slowly moving the handle across the surface of the chart, the visual fields may be examined for 95 degree on each side of fix ation. A telescope through the back of the hemisphere allows for constant observation and control of the patient's eye. In it there is a light of variable size,fixation point. A slide allows the target to vanish and reappear noiselessly. The projected targets are ellipses of varying size from 1/16 mm to 64 mm and are easily changed by stops. A series of neutral filters permits geometric reduction in the luminosity of the targets from 100 milliamberts to 3.16 mill- ilamberts. The basic luminosity of the tar get has been fixed at thirty-three times that of the background and a photometric device is incorported in the instrument to ensure the constancy of this ratio. If a perimeter does not automatically keep this contrast between background and target lumi nosity a constant, it is unusable for exact relative perimetry. Previous projection per imeters have not done so. Hitherto such a constancy has only been found with perime ters using paper targets.
Because of the importance of maintaining this ratio of background to target luminos ity, the photometric adjustment of the per imeter should be effected before each exami nation, and after the patient is seated before the instrument since lighter or dark er face and clothes can alter the background luminosity by several percent. 59
5. The Correlation Between Optic Nerve Head and Visual
Field Defects
Several large studies reported that the ability to
correctly predict the presence of established field
loss on the basis of disc appearance ranges from 66%
to 85% (Shutt, 1967? Drance, 1976,1978? Hos
kins,Gelber, 1975? Hitchings, Spaeth, 1977). Inmost
cases, disc changes precede detectable field loss
(Drance, 1978 ). In conclusion, even the correlation
between optic nerve head and visual field defects in
glaucoma is not perfect, but optic nerve head changes
may possibly have their greatest effect in the early
stages of glaucoma.
Color VisioD-Pefect and Glaucoma
Color vision defects originate from two completely dif ferent causes. When the defect is characterized by a pri mary abnormality of color vision without organic disease,it
is usually called a "congenital", or hereditary defect.
Contrary to the congenital defects, color vision abnormali ty that accompanies another disorder is usually called an
"acquired" defect.
A number of differences between congenital and acquired color vision defects are summarized in Pokorny & Smith's
"Congenital and Acquired Color Vision Defects"(p.72). 60
Table 5 is the summary of these differences between conge nital and acquired color defects.
Table 5
Differences Between Congenital and Acquired Defects
Congenital Acquired
Usually color discrimination Often no clear cut area of loss in specific areas discrimination loss
Less marked dependence of Marked dependence of color color vision on target vision on target size and size and illuminance illuminance
Characteristic results Conflicting or variable obtained on various results may be obtained clinical color vision on various clinical color tests. vision tests.
Many object colors are named Some object colors are correctly or predictable named incorrectly error are made
Both eyes usually affected Two eyes sometimes affected to some degrees to different degrees
Usually no other visual Sometimes a visual complaint complaint e.g. reduced visual acuity or field loss
Defect does not change Defect may show obvious progression or regression
Much evidence (Krill & Fishman, 1971, Pokorny & Smith,
1979) shows that acquired color vision defect also develops in glaucomatous patients. Moreover type III acquired violet- yellow defect is the most frequent defect. It is 61 characterized by mild or moderate loss of discrimination on the violet- yellow axis of the chromaticity diagram. In other words, it is "confusing" to distinguish colors in the violet region of the chromaticity diagram for the early stage glaucomatous patients. Meanwhile the visual acuity deficit is also similarly mild or moderate. Type III defect also relates to aging which primarily causes discoloration of the lens and may also involve the retina and pigment epithelium changes. When glaucoma is in an advanced stage, the vision is dichromatic and there is a neutral zone at about 550nm(Pokorny & Smith, 1979, p.80).
Clinical Color Vision Testing Methods
Clinical tests are designed for easy and rapid evalua tion of color vision. Some of the typical tests used for clinical examination of color vision will be reviewed in this section to help us understand the purpose, accuracy, and limitation of these tests.
1. Plate Test---
It is the most popular color vision test, because
it is quick and easy to use. In a plate test the
observer is usually required to read or identify a
figure or a digital number embedded in a background.
The Ishihara plate test is the most popular screening
test for congenital protan and deutan defects, and it is generally accepted as the best pseudo isochromatic
test for this purpose. In this test, an introductory plate is*provided for both the standard section of the test (24 plates) and for the illiterate section (12 plates). No scoresheet is provided, and the instruc
tions to the examiner are minimal. The Ishihara plate test is excellent for screening patients with both slight and severe red-green defects from the normal population. However, compared to the Nagel, it fails to detect many red-green anomalous observers.
Farnsworth-Munsell 100-hue test (FM 100-hue test)---
The 100-hue(Farnsworth,1943) test was designed to test hue discrimination among persons with normal col or vision and to evaluate chromatic deficiency in observers with congenital color vision defects. In
1956, the 100-hue test was applied as an acquired col or vision defects test by Francois and Verriest
(Pokorny and Smith,1979,p.93)
In the 100-hue test, there are four boxes contain
ing 85 movable color caps which are numbered from 1
(Red) to 85 (Redish Purple) (Box I numbers 1-22; Box
II numbers 23-43; Box III numbers 44-64; Box IV num bers 65-85) on the back, according to the ideal color order of hue circle. The colored caps are randomly ordered on the upper lid in each box. Only one box is presented at a time. The subject is required to arrange these colored caps in order according to the two end fixed colored caps of the box. Under usual testing circumstances, the subject is allowed 5min/box to complete the task. Then the cap sequence is recorded, using the numbers on the reverse side. Com pared with standard cap sequence numbers, the error score for each cap is calculated as the sum of the absolute difference between adjacent caps, minus 2
(note: the sum of the absolute difference between standard sequenced adjacent caps is 2). Then the error score for each cap is plotted as a distance on a radial line. On the inner circle, the standard sequence numbers are marked in equal distance (as shown in Fig. 26).
Anomaloscope Test---
An anomaloscope is an instrument designed to evalu ate specialized color matches and to diagnose color defect in a large population. It contains one test color and three primaries. Usually, the observer is told to adjust the three primary lights and make one primary and the .test color to match the mixture o>f the remaining two primaries. For the "normal" observers, they can match all hues by appropriate adjustment of three primary lights. Since the purpose of the anoma- 64
/S' 3 0 /7 /// 19 /6 « /8 yf Vs y* yf if yr v &
Figure 26: Section of a subject's profile illustrating how error scores are plotted.
loscope is to screen out those observers with color
defects (anomalous trichromat or dichromat) from nor
mal observers, two primary color matches are made for
relatively quick and easy use instead of three prima
ries color matching Appendix D.
Three color matching equations are commonly used in
anomaloscopes for measuring different types of
acquired color defects. The position and width of the
Rayleigh equation match(the conventional Rayleigh
match contains one test color and two primaries) allow
• Type I acquired red-green defect shows a moderate or 65
differentiation of the type I8 and type II* acquired
color vision defects. The Engelking Trendelenburg and
Pickford Lakowski equations are useful in evaluating
the type III acquired color defect.
A Possible Color Vision Testing Method Applied to Glaucoma Petec.ti.pn..,at .Early S.tage
As has been mentioned before, color vision defects on glaucomatous patients start from violet yellow defect which is type III acquired color vision defect. Therefore, there is no doubt that the plate test is not suitable for early glaucomatous detection.
The FM 100 hue test is a very, sensitve clinical test of a minimal defect of chromatic discriminative ability. It can reveal a faint violet- yellow defect with its monopolar bulge character which is a valuable guide to trace the course of the defect. A disadvantage of the FM-100 hue
severe loss of the chromatic discrimination on the red- green axis and also accompanies a parallel defect in vis ual acuity. In an advanced stage, a totally color blind ness occures in the affected visual field. It is primarily a retinal disease involving the photoreceptors of the posterior pole.
• In addition to the signs revealed in Type I acquired color vision defects, Type II also shows a mild violet yellow loss. The optic nerve involvement such as optic neuritis, retrobulbar neuritis, optic atrophies; and optic nerve abnormalities such as malformations of the optic disc, tumors in the optic nerve or chiasm are the common causes of creating Type II color vision defect. (Congenital and Acquired Color Defects; Pokorny and Smith, 1980) 66
test is that , in general, it is rarely performed well by the children under 12 or some adult patients. To those observers with visual acuity of 0.2 or less , the FM
100-hue test also presents some degree of difficulty.
The anomaloscope is also a very sensitive clinical color vision test instrument. By using different primaries, it can detect different types of acquired color vision defect.
In comparsion with the FM 100-hue test, the anomaloscope is even more sensitive and accurate for detecting color vision defects. It also is less affected by the age of the observer (Kinnear,Aspinall, Lakowski,1973; Airaksi- nen,Lakowski,Drance,Price, 1986). APPENDIX B
STANDARALIZING THE COLOR MATCH
The following steps show how to obtain the match :
1. Use the computer to set a proper green light on the
left-half side of the bipartite field while making the
luminance comfortable to the observer.
2. Using flicker photometry produce an equal luminance of
violet and green on the same half field.
3. The above flicker match is done by 2 optional control
pots which are described in the previous sections.
One pot controls the motor speed, the other pot con
trols the violet luminance.
4. Then the computer memorizes the values of the voltages
applied to both the green and the violet channels and
counts them as 1 unit individually. For example, if
the computer senses 2V reference voltage on the green
channel and senses 2.5V from the control pot of the
violet channel for matching the green, then the com
puter counts that 1 unit of green as 2V and 1 unit of
violet as 2.5V. In order to produce the same amount
of light, the violet needs a higher reference voltage
- 67 - than does the green . In other words, under the same energy conditions, the green seems more bright than the violet. Now, no matter hotf the ratio of the violet and the green changes, the total amount of luminance will not change on the left-half side of the bipar tite field. This means that if the green luminance decreases 1/x of one green unit, then the violet lumi nance increases (2- 1/x) of one violet unit. These calculations and controls are performed by the comput er.
Following the above procedure the brightness-match test is made. Keeping the values of the green-violet on the left-half side of the bipartite field, the observer is asked to use pure yellow on the right- hand side of the bipartite field to make the bright ness match. Therefore the mixture side is matched to the yellow side to determine the luminance of the mix ture side.
After the brightness match is done, the yellow and blue channels should be used in the final 4-primaries match. In this step, one of the two control pots acts as the yellow and blue ratio control. Meanwhile the other pot performs the simultaneous luminance control with the same ratio on both yellow and blue. In other words, this pot operates like an "electronic neutral density filter" for this case. Therefore, the two con trol pots are responsible for the color and luminance control on the right-half side of the bipartite field.
In this way, the whole process can be done very accu rately by the computer.
At this point, the observer should increase the motor speed until the flicker is not seen on the left-half side of the bipartite field. No matter how he/she changes the green-violet ratio by turning the one optional control pot,the brightness will not be changed. In the meantime, the observer also adjusts the yellow/blue ratio and its luminance making both fields match.
After repeating the above procedure several times in order to obtain an accurate match, all the data from the match are stored on a disc. Several normal observers are needed in order to get a midmatch point.
By averaging the above data, the yellow and blue ratio is obtained for normal people. The two end points of green and violet are also averaged from the flicker matching data for normal people.
The fixed yellow and blue ratio and the two end points of green and violet are put into the computer program as parameters. They are not changed unless it is nec essary. APPENDIX C
THE CONTROL PROGRAM
As the following flow chart shows the control program plays the most important role in the entire system. In the beginning, the four channels have to be initialized by put ting proper values into the control program. These values are the two end points of the violet and the green and val ues for the ratio of yellow and blue. The program causes the computer to read values from control pots, send calcu lated values to the inputs of the control circuit, monitors the push buttom to determine whether it is pressed or not and stores data.
- 70 - 71
Start; Loading subject's name, age, sex, & date
Initialization for 4 channels
Read values from two control pots & send out the calculated results to proper inputs of control circuits
^ — Has No the push buttom been pressed
Yes
/ Store x Yes data into compute and display "need more test"
No
END 72
100 GOSUB 1300 110 DIM YY(10),C(10),GB(10),BH(10),K(3)rB(10),G(10) 120 PA=56576:PB*56577:DA=56578:DB*56579 130 HI=7:LO=3 140 NUM“0:AD=4:GOSUB 1000:AD=5:GOSUB 1000 150 AD=6:GOSUB 1000:AD=9:GOSUB 1000 160 GA=80:BA=136 170 MH=0:BY=109:YI=6 180 AD=0:GOSUB 1200:GOSUB 1100:NF=X:CK=NF/200 185 ADs2:GOSUB 1200:GOSUB 1100:BI=X 188 IF BI>255 THEN BI=255 190 NUM=BI:AD-4:GOSUB 1000 200 NUM=INT((2-BI/BA)*GA):GI=NUM:IF NUM>255 THEN NUM=255 210 AD=9:GOSUB 1000 212 NUM=BY*CK:AD=6:GOSUB 1000 214 NUM=YI*CK:AD=5;GOSUB 1000 225 PRINT"YI="YI;"BY="BYiPRINT 230 PRINT"GI="GI;"BG="BI?"CK="CK 240 FLAG=PEEK(56589) 250 IF FLAG<>16 GOTO 180 253 GBR=(GI/GA)/(BI/BA) 255 G(MH)=GI:C(MH)=CK:GB(MH)=GBR:B(MH)=BI:BH(MH)=BY: YY(MH)=YI 260 GOSUB 1500 280 PRINT "MORE TEST? Y OR N" 290 INPUT"";D$:IF D$="" THEN 290 310 IF D$="N" THEN 340 320 IF D$="Y" THEN 330 MH=MH+1:GOTO 190 340 PRINT:PRINT:PRINT"END OF TEST" 400 OPEN 3,8,2,"0:"+A$+"fS,W" 410 PRINT#3,A$","AG","B$","YE","MO","DY","MH 430 FOR L=0 TO MH:PRINT# 3,YY(L)","C(L)","GB(L)"," BH(L)","B(L)","G(L):NEXT 440 CLOSE 3 520 PRINT"NAME:"A$ 530 PRINT"AGE:"AG 540 PRINT"SEX:"B$ 550 PRINT"DATE:"YE","MO","DY 555 PRINT 610 PRINT 620 PRINT" YY B1 C G/B2 G B" 630 FOR L=0 TO MH 650 PRINT""YY(L)TAB(4)BH(L)TAB(10)C(L);GB(L);G(L);B(L) 660 NEXT 680 END 990 AD=9:FOR NUM=0 TO 255 1000 POKE PA,HI:POKE DA,63:POKE PB,AD 1010 POKE DB,255:POKE PA,LO:POKE PB,NUM 1020 POKE PA,HI:RETURN 1050 POKE PA,HI:POKE DA,63:POKE PB,AD 1055 FOR N-0 TO 2:NEXT 1060 POKE DB,255:POKE PA,LO:POKE DB,0 1065 FOR N-0 TO 2:NEXT 1070 POKE PA,HI:X=PEEK(PB):RETURN 1100 Z»(K(l)*K(2)*K(3))/3 1110 COUNT-1 1120 FOR 1-1 TO 3 1130 IF K( I )>Z THEN COUNT-COUNT+1 1140 NEXT 1150 ON COUNT GOTO 1160,1160,1180 1160 X-INT(Z) 1170 RETURN 1180 X-INT(Z)+1 1190 RETURN 1200 FOR L=1 TO 3 1210 GOSUB 1050 1230 K(L)=X 1240 NEXT 1250 RETURN 1300 INPUT"NAME";A$ 1310 INPUT"AGE";AG 1320 INPUT"SEX";B$ 1330 INPUT"DATE";YE,MO,DY 1340 RETURN 1500 S-54272 1510 FOR L-S TO S+24:POKE L,0:NEXT 1520 POKE S+5,9:POKE S*6,0 1530 POKE S+24,15:POKE S*l,128 1540 POKE S+4,33:FOR K-0 TO 40:NEXT 1550 RETURN 74
In the program as shown above, line 100 refers to line
1300 where it starts to load the subject's personal infor mation such as name, age, sex, testing date, and ends’ at line 1340. In this routine, all the test results, stored in the disk, will be under the subject's name. In other words, the subject's name is the data file name. Based on this, a data base is built.
Line 110 is a command of reserving memory space. All the test results or a temporary data storage in a routine are stored in these memory spaces. According to the com puter itself, the address of the I/O port and the data-in or data-out of the I/O register are defined in line 120, where PA is the address of port A of the I/O register, and
PB is the address of port B of I/O register. Because of % the operation of the computer, port A only provides one bit as a control line. Under a proper digital signal vari ation at the proper time on this line, the computer is able to communicate with the outside environment through port B.
Line 130 indicates that two proper numbers should be put into port A of the I/O register. These are put in at dif ferent times in order to get a proper digital control sig nal on the control line at port A. Thereafter, the ini tialization for the four channels is done on line 140 and
150 through a subroutine starting on line 1000. 75
In line 160 and line 170, GA, BA, BY and YIare the matching values of the 4-primaries of the anomaloscope
from three very experienced observers. These values are permanently put into the program and serve as a fixed
yellow/blue ratio and two end points of green and violet.
MH here functions as a counter whose major work is counting
the frequency of the data being stored in the reserving memory space. This implies that the computer needs to know how many "packages" of data in the sequential data file should be transferred from the disk to the computer when
ever it is needed in the future.
From line 180 to line 230, the computer starts to "read"
the values of the two control-pots and sends the calculated values to the inputs of the brightness-control circuits and
then prints out the results on the screen. When a "criti
cal point" is found , a push buttom should be pressed by
the examiner. Here, on line 240, the flag reflects whether
the "critical point" is found or not. When the push buttom
is pressed, the value of flag is 16, otherwise, it is 0.
Line 250 determines whether the computer continues to moni
tor the flag or stores the data in the reserved memory
space. The decision is made by line 250. If the flag val
ue is 16, then the data is stored and a ring (provided by a
subroutine from line 1500 to 1550) is generated. However,
if the flag value is not 16, the computer has to do the 76 whole subroutine which starts on line 180 again, until the flag value is 16 (or the push buttom is pressed).
As is shown in the program flow chart, when the examin er finishes the test by typing "N" after the "more tests?" is displayed, the entire data in the reserved memory space are automatically transferred to the disk and these data are also displayed on the screen. Line 280 through line
680 serve the whole procedure. In this program, two very important subroutines, lines 1000 to 1020 and line 1050 to
1070, serve as "reading" (values) from control-pots and
"writing" (values) to the brightness control circuit inputs routine. Lines 1100 to 1190 and lines 1200 to 1250 perform the averaging subroutines which can minimize the possible noises from the two control- pots before these data are used as parameters in calculations. APPENDIX D
TRICHROMATIC COLOR THEORY
Many researches have investigated the physiological basis of color vision. One of the earliest findings reported was that under scotopic levels of illumination, when only the rods are activated10
The first hypothesis regarding color vision was suggest ed almost 200 years ago by Thomas Young (1773 -1829).
Young suggested that only a few different types of retinal receptors, operating with different wave length sensitivi ties, would be necessary to allow human to perceive the number of colors they do. He further suggested that per haps as few as three would do. His theoretical notion was revived in the 1850s by Helmholtz. At that time, Helmholtz and Maxwell showed that normal observers need only three
10 Some evidence indicated rods were found predominantly in animals that are active during the twilight hours (or in conditions of dim illumination), while cones are found predominantly in the retinas of animals that are active during daylight. It was suggested that cones function to provide photopic or daylight vision (includ ing the'perception of color), while rods provide scotop ic or twilight vision.
- 77 - 78
primaries to match any color stimulus. These data were
taken as evidence for the presence of three different
receptors in the retina. Since the usual color matching
primaries consisted of a red, a green and a violet, it was
presumed the there were three types of receptors, one
responsive to red, one to green, and one to violet.
Although the data from color mixing and color defects
seem to support a trichromatic theory of color vision, direct physiological evidence for three cone pigments did
not appear until the 1960s. The most elegant measurements were done by Marks, Dobelle, and MacNichol (1964) and Brown
and Wald (1964). Their procedure was conceptually simple, but technically quite difficult. It involved a device
called a Microspectrophotometer. With this device a tiny spot of monochromatic light is focused upon the pigment- bearing outer segment of a cone. Now tiny amounts of light of various wavelengths are passed through the cone, and the
amount of light absorbed at each wavelength is measured.
The more light of a given wavelength Such measurements were taken using cones from the retina of goldfish, monkeys
and finally on cones from the retina of humans. The
results were unambiguous. There were three major groups of
cones. One had a maximum absorption in the range 445-450
nm, another in the range 525-535 nm and the third in the
range 555-570 nm. 79 QPPQNENT-CQLQR THEORY
In the late nineteenth century, a German physiologist
Ewald Hering was not completely satisfied with the trichro matic theory of color vision. He developed a viewpoint about color vision that was largely based upon the subjec tive appearance of colors rather than upon physical experi ments like those of Maxwell, that had involved only color matching. It seemed to him that human observer acted as if there were four, rather than three, primary colors. For instance, when observers were presented with a large number of color samples, and asked to pick out those which appear to be pure (defined as not showing any trace of being a mixture of colors), they tended to pick out four, rather than three colors. These unique colors almost always included a red, a green, and a violet, as trichromatic theory would predict; however, they also include a yellow
(Bornstein, 1973). Boynton and Gordon (1965) showed that with the color names red,yellow, green and violet, English- speaking observers can categorize the entire range of visi ble hues.
Hering also looked at another aspect of the subjective experience of hue. He noted that certain color combina tions were never reported by observers. For instance, one could not have a yellowish-violet, nor could one have a greenish-red. This led Hering to suggest that the four primaries were arranged in opposing pairs. One pair would signal the presence of red or green, while a separate pair would signal violet or yellow. This might be accomplished by having a single neuron whose activity rate increased with that of one color (red) and decreased in the presence of its opponent color (green). Since the cell's activity could not increase and decrease simultaneously, one could never have a reddish-green; A different opponent process cell might respond to violet and yellow. A third unit was suggested to account for brightness perception.
At the time when Hering first suggested an opponent- color mechanism for the neural encoding of hue information, there was no physiological evidence to support such a spec ulation. The first evidence that different wavelengths of light could cause opponent effects in neural response was offered by Svaetichin, who inserted an electrode into the cell layers of the retina of the goldfish. When he record ed the responses to light, he found that they varied depending upon the wavelength. He found that not only did the strength of response vary as one changed the wavel ength, but also more importantly the polarity of the elec trical response was different for long and short wavel engths. Figure 27 shows the pattern of responses recorded by Svaetichin and MacNichol(1958). Notice the spectral sen sitivity of the unit marked. 81
♦ 6 4-4 + 2
400 500 GOO 700 (/) Wavelength of 75 stimulus (nm) o> u e Blue-yeilow response 2 +6 0>C + J a *2 600 c 0 400 500 700 2 -2 2 -■> "O — 6 5 3 Rea-green response « +4 | +2 n 0) 0 400 500 600 700 c -2 - 4 -6 -8 Luminance (L) response
Figure 27; Graded electrical response of retinal cell to various wavelengths of light
For R-G and B-Y, the cell marked R-G would respond with a large positive signal if the unit is stimulated with a long-wave length light (650 nm). This positive response would signal red. If the unit was stimulated with a green ish hue (500 nm), it would give its negative response. If the unit was simultaneously stimulated with both red and green, the positive and negative response would cancel each other and no signal would result. It also displayed the same result to the B-Y cell. Notice the cell marked L-in 82 the figure. It simply responded to the intensity of the light regardless of wavelength. This could be the black- white cell hypothesized by Hering.
How can a four-primary, opponent-color system exist when one has already provided physiological and psychophysical evidence indicating that the retina operates with a three- color pigment system? The opponent-color and trichromatic theory competed for a long time. The psychologist G. E.
Muller and the physicist Erwin Schrodinger (Boynton, 1979) worked out plausible zone theories to handle the problem.
According to such models, the trichromatic first stage leads next to an opponent-color stage. For further stud ies of photo-receptors, horizontal cells, lateral genicu late nucleus in electrophysiology, the Muller and Schro- dinger's zone theories were accepted by most of the investigators in this area. Actually, in 1974, Hurvich and
Jameson suggested a NEURAL WIRING DIAGRAM (Fig. 28) that indicated the way in which cones, each containing only one of three pigments, would produce opponent responses at the retinal level.
By now, the two most important theories, trichromatic theory and opponent-color theory, have been introduced in the beginning of this chapter. A more detailed discussion about this, such as "how do those signals project to the brain?" is not in the scope of this study. 83
f "Blue"\ '"Green") I "Red") cones ' Cones cones (440 nm) (530 nm) (570 nm)
W-BIk B-Y R-G
Figure 28; Schematic diagram indicating how a three- pigment system might be connected to produce an opponent-process neural response
COLOR MATCHING AND THE COLOR EQUATION
In 1758, a German mathematician Tobias Mayer produced a color classification tht to some extend is still in use.
He chose red, yellow, and violet as his primary colors. In viewing transitions from one color to another, each sepa rate step must be distinguishable but not so large as to appear as a sudden change. Mayer concluded that the num ber of stages was 12. In other words, he felt that 12 transitions between colors were needed to get the smallest detectable stage. This means that the colors mixed from the 3 primaries (each primary has 12 units) have 91 combi 84 nations. This total of 91 tiny distinguishable colors was built into "color triangle" (Fig. 29) with the 3 primaries on the corners. This is the origin of the famous triangle of colors used later by Young and Maxwell and still found in elementary science textbooks. The 91 different colors in this scheme seem rather small compared to nature's mul tiple hues. The reason is that the system doesn't include brightness in the various hues.
Figure 29: Triangular classification of colors by Mayer using red, yellow and blue primary colors. Color graduations take place in steps of one part on a scale of twelve parts. 85
It (the color triangle) compounds only varieties of the pure saturated primaries. Mayer recognized this require ment and allowed the primaries to move toward white to produce desaturated colors. Therefore, on the scale of 12 he diluted each primary toward white by one step so that there was one part white plus eleven parts color. Also, this resulted in fewer separate colors because of the diluted primaries. Under this condition, the diluted pri maries then formed a new color triangle containing 78 sepa rate color elements. Then Mayer repeated the process and finally gathered 91 ♦ 78 + 66 * 55 ♦ ... ♦ 1 = 455 color elements. This was a number that Mayer recognized as a pyramidal number of side 12. The base of the pyramid is the original color triangle and its apex is white. A simi lar argument applied to darkened shades and yields another pyramid with a black apex. Therefore, there were 819 (91 ♦
2 * (78 + 66 ♦ 55 +...♦ 1) =819 ) separate colors altogeth er, presumably sufficient to represent a great variety of subjects.
So far in this history of the underlying problem was that of determining the composition of mixed colors. The physicists always assumed that mixed color of lights behaved the same as mixed pigments. In the middle of the
19th centruy, this concept was refuted by Helmholtz. In his experiment, he used a viewing telescope to observe the 86 mixtures of spectral colors which were created by a V-shape slit and a prism. Helmholtz's results were amazingly dif ferent from his predecessors' assumption. A very simple example was that Helmholtz mixed yellow and violet light and resulted in white. In contrast, the compound of yellow and violet pigment was green. This showed that the mix tures of pigments and mixtures of lights obey quite differ ent laws.
Under Grassmann's prompt, Helmholtz further studied to the problem of spectral complementary color. During that time, a superior color mixing method was published by Leon
Foucault. Helmholtz employed the new method and determined seven pairs of complementary spectral colors. In addition to these results, Helmholtz also estimated the relative luminances of the complementary colors and pointed out that they need not be equally bright. Combining the complemen tary spectral colors and their relative luminances, Helm holtz proposed a hypothetical curve of the locus of spec tral color (Fig. 30). This curve served, in effect, a heuristic function compared to the original color triangle.
In the mean time , the young James Clerk Maxwell was also a first year college student at Cambridge as was Helm holtz. He was influenced by his professor J. D. Forbes very much. Maxwell said,, " the experiments of Professor
J. D. Forbes, which I witnessed in 1849, first encouraged 87
Violet Purple
Figure 30; Helmholtz's hypothetical curve of the locus of spectral colors
me to think that the laws of this kind of mixture might be discovered by a special experiment." At that time Forbes had attempted to form color equations and had made the surprising discovery that yellow and violet yielded grey rather than green on the rotating disc. Forbes ,however, did not form color equations until four years later.
Meanwhile Maxwell chose 3 primaries, vermilion (V), ultra- marine (U) and emeraldgreen (EG) to form a nuetral grey which was "perfectly indistinguishable" from some combina tion of snow white (SW) and ivory black (BK) on his rotat ing colored papers. From the scaled color sector, he formed the equation :
0.37V ♦ 0.27U + 0.36EG = 0.28SW ♦ 0.72BK (D.l) 88
Then he kept two variables on the left hand side of the equation (D.l) and substituded for the third as in the fol lowing case, for instance, a brighter grey equation was:
0.34PC ♦ 0.55U ♦ 0.12EG * 0.37SW ♦ 0.62BK (D.2)
where PC ■ pale chrome
There is no doubt that the above two equations are just an achromatic match. Of course, it is certainly possible to perform experiments that produce a definite color rather than grey. An example of a color match was:
0.39PC + 0.21U ♦ 0.40BK - 0.59V ♦ 0.41EG (D.3)
Following the above experiments, Maxwell tried to use this color construction mechanism to predict new color equations. First, he placed the three standard colors, vermilion, ultramarine, and emeraldgreen at a vertex of an equilateral triangle. All colors composed of these three must be points within the triangle. Therefore, through equation (D.l), Maxwell found the "white" point to be the center of gravity of the three standard color points.
Next, he used (D.l) as a standard equation, and substituted equation (D.2) to equation (D.l). Because of different brightnesses on the right hand side of this equations, the 89 white values must-be normalized and recalculated to weight the ratios (or coefficients) of each color. He finally located the position of PC (pale chrome) outside of-the triangle in terms of the standard colors from his color equation (D.2). Notice that in Mayer's color pyramid, the hue, saturation, and brightness required a 3-dimension space. But with this normalization procedure. Maxwell had effectively succeeded in plotting the 3-dimensional data in
2 dimensions. This concept was crucially important to the emergence of the modern chromaticity diagram.
After the results from 10 observers in his experiment in
1854, Maxwell's predictions of his color construction agreed well within the observational limits. He concluded the following:
1. That the human eye is capable of estimating the likeness of colors with a precision which in some cases is very great.
2. That the judgement thus formed is deter mined, not by the real identity of the col ors, but by a cause residing in the eye of the observer.
3. That the eyes of different observers vary in accuracy, but agree with each other so near ly as to leave no doubt that the law of col or vision is identical for all ordinary eyes(Sherman, p.169, 1981). 90
THE CIE SYSTEM
Even though Maxwell did these most important psychophy
sical experiments on color matching more than 100 years
ago, it was not until 1931, that color could be precisely
specified on a scientific basis. In that year, the Inter
national Commission on Illumination (CIE) adopted a system
of color specification which has lasted to the present
time. Their data was based on data similar to Maxwell's measurements and calculations. A standard set of color- mixture values based mainly on extensive experimental
investigations by W. D. Wright and J. Guild in England, was
adopted. The idea was:
to reduce any spectral radiance distribution to only three variables, and to state that, for col or vision, any two stimuli describable by the same values of each variables, no matter how physically different, would be defined as color- metrically identical.(Boynton, 1979)
Therefore, the three spectral primaries for color-matching may be described by an equation:
c(C) ♦ r(R) = g(G) ♦ b(B) (D.4)
where (C) = test color,vector (R), (G), (B) = primaries,vectors c, r, g, b - scalar, which might be negative 91
The above equation should be read as : c units of test color (C) plus r units of the red primary (R), additively mixed to one half of the field (bipartite), exactly matches
(») g units of the green primary (G) plus b units of the violet primary (B), additively mixed to the other half of the field. For all the algebra work such as additive, mul tiplicative, associative and distributive laws can apply to the above color equation, therefore, the color-matching behavior is predictable.
By now, everything goes smoothly. However, in order to have the universal adoption by both commerce and science, a method of specifying colors is referred to the visual properties of a standard observer. To specify the color vision of the Standard Observer, the color-mixture results of a group of observers for a set of rigorously specified viewing conditions (2 degree field with dark surroundings) were averaged and smoothed. The curves were called color mixture functions and were accepted by the CIE in 1931.
The values r* , g*, and b&shown in the curves (Fig. 31) are known as distribution coefficients.
A concept called tristimulus value is introduced in order to provide a colorimetric specification of any stimu lus light. It is necessary to evaluate its effectiveness with respect to each of the three color mixture functions.
The tristimulus values are defined as follows: 92
310 20 ( r,g ) - chromaticity diagram 300
340
490' • 0 5 ■ 3/3 • (E) 4 /3 A . .too
-1.5 . o s 4 3 3 . (8) 05
-0.5 ■
Eig.ur.e-31: Chromaticity diagram of CIE 1931 standard colo rimetric observer in the system of real primary stimuli R.G.B.
R = J ea i* d*
G = J*EA gA d* (D. 5)
B “ *****
(where E the radiance distribution in the stimulus.)
Here E must be measured in physical energy units for every wavelenggth throughtout the visible range of the spec- trum. 93
The Chromaticity Diagram
Depending on the radiance level of the stimuli, the
tristimulus values can have any magnitude. Since hue and
saturation are approximately independent of luminance,
therefore a two-variable scheme that related to trichromat
ic units can be developed while at the same time factoring
luminance out of the system. This Chromaticity Coordinates
is defined as follows :
r = R/ (R ♦ G ♦ B)
g=G/(R+G+B) (D.6)
b = B/ (R ♦ G ♦ B)
Unfortunately, if we select any three real primary col
ors, a number of perceptual and mathematical problems
result. The major perceptual problem is the fact that
there are some colors that cannot be accurately represented within the triangle. Thus a pure spectral yellow can not
be represented, since as we know no color mixture can be as
saturated as the pure spectral color itself. To solve this problem, CIE selected three imaginary primary colors. They
arranged the primaries at the corners of the triangle shown
in Fig. 32. The imaginary primaries are more saturated
than it is possible to get with any real colors. 94
10 (V)
08 '910 130 ( x , y ) - chromaticity diagram 06 \i» 300 y 990 04 .900 • (E) 630 ’0 0nm 02
3*0, 430 (X) (Z) 02 0.4 0 6 0 8 1.0 X
Figure 32: Chromaticity diagram of CIE 1931 standard colo rimetric observer in the transformed system of imaginary primary stimuli X,Y,Z. REFERENCE
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