Simple Multispectral Photography and Additive Color Viewing

Home , Additive color

DONALDS. ROSS* Inte~nutionalImaging Systems Mountain View, CA 94040 Simple Multispectral Photography and Additive Color Viewing

FIG.1. 1% Multispedral Camera, Mark 1.

High-quality imagery is provided for routine interpretation, research and training without excessively stringent and costly specifications.

INTROD~ION technique is used is that with black-and-white films and conventional processing, the bene- OF THE literature on multi- Mspectral photography and additive view- fits of natural- and false-color films can be ing systems discusses work and equipmeht obtained rapidly at less cost. As normal black- and-white negative and positive processes are which is either very com~lex#Or apparenthi very simple. At one end of the scale, tech- used, more exposure latitude is possible. niques of microdensitometry, pattern recog- Moreover, a Eluch greater flexibility & avail- nition, image processing by computer, quar- able to the user in selecting hue and bright- ter-million dollar cameras and optical viewers, ness levels in the image being analyzed, to and similar sophisticated approaches are de- emphasize best the phenomena of interest. Where adequate sensitometric control is avail- scribed. At the simple end, many systems based on three or more off-the-shelf 35-mm able, multispectral images can be used for or hand cameras and conventional abridged s~ecbo-~hotomedth more ac- slide projectors have been discussed. curacy than is possible with color films.1 Either way, the impression is given that NEED FOR PRACTICAL SYSTEM much remains to be done before standard, economical multispectral photography and The scope and complexity of mult&~ectral additive display will be feasible for everyday research and development, and the technical rouljne tasks. is not the case; simple magnitude of programs such as ERTS, tend to production equipment is available commer- intimidate the average user in committing cia]ly and has been put into use in the united himself to simplified multispectral equipment States and many foreign countries during the and procedures. Certainly, a great deal of last three years. intensive research remains to be done to wring the utmost value from the potentials REASONSFOR of remote sensing; but this will be restricted M~TISP&CTRALPHOTOGRAPHY to a few centers, small groups of specialists AND ADDITIVE COLORVIEWING and large investments in facilities and equip- The chief reasons why the muItispectra1 ment. Simple systems must be developed to certain specifications, but care should be exer- Q presented at &e ~~~~~lConvention of he American Society of Photogrammetryin Wash- cised before accepting many of the ~utionav ington, D.C.,Mad 197'9. statements voiced by some, who wonld per- 583 ABSTRACT:A multispectral camera and additive color viewer system must satisfy certain optical, mechanical and performance specifications if it is to produce imagery of a quality, and in a form useful for aerial and earth-resources surveys, research, or training applications. The equipment shoz~ldprovide timely results on an economical basis in a convenient, standard form. Published information describes systems which are either prohibitively expensive for the average user, or simplistic to a degree which fails to meet the requirement. Some technical parameters have determined realistic specifications for an economical production multispectral system proven in wide use in the United States and many foreign countries.

suade us that excessively stringent conditions Multi-camera arrays use separate rolls of must be satisfied to make even simple multi- film, often of different types, in each camera. spectral systems efficient. Control over il- Processing times are usually different among lumination to a few percent in viewers, reso- the spectral bands, and accurate sensitometric lution and registration four or five times bet- control measures are multiplied by the num- ter than visual acuity, establishing exact GIE ber of negative films and positive prints in- color coordinates, narrow-pass camera filters volved. Discrepancies are apt to occur. The and dozens of spectral bands, are but a few headaches of search and retrieval in finding examples of requirements which are purely matching sets of spectral images among a academic for 99 percent of the image ana- number of rolls of film is, of course, well- lyst's work. What, then, are the parameters known. for establishing within economic limitations a multispectral camera-viewer system for every- MULTIPLE PROJECTORS day use? Here are some of the key determin- Several commercial projectors are usually ing factors. adapted in some kind of common mounting to project the spectral images in registration on a screen. Few projectors have built-in 70- mm roll-film facilities, and individually cut film chips must be mounted in each projector. Few projectors have lenses of matched Simplicity and economy are generally focal lengths, low distortion, good color cor- closely related. At first glance, the simplest rection, or project a uniformly flat field. solution is to use several good-quality com- Aligning images from three or four projectors mercial hand cameras and slide projectors in x, y and rotation, while attempting to cor- to form a multispectral system, and a number rect for scale differences in the camera by the of experimenters have taken this route. How- projector, are formidable tasks. It can be ever, it is soon found that the technical op- done, but few image sets can be handled at erating problems become quite complicated, one working session. the system is anything but simple, and stand- As with multi-cameras this solution is not ardized results are impossible to obtain. efficient or convenient. Among the major faults is camera mounting; image rotation is almost inevitable be- SIMPLE MULTISPECTRAL tween the several cameras and has to be cor- C4MERA AND VIEWER rected somehow during projection, as the eye The major problems discussed above are is very sensitive in detecting rotational mis- eliminated by the production Camera and registration. The most common problem is the Viewer system illustrated in Figures 1 and 2. difference between the scales of images taken The four lenses in the camera are mounted in different spectral bands. Production hand- in the same lens cone, ensuring absolute camera lenses of the same make, even though boresighting and completely obviating rota- well color-corrected, can vary by a few per- tion of images relative to each other, as all cent in focal length from lens to lens, and be- 3%-inch square images are exposed on a tween spectral bands. These errors must be single roll of 9%-inch film. The lenses have corrected if several images are to be regis- been selected in matched sets by their meas- tered as a composite. ured focal lengths in the blue, green, red and As with any other aerial photographic op- eration, elements of the system extend from image acquisition through to data reduction, and involve equipment, techniques and re- actions of the human analyst. These interact- ing elements require further discussion, to define key parameters.

CAMERA mTER BANDS How critical is the spectral position and band-pass of a filter; must special filters be used for detecting particular subjects? In general, the answer is that normal three-coIor separation filters and an infrared record will satisfy almost aU multispectral requirements. Terrain and ocean subjects have no unique spectral signatures in the same meaning that spectroscopists use; there are no narrow-band spectral spikes distinctly identifying elements and coIors one from the other, hut rather a gradual change in reflectance values, usually undulating at low amplitudes of radiance across the spectrum which the photo material FIG.2. 1% Mini-Addcol Additive Color Viewer. is capable of recording. An exception is the strong but broad-band infrared reflectance of broad-band infrared reflectance of chlorophyl- infrared spectral regions, eliminating the need lic substances. for scale corrections in viewing. Many subjects have marked reflectance dif- The characteristics of the film emulsion" ferences according to season, qualiiy of illumination, time of day, atmospheric Mid and are such that processing in conventional de- Rayleigh effects, polarization, rainy and velopers permits accurate color separation drought conditions, and other variables which and matching gammas for the blue, green and red images. The higher gamma in the may be unknown, unpredictable and usually cannot be accounted for. For exampIe, spec- infrared region accentuates chlorophyll effects. tral measurements made on a leaf may he quite different from those made on a tree fuIl All images remain together during repro- of the same leaves, or the wind may blow, duction, print processing and display, and the turning the underside of many leaves toward search and retrieval problem is minimized. the spectroradiometer, yielding a still difFer- The Viewer shown in Figure 2 only re- ent spectral measurement. The same plant quires x, y image motions for registering the species in a single scene may show large four images; however, a special adapter plate changes in spectral reflectance, because of is provided for ERTS, or other 70-mm im- changes in growth caused by differences in agery, in which images can also be rotated elevation, prevailing winds, local temperature for registration, Each of the four projection and available moisture. lenses is fitted with blue, green, and red Because of these unpredictable differences filters, as well as a clear position, enabling in spectral reflectance, coupled with unknown false-color combinations to be formed which atmospheric effects, using many, or narrow are not possible with color films. Other ver- spectral filter bands complicates rather than sions of this viewer have projection capabili- clarifies multispectral interpretation, and such ties. systems have been largely abandoned except for research under controlled conditions, For general interpretation, experience shows that " Kodak Infrared Aerographic Film 2424, recording the conventional blue, green and which is sensitive from the ultraviolet through red primaries satisfies all major multispectral the visible region, and into the infrared to about filter requirements, The addition of an infra- 900 run. red record permits the reconstitution of natural, false-color IR and a wide range of form and background. Recording spectral in- color effects. formation is also a part of this condition. At A set of spectral bands which yields ex- the limits of optical resolution, the lens fails cellent results is shown in Figure 3. The 57A to resolve an object point as a coherent filter has a passband similar to the ERTS-1bundle of energy, or at the limit of film reso- green RBV Camera, and the 25 filter to the lution the imaged point is scattered among RBV red record. The 88A extends further into grains of silver halide, or both effects com- the infrared than the equivalent ERTS-1RBV. bine. But the image carries spectral as well as spatial information. If spatial resolution IMAGE FORMAT AND RESOLUTION limits are exceeded, spectral energy informa- The camera (Figure 1) takes four 3%- tion is degraded, and the accuracy of spectral inch square images; each having an area 240 recording is affected. percent larger than a 70-mm foimat. A Multispectral photography should be flown magnification of 2.6x enlarges the image to at a scale large enough to ensure that impor- 9 inches square on the screen of the viewer tant ground spectral detail is imaged within shown in Figure 2. A 70-mm format would system resolution limits. have to be enlarged 4x to become 9 inches The contrast of subjects may also vary be- square. The greater magnification not only tween spectral bands, indirectly affecting ap- emphasizes the appearance of granularity, parent spatial resolution. A white object re- but requires a higher initial film resolution to flecting green and red energy equally, placed record the same spatial detail at the same on a green background, will have a lower image scales on the viewer screen. contrast ratio in the green than in the red Multispectral systems use Kodak 2424 In- spectral image. As the green background has frared Aerographic emulsion for the IR rec- little red reflectance, the red spectral band ord, which resolves 80 line/pairs/mm with a sees the white object as contrasting strongly high-contrast target and 32 l/p/mm at target against its background, but in the green contrast of 1.6:l. These are limiting factors record the contrast ratio will be reduced and are independent of lens focal length. The considerably by the energy reflected from resolution of the eye is 7 to 10 I/p/mm. the green background and the white object. At a magnification of 2.6x, 32 l/p/mm It is feasible, therefore, to obtain more become 12 I/p/mm on the viewing screen, than adequate spatial and spectral resolution but at 4x this drops to 8 I/p/mm. Retention from a simple optical-film combination, which of the higher resolution is important because will satisfy the great majority of multispectral the resolution of spatial detail is intimately interpretation tasks, as well as training and connected with the subject contrast, color, research.

400 450 500 550 600 650 700 750 800 Wavelength (nm)

FIG. 3. Typical Wratten filters used in the 12s Multispectrai Camera, Mark 1. Interference filters block the longer wavelength transmission of the blue, green and red filters. Other filters and passbands may be selected, such as an 89B on Band 4 which has 11 percent transmission at 700 nm and opens rapidly into the infrared region. SIMPLE MULTISPECTRAL PHOTOGRAPHY AND ADDITIVE COLOR VIEWING 587

tropical regions haze and strong chlorophyll Multispectral camera exposures (where the effects may require Band 1 to be exposed at same shutter exposes all bands at a given f/8, and Band 4 at fll6, at altitudes of speed) is adjusted by setting lens apertures. 10,000-20,000 ft. Exposure in each band should be sufficient Exposure determination is no more com- to place the subject reflectance range of in- plicated than with color aerial photography, terest on the straight-line part of the film but should receive the same attention. Once response curve for each spectral band. The a satisfactory balance is found, a table such balance should be such that a white object, as that in Figure 4 may be used. Aerial ex- reflecting equally throughout the 400-nm to posure calculators should be used only for 900-nm region has the same normalized determining sun altitude because film speed density in the processed negative. If exposure data and other factors are calci~lated for imbalance is excessive, some spectral records conventional aerial ~hotographyarid are not will have little density and others too much, applicable. making a reproduction balanced for viewing FILM PROCESSING, GAMMA AND clifficult to print. EXPOSURE BALANCE Based on extensive operational experience, the normal aperture settings for 2424-type Kodak 2424 emulsion, if processed as rec- film at 1/400 second are indicated in Table 1. ommended here will reproduce the blue, With certain conditions and subjects, a green and red images at almost the same change in aperture relationships may be ad- gammas (1.60), providing excellent color visal~leto optimize the multispectral records, separations. The infrared band has a higher or to facilitate reproduction. For example, in gamma, but this is advantageous in detecting and emphasizing reflectance variations in this invisible region. However, in balancing ex- TABLE1. NORMALAPERTURE SETTINGS FOR posure of the IR band relative to the others, K~DAKAENOGRAPHIC FILM 2424 AT the higher IR gamma must be taken into ac- 1/400 SECOND count. As the gammas between the first three bands and the IR are in the proportion 1.60/ Altitudes to: 1.80 = 0.89, the density of the white object 20,000 ft. 65,000 ft. in the IR band, multiplied by .89 should be Band 1-47B, blue f/5.6 f/8 equal to densities for the same object in the Band 2-57A, green f/4 f/5.6 other bands when relative exposure balance Rand 3-25, red f/4 f/5.6 has been achieved for all four. Film process- Band 4-88A, infrared f/ll f/ll - -- ing data are given in Figure 5.

FILhl POSITIVE The positive roll film transparency for ad- Expo~u~e ditive color viewing generaIIy should be Increase Shutter R~~~I~~~-g- Speed printed on a high-contrast emulsion such as r/sfopg FROM Kodak 2430 Aerial Duplicating Film, proc- Nominal '1400 essed to a gamma of 2.0 or more. The prod- uct of the film negative and positive print 1- - 1120~ gammas determines the degree of chromatic saturation obtainable in additive viewing, and 2- %- '/ID" should be 3.0 or more, as in most aerial color films.

ADDITIVE COLOR VIEWING Viewing any kind of color reproduction in- :'ill LLLVATIO:I ABOVE HORIZON volves many physio-psychological factors, and ',oIat altltudr tahlci for latltudo, t1r.p of day and month it is nat~~ralthat specification of additive ill" 1"l"O 1" ~xno;,," ralrllniori ;rrh a5 ti,, Kodai nprlri color display and control leads to c:ontroversy. "0"0 " T)." r,rr" 1s di.rlvr.d irn,,, $""Lh,, Frl t, The inclination, in making trade-offs between ,I# <$ ',,,"#8, 'Pnr,al Cd!W?, ' X""\,,r# ?,,It, solar AIt,,,,,i@' , economy and precision, is to lean heavily to- 71 n' 1, I 'hi. ward precision to an extent not justified bv Frc:. 4. Relation between solar altitude and ap- facts. proxirnntc: changes reclnireil in f/stops or shutter In the first place, visual evaluation of color \pc,cd for average contlitions at 10,000 fcet. is a subjective process; a relative rather than Relative log ErW.eurei Increaaing Scene Brightness t

FIG. 5. Spectral response curves for Kodak Infrared Aerographic Film 2424 ( Estar Base), Mil Designator 11-24-74f, D-19 developer, 12 min 68°F (20°C). Exposure sec., EG&G MK VI Sensitometer, lamp calibration traceable to NBS Standard. Straight-line regional gammas: Band 1, 47B, blue, 1.60; Band 2, 57A, green, 1.66; Band 3, 25, red, 1.60; Band 4, 88A, IR, 1.80. Similar results are obtained with 641 chemistry in Versamat processors, 2 racks in developer, 5 feet per minute, 85°F (29°C). an absolute function. Perception and accept- films are purposely designed to improve con- ance of natural color images are conditioned trast rendition, and a series of haze filters by the individual's memory of object colors may be used to suppress excess blue. The seen every day. The adaptability of the eye color transparency is thus deliberately shifted and memorv is such than even two-color from real values to a color balance closer to (blue-green and orange) additive systems ap- everyday appearance, and the interpreter's pear to synthetize a wide range of color. memory of grass as green, not blue-green. In Filters used in multispectral photography additive viewing, the effect of using a haze are designed to recreate natural and color- filter is given by adjusting relative intensities infrared or other effects, but the same filters between the blue and green bands. do not have to be used in additive color pro- The eye can discriminate about 150 color jection to synthetize color imagery completely hues, and several million color differences effective for viewing and analysis. An abso- under test conditions. In ordinary color view- lute color match between the originalu and ing other factors control the situation. Ap- the image is seldom possible, or even desira- parent hue, saturation and brightness are af- ble. fected by the visual angle; that is, as detail It is well-known in color work that a pre- becomes small and its shape varies, different ferred renroduction of colors is usuallv re- hues may actually seem identical. Even large quired, where departure from equality of ap- areas of the same hue, but displaced by some pearance between the original and the image distance from each other on a viewing screen is necessary for optimum perception of the can seem different against different color scene. This applies to paper prints, projected back~rounds.u color transparencies, color television, and ad- Although CIE tri-stimulus measurements of ditive color viewing systems. color images may be read off the viewink For example, the ground viewed from an screen, for technical reasons the standard tri- aircraft at altitude appears blueish because stimulus spectral light sources cannot be re- of atmospheric light-scattering in the blue produced exactly in an additive projection region. Some color hues are altered and con- system, and it is unrealistic to specify that trast reduction occurs. Natural color aerial this be done. It should be remembered that 'NCE

NANOMETERS

SPrcrmnL cn~~KTErlsTlc~or; KELTTEN N0.58 FILTER-VS VOLTAGE VAR~AT~ONSDVY QUAPTZ HaLocrQ ~sowL SCALE FACTOR 31 6=- -. DATE/P~~&L~ RECORDED ON LA'MMA 4C l t\TIFI<. I\CORPOdArLI>\MODEL 3000 SCANNING SPECTIORADIOMETEII II &

Frr:. 6. Spectral radiometric n~eas~~rrnlentson two filters where light-so~~rcevoltage is varied. Note that peak wavelengths and hand-pass half-power points cIo not change with voltage. The IIppcbr i1iazr:irn was for a Wrattcn No. 47A filter and the Iower for a NO. 58. CIE coordinates measure a sensory impression Note that peak wavelengths and half-power of color and are not necessarily a souna points do not shift. guide for exposing ~hotographiccopies of It is often specified that illumination must the screen image. Photo materials are likely be controlled in a series of very small gradua- to respond to spectral radiant energy in a dif- tions, such as 5 percent, accurate to *5 per- ferent way than the eye, and will record a cent. The cost of meeting such specifications different color balance. is needlessly exhorbitant for normal viewing Color de-saturation by bleeding white light purposes. Response of the human eye and into a spectral channel has been advocated photographic materials tends to be logarith- by some experimenters. The value of this mic rather than linear. The relationship of measure is open to question. Studies by percent transmission to optical density is Winterberg and Wulfeck found that more plotted in Figure 7. At one end of the scale, targets" were detected and more detail was the difference between 100 percent transmis- resolved in complex scenes by additive color sion and 95 percent (AD = 0.02) is equiva- viewing than with natural color films; this lent to putting a piece of plain glass in the was attributed primarily to enhanced chro- optical path. More useful illumination step maticity and brightness contrast of additive increments for both visual and photographic spectral records, effects opposite to de-satura- recording would be calibrated voltage set- tion. tings which control illumination in the equiv- alents of one-half lens aperture stops; such ILLUMINATION CONTROL as 100, 70, 50, 35 percent, and so forth, as In additive viewing, illumination is con- in Figure 7. trolled simply by varying lamp voltage. It is not necessary to go to the cost and complica- IMAGE REGISTRATION tion of having a large series of neutral-density Excessively exacting precision for multiple- filters. The color temperature of the lamp image registration is frequently said to be changes but the spectral filter still continues necessary for useful viewing and interpreta- to transmit light within its designed pass- tion. band; only the amount of energy transmitted Multiple image registration should be ac- is changed, which would also occur with the curate enough to avoid visible color fringing neutral-density filters. Figure 6 shows radio- in an aerial photo image viewed at a normal metric measurements of the effects of large distance. Under optimum conditions of il- lamp voltage changes for two Wratten filters. lumination and contrast, the eye can resolve

% TRANSMISSION

FIG. 7. Optical density versus percent transmission. The sensitivity of the eye tends to follow the curve. Points equivalent to one-half aperture stop differences are marked (V) for photographic exposure control purposes (AD - 0.15). about one arc-minute of subtended angle; to be part of the loop in translating image but normal resolution is about 12 arc-minutes. information into useful data. At least, the One arc-minute is about 0.005 inch at 18 multispectral system enables the familiar inches distance in a photograph. Seven to ten color world to be re-created, into which un- line/pairs/mm at 10 inches is also quoted; real color hues and false balances can be in- this is a target bar and space width of 0.004 troduced to enhance subject matter; but we inch. In a viewer of the type shown in Figure all have a learning process ahead of us to 2, the maximum image registration error on exploit fully these techniques. the screen is normally within f0.0025 inch. With aerial photo images this is impercepti- ble and must be measured with high-contrast Simplified production equipment is now projected grid images. available for multispectral photography and As scale and rotation differences have been additive color display. Standardized results avoided in the camera, only small x, y image can be obtained for comparing results oh- motions are needed for registration. Four tained among experimenters. High-quality spectral images of a scene can be registered imagery is provided for routine interpreta- in the viewer within 20 or 30 seconds, and tion, research and training without adherence as all images are on one roll of film, rapid to excessively stringent, largely academic and scanning of many is possible during a short costly specifications. The systems use black- working period. Special registration marks and-white negative and positive films and are not needed, as the images provide a conventional processing. Superior natural- wealth of detail serving the purpose. and false-color effects not obtainable with color films are created and controlled by the MULTISPECTRAL IMAGE INTERPRETATION user. The user is the final arbiter; but his Rapid growth in the remote sensing field success in converting image information into has introduced many to aerial and multispec- useful data still depends primarily on the tral photography for the first time. Some have depth of specialized subject knowledge he been led to expect the image information brings to the task. Multispectral systems related to their specialties to be presented in uniquely aid his subjective judgement proc- clear, unmistakable form as the knobs are esses. turned. Alas, there are few if any unique, spectral signatures in nature. Multispectral photography and additive 1. Manual of Color Aerial Photography, Ameri- can Society of Photogrammetry, 1968. color display provide more powerful, flexible 2. The Reproduction of Color, R. W. G. Hunt, tools than we have had before, but as has John Wiley & Sons, Inc. always been the situation, the interpreter not 3. The Measurement of Color, W. D. Wright, only must have expertise in the subjects he Hilger & Watts. is analyzing, but familiarity with their ap- 4. Principles of Color Technology, I?. W. Bill- pearance in an aerial photograph. In the meyer, Jr. and Max Saltzman, John Wiley & earth sciences, foresters, geologists and agro- Sons, Inc. nomists have been forerunners in photo in- 5. Principles of Color Photography, R. M. Evans, terpretation. Much of their skill has been W. T. Hanson and W. L. Brewer, John Wiley & Sons, Inc. founded on relating personal knowledge of 6. The Physical Aspects of Azrial Photography, ground truth with its appearance in the aerial G. C. Brock, Dover Publications, Inc. photograph. For a long time to come, the 7. Photographic Considerations for Aerospace, human and his stored experience are going Itek Corporation.

Two Meetings in October-see pages 616 and 629.