Species Richness in Fluctuating Environments

ELECTRONIC TRISTIMULUS CHROMATIC MEASUREMENT SYSTEMS

D. TOMTSIS, K. SAPALIDIS, C. KATSANOS*

TEI OF WEST MACEDONIA, * PATRAS UNIVERSITY, MACEDONIA RESEARCH CENTRE, DEPT. OF ELECT. ENG. AND COMPUTER TECH., KOILA, KOZANI, PATRA, GR-50100, GREECE , GR-26441, GREECE,

Abstract: Three practical electronic systems were designed to measure colour or chromaticity. The development of all systems was based on tristimulus detection, which offers both dominant wavelength and saturation information. Such colour or chromatic measurement systems are generally used in colour vision systems or robotic vision applications and in manufacturing and testing of CRT displays as a performance verification tool. In general, the chromatic approach provides a powerful and flexible measurement method with attractive advantages over intensity and two wavelength modulation techniques.

Keywords: chromatic modulation, chromaticity.

1. Introduction with two photo-detectors. Systems which make use of There are many situations in which the optical a spectrometer are expensive, optically inefficient and, measurement of a physical parameter would be of require excessive data processing. Systems using benefit to industry. For example, optical systems can filters and photo-detectors are less expensive, but are be used safely in hazardous environments and also in wasteful of optical power. Such systems may also be areas where high magnetic or electrical fields make used for wavelength encoding detection but the electronic systems inadequate. However, the uptake by spectrometer approach leads to improved resolution industry of optical technology has been slow either only at the expense of optical power whilst the narrow- because of a lack of confidence in the technology or band filtered photodiodes only have a limited range of because optical monitoring tends to be specialized, operation. expensive and insufficiently robust to withstand the Many of the difficulties inherent in the spectral or two industrial environment. wavelength monitoring methods may be overcome Of the methods available for modulating light (e.g. using a chromatic measurement system. A number of amplitude, phase, polarization changes) intensity sensors based upon this approach have been developed modulation offers the advantages of inherent [1, 2] and shown to possess attractive advantages. simplicity. However, conventional methods involving Such systems measure changes in the spectral absolute intensity have associated problems. The most signatures of polychromatic light using carefully basic intensity monitoring systems use only a single configured combinations of broad band detectors. The photodiode to produce an output but these systems technology evolves from the study of colour vision [3]. tend to be sensitive to spurious changes in intensity The chromatic approach provides a powerful and resulting from variations in the light source or other flexible measurement method whose full potential is components within the system. Removing these still being explored. spurious effects is difficult and leads to complicated This contribution seeks to describe some of the and expensive systems. fundamental aspects of chromatic measurement Wavelength monitors attempt to deduce the state of a systems, which are relevant for optical sensing system by taking the ratio of intensities at two applications. These are based upon the understanding different wavelengths. In principle, the two that although the considerations derive from the wavelengths should be chosen tο be close enough concepts of colour science [4, 5, 6, 7], e.g. together to ensure that spurious signals affect both chromaticity diagrams, dominant wavelength etc, they wavelengths equally. However, the modulator needs tο need to be adapted in fundamental1y different ways affect only one of the wavelengths so leading to for their advantages to be ful1y exploited. The work conflicting demands. Such wavelength modulated presented in this paper, which is based on tristimulus systems may be constructed using spectrometers or by chromatic modulation, is an extension of our previous using two narrow-band optical filters in conjunction work [8] on distimulus chromatic modulation. overlapping spectral responses. The output of each 1.1. Optical Spectrophotometry detector may then be expressed as [9] Spectrophotometry is a technique for determining the spectral energy of an electromagnetic wave. This Vn =  P()Rn()d (0) distribution is represented as a record of intensity against wavelength. To achieve this, comparison of where P() is the spectral power distribution in the optical signal and Rn() is the intensities is performed using narrow-band, non- th overlapping filters or detectors centered on wavelength responsivity of the n detector and λ is the wavelengths situated at regular intervals throughout wavelength. Each detector output may also be the spectrum. At each wavelength, a comparison is intensity normalised according to: made between the incident light and a working Vn un  (0) standard. There are a large number of designs for V spectral photometry, which provide varying degrees of n T resolution and accuracy. However, they all require a where light source, a method of dispersing light into a spectrum, and a means of detecting and comparing the u1 + u2 + … + u(n-1) + un = 1 (0) intensities from the sample and the standard beams. In such a way, chromaticity maps may be formed in

Spectrophotometry is the most precise method of terms of the coordinates u1, u2, … u(n-1). describing the spectral profile and thus the System simplification and implementation economy is measurement of color of the incident light. However, therefore possible by reducing the amount of the interpretation of this spectral data is complex, and information acquired and which may be achieved together with the high cost of spectrophotometers, through the use of two rather than n detectors. The makes it an unattractive technique for general color case of n=2 leads to a distimulus chromaticity map on measurement. which either chromaticity coordinate (u1 or u2) may be In this case, it is necessary to reduce this amount of used to completely describe changes in optical signals. data to only a few parameters which are able to Any two detectors with different but overlapping provide spectral profile information and also a spectral responsivities may be used to form a perceptual correlation between the color of light distimulus measurement system. The chromatic model detected and the observer, a method which is representing this mathematical formalism is called LXY commonly known as color matching. and provides the relative magnitudes of the distimulus

values (i.e. X=u1; Y=u2). 1.2. Color Matching Color matching is important both for commercial and scientific purposes. Its importance lies in the reproduction of perceived color, for example, in photography and television. The standard method of color matching is by using a human observer presented with a color sample together with three controllable color sources matched to the human eye responses. The observer adjusts the intensities of the sources and forms the basic color components of the sample. Color vision is basically a function of the responses of three different spectral weighting functions. Hence, it is expected that color can be extracted from the spectral Fig. 1. Chromaticity diagram for distimulus detection. data using similar functions. The advent of cheap electronic detector systems that have spectral The two-dimensional nature of the chromaticity responses similar to the human eye have made this diagram, shown in fig. 1, implies that in general, the approach to color matching a preferred technique for status of the measurand is over-specified in being colorimetry. defined by two coordinates, u1, u2.

Since the ratio υ1 / υ2 provides normalization with 2. Chromatic Modulation Theory respect to signal intensity the distimulus method The essence of chromatic modulation is the utilisation preserves the intensity-independent nature of of polychromatic light for sensing spectral changes by chromatic monitoring. Also, since the signals υ1 and u2 monitoring the total profile of an optical signal within are derived from overlapping detector responsivities a spectral power distribution. Chromatic changes can there is a degree of inbuilt immunity to extraneous be monitored by a number (n) of detectors with spectral noise in the output υ1 / υ2.

The determination of color using three detectors is wave1ength basis in a numerical approximation to eq. called tristimulus detection [10]. The human eye and 1 ie; many computer vision systems are examples of 700nm tristimulus chromatic detection systems. Tristimulus X =  x()S() (0) detection is not reliant on the detection of a complete 400nm spectral power distribution. It is sufficient to measure three attributes of the spectral power distribution. The 700nm three detector (tri-stimulus) system can, therefore, be Y =  y()S() (0) used to measure those spectral signature changes, 400nm which lead to a change in dominant wavelength and 700nm saturation. Z = z()S() (0) 400nm

3. Electronic Tristimulus color Where S(λ) is the incident spectral intensity and x(λ), measurement systems y(λ), z(λ) are the spectral responsivities of the The practical measurement of the tristimu1us values detectors. Χ, Υ and Ζ, and their subsequent processing to obtain Finally the co1or coordinates, x and y are ca1cu1ated the co1our coordinates x and y, is discussed in this using eq. 2. This system is ab1e to produce very section. Three electronic color measurement systems accurate co1or measurements (there is a 95% are presented. The first uses a computer to calculate confidence that any measurement wi11 1ie within the color coordinates from a measured spectral 0.0001 of the true va1ue). The main disadvantages of signature. The second system combines direct optical the system are that each spectral scan takes three measurements of the tristimulus variables Χ, Υ and minutes and that the system is quite cost1y to X+Y+Ζ. The final system measures the tristimulus imp1ement. variables individually, and uses a computer to calculate the color coordinates, although in this case 3.2. Α direct reading chromaticity meter the detectors do not have the same spectral Light is gathered into a fiber bund1e from a camera responsivities (x, y and z) as the standard CIE head, which is focused on a region where co1our is to observer. be measured. The fiber bund1e is sp1it into three such that the 1ight is incident on three optica1 fi1ters, 3.1. Obtaining color coordinates from a giving the tristimu1us va1ues Χ, Υ and Χ+Υ+Ζ. The spectral signature fi1ter responsivities were matched to the CIE spectra1 This section describes the operation of the developed responsivities, Χ, Y and Ζ, using a computer program colorimetric measurement system, which was designed which chose the best combinations of different fi1ter to measure the color properties of visible light sources types and thicknesses from a database of readi1y and optical filters. obtained optica1 fi1ter spectra1 transmission The system uses a monochromator, with wavelength characteristics. selection controlled by a microcomputer. The optical intensity at each selected wavelength is measured using a photomultiplier tube, the output of which is interfaced to the microcomputer. The microcomputer is programmed to scan the whole optical spectrum, and to store the outputs from the photomultiplier tube. In this mode of operation, the microcomputer controlled monochromator system behaves as a spectrum analyzer, and hence the stored photomultiplier responses give the spectral signature of light incident on the monochromator. The measured spectral signatures are processed in order to compensate for the spectral gain characteristics of the monochromator and photomultiplier, hence producing the true spectral signature of the incident light. The microcomputer calculates the tristimulus values Χ, Υ and Ζ from the Fig. 2. The Direct Reading Chromaticity Meter stored spectral signature. This is achieved by multiplying the CIE standard observer responsivities The intensity of the 1ight passing through each of the by the captured spectral signature on a wavelength by fi1ters is converted to an e1ectrica1 signa1 by a sing1e photomu1tip1ier tube. The 1ight intensity measurements made with the photomu1tip1ier are digital converters (ADC’s). Α microcomputer uses the taken at different time interva1s such that the Χ, then ADC digitization results to calculate and continuously the Υ and fina11y the Χ+Υ+Ζ va1ues are measured, display the chromaticity coordinates. It should be this being achieved by cutting ho1es in a rotating noted that this system is not designed to measure timing disk, shown in figure 3, such that the Χ and Υ colour, but chromaticity (i.e. the spectral responsivities fi1ters are individua11y uncovered and then the Χ, Υ of the three photodiodes were not the same as the CIE and Ζ fi1ters are simu1taneous1y uncovered, as the Χ, Υ and Ζ detectors). The circuit diagram of a disk rotates. photodiode with the associated transresistance The output from the photomu1tip1ier tube is digitized amplifier is shown in figure 4. using an ana1ogue to digita1 converter, and a digita1 processing system is used to convert the measured tristimu1us va1ues into co1or Coordinates.

The main advantage of this system is that the measurements may be acquired more rapidly than those taken with the system described in the previous section. The disadvantage of the system is the practical difficulty in obtaining filters with the exact responsivities of a CIE standard observer. Deviations in the filter responses lead to errors in the measured color coordinates, which may be particularly Fig. 4. Photodiode and transresistance amplifier noticeable with some spectral sources. However, if it combination, where ip, Rf and Vp are the values of is sufficient to make chromaticity measurements not photocurrent, feedback resistor and output voltage respectively based on the CIE standard observer, the deviation of the filter responses from the CIE standard observer is The photocurrent is related to the incident light not a problem. Α color space may be defined using intensity on the photodiode by equation 7. three detectors of known optical responsivities a, b and c. The tristimulus values Α, Β and C could be directly i p = K D()I()d (0) obtained using standard filters.  v = i R  R K D()I()d p p f f  (0)

Where Κ is a proportionality constant. The voltage output of the transresistance amplifier is given by eq. 8.

Eq. 8 shows that Vp depends directly on the spectral power density distribution of the light incident on the photodiode. Hence using three ADC’s to measure the

values of Vp for three photodiodes with different spectral responsivities gives a digital representation of the tristimulus values. The tristimulus values are manipulated using eq. 2 to give the chromaticity coordinates. The main advantages of this system are that Fig. 3. Front View of the Rotating Timing Disk chromaticity measurements may be obtained very rapidly (12 ms). Also, compared to the two previous 3.3. Α Continuous tristimulus measurement systems, the optoelectronic components are cheap. system The main disadvantages with the system are that the optical responsivities of available photodiodes are The system described in this section makes use of limited, and the accuracy is dependent on the stability three photodiodes with different spectral responsivities of the three individual photodiodes whose thermal and as the basis of a tristimulus measurement system. The ageing characteristics will be different. The limited three photodiodes produce photocurrents with values range of photodiodes constrains the design of different depending on both the spectral signature, and the chromaticity diagrams, although in theory it is possible intensity of the incident light. The photocurrents are to use photodiode and optical filter combinations to converted into voltages, using transresistance derive specific detector spectral responsivities. amplifiers, which are digitized using analogue to

4. Conclusion Three practical electronic systems were designed to measure color or chromaticity (i.e. not based on the CIE detector responsivities Χ, Y and Ζ). The development of all systems was based on tristimulus detection, which offers both dominant wavelength and saturation information. It was noted that although the computer controlled monochromator system produced the most accurate color measurements it was expensive and slow. The system based on three photodiodes was the fastest and cheapest of the three measurement systems, however, it did not provide true color measurement. Such color or chromatic measurement systems are generally used in color vision systems or robotic vision applications and in manufacturing and testing of CRT displays as a performance verification tool.

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