Tristimulus response

The GIE Standard Observer. The CIE standard observer is a set of curves giving the tristimulus responses of an imaginary observer representing an average population for three primary colors arbitrarily chosen for convenience. The 1931 CIE standard observer was deterrnined for 2° foveal vision, while the later 1964 CIE supplementary standard observer appHes to a 10° vision a subscript 10 is usually used for the latter. The curves for both are given in Eigure 7 and the differences between the two observers can be seen in Table 2. The standard observers were defined in such a way that of the three primary responses x(X),jy(X), and X), the value ofjy(X) corresponds to the spectral photopic luminous efficiency, ie, to the perceived overall lightness of an object. 

The result is finalized response curves for the Standard Observer, also called "Tristimulus Response curves", and is shown as follows ... 

Our next step is to define colors in terms of tristimulus responses. We know that we can define the energy of any spectral curve as a summation of intensities times wavelengths, i.e.-... 

Therefore if we take the spectral curve, and multiply it by the overlap of each tristimulus response curve, we get TRISTIMULUS VALUES, i.e.-... 

Once we have done this, we now have our three primary colors in the form of standard lamps, and can proceed to determine Items 1,2 3, given in 6.7.16. To do this, we vary the wavelength of the monochromatic light, and determine relative amounts of red, green and blue light required to match the monochromatic color. This is done, as stated before, for about 5000 observers. The result is finalized response curves for the Standard Observer, also called "Tristimulus Response curves". 

Let us now summarize the results we have achieved. We have measured the luminosity response of the human eye, in terms of photopic and scotopic behavior. We also defined a "black-body" and its wavelength emission, stipulating its absolute temperature. We then defined Standard Sources. We next designed a Color Comparator and then determined the transmission characteristics of three (3) filters required to duplicate the response of the three color preceptors of the human eye. These we called the tristimulus response of the Standard Observer. 

We finally arrive at the result we want, since we can now set up "Tristimulus Filters" to use in defining colors. We can now define "y as our standard luminosity curve for the human eye (photopic vision). Note that x, the red tristimulus value, has a certain amount of blue in it in order to duplicate the response of the red preceptor in the retina. 

Tristimulus colorimeters are used to measure color in certain food applications. They combine light source, filters, and photodetectors to reproduce the CIE Standard Observer response functions (Fig. F5.1.8). Colorimeters, having broad band-pass filters, do not measure the spectral data. Without spectral data they cannot offer the choice of either observer or... 

A matrix Tc is applied to the tristimulus data. The result will be the response of the cone receptors denoted by L, M, S that respond to light in the long, middle, and short part of the spectrum respectively. Since the response of the receptors is proportional to the logarithm of the intensity, the logarithm is applied at the next stage. After the logarithm is applied, the data is transformed by Tp such that one of the coordinate axes is the achromatic axis and the other two axes are the red-green and yellow-blue axes respectively. 

The first is to imitate the analysis made by the eye in terms of responses to three stimuli. This technique, known as "tristimulus colorimetry," sets out to measure X, Y, and Z directly. 

Tables 3 and 4 present the tristimulus coordinates and the corresponding absorption coefficients of the different samples presented in Figs. 2 and 3. Some observations can be drawn. Except for the industrial product, the corresponding colour characteristics are rather close, even if the visible spectra show some differences. This is particularly true for the tristimulus method which gives an integrated response more adapted for dyes than for water or wastewater. The ISO method seems to discriminate more efficiently the colours but needs a more complete comparative study for each absorption coefficient.

In a tristimulus colorimeter, three or four filters duplicate the response of the standard observer. The filters, which correspond to the three primary colours in the spectrum (red, green and blue), can be combined to match most colours. The more sophisticated instruments carry a fourth filter to simulate the blue part of the CIE [[X bar]] function shown in Fig. 5.1. The other essential parts of a tristimulus colorimeter are a white light source, an array of photometers and, nowadays, a computer or an interface to one, as shown in Fig. 5.2a. The computer can collect responses as well as carry out data transformations between CIE and other colour scale systems or between different standard white light sources or white diffusers. Hunter and Harold (1987) give a good summary of the transformation formulae employed. Data from a colorimeter is given as a three-point output, commonly CIELAB, HunterLab or 7, x, y. 

The three different color-response mechanisms in the human eye are the scientific basis for the measurement of color that is standardized in the Cl E Standard Observer (1931). This system specifies color according to X, Y, Z tristimulus values and. v. , z chromaticity coordinates. Several alternative scales were subsequently developed to provide a better adaptation to the color differentiation ability of the human eye [9.26]. 

Integration (colorimetry) n. The process of summing the products of the three values the illuminant, the observer response characteristics and the reflectance or transmittance of an object at specified wavelengths in order to obtain the tristimulus values. 

Luminous lu-m3-n9s [ME, fr. L luninosus, fr. lumin-, lumen] (15c) adj. (1) Adjective used to imply dependence on the spectral response characteristic of the Standard Observer defined in the CIE System. Thus, the luminous reflectance or the luminous transmittance is described by the Y-tristimulus value in the CIE System. The adjective is applied to many measures of light, such as... 

Observer, standard n. The spectral response characteristics of the average observer defined by the CIE. Two such sets of data are defined, the 1931 data for the 2° visual field (distance viewing) and the 1964 data for the annular 10° visual field (approximately arm s length viewing). By custom, the assumption is made that if the observer is not specified the tristimulus data has been calculated for the 1931, or 2° field observer. The use of the 1964 data should be specified. 

The tristimulus filter colorimeter was apparently developed in order to retain some of the advantages of the spectrophotometer and at the same time to decrease the labor needed to calculate the numerical specifications of color. They are generally more portable than the spectrophotometers, cost less, and require much less interpretation of the results. All of the data of the tristimulus colorimeters can be interconverted to any of the standard systems of color measurement. Their accuracy depends upon the fit between the theoretical response of the C.I.E. primaries and the response achieved by the combination of light source, filter, and photocell. The fit is in no case perfect, and is usually worst in the small lobe of the X primary. 

Natural Language under the Phonetical Tetrahedron. We may return to the tristimulus triangle of vowels and its extension to a tetrahedral pyramid. The summit is responsible for a neutral plos-sive and the bottom carries vowels. In between at medium heights more... 

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