Spectral tristimulus values

Figure 3. Spectral tristimulus values according to the 1931 CIE standard observer. These values are dimensionless.

They are represented as coordinates in a color plane. The chromaticity coordinates x and y are used to specify the saturation and hue of any color in the CIE chromaticity diagram. See Figure 4 a for illumination D 65. The CIE spectral tristimulus value y (2) corresponds to the lightness sensitivity curve of the human eye. Therefore, a third color variable is specified in addition to x and y, namely the CIE tristimulus value Y, which is a measure of lightness. 

A block copolymer consisting of three AnB ,An blocks. The instrument for color measurement based on spectral tristimulus values. The color is expressed in terms of three primary colors red, green, and blue. 

From the above discussion, it is clear that given a CIE standard source, object, and spectral tristimulus values, one can easily measure color. The instrument developed for such color measurement is called a tristimulus colorimeter. A tristimulus colorimeter measures color in terms of three primary colors red, green and blue, or more properly stated, in terms of three tristimulus values. Many different color scales have been developed to describe the color numerically in terms of lightness and hue. One of the most widely accepted systems is known as the L, a, b tristimulus system. Figure 6-10 illustrates L, a, b color space. The coordinate L is in the vertical direction and corresponds to lightness. A perfect white has an L value of 100 and a perfect black a zero. The variables at and bt identify the hue... 

TABLE I Spectral Tristimulus Values for Equal Spectral Power Source... 

The spectral characteristics of the source, photocells, and the three filters are such that approximate I.C.I. tristimulus values may be calculated (5) and from these a specification in terms of luminous reflectance, dominant wave length, and purity can be obtained. Hardy has cautioned (3), however, that the usefulness of such an instrument as a tristimulus colorimeter depends upon the standardization and constancy of the spectral characteristics of the light source, cell, and filters. 

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

The most successful quantitative expression of color is that known as the CIE (Commission Internationale de l Eclairage) system [13]. This methodology assumes that color may be expressed as the summation of selected spectral components (blue, green, and red hues) in a three-dimensional manner. The CIE system is based on the fact that human sight is trichromatic in its color perception, and that two stimuli will produce the same color if each of the three tristimulus values (X, Y, and Z) are equal for the two. Detailed summaries of the CIE and other quantitative systems for color measurement are available [14,15],... 

In Eqs. (7)—(10), 5(A) is the spectral power distribution of the illuminant, and R A) is the spectral reflectance factor of the object. Jc(A), y(A), and 5(A) are the color-matching functions of the observer. In the usual practice, k is defined so that the tristimulus value, Y, for a perfect reflecting diffusor (the reference for R A)) equals 100. Using the functions proposed by the CIE in 1931, y(A) was made identical to the spectral photopic luminous efficiency function, and consequently its tristimulus value, Y, is a measure of the brightness of objects. The X and Z values describe aspects of color that permit identification with various spectral regions. 

Colorimetry [1.17] - [1.19]. The principles of colorimetry are based on the fact that all color stimuli can be simulated by additively mixing only three selected color stimuli (trichromatic principle). A color stimulus can, however, also be produced by mixing the spectral colors. Thus, it has a spectral distribution, which in the case of nonluminous, perceived colors is called the spectral reflectance q (2). After defining three reference stimuli, the trichromatic principle allows a three-dimensional color space to be built up in which the color coordinates (tristimulus values) can be interpreted as components of a vector (CIE system for standards, see Table 1, Colorimetry CIE = Commission Internationale de l Eclairage). For uncolored illumination the three CIE tristimulus values depend on the spectral reflectance as follows ... 

Instrumental Color Measurements. When treated samples showed AATCC ratings of 4 or less, they were measured against the untreated samples on the Sargent-Welch SP8-200 UV-vis spectrophotometer with a diffuse reflectance sphere attachment. The instrument is interfaced with a Hewlett-Packard HP-85 computer and run by color measurement software. Spectral reflectance data at 10-nm intervals, tristimulus values, chromaticity coordinates, and CIELAB color difference were determined for these samples (21). 

In the human visual system, the JC f and Z tristimulus values are calculated from spectral power as given in the following equations (CIE, 1986). 

Weighting factors published in the American Standard Test Method, ASTM E 308-85, were used to calculate tristimulus values from spectral reflectance data. The weighting fac-... 

The tristimulus colorimeter is the simplest means for instrumental color specification. A light source, filters, and a photodetector are combined such that together they yield a direct evaluation of the tristimulus values X, Y, and Z. A spectrophotometer may also be used for color specification. That is, a spectral reflectance or transmittance curve (R) can also be converted into X, Y, Z data. The curve R versus wavelength 2) is integrated over the visible range, with the spectral emission of an illuminant (E), and the standard observer functions (x,y,z). The areas under the resulting curves yield the values of X, Y, and Z that is ... 

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... 

Metameric pair n. A pair of colors, which match when viewed in a described way but which do not match if the viewing conditions are changed, thus, a metameric pair of samples exhibit the same tristimulus values for a described set of viewing conditions (observer, light source, geometry of the illumination, and viewing arrangement) but have different spectral distributions. Hence, they exhibit a match, which is conditional only. 

Reflectance, luminous n. Reflectance derived firom radiant flux by evaluating the radiant energy reflected according to its action upon a selective receptor (such as the eye), whose spectral sensitivity is defined by a standard relative luminous efficiency function. In popular usage, the term is used to describe the Y tristimulus value in the CIE system. 

Venable, W. H., Accurate tristimulus values from spectral data. Color Res. Appl. 14(5), 260-267 (1989). 

Tristimulus Computation Data n Products of relative spectral-energy distribution of an illuminant multiplied by each of the three color matching functions in the CIE System designated as EcX,Ecy,EcZ, for example, for Illuminant C and the color mixture data for the standard observer at designated wavelengths. See Tristimulus Values. 

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