Light tristimulus values

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. 

The R,G,B values of the Berger formula measured by tristimulus colorimeters are linearly related to the X,Y,Z tristimulus values of the CIE system. The Stensby formula incorporates the L,a,b tristimulus values of the Hunter system. In the CIE 1982 formula, x and yn are the chromaticity coordinates of the D65 (2° or 10° observer) light source. 

The Y tristimulus value may be taken as the relative lightness of the tablet... 

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. 

Methods of Determination. Lightness. The white pigment powder is compressed in a suitable powder press to give an even, matt surface. The CIE tristimulus value Y is measured with color measuring equipment. For standards, see Table 1 ( Lightness ). Apparatus spectrophotometer or tristimulus colorimeter, powder press, white standard. 

When determining the tinting strength of inorganic pigments, the tristimulus value Y (lightness) is usually used [1.37]. 

Evaluation of Color Changes (see Section 1.3.2). Color changes are differences in lightness, chroma, and hue such as exist between identical samples with different histories. Evaluation methods are particularly suitable for the determination and evaluation of color changes that occur following physical and chemical stresses on pigmented materials. The CIE tristimulus values X, Y, and Z of the samples or of different test locations on a sample are determined with a colorimeter. The CIELAB... 

Tristimulus values X, Y, and Z are computed for a primary light source with power spectrum L(X) from the color-matching functions x, y, and z as follows ... 

By creating scales for hue, lightness, and chroma, we can now measure color numerically. Other methods for expressing color numerically were developed by the Commission Internationale de l Eclairage (CIE), an international organization concerned with light and color. The two most widely known of these methods are the Yxy color space, devised based on tristimulus values XYZ defined by CIE, and... 

Color The color of the film can affect the consumer acceptance of coated produce (60). Five different color systems XYZ, Yxy, L a b, L C H° and Hunter Lab are used to express absolute chromaticity. Color system XYZ is measured as tristimulus values of color and forms the basis for most calculations. In the color system Yxy, Y is a lightness factor expressed as a percentage based on a perfect reflection of 100%. x and y are chromaticity coordinates defined by the following equations ... 

The degree of the stimulation to the red sense organ is denoted by X, to the green by Y and to the blue by Z. We call the values "tristimulus values". The value of Y is also used to denote the brilliancy of the light, because the relative luminosity curve of the pyramids coincides with the stimulus curve of the green sense organ relatively. The values ... 

The original 1931 CIE Y, x, y system of colour measurement is not visually uniform (Fig. 3.4a). Constant hue and chroma are distorted and equal visual distances increase several-fold fi om purple-red to green. Improved spacing has been accomplished by both linear and non-linear transformations of Y, x, y (Bems 2000). Near uniform colour spaces of practical importance are the Hunter and the CIELUV and CIELAB spaces. In the Hunter (1958) Z, a, b colour space the lightness co-ordinate L is the square root of the tristimulus value T, and a, and b are the red/green and yellow/blue opponent co-ordinates. The 1976... 

Photoelectric colom measuring instruments can be divided into two classes, trichromatic colorimeters and spectrophotometers. The most successful of the early trichromatic colorimeters was developed in the 1940s by Hunter (1958). It comprised a light somce and three wideband red, green and blue filters to approximate CIE standard illuminant C and the 2° observer. The tristimulus values obtained were transformed into Hunter L, a, b colom space. Until the advent of the computer and the photodiode such... 

X, Y, and Z are the tristimulus values of a light, i.e. they are the amounts of three reference stimuli needed to match exactly the light under consideration in a trichromatic system. 

Objective color measurement is based on tristimulus colorimetry. " This instrumental method measures tristimulus values of light transmitted by a sample and compares the results to the values transmitted by distilled water. The results can be recalculated to the platinum-cobalt scale referred to in the previous standards. The results can also be interpreted by normal methods of color measurement to yellowness index, color, color depth, etc. 

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 ... 

Since many paint layers already exhibit inhomogeneities in such dimensions for the calculation of color data (tristimulus values and chromaticity coordinates), it is necessary to use mean reflectance values derived from three to five individual measurements of different sample areas. For individual measurements, the sample surfaces, especially in the case of paints containing plate-Hke pigments (light interference and/or color variable pigments), should have varying orientations to the adjustable measuring aperture. 

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. 

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