Chromaticity chart

It will be apparent that the fully saturated colours are situated on the spectral loci curve, and saturation decreases as the central white portion is approached. A colour inside the chromaticity chart may be looked upon as the hue of the dominant wavelength, together with a measure of desaturation caused by the addition of surplus light of other wavelengths. With the additive primaries this tends to promote whiteness by increasing the spectral distribution of the reflected or emitted light. 

Hue and purity alone are not enough to describe the subjective sensation of colour. The element of luminosity must also be taken into account. I hus the difference between the orange represented by = 0-56 and y = 0-41 on the chromaticity chart, and a dark brown with the same co-ordinates, is that one might have a luminosity of 25 per cent and the other 8 per cent. The complete specification of a colour, therefore, requires a third dimension and, in the C.I.E. system it is a mathematical conception that the X and Z stimuli have hue but no luminosity, and the whole of the latter attribute is contained in the Y component. Thus when the whole of the incident light is reflected, the value of T is 1 and the colour is white. For lesser degrees of luminosity the values of F are fractions of unity, until at zero the colour is black because there is no reflection or transmission of light. A three-dimensional colour solid is illustrated in Fig. 26.17 in which the base is black, the apex white, and in any horizontal plane saturation decreases as the location of the colour becomes nearer to the Y axis. 

In 1942 MacAdam J. Opt. Soc. Amer., 1942, 32, 247) published the result of an extensive investigation of visual sensitiveness to colour. Many observers were asked to make matchings of a range of shades with a trichromatic colorimeter, and their readings were converted into x and y coordinates on a chromaticity chart. The spread of readings for individual colours fell within the ellipses shown in Fig. 26.21. The important feature is that the eye is obviously less sensitive to variations in colour in the green... 

Fig. 26.21 Optical sensitivity in different regions within the chromaticity chart...

Figure 11.7 The chromaticity chart with the incandescence curve added. The temperature of the illuminants are indicated in K. D65, B, and A are standard CIE illuminants. 

Methods are described for determining the extent to which original natural color is preserved in processing and subsequent storage of foods. Color differences may be evaluated indirectly in terms of some physical characteristic of the sample or extracted fraction thereof that is largely responsible for the color characteristics. For evaluation more directly in terms of what the observer actually sees, color differences are measured by reflectance spectrophotometry and photoelectric colorimetry and expressed as differences in psychophysical indexes such as luminous reflectance and chromaticity. The reflectance spectro-photometric method provides time-constant records in research investigation on foods, while photoelectric colorimeters and reflectometers may prove useful in industrial color applications. Psychophysical notation may be converted by standard methods to the colorimetrically more descriptive terms of Munsell hue, value, and chroma. Here color charts are useful for a direct evaluation of results. 

Funt et al. (1991, 1992) use a finite dimensional linear model to recover ambient illumination and the surface reflectance by examining mutual reflection between surfaces. Ho et al. (1992) show how a color signal spectrum can be separated into reflectance and illumination components. They compute the coefficients of the basis functions by finding a least squares solution, which best fits the given color signal. However, in order to do this, they require that the entire color spectrum and not only the measurements from the sensors is available. Ho et al. suggest to obtain the measured color spectrum from chromatic aberration. Novak and Shafer (1992) suggest to introduce a color chart with known spectral characteristics to estimate the spectral power distribution of an unknown illuminant. 

It is now possible to define chromaticity with x and y co-ordinates on a two-dimensional chart, because, as explained, Z=l— X+ F). 

Figure 11.6 Chromatkity diagram or chart. Color as encoded in a VIS spectrum is described by plotting the chromaticity coordinates X and y points on the chart. The outer band contains spectral colors (ROYGBIV), and the connecting line consists of nonspectral colors, such as pink and purple. A colored version is found in the color insert. 

Their concentrations in the effluent are measured by a detector such as a thermal-conductivity cell. The recorded concentrations are displayed on a strip chart with time as the abscissa. From the positions and areas of the peaks the identities and relative concentrations of the constituents in the sample may be determined. An example of a gas chromat( aph combined with a mass spectrometer is the Clarus 500 GC Mass Spectrometer. 

For color development work, a colorimeter that provides the tristimulus value and calculates the color difference is simply not enough. A more sophisticated instrument such as a spectrophotometer is generally required. A spectrophoto-meter, when used with a chart recorder or CRT, provides a complete spectral reflectance curve covering the entire range of the visible spectrum from 380 to 700 nm. Additionally, it provides the percentage reflectance value at each 20-nm interval and computes the tristimulus values, chromaticity coordinates, and color difference values. 

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