By convention x and y are plotted on the diagram that features the horseshoe-shaped spectrum locus. Dominant wavelength is obtained by drawing a line from the illuminant through the sample to the spectrum locus as shown in Figure 4 and represents visual hue. Purity is obtained by the equation a/(a + b) and represents visual chroma or saturation. Samples located nearer the spectrum locus are more saturated. Colors placing below the illuminant at the horseshoe base are considered nonspectral colors, and a complimentary dominant wavelength is calculated by drawing a line from the sample through the illuminant to the spectrum locus. Purity is determined as for spectral colors. The horseshoe spectrum locus represents two dimensions of color and the third is represented by Y as shown in Figure 5. The plane of the horseshoe rises as the color becomes lighter. Spectrum colors on the spectrum locus have a low lightness factor.
The Hunter L, a, and b color scale is an opponent-type system and has the advantage of being more uniform than the CIE system. It has proved popular for measuring food products. This scale was developed in 1958, allowing computation and direct readout from the dials of the colorimeter (3). Readout is as Lh, aL, and bL, and the functions are related to the CIE X, Y, and Z as
The a and b chromaticity dimensions are expanded so that intervals on the scale for a and b approximate the visual correspondences to those of the 100-unit lightness scale Lh. This color scale is based on Hering's theory that the cone receptors in the eye are coded for light-dark, red-green, and yellow-blue signals. It is argued that a color cannot be both red and green at the same time; therefore, a is used to represent these; —a represents greenness and +a represents redness. Similarly, —b represents blueness and + b represents yellowness. Lightness is represented by Ll (Fig. 6). Hue is calculated as the angle going counterclockwise from +a and hue angle is calculated as tan-1 b/a. Chroma is calculated as (a2 + b2)1'2. Color difference
is calculated as AE = [(Lx - L2)2 + (a, - a2)2 + (i>i -&2)2]1/2> but this value gives no indication in which dimension the difference lies. The Hunter color scale was first developed for use with reflectance and later adapted for transmission measurements. Confusion arises with luminosity of dark liquids such as dark fruit juices. This prompted adaptation and revision of scales for use at low luminosity levels (6). These scales are useful when the ratio of absorbance to transmittance is high.
The three-dimensionality of color is an important concept for measuring the color of a food. Examination of the wavelength distribution along the spectrum locus makes it clear that color is not equally visually spaced with this system. The elliptical nature of the color spaces prove troublesome when trying to match or establish specifications for color. Much attention has been devoted to providing more uniform color spaces and the CIELAB (1) system with parameters L*a*b* was designed to do this. Another color system was called CIELCH with parameters L*C*H* (9). The CIELAB system seems to be gaining more prom-inance. Color-matching computers are now available and are much used in textile and other industries. It may be important to determine whether a color difference exceeds the just-noticeable difference (JND). Computers have removed the computational labor of the more accurate calculations of differences.
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