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The cubical CIE Lab color space 

The cubical CIE Lab color space 

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A most common problem encountered in radiosterilization of solid drugs is discoloration or yellowing. By pharmacopoeia method, discoloration can be assessed by measuring absorbance of solutions of irradiated solid samples at 450 nm. We propose to evaluate discoloration of solid samples directly by recording their diffuse reflectance spectra. Furthe...

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... subscript n refers to the tristimulus values of the perfect diffuser for the given illuminant and standard observer. The exponent 1/3 in equation for tristimulus values greater than 0.008856 provides the nonlinearity mentioned earlier. For values less than 0.008856, linearity piece is kept so as to avoid difficulty during conversion of Lab to XYZ Also, L * designates lightness which is equivalent to luminance like aspect of reflective color. The chrominance is described by two variables a * and b * representing red vs. green and yellow vs. blue axes, respectively. The spatial representation of CIE Lab color space is shown in Fig. 3. From CIE Lab values of unirradiated and irradiated samples color difference ( $ E Ã ab ) was calculated using following equation (13) The subscripts 1 and 2 refer to the irradiated and unirradiated samples, respectively. The $ E Ã ab value is useful for quantitation of color difference and for defining color tolerance limit. The dominant wavelength of a color correlates in an approximate way with what would be called as the hue of the color as observed under everyday conditions. It can also be defined as the wavelength of the spectrum band, which, when mixed with some specified achromatic stimulus, matches the given color. The derivation of dominant wavelength form the chromaticity coordinate is carried out by drawing a straight line through the point representing the achromatic color D65 and the point S representing the color to be evaluated, and this line is produced in the direction D65 to S to intersect the spectrum locus. The wavelength of the intersection is the required dominant wavelength ( λ d ) of the given color as depicted in Fig. 4. The excitation purity of any color possessing a dominant wavelength is an exactly defined ratio of distances in the chromaticity diagram indicating how far the given color is displaced from the achromatic color toward the spectrum color (Fig. 4). Excitation purity is the degree of saturation relative to the most concentrated form. The excitation purity calculated from trichromatic values is defined by The yellowness index shows a degree where the hue leaves white or achromatic color toward yellow. If it takes a negative value, it moves in the blue direction. The yellowness index for 2° observer and D65 illuminant is calculated by following ...

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... In other interpretations, DW represents the principal wavelength of the colour, i.e. DW correlates with the colour's hue [40]. In general, the larvae processing resulted in fluctuations in DW around the steady-state value of 577 nm (Table 3), pointing to the presence of yellow pigments. ...
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