L, a, b color

In the work with cottonseed flours, we used the Hunterlab color meter D25D2A and expressed these measurements as Hunter L, a, b color values. These are coordinates of the three-dimensional opponent-color space shown in Figure 2. The L value measures lightness, or the amount of light reflected or transmitted by the object. The a and b values are the chromaticity coordinates from which information about hue and saturation can be obtained. The a value measures redness when plus and greenness when minus. The b value measures yellowness when plus and blueness when minus. 

Figure F5.1.10 The Hunter L,a,b color space was designed for measuring color differences. Figure courtesy of GretagMacbeth. This black and white facsimile of the figure is intended only as a placeholder for full-color version of figure go to http //www.currentprotocols.com/colorfigures...

DE = (dL 2 + da 2 + db 2)1/2 The DE for the Hunter L,a,b color scale would be calculated using the same basic equation. However, one DE formula cannot be converted into another formula The Hunter DE and the CIELAB DE do not correlate, so whatever color difference equation is selected... 

Many single number color scales have been developed for foods by converting X, Y, Z or L, a, b color specifications to a number indicating grade or quality on the desired scale. Such a scale was developed by Hunter to measure the citrus redness CR, and the citrus yellowness CY of the juice. The formulae are derived as follows ... 

As pink or red—fleshed grapefruit ripen, the color of the juice loses the distinct coloration and gives a juice that is definitely amber with a very slight brownish cast. Mixtures of white-fleshed and pink-fleshed grapefruit have a dull appearance and may look grayish to dull amber. There are no visual color standards for grapefruit juice products. However, color characteristics may be measured on tristimulus colorimeters using the L, a, b, color notation. 

Another perceptually approximately uniform color space is the CIE L a b color space (Colourware Ltd 2001 International Commission on Illumination 1996). This color space was intended for use with surface colors. The three coordinates L, a, and b are computed from the tristimulus values X, Y, and Z as follows ... 

L describes lightness and extends from 0 (black) to 100 (white). The a coordinate represents the redness-greenness of the sample. The b coordinate represents the yellowness-blueness. The coordinates a and b have a range of approximately [—100,100]. Notice the cube root in the above equation. The cube root was introduced in order to obtain a homogeneous, isotropic color solid (Glasser et al. 1958). Visual observations correspond closely to color differences calculated in this uniform coordinate system. A transformation of the sRGB color cube to the L a b color space is shown in Figure 5.5. 

Figure 5.5 sRGB cube visualized in L a b color space. 

Output images that were generated with this operation are shown in Figure 11.9. Note that here, the output colors may not have the same lightness. Of course, we could also adjust the intensity of the output pixel such that the lightness is identical to the input pixel. Alternatively, the color shift could also be performed in a different color space such as the L a b color space. 

Fig-4 L a b color space (a) and color circle and hue angle (b) shown on an a b chro-maticity diagram... 

The L a b color space is a visually uniform color space and is quite useful for designing devices suitable for visual colorimetry. The angle originating from the positive direction of the a axis expresses the hues themselves and is defined as the hue angle, hab, expressed by Eq. 29 [13] ... 

To make use of their merits and to erase their drawbacks, we carried out the multicolor visual colorimetry over a large concentration range of protein under the optimum mixing ratio of tetrabromophenol blue and eosin B using L a b color coordinates based on DCA. 

Figure 6-10 The Hunter L, a, b Color Space. Source From Hunter Associates Lab., Inc.

The L a b color space (also know as CIELAB) is presently the most popular color space for measuring object color and is used in many fields. In this color... 

Figure 2.11. Illustration of the color solid for the CIE 1976 (L a b ) color space (Minolta, 1993).

Figure 24.1. CIELAB L a b color difference plot on samples of PCR, correcting color let down in natural PCR with 3 and 6% correcting colorant. The plot shows that the best mix ratio falls between 3 and 6%.

L Tolvaj and O Faix. Artificial Ageing of Wood Monitored by DRIFT Spectroscopy and CIE L[ ]a[ ]b[]Color Measurements. I Effect of UV Light. Holzforschung 49 397 04, 1995. 

ARIAS R, LEE T-C, LOGENDRA L and JANES H (2000), Correlation of lycopene measured by HPLC with the L, a, b color readings of the hydroponic tomato and the relationship of maturity with color and lycopene content , J Agric Food Chem, 48, 1697-1702. 

The color measurement of fried foods is traditionally done either instrumentally or visually through trained panelists. Human evaluation is quite subjective, tedious, and subject to error of perception. Hence, it is least recommended for the color determination of fried foods. The instrumentation procedure involves the use of color spaces and numerical values to create, visualize, and represent color in 2D and 3D spaces (Trusell et al., 2005). Food color is usnally measured using the L a b color indices. This is the international standard color measurement technique, adopted by the Commission Internationale d Eclairage (CIE) in 1976. L is a measure of luminance or lightness on a scale of 0-100, a is a measure of greenness to redness, and b is a measure of blueness to yellowness. This technique provides measurement similar to how human eye perceive colors (Hunt, 1991). 

To identify the crystalline phases that were present in the raw and fired samples. X-ray diffraction patterns were collected using a conventional powder technique in a Siemens Diffractometer (D500 mod) employing Cu K.a Ni-filtered radiation. To define the color developed about the samples, a UV-Vis spectrophotometer with analytical software for color measurements (PERK-fN ELMER Spectrometer Lambda 19, UV/VIS/NIR, Standard Observer 10°) has been used. L, a, b color parameters have been measured following the CIE (Commission International de I Eclairage) colorimetric method. In this method, L is the lightness axis (black 0)—>white (100)), a is the green (-) —>red (-H) axis, and b is the blue (-) — yellow (-r) axis. 

L a b color space A method of depicting color that is uniform and recommended by the Comission Internationale d Eclairage for standard color description. L - psychometric lightness (0 = black 100 = white) a psychometric red ( +ve values), green (-ve values) b psychometric yellow (+ve values), blue (-ve values). 

CIE L a b color values for 5.0% blown film samples are shown in Table 2. Relative color strength and dispersion results are shown in Tables 3 and 4, respectively. Tint strength test results appear in Tables 5 and 6. 

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