L* a* b* color space

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...

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] ... 

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).

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). 

As polystyrene substrates have a very high degree of whiteness it is particularly suitable for studying the change in hue of the sample as a function of the spectral irradiance distribution. A hue such as yellow is uniquely identified by a point in the L, a, b color space. The displacement of this point as a function of the cut-on filter used to modify the source spectrum indicates the progress of discoloration. Therefore, when changes in hue of the irradiated samples in a—b color space is plotted for samples exposed behind each filter, the general trend of the process becomes apparent. With polystyrene, it is clear that the... 

A commonly used and universally accepted system for describing color is the L a b color space, or CIELAB, developed by CIE, the International Commission on Illumination. The color space shown in Figure 111 is a three dimensional sphere comprised of three numerical coordinates ... 

L a b Color Space, Konica Minolta Sensing, Inc., Precise Color Communication, 2007 graphic reproduced from Reference I. 

Figure 3. A flow chart of the color acquisition procedure. The RGB signals of a white reference and samples were first acquired by a camera sensor. Colors of samples were then acquired and converted from RGB color coordinate to L a b color space for color identification.

A CMVS was designed and evaluated on colors perceived by human. The machine vision measured tri-stimulus values in R,G,B color coordinate were transformed to L a b color space for better representation of human perceived colors. The CMVS measurements were evaluated using Munsell standard color chips, a colorimeter, and a sensory panel. Excellent correlations were found between the CMVS and the colorimeter, which is designed to quantify human perceived colors. The CMVS s ability in quantifying human perceived colors was evidenced by accurate measurement of Munsell standard color chips and the excellent correlations with the sensory panel perceived colors (R>0.90). From the experiments, it was possible to resolve up to 0.7 CIE unit color difference of the tested colors. 

From the above discussion, it is clear that given a CIE standard source, object, and spectral tristimulus values, one can easily measure color. The instrument developed for such color measurement is called a tristimulus colorimeter. A tristimulus colorimeter measures color in terms of three primary colors red, green and blue, or more properly stated, in terms of three tristimulus values. Many different color scales have been developed to describe the color numerically in terms of lightness and hue. One of the most widely accepted systems is known as the L, a, b tristimulus system. Figure 6-10 illustrates L, a, b color space. The coordinate L is in the vertical direction and corresponds to lightness. A perfect white has an L value of 100 and a perfect black a zero. The variables at and bt identify the hue... 

Figure 6-10. L, a, b color space. (Courtesy of Gretag Macbeth.) See color insert. A full color version of this figure can be viewed on DVD included with this book.

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