Hunter Lab
L is a correlate of lightness, and is computed from the Y tristimulus value using Priest's approximation to Munsell value:
where Yn is the Y tristimulus value of a specified white object. For surface-color applications, the specified white object is usually (though not always) a hypothetical material with unit reflectance and which follows Lambert's law. The resulting L will be scaled between 0 (black) and 100 (white); roughly ten times the Munsell value. Note that a medium lightness of 50 is produced by a luminance of 25, since
a and b are termed opponent color axes. a represents, roughly, Redness (positive) versus Greenness (negative). It is computed as:
where is a coefficient which depends upon the illuminant (for D65, Ka is 172.30; see approximate formula below) and is the X tristimulus value of the specified white object.
The other opponent color axis, b, is positive for yellow colors and negative for blue colors. It is computed as:
where Kb is a coefficient which depends upon the illuminant (for D65, Kb is 67.20; see approximate formula below) and Zn is the Z tristimulus value of the specified white object.
Both a and b will be zero for objects which have the same chromaticity coordinates as the specified white objects (i.e., achromatic, grey, objects).
Read more about this topic: Lab Color Space
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