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# 17-23-076

CIE 1976 L*a*b* colour space

CIELAB colour space

three-dimensional, approximately uniform colour space produced by plotting in rectangular coordinates L*, a*, b*, quantities defined by the equations:
L* = 116 f (Y/Yn) − 16
a* = 500 [f (X/Xn) − f (Y/Yn)]
b* = 200 [f (Y/Yn) − f (Z/Zn)]
where
f (X/Xn) = (X/Xn)1/3 if (X/Xn) > (6/29)3
f (X/Xn) = (841/108) (X/Xn) + 4/29 if (X/Xn) ≤ (6/29)3
and
f (Y/Yn) = (Y/Yn)1/3 if (Y/Yn) > (6/29)3
f (Y/Yn) = (841/108) (Y/Yn) + 4/29 if (Y/Yn) ≤ (6/29)3
and
f (Z/Zn) = (Z/Zn)1/3 if (Z/Zn) > (6/29)3
f (Z/Zn) = (841/108) (Z/Zn) + 4/29 if (Z/Zn) ≤ (6/29)3
and X, Y, Z describe the colour stimulus considered and Xn, Yn, Zn describe a specified white achromatic stimulus.

Note 1 to entry: Approximate correlates of lightness, chroma and hue can be calculated as follows:
CIE 1976 lightness
L* = 116 f (Y/Yn) − 16
where
f (Y/Yn) = (Y/Yn)1/3 if (Y/Yn) > (6/29)3
f (Y/Yn) = (841/108) (Y/Yn) + 4/29 if (Y/Yn) ≤ (6/29)3
CIE 1976 a, b chroma CIE 1976 a, b hue angle
hab = arctan (b*/a*)