## 17-157 CIE 1976 L*a*b* colour space

3-dimensional, approximately uniform colour space produced by plotting in rectangular coordinates L*, a*, b* quantities defined by the equations:

L* = 116 ƒ(Y/Yn) - 16

a* = 500 [ƒ(X/Xn) - ƒ(Y/Yn)]

b* = 200 [ƒ(Y/Yn) - ƒ(Z/Zn)]

where

ƒ(X/Xn) = (X/Xn)1/3                                                 if (X/Xn) > (6/29)3

ƒ(X/Xn) = (841/108) (X/Xn) + 4/29                if (X/Xn) ≤ (6/29)3

and

ƒ(Y/Yn) = (Y/Yn)1/3                                                 if (Y/Yn) > (6/29)3

ƒ(Y/Yn) = (841/108) (Y/Yn) + 4/29                if (Y/Yn) ≤ (6/29)3

and

ƒ(Z/Zn) = (Z/Zn)1/3                                                    if (Z/Zn) > (6/29)3

ƒ(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.

Equivalent term: "CIELAB colour space"

NOTE Approximate correlates of lightness, chroma and hue may be calculated as follows:

CIE 1976 lightness

L* = 116 ƒ(Y/Yn) - 16

where

ƒ(Y/Yn) = (Y/Yn)1/3                                                 if (Y/Yn) > (6/29)3

ƒ(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*)