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/Y_n) - 16

a* = 500 [ƒ(X/X_n) - ƒ(Y/Y_n)]

b* = 200 [ƒ(Y/Y_n) - ƒ(Z/Z_n)]

where

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

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

and

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

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

and

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

ƒ(Z/Z_n) = (841/108) (Z/Z_n) + 4/29                  if (Z/Z_n) ≤ (6/29)³

and X, Y, Z describe the colour stimulus considered

and X_n, Y_n, Z_n 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/Y_n) - 16

where

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

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

CIE 1976 a, b chroma

CIE 1976 a, b hue angle

        h_ab = arctan (b*/a*)

See also CIE 15 Colorimetry

See also ISO 11664-4:2008(E)/ CIE S 014-4/E:2007 Colorimetry - Part 4: CIE 1976 L*a*b* Colour Space