The CIE L*a*b colour model

In 1976, another development step was taken, which led to the creation of the CIE L*a*B colour model, which fixes some of the problems the CIE Yxy colour model had: all perceptible colours are clearly determinable within the CIE L*a*b colour model by vector. The L-axis, which can only be represented in a three-dimensional view, contains the different levels of bright-
ness; all colours with the same brightness are on one layer.

It is similar when depicting it within the L*a*b colour space, which consists of the two-colour axes a and b: On axis a, colour components can be arranged between red and green. On axis b, colour components between yellow and blue can be arranged, while axis L (for luminance) is responsible for the brightness of the hues. The colour Pantone 3005 U, which is a rich blue equivalent to the RISO blue, represented as colour values would be L 45, a -13 and b -49. So, it is moderately light as well as a composite of a light green and a moderate blue tint. This colour information is just as explicit as classifying them into an RGB or CMYK scale:

Pantone 3005 U
in RGB: R 63 G 120 B 193
in CMYK: C 92 M 44 Y 0 K 0
in L*a*b: L 45 a -13 b -49

The ICC profile provided in L*a*b is ultimately a target/actual comparison between colour values, a comparison of the ideal state and reality.

1 The CIE L*a*b colour model with the colour axes a, b and the brightness axis L