A way of writing any colour as three numbers: L* for lightness, a* for green to red and b* for blue to yellow, designed so that equal distances look roughly equally different.
The CIE 1976 L*a*b* colour space, defined in the CIE's colorimetry reference. Each value is calculated from CIE XYZ relative to a stated reference white through a cube-root function. Its cylindrical form, LCH, replaces a* and b* with chroma (C*ab) and hue angle (hab).
A CIELAB value is incomplete without its reference white. Print and ICC profiles use D50, many screen tools use D65, and the same colour has different a* and b* values in each.
A mid-grey is L* 50, a* 0, b* 0; a strong red has a large positive a*, a strong blue a large negative b*.
It is the space in which colour differences (ΔE) are calculated and in which paint, print, plastics and textile colours are specified and checked.
CIELAB values are calculated from a measurement. A spectrophotometer gives a reflectance curve, which is combined with an illuminant and a standard observer to give XYZ. Each of X, Y and Z is divided by the value for the reference white and passed through a cube-root function. L* comes from Y alone, a* from the difference between the X and Y terms and b* from the difference between the Y and Z terms. The result is meaningless without its conditions, so a full statement reads, for example, CIELAB D50/2°. Graphic arts and ICC profiles use D50; CSS lab() also uses D50; many display tools use D65.
| Colour | L* | a* | b* | C*ab | hab |
|---|---|---|---|---|---|
| Scarlet #ff2400 | 54.6 | 76.2 | 67.7 | 101.9 | 42° |
| Emerald Green #50c878 | 72.5 | −51.3 | 30.3 | 59.5 | 149° |
| Mustard #ffdb58 | 88.3 | −1.5 | 66.8 | 66.8 | 91° |
| Cobalt Blue #0047ab | 32.8 | 22.5 | −58.4 | 62.6 | 291° |
| Grey #808080 | 53.6 | 0.0 | 0.0 | 0.0 | none |
Scarlet has a positive a* (76.2) and Emerald green a negative one (−51.3): that axis runs from green to red. Cobalt blue's b* of −58.4 puts it far down the blue end of the yellow-to-blue axis. Grey has a* and b* of zero, so it has no chroma and no hue angle. C*ab and hab are the same point written as a distance from the neutral axis and an angle around it.
How this was calculated: sRGB to XYZ to CIELAB with the D65 white of sRGB as reference. Print and ICC work quote CIELAB relative to D50, which gives different numbers for the same colour.
A colour standard is passed between companies as three Lab numbers with nothing else.
The receiver assumes a different illuminant, observer or measurement geometry and manufactures to a different colour while meeting the numbers. A specification states all three alongside the values and the ΔE formula for the tolerance.
Two colours are blended, or a gradient is drawn, in CIELAB.
Blue shifts towards purple on the way to white or grey, because lines of constant hue are curved in that part of the space. OKLab was introduced largely to correct this.