Page 107 - Graphic Design and Print Production Fundamentals

4.4 Lab Colour Space and Delta E Measurements

Alan Martin

The CIE (Commission Internationale d’Eclairage or International Commission on Light) is a scientific
body formed by colour scientists in the 1930s that has provided much of the fundamental colour
knowledge we possess today. Three core definitions provided by the CIE are the standard observer,
the Lab colour space, and Delta E measurements. The latter two are particularly important for colour
management.

The Lab Colour Space Revisited

In section, 4.2, we mentioned the Lab colour space as a natural outgrowth of understanding the function
of opponency in human vision. It’s comprised of three axes: L represents darkness to lightness, with
values ranging from 0 to 100; a represents greenness to redness with values of -128 to +127; and b
represents blueness to yellowness also with values from -128 to +127.

Notice that there are no negative values on the L axis as we can’t have less than zero light, which
describes absolute darkness. The L axis is considered achromatic meaning without colour. Here we
are dealing with the volume rather than the kind of light. In contrast, the a and b axes are chromatic,
describing the colour character and the type of light.

The standard two-dimensional depiction is of only the a and b axes, with a as the horizontal axis and b as
the vertical axis. This places red to the right, green to the left, blue at the bottom, and yellow at the top.
If you found our previous mnemonic aid of POLO helpful, you can use RGBY to remember the colour
pairs. For correct placement, remember that red is on the right, and blue is on the bottom.

Colours are more neutral and grey toward the centre of the colour space, along the L axis. Imagine
that equivalent values of the opposing colours are cancelling each other out, reducing the saturation and
intensity of those colours. The most saturated colours are at the extremes of the a and b axes, in both the
large positive and negative numbers. For a visual depiction of the Lab colour space, open the ColorSync
application found in the Utilities folder of any Macintosh computer and view one of the default profiles
such as Adobe RGB.

Now it’s time to explore the practical application of this colour map for the comparative analysis of
colour samples. We can’t make any progress in evaluating our success in colour matching unless we have
a frame of reference, some yardstick to determine how much one colour sample is similar or different
from another. That yardstick is the Delta E measurement.

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