Lab Colour Space and Delta E Measurements

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

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.

Delta, the fourth letter of the Greek alphabet and symbolized as a triangle, is used in science to indicate difference.?Delta E?is the difference between two colours designated as two points in the Lab colour space. With values assigned to each of the L, a, and b attributes of two colours, we can use simple geometry to calculate the distance between their two placements in the Lab colour space.

How do we do that? It looks a lot like the formula used to determine the long side of a right triangle that you may remember from high school geometry. We square the difference between each of the L, a, and b values; add them all together; and take the square root of that sum.