Page 102 - Graphic Design and Print Production Fundamentals

90 Chapter 4. Colour Management in the Graphic Technologies
Lab Colour Space

Once the opponent nature of colour interpretation was understood, colour scientists were able to create
a model colour space based on the opposing pairs. This is the Lab colour space (see Figure 4.4). The
Lab variation of interest to us is officially called CIELAB, and all references in this textbook to Lab
will mean CIELAB. Additionally, references to L, a, and b in this textbook are equivalent to the L*, a*,
and b* units of the CIELAB colour space. Each of the opposing pairs provides one axis of this three-
dimensional colour space. L is the axis for darkness to lightness; a is the axis for greenness to redness;
and b is the axis for blueness to yellowness. By providing a value for each of the L, a, and b attributes,
a colour is accurately and uniquely located in the colour space. The tremendous utility of the Lab colour
space is that it allows for the mathematical description of a colour in a non-ambiguous and meaningful
way.

Psychology of Colour Perception

We come to the last of our three science P’s: psychology. After the trichromatic response is passed
to opponent interpretation in the physiology of our optic systems, the final engagement of colour
perception occurs in our minds. This interaction complicates and potentially confounds our objective
measurements, so it is critical to be aware of the typical issues that the filter of our mind brings to the
arena of colour perception.

Colour Constancy

Colour constancy is how our mind adjusts our colour perception to discount or remove the effects of an
overall colour cast due to a coloured illuminant. If we see someone wearing a yellow sweater illuminated
by a bluish cast of light, we still ‘see’ a yellow sweater, even though we have no trouble identifying
the colour in the sweater as green if it is isolated from the scene. In our mind, we remove the blue