Page 100 - Graphic Design and Print Production Fundamentals

88 Chapter 4. Colour Management in the Graphic Technologies
Trichromacy

Figure 4.3 Rods and cones (adapted by Ken Jeffery)

We call it ‘visible’ light because it is the portion of the electromagnetic spectrum that our eyes are
sensitive to. The two types of receptors in our eyes are cones and rods (see Figure 4.3). The cones
respond to colour and the rods come into play in low-light situations when we see only varying
shades of grey. There are three types of cones, each sensitive to approximately one-third of the visible
spectrum. We characterize those segments of the spectrum as red, green, and blue, and this three-colour
or trichromatic response by the cones is where all colour experience begins. Every colour we perceive
comes from mixing varying volumes of the red, green, and blue signals from the three types of cones in
our eyes.

The Additive Primaries

We refer to the red, green, and blue colour set (RGB) as the additive primaries. When we combine or
add all three of these, we get the entire spectrum and, thus, white light. This is the primary colour set
involved whenever we talk about the transmission of light, such as the display on a computer monitor, a
tablet, or from a projector. For this reason, red, green, and blue are also referred to as the transmissive
primaries.

The Subtractive Primaries

What happens when we project two of the three additive primaries on top of each other? This is the same
as removing or subtracting one of the additive primaries from white light. Let’s start with red and green.
Though not at all intuitive, if you have any experience with mixing paint or inks, the combination of
red and green light produces yellow. Remember that we are adding light to light, so the production of a