How We See Colour

In this section I will explore the effect that your perception of colour has on your editing ability.


FIGURE 1 Visible light to the human eye is just a small band in the vast spectrum of radiation energy where the only difference is in the wavelengths.

There is no colour in light any more than there is the feeling of pain in a sharp object; both are responses our senses have upon coming into contact with each. At the back of the eye are receptors (cones for colour and rods for intensity) that are sensitive to three main wavelengths which we register as the primary colours of Red, Green and Blue (RGB). Different wavelengths will trigger different amounts of these three basic colours and from the resulting combinations the eye can form millions of colours. Some animals may see light with the same values as humans, but apparently many see colour very differently. A few are even able to see into the infrared region of light wavelengths, which would be useful for night vision, so you could argue that their primary colours may well be described as IGB instead. Why do we have three primary colours and not two or four? There is nothing in physics to say that there must be three, it has just been decided for us by evolution as the best choice to give a wide range of colours. Our eyes and brains register and 'see' RGB, so to humans everything is in RGB values.


FIGURE 2 Here we see how the primary colours of visible light have varying wavelengths.

When making electrical equipment which emit light (such as scanners, TVs, monitors, etc) what stops us using Cyan, Yellow and Infrared as primary colours, as shown by the top row of pointers above? Nothing actually, but they would be of limited use. If a scanner was set to use these primary colours you could not use Blue in the images as no amount of Cyan, Yellow and Infrared will ever make up Blue and the Infrared would be invisible. If we expect to get as close as possible to the full width of colours that our eyes can see, then we had better place two of the primary colours at the two extremes of Red and Blue. The logical place for the third primary colour is in the middle, which is Green just as the human eye is constructed, as shown by the bottom row of pointers above.

FIGURE 3 A common misunderstanding is that the primary colours are Red, Blue and Yellow.  This is based on most peoples' experience with paint, so they are really thinking of Magenta (light red), Cyan (light blue) and Yellow.

Another common misunderstanding states that quality scanners scan with the primary colours of CMYK. Cyan and Yellow are no problem. They may not be human primary colours, but the separate light wavelengths for Cyan and Yellow do exist. But where is the Magenta in the colour spectrum? This can not be a primary colour as there is no wavelength of light for it. Magenta is a secondary colour made from a mix of Red and Blue so to scan in CMYK we would really be scanning in C(RB)YK instead. The main problem though is just where in the spectrum is Black light? It does not exist. Black light equals no light and no scanning equipment can deal with that. The laws of physics apply to high level scanners as they do to low level - if you are scanning for humans then it had better be in RGB. The CMYK scanners do in fact scan in RGB but they convert to CMYK on the fly before the image reaches Photoshop. The Black (K) is generated by extracting values from the main CMY colours.

We see colour in Red, Green and Blue values but that does not mean that we are therefore equally sensitive to all three. We have evolved on a planet awash with blue light and as a result evolution has given us eyes that are only half as sensitive to blue as they are to red and green to help compensate. This 40% Red, 40% Green, but only 20% Blue sensitivity in the cones at the back of the eye results in us seeing blue as a dark colour. This is of benefit in nature, but with the invention of printing it has resulted in a real problem. We just can not produce blue dyes, inks and paints that are of a strong enough colour to compete with the extra sensitivity we have to red and green. For some reason we also have a stronger reaction to green than to red. The combination of RGB makes white, but of the three, green is the brightest, followed by red, and blue being the darkest.

The secondary colours of Cyan, Magenta and Yellow (CMY) are made by mixing 2 of the primary colours for each. Magenta is formed with red and blue, and as more cones in the eye are used we see it as a lighter colour than red or blue. Cyan is formed with green and blue and for the same reason it too is a lighter colour. Yellow is formed with red and green and since this involves most of the cones in the eye it is therefore the lightest colour of all, with the exception of white.