Way back in 1968, the year before the BBC first broadcast TV in colour, the technology programme Tomorrow’s World showed a cunning demonstration of an illusion appearing to show colour on a monochrome TV image. It went something like the Lilac Chaser colour illusion below.

The ‘lilac chaser’ shown here and detailed on Wikipedia is a simple example, and there are collections such as that at Archimedes’ Laboratory here. They emphasise that, no matter what physicists might claim, human colour vision is pure perception, a construct of our brain in response to physical stimuli transduced by specialised nerve cells.

Physical measurement of colour can be expressed in its chromaticity, a point in the CIE 1931 chromaticity space above, for example. Although this attempts to include all the chromaticities which can be perceived as colours by humans, it’s a physical not perceptual model. There’s a great deal more to human colour vision than mere chromaticity.

We often use different properties to characterise different colours as we perceive them. Among these are hue, brightness, lightness, colourfulness, chroma, saturation, and colour relations. Each has formal definitions given in various standards concerning colour, but one of the best ways of remembering their relationships is through equations:

• chroma = colourfulness / brightness of a white at the same illumination
• saturation = colourfulness / brightness
• lightness = brightness / brightness of a white at the same illumination
• saturation = chroma / lightness

Hue is the type of colour, such as red, yellow, etc., or a mixture of colours forming an intermediate. These colours are commonly arranged in the familiar circle or wheel with adjacent colours merging. One measure of hue is then expressed as the angle subtended within the circle, although there’s no evidence that our brains handle hue in that way. The angle is related to the wavelength of the light, although as there’s no easy way to convert between angle and wavelength that’s seldom attempted.

Chroma is the degree of departure of the area of colour from an area of grey with identical lightness, and is in effect the amount of hue present in the colour. In most cases this isn’t easy to separate from lightness, as very pale colours necessarily have relatively low chroma. You can gauge the chroma by comparing the original colour with a matching patch which has been desaturated to grey.

Lightness is the perceived brightness of a non-white area of colour compared with that of a perfectly white area. It thus equates to what’s often called ‘tone’ or ‘value’ in visual art. You can get an idea of the lightness ‘signal’ of an image by desaturating it to greys, then adjusting the brightness and contrast until the whitest white in the image is shown as pure white, and the deepest dark approaches a pure black.

Unlike brightness, which is a more absolute perception of light intensity or luminance, lightness is relative because our visual systems have quite good lightness constancy. This is achieved in part by constricting and dilating the pupil to adjust the aperture of the lens, just as cameras do, although there’s also central processing in the brain to adjust lightness. However, when it’s very dark or very bright, these mechanisms cannot fully compensate, so our whole visual image on a dark moonless night is very dark indeed.

However we care to define these characteristics, human colour vision seldom seems to work by anyone’s rules. Take the perception of lightness, which should be one of its simpler features.

In the Checker Shadow or grey square illusion and its relatives, two areas of identical luminance/luminosity appear to have different lightness (or brightness). This is presumed to be the result of perceptive intent to maintain lightness constancy under shadow: we perceive that the darker grey squares are all the same level of lightness, so a ‘white’ square in shadow must therefore appear to be lighter than a grey square in full light.

The Helmholtz-Kohlrausch illusion is observed when an area of colour is viewed against a background (or surround) of the same luminance/luminosity but without hue or chroma, a grey. This illustrates the confounding effect of hue and chroma on lightness perception.

There are several illusions showing the effects on perception of simultaneous contrast. Concentrating on those most relevant to lightness, the lightness of an area is influenced by that of its background, surround, or adjacent areas. A simple example of this is to view the same colour (or luminance/luminosity, if grey) on a darker and lighter background; the darker background will make the colour/grey appear lighter still, while the lighter background will make it appear darker.

Simultaneous contrast effects are also perceived when an area of the same colour or grey is viewed against changing or different background colours or greys, as shown above.

The interaction of chroma with lightness or luminance/luminosity makes separate study very difficult. Chroma constancy is part of the more general colour constancy, but hasn’t been studied on its own as much as lightness or colour constancy have.

For example, these two test strips feature areas which are set at identical lightness/brightness, but whose luminance/luminosity differs. The first strip uses lightness/brightness fixed at 50%, the second at 75%. However the different coloured areas appear to have different levels of chroma, with the red and blue squares apparently higher in chroma than the others. This matches the differences in luminance/luminosity. This is thoroughly confusing, and illustrates how difficult it is to isolate chroma effects.

Fascinating as these illusions are, what are we to learn from them? Most importantly that artists need to depict what they see by producing what looks to them to be an accurate representation of the image in their mind. If you’re trying to paint a faithful realist representation of a landscape, then look at the landscape and paint what you see. They also underline the first message, that there’s a great deal more to human colour vision than mere chromaticity.

Reference

Mark D Fairchild (2013), Color Appearance Models, 3rd edn, Wiley, ISBN 978 1 119 96703 3.