Perception

Perception of Colours

Try looking at the diagram above from different distances. Do the colours appear to remain the same?

The picture on your TV set is made of tiny red, green and blue spots or lines. You can see this if you look briefly with a lens. But if you find regions where the picture is bluish, greenish, reddish, yellowish, purplish, etc, what happens to the apparent colours of the spots? The following pictures give some idea of the way a shadow mask cathode ray tube operates and how the colours mix. The density of the three primary colours is graded across the triangle. When the primary colours are brightest at the tips of the triangles they are seen pure. When the primary colours are brightest at the sides of the triangles the tips show a mixture of two primary colours, which are complementary to the primary ones.

The colours produced by three-colour TV or slide film can never be perfectly correct, for the following reasons. When light falls on a tri-colour TV camera or a tri-colour film, the colour and brightness information at each point is encoded into three values, corresponding to the responses of the three types of receptors. Unfortunately, for each set of three values there are many possible variations in the incoming light, which are therefore indistinguishable by the system..

Obviously not all of these can be reproduced properly, even if one of them can. After processing the data, these values will cause the three TV phosphors, or the three film dyes, to produce or transmit light, according to their respective spectra. In general, these will not add up to make the same spectrum as the original light source. When the resulting spectrum is detected by the eye, it will generally give a different result from the original one.

The picture below, of sunlight reflected from a compact disc, shows that the smoothly graded solar spectrum has been replaced by a much more limited range of colours.

In the pictures below the brightness of the squares varies vertically, while the brightness of the background varies horizontally – nothing varies diagonally.

As well as these static effects, which are well illustrated by many of the pictures of Victor Vasarely, there are effects caused by fatigue of the receptors, which can produce after images when pictures are coloured strongly or have strong black and white patterns. Marcel Duchamp’s "Fluttering Hearts" is an early example of this type of "Op Art". Some pictures by Bridget Riley show related effects.

The differences so great that people often prefer to use only daylight when comparing colours. The difference between incandescent light can be seen well around dawn or dusk, when lights are on in houses. In spite of the effects of different types of illumination, people have no trouble in distinguishing, white, grey and coloured objects. Under these various types of illuminations, people can also distinguish pure white from very pale tints. It is as if the concept of whiteness is built in, and can survive moderate changes of illumination.