Colour Perception And Illusion

A colour illusion takes place when the colour we see gives information which, if tested otherwise than by vision, would be found to be misleading or confusing or even meaningless. An illusion is also set to occur when colour changes for no reason immediately apparent to the observer. A colour illusion causes us, for the moment at least, to be deceived by what we see; but the extent to which we are deceived depends on the extent of our knowledge and experience. What is an illusion for one person may not be for another.

Colour is a sensation in the brain and exists only when there's someone present to experience the sensation; and since it is usually caused by the reflection of light waves from surfaces onto the eye it changes with every change of light or surface. Colours become bluer, yellower, darker or lighter according to whether, for example, they are seen in open daylight, in artificial light, in twilight or in sunlight, and they are hardly ever exactly the same for any length of time.

Constancy

The brain achieves an extraordinary feat when it gives a consistant and uniform appearance to the things we see in so many different lights and contexts. The brain's faculty for giving continuous order to the mass of unassimilated data that it is presented with by the senses is called constancy.

The brain's attempt to recognize beneath the changing conditions of the outside world, certain surface qualities that are relatively constant can be described as colour constancy. It's success depends upon stored experience and the evidence not only of sight but also of our other senses.

Illusion is, physiologically speaking, a glimpse behind the scenes at unassimilated sense data which has not, as it where, been put into proper perspective by constancy.

Colours often appear to change when placed one against another. Many people will see the two squares here as one bluish and the other yellowish. They are, in fact, exactly the same colour. This has been an illusory change in Hue.

Illusory change in Lightness

The colour in the centre square appears darker on the light surface and lighter on the dark surface. This has been a change in Lightness or Tone Value.

Illusory change in Saturation

A colour of low saturation, or colour intensity as it is sometimes called, placed on a surface colour of even lower saturation, tends to look more saturation than it is. The same colour placed on a surface of higher saturation in colour will appear to be less saturated than it is. The classic work on the subject is that of Michel Eugene Chevreul, chemist and director of the Goblin Tapestry works. Colours appeared to change when woven into their tapestry and Chevreul wrote on what he called the Law of Simultaneous Contrast. CHEVREUL, M.E., (1839) De la Loi du Contraste Simultané des Couleurs. Paris. This was a proufound influence upon the Impressionists. It was not translated into English until 1854. (Martel's Principles of Harmony and Contrast of Colours.) The finest examples of colour illusions, however, are those in The Interaction of Colours by Josef Albers of the Bauhaus and Yale University.

Change of apparent size

Some colours appear to change the size of the shape they are in. Here, the yellow square appears larger than the blue square, although they are both the same size. This is because Yellow has a lot of white in it, so that it is received on the retina of the eye by three different receptors, the red receptor cones and the green receptor cones.

Landscape

In this landscape, the eye sees the fields in the foreground as green and the hills in the distance as blue, but no one supposes that the grass on the hills is in fact blue. We take it for granted that if we'd climbed the hills we would find them to be green, and we are well aware that a particular quality of the grass, that of greenness, is constant. The faculty of the brain to discern the constancy of things in relation to changing conditions allows us to remain undisturbed by the many colour illusions which meet the eye. The blueness of the grass on the hills is not an illusion to the average person. One is not deceived by it. It has been learnt from experience that distance often makes things appear bluer.

Simultaneous contrast

Colours can appear to change by placing them against different colours. A colour will look less colourful if placed against a more colourful hue. This is an illusion of saturation. Another colour will look darker on a light surface, and lighter on a dark surface. This is an illusion of tone value. Yet again, a colour placed on a warmer suface will look colder than it is normally, and warmer on a colder colour. These are illusions of hue.

Metamerism

The colour of a fabric can match that of another under one light but not under another. On the left are two green samples seen under daylight. On the right the same two samples are completely different under another light such as tungsten. This illusion can cost a client a lot of money if the colours haven't been checked under both lights. This is because the two samples are from different batches of the 'same colour' dyed with completely different chemicals.Two samples of textiles that match under one light but not under another one called a matameric pair

After-Image

Union Jack by After-Image

Here is another flag that changes by after-image. Look at this for twenty seconds without moving your head or shifting your eyes, then look at the grey square next to it and the unknown flag becomes the Union Jack!