COLOURS (Part 3 of 3)

Colour Vision in Animals

Since animals cannot answer questions about the colours they perceive, scientists have had to develop experiments to find out whether animals can be trained to make choices on the basis of colour If an animal's food is always placed under a red square instead of a green square and if the animal consistently looks under the red square when it is hungry, scientists conclude that the animal can distinguish between red and green. Since monkeys and apes can be trained in this way and, in addition, their retinal cells contain colour-sensitive chemicals, researchers are convinced that these primates have colour vision.

Non primate mammals tend to be insensitive to colour differences, but it is not certain whether this means that they cannot perceive them or that they do not regard them as important. Cats, for example, are commonly assumed to be colour-blind, but claims have been made that they can be trained to discriminate between some colours, among them blue and green, by first linking the colours with position. In any case, their sensitivity to colour is not great.

Birds have good colour discrimination, somewhat similar to that of humans. Many of their behaviour patterns for example, the identification of their mates or of their prey are based on the recognition of colour Fish can also discriminate among colours Bees have colour vision similar to human colour vision, except that it includes ultraviolet wavelengths too short for humans to perceive, and it excludes the red end of the spectrum, visible to humans.

Techniques for Reproducing Colour

When early humans painted pictures on cave walls, they used the pigments in coloured earth and clay to give colour to their creations. Red, black, and yellow pigments were the easiest to find. Then humans learned how to create new colours by mixing these three pigments. Other pigments, it was discovered, produced other colours, such as orange, brown, and blue-black.

As humans learned how to make pottery and carvings and to weave fabrics, they also learned how to apply colour to these new objects. Pigments had to be applied to pottery and carvings in a form that would adhere to their surfaces. The materials and techniques originally employed were closely related to those that had been used in painting pictures on flat surfaces. Naturally available pigments were mixed to get new colours

From the beginning, however, making dyes for woven fabrics involved chemical processes. At first, parts of plants were boiled to separate coloured chemicals. The cloth was then soaked in the resulting coloured solutions.

Today chemical reactions are used in various ways to produce new dyes. The changes in colour that result from these reactions are a consequence of changes in chemical structure and cannot be explained by the laws of colour mixing.

Modern techniques for printing pictures in full colour can become quite complicated. They are based on colour separation at one stage and subtractive colour mixing at another and may also include additive colour mixing. The three basic colours of colour printing yellow, cyan, and magenta can be mixed in various proportions to duplicate almost any colour

The first step in colour printing is to scan the original photograph or piece of artwork with a light beam that gets split into three beams after it has passed through, or has been reflected from, the original document. Each beam then strikes a photocell that is covered with a filter coloured to match one of the additive primary colours In this way each area of the original is separated into its three colour components. Four computers are used to correct the colour via electric currents that are fed into the computers from the photocells.

Each computer corresponds to one of the additive primary colours and one computer is reserved for the black component, which is computed from the other three signals. Exposing lights manipulate the modified currents from the computers and then expose the corrected colour separations on film or paper.

In half-tone printing, the colour is applied as an array of tiny dots. Where both cyan and magenta ink are applied, some of the dots will be cyan and some magenta. Printed side by side, both colours are reflected, and they mix additively to form blue. Where yellow and magenta are reflected in equal strength, they mix additively to form red. Intermediate shades can be formed by varying either the size or the number of the dots and by printing the dots on top of one another.

Colour television also works by first separating colours into their additive primaries and then recombining them by additive mixing of coloured dots. In some cameras, mirrors separate the light into three beams. The first beam passes through a blue filter, the second through a green filter, and the third through a red filter. Three camera tubes then record the colour information in the form of electromagnetic signals, which are beamed to the television receiver. Consumer cameras may use a single detector chip, known as a charge-coupled device (CCD). Red, green, and blue filters are affixed directly to the chip.

A colour picture tube contains three electron guns, one for each colour The back of the television screen is coated with tiny dots of chemicals called phosphors. When a phosphor is hit by an electron, it gives off wavelengths of light. Most colour television sets contain three kinds of phosphors that give off blue, green, and red light, respectively. A screen called a shadow mask lies behind the phosphor layer. The electron gun that receives the blue signal fires electrons toward the phosphors. The shadow mask screens the red-emitting and green-emitting phosphors from these electrons so that only the blue-emitting phosphors are activated. Similarly, the electron gun that receives the red signal is screened from all but the red-emitting phosphors, and the one that receives the green signal is screened from all but the green-emitting phosphors. The colours mix additively to form a wide range of colours

Colour photography is a blend of chemistry and the additive and subtractive principles of colour mixing. A typical colour film consists of three layers of chemicals, each of which is sensitive to one of the additive primaries. The top layer contains chemicals that react to blue light only. Below it lies a yellow filter that absorbs all blue light passing through the top layer. Green and red light pass through the top layer and the filter to reach the middle layer, which is sensitive to green and blue. Since the yellow filter has stopped all the blue light, only green light affects the middle layer. Red passes through this layer to the bottom layer, which is sensitive to red and blue and partly to green. Since the blue light and the green light have already been filtered out, only the red light can cause a chemical change in the bottom layer. In this way, three records are made of the scene, each containing information contributed by one of the additive primaries.

The three records may be combined into a colour transparency, which is dyed in appropriate combinations of the subtractive colour primaries magenta, cyan, and yellow. White light may be shined through the transparency and the resulting image projected onto a screen. If the transparency has been dyed with cyan and magenta only, the cyan absorbs red light, the magenta absorbs green light, and the light transmitted and projected onto the screen is blue. If the transparency has been dyed with yellow and a small amount of magenta, no blue can pass through it, and some but not all of the green is absorbed, leaving a mixture of wavelengths that produces a sensation of orange.

A photographic technique called holography uses laser light to produce a three-dimensional image of a subject. A record called a hologram is made of the interference pattern between laser light that has bounced off the subject and light coming directly from the laser. When laser light is shined through the hologram, light deflected by the hologram reconstructs the image of the subject. If an observer walks around the hologram and views it from different angles, he can see the changing perspective just as if the original subject were still there. Techniques are being developed to obtain holographic images using laser light of three different colours, so that a full-colour image can be produced. A problem not yet overcome is that ghost images appear to the side of the actual image.

Ultimately, holography may be used to produce three-dimensional television.

Response to Colour

On the whole, people tend to regard blue and green as cool, quiet colours, while yellow and, especially, red are considered warm colours Individual colour preferences may be based on this general difference in responses to colour Tests have shown that the colour preferences of children tend to shift from warmer to cooler colours as they grow older.

Other tests have shown that blue is the most widely preferred colour, with red, green, violet, orange, and yellow as runners-up. However, variations on these colours have produced different rankings. Yellowish green usually ranks below a relatively pure green.

It has also been shown that many people prefer pure, or saturated, colours

Assisted by Roy S. Berns, Director of the Rochester Institute of Technology's Munsell Colour Science Laboratory in New York.

BIBLIOGRAPHY FOR COLOUR

Anderson, L.W. Light and Color, rev. ed. (Raintree, 1988).

De Grandis, Luigina. Theory and Use of Color (Prentice, 1986).

Falk, D.S. and others. Seeing the Light: Optics in Nature, Photography Color, Vision, and Holography (Harper, 1986).

Hoban, Tana. Of Colors and Things (Greenwillow, 1989).

Kuehni, R.G. Colour, Essence and Logic (Van Nostrand Reinhold, 1983).

McLaren, K. The Colour Science of Dyes and Pigments, 2nd ed. (Hilger, 1986).

Nassau, Kurt. The Physics and Chemistry of Color (Wiley, 1983).

Norman, R.B. Electronic Color: The Art of Color Applied to Graphic Computing (Van Nostrand Reinhold, 1990).

Rossotti, Hazel. Colour (Princeton Univ. Press, 1985).

Thorell, L.G. and Smith, W.J. Using Computer Color Effectively (Prentice, 1990).

Williamson, S.J. and Cummins, H.Z. Light and Color in Nature and Art (Wiley, 1983).