COLOURS (Part 2 of 3)

Colour Classification Systems

People who make, sell, or use nail polish, lipstick, paint, ink, and many other products deal with very small variations in colour Colour classification systems have been developed that enable them to specify and obtain the precise colours they want. Some of these systems show how colours differ in ordinary daylight. Others calculate the wavelengths of light that pass through filters of different colours when a special light source, such as a tungsten lamp, is used.

The Munsell system arranges colour samples according to three qualities hue, value, and chroma. Hue is what is usually meant by the word colour Red, blue, green, and yellow are hues. The Munsell system divides all hues into ten categories: yellows, green-yellows, greens, blue-greens, blues, purple-blues, purples, red-purples, reds, and yellow-reds. Hues are often arranged in a circle. Value is the Munsell term for the lightness of a coloured sample. A yellow material may be light while a blue material may be dark. A series of greys, from black to white, best define value. Chroma defines the amount of hue in a given sample. The word chroma is related to the word chromatic and describes colours ranging from grey to vivid hues. A brick and a ripe tomato, for example, may have the

same red hue and the same value. Their difference in colour is a difference in chroma.

Thus colours can vary in hue, value, and chroma.

In addition to the Munsell system, there are many colour classification systems that relate to colour perception. Two examples are the Optical Society of America Uniform Colour Scales (OSA-UCS) and the Swedish Natural Colour System (NCS). The NCS system arranges colours based on the perceptions of white, black, red, green, yellow, and blue, with only four perceptions for a given colour A purple, for example, may consist of white, black, red, and blue. With experience, one can assign percentages to each perception.

There are also classification systems based on colourant mixtures. These systems are very useful to help visualize how colours mix together. Examples include the Ostwald System and the Pantone Matching System.

Finally, numerical systems have been developed based on the knowledge of the eye's physiology and extensive experimental studies. These numerical systems have been standardized by the Commission Internationale de l'Eclairage (International Commission on Illumination), or CIE. A colour is specified by three numbers relating to the eye's three colour receptors. To specify a colour, one first performs physical measurements on how much a coloured material will reflect or transmit light across the visible spectrum.

Computations follow using this wavelength information to arrive at the three numbers. This system is used to specify the colour of most man-made products and forms the basis for standardizing present and future colour television signals, including high-definition television (HDTV).

Colour Chemistry

Subtractive mixing is based on the way matter affects light. Somehow a beam of white light is changed when it meets certain kinds of matter. Some of the light stays in the matter is absorbed. As a result, the light that emerges is reflected or transmitted has a different colour This happens in part because of the way matter is constructed.

All matter consists of atoms. Each atom contains a dense, heavy centre called a nucleus and one or more electrons that are in continuous motion around the nucleus. According to atomic theory, distinct quantities of energy are available to each of these electrons. An electron can have the quantity of energy dictated by one or another of the atom's energy levels, but it cannot have an intermediate quantity.

Transition element, any of various chemical elements that have valence electrons in two shells instead of one.

Sometimes an atom has two electronic energy levels whose difference is equal to the amount of energy possessed by a light quantum associated with a certain wavelength. This is a characteristic of a series of chemical elements called the transition elements. In chemical combination the atoms of these elements can absorb visible light. The light energy boosts the electrons into higher energy levels. The electrons then dissipate this energy in the form of heat and return to their normal energy levels. The compounds of the transition element cobalt, for example, are known for the brilliant blue that is left after they absorb and dissipate red light.

Many kinds of molecules, or combinations of atoms that form a chemical substance, have electronic energy levels that lie close together, a situation similar to that of the transition elements. The molecules that can absorb and dissipate visible light usually contain many double bonds. These light-sensitive coloured molecules make up a very important group of chemicals. The green pigment chlorophyll, found in the leaves of plants, absorbs light energy that is then converted to food energy. Four types of similar light-sensitive molecules are involved in human vision, each sensitive to a different range of wavelengths.

Dyes are another group of chemicals that often contain many double bonds. The exact structure of a given dye determines the energy levels available to the electrons and, therefore, the wavelengths that the dye will be able to absorb. For example, when the molecules of a substance can be linked chemically with a textile fibre, the substance can be used as a dye.

Colour Perception

In a psychological sense colour can exist without light. People in a completely dark room can "see" colour by shutting their eyes tightly. When they do this, coloured spots called phosphenes seem to appear in front of their eyes. Phosphenes have also been produced by direct stimulation of the brain and by stimulation of the eye with pressure or electricity.

Persistence of vision in another example of colour perception in the absence of a physical stimulus. When people watch a motion picture, they are actually observing a series of rapidly projected still pictures. During the very short interval between pictures, a person retains an image of the preceding picture. This image blends into that of the following picture, giving an impression of continuous motion. The retained image is called a positive after-image Similarly, if people look at a patch of one colour for about 30 seconds and then look at a blank sheet of white or grey paper, they will probably see a patch of colour that is the complement of the original colour This is called a negative after-image

When light reflected from an object enters a human eye, it passes through the cornea, the pupil, and the lens and lands on the retina. The retina contains two kinds of light-detecting cells. These cells are called rods and cones. The cones are colour sensors. The rods make night vision possible.

Young, Thomas (1773-1829), English physicist and physician, born in Milverton, Somerset; discovered interference of light; offered red-green-violet theory of light perception; professor of natural philosophy at Royal Institution and foreign secretary of Royal Society; helped decipher text of Rosetta stone.

In the early 1800s Thomas Young advanced a theory of human vision that was later elaborated by Hermann von Helmholtz. A modern version of the Young-Helmholtz theory states that the eye contains three kinds of colour receptors, or cones. One kind has greatest sensitivity to green light, another to red light, and the third to blue light. According to this theory, any other colour stimulates more than one kind of cone in varying amounts, depending on the mixture of wavelengths in the colour

Four different light-sensitive chemicals are found in the human eye. Rhodopsin, located in the rods, seems to be limited to black-and-white vision. The other three, called iodopsins, are involved in colour vision. However, eye chemistry alone does not account for man's ability to identify colours A part is also played by the brain, which may contain separate colour-detection centres

Hering, Ewald (1834-1918), German physiologist and psychologist, born in Alt-Gersdorf, Saxony; advanced theory of four colours occurring in pairs as opposed to three-colour theory of Helmholtz.

In the 1870s Ewald Hering suggested that there were four primary colours blue, green, yellow, and red. He arranged these on a circle, with red opposite green and blue opposite yellow. The circle could be filled in with intermediate colours Hering considered colours opposite each other to be opponents. He regarded white and black as a special pair of opponents. Hering's theory agrees with the common notion that red and yellow are perceived psychologically as warm colours, while blue and green, their opponents, are regarded as cool colours The three cone receptors hypothesized by Young and Helmholtz form the first stage of colour perception. Electrical signals generated by the cones combine in the retina to yield opponent signals as suggested by Hering. There are three signals: white-black, red-green, and yellow-blue. Thus colour vision begins with a two stage-process. Signals sent to the brain along the optic nerve are coded into these opponent signals.

A series of experiments performed by Edwin H. Land in the 1950s called both theories into question. Land demonstrated that a wide range of colours could be produced from a mixture of only two colours He photographed the same scene twice once with light having long wavelengths, and once with light having medium wavelengths. He called the two types of photographs that resulted the long record (for long wavelengths) and the short record. When Land projected the two records onto the same screen using red light to project the long record and white light to project the short record the image on the screen seemed to have a full range of colours Land suggested that some of the colours seen whose wavelengths were absent were perceived through a process involving the comparison of surrounding colours He surmised that the total combination of colours in a scene played an important part in colour vision. His experiments can also be interpreted as proof for the two-stage theory of colour vision.

Abnormal Colour Vision

Some people suffer from abnormal colour vision, or colour blindness. When asked to pick out chips that have the same colour a process called colour matching such people may pair chips that do not look at all alike to most observers. Abnormal colour vision is usually inherited, but it may also result from a chemical imbalance in the body or from eye injuries.

Scientists classify colour blindness into three major types. The most severe, as well as the most unusual, is mono chromatism, or total colour blindness. A person who is completely colour-blind cannot distinguish individual hues. To him all colours match with greys of the same lightness.

Dichromatism, or partial colour blindness, is a more widespread abnormality. Some dichromatic people confuse red, green, and grey but are able to distinguish blue and yellow. Others cannot see the longest wavelengths of light the red end of the visible spectrum. Rare forms of dichromatism include the inability to distinguish among blue, yellow, and grey and the inability to see light of very short wavelengths the violet end of the spectrum.

Normal colour vision the vision that most people possess is trichromatic. The most common type of colour blindness is a variation of normal colour vision called anomalous trichromatism. The whole range of colours visible to people with normal colour vision is also visible to people with this condition, but they match colours that do not appear the same to people with normal colour vision. For example, in a test in which mixtures of red and green light are varied to match a yellow sample, a person with green-weak vision adds more green light to match the yellow than does a person of normal colour vision. Green-weak and red-weak vision are the most prevalent forms of anomalous trichromatism; the blue-weak form is extremely rare.