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Historical perspectivePrior to the 17th century, very little was known about the nature of light and colour because university teachings were derived from the Greek philosophers, who were good at inventing theories but sadly lacking when it came to testing them by practical experiment. It was generally accepted that light was inherently white and the objects that it interacted with added colour to it. During the late 19th and early 20th centuries, commercial colour printing became well established and various processes for colour photography were developed. By the 1940s, most of the basic systems used in colour photography had been developed, although the processes continue to be improved even today. The next major development was colour television in the 1950s, and in the 1980s and 1990s computer printers with colour capability brought personal colour printing within everyone's reach. The next major step in the recording and reproduction of colour images will undoubtedly be the popularisation of digital photography, when electronic cameras and colour printers become good enough, and affordable enough, to replace conventional film. The observer as part of the system Most colour systems contain three basic elements: A source of illumination (light source) whose spectral content can affect how colour is perceived. Apart from street lighting, which has a gross effect upon colour, the main everyday sources of illumination are "white" light such as daylight, incandescent lamps and fluorescent lamps. Even these have widely differing spectral content An object (or objects) upon which the light falls, which reflects light and usually modifies the spectral content An observer, whose eyes receive light reflected from the object(s). Each person's perception of colour is different and standards for colour can only be derived by averaging the results from a large number of observers The majority of everyday objects are viewed by reflected light, but in some cases the object may be self-luminous, for example a television screen or a firework display When assessing colours, all the above factors must be taken into account. Attributes of light and colour Colours have three main perceptual attributes: Qualitatively, hue is whether a colour is red, orange, yellow or green. Although we may think of a pastel colour, such as pink, as being different from red, in fact the two have the same hue, it is just that the red in pink is diluted with white. Hue can be defined as the dominant wavelength of a light source or the light reflected from an object. To get around this, the spectrum can be made into a colour wheel in which the purple colours join the ends of the spectrum. The hue is then defined as a hue angle, with the spectral colours ranging from 0-degrees (red) to 267-degrees (violet), and the purple colours occupying angles between 268- and 359-degrees. This allows all hues to be defined quantitatively, and it also ties in neatly with colour television, where hue is encoded as the phase angle of the colour subcarrier. Colourfulness This is the degree to which the hue is apparent. The pure colours of the spectrum are very colourful, but pastel colours are less colourful because the predominant colour is diluted with white. Light sources will also appear less colourful the less bright they are, and reflecting objects will appear less colourful if they contain black or the illumination is less bright. Brightness This is the extent to which an area appears to emit light. The sun is very bright, as are some man-made light sources. Light reflected from a white area may be bright. Reflected brightness from greys and browns may be medium, and black very low. The objective term corresponding to brightness is luminance. The brightness of objects reflecting light varies with the illumination, so a piece of grey paper viewed outside on a sunny day may appear brighter than a piece of white paper viewed inside on a dull day, even though we know that the white paper is "lighter" than the grey paper. In order to compare the relative brightness of colours under differing illumination, the term lightness is used. This is the relative brightness of a colour compared to a white area under similar illumination. The eye A basic understanding of how the human eye works is essential to the understanding of colour perception and colour reproduction. Light entering the eye first passes through the conjunctiva, a thin transparent membrane that is not part of the eyeball. Next it passes through the cornea, which is a transparent window in the otherwise opaque outer layer of the eyeball, the sclera or white. It then passes through the pupil, the circular hole in the iris, the coloured part of the eye. Light passing through the pupil is focussed by the lens, which is held in place by a ring of ligaments connected to the ciliary or focussing muscle. When the ciliary muscle is relaxed, the lens assumes a fatter shape with a shorter focal length. When the ciliary muscle contracts, the suspensory ligaments pull the lens into a flatter shape with a longer focal length. In this way the images of objects at any distance can be brought into focus on the retina at the back of the eye. The image formed on the retina is inverted. Colour response of the eye It is impossible to measure the response of the eye directly, since one cannot get at the outputs. All that we can do is average the subjective responses of many observers under experimental conditions or carry out post-mortem experiments on the pigments in the retinal cells to determine their spectral response. The problem is made more difficult by the way in which the eye adapts to different light levels. Firstly, the iris can vary the amount of light falling on the retina over about an 8 to 1 range. Secondly, it is generally accepted that the rod cells depend for their operation on a compound called visual purple, which is broken down by light. Thirdly, the sensitivity of the cone cells is reduced by a brown pigment that builds up at high light levels. This is why it takes several minutes to become adapted to bright sunlight after being in the shade (much longer than it takes the iris to close down). With the increase in the use of digital images, consumers can now expect better colour recording and reproduction. The software-supporting, top of the range digital camera's and printers has become increasingly user-friendly and affordable, thus allowing a better standard all-round. Although not all aspects of colour is covered in this article, the key topics discussed will help develop a better understanding of the concept of colour and the developments in colour technology. Compiled by Paolo Formenti