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Candela Candela
A measure of brightness. Scientists first determined brightness by measuring the light given out by a ‘standard candle’. That is how the term candela came to be used for brightness. Since then the candle itself has been replaced by more scientifically accurate measurements.
Candle power
The unit first used for measuring the light output of electric lamps. Before electric light, people used candles for lighting, and so they wanted a simple way of comparing electric lights to candles.
Colour
Light that reaches the eye and that does not appear white. Colour is very important and gives us much of the information our brain uses.
Not all animals see in colour. For example, dogs see in black and white. (See also: Colour mixing; Colour vision; Hue; Saturation.)
Colourants
Substances that are used to change the colours of something, such as dyes and pigments. They are used in paints.
Green
Blue
Colour blindness
A condition in which some people cannot tell the difference between two kinds of coloured light, often red and green. It is thought that this problem lies in the brain rather than in the eye.
Such people do not see red or green, but only yellow, blue and grey. People with this problem still describe colours as red and green, but that is based on experience of what other people tell them.
Colour blindness is thought to be an inherited problem. It mainly affects men.
Colour filters
Sheets of glass or plastic that
are designed to stop certain light waves from going through them. They are widely used in photography. For example, ultraviolet (UV) filters help prevent a photograph appearing
too blue; a polaroid filter helps
stop unwanted reflections from water, glass and metal. A blue
filter is sometimes used for indoor photography with daylight film. It compensates for the fact that indoor light is redder than daylight. A red filter may help a black-and-white photographer get stronger sky
tones. An orange filter will help a black-and-white photographer show red flowers against green leaves, while a green filter will make dark leaves appear lighter in tone.
Colour mixing
The eye can distinguish ten million colours. All colours are produced by mixing colours in one of two ways: by adding or by subtracting. To get colour mixtures by adding, new sources of light go in, for example, by adding beams of light. In subtracting, some light is taken away by using a filter.
Newton’s colour circle shows how adding light beams of the same intensity can produce new colours. Adding complementary colours (those opposite each other in the circle) produce white. Other colours can be obtained by mixing coloured light beams. A mixture of red and yellow light produces orange, for example.
The basic, or primary colours, from which all other colours can be produced, are red, green and blue. When the three primary colours are added together in equal amounts, white is produced. Mixing colours by adding is what gives colours to the pictures shown on TV.
Red
Yellow
Magenta
 Colour mixing – The two ways of mixing colours are additive (left) and subtractive (right).
Cyan
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