Focus on Pictures

Photography of shiny objects

Colour of light

('colour temperature') - Obviously the colour of objects needs to be correctly reproduced. Colour rendering depends on a number of factors, such as:

Color temperature chartLet us consider the colour of light. As we all know from looking at the rainbow white light is made up of a large spectrum of colours. At sunrise and sunset the outside light is reddish, with the sun high in a clear sky it is bluish but white when the sun lights the earth through a layer of clouds. The light from normal (tungsten) light bulbs is yellowish or orange tinged, the light emitted by halogen light sources is closer to white, photographic flash lights emit white light. However, if you use a flash light in a space with a coloured wall, the light reflected from the wall will not be white and will influece the perception of all objects illuminated with the reflected light. Similarly in e.g. a church with stained glass windows, the filtered light will create a colour cast that affects all objects in its interior. Usually we are not conscious of the more subtle variations of the colour of light as the brain adapts to it, so that after a short while a colour cast is filtered out and goes unnoticed. The material that we use to take a photograph (film, CCD or CMOS) does not adapt and mercilessly records any colour cast .

Colour of light - An iron rod when heated starts to emit red light, then it turns more yellow, subsequently white and at an even higher temperature blueish. One can correlate the colour of the light emitted to the thermodynamic temperate of the object emitting the light; temperature is expressed in Kelvin (K) (or in mired, see below). The light emitted at a certain temperature is thus called the temperature colour. Interestingly, in everyday language red is called a warm colour, and blue a cold colour.
The figure on the right shows the colours of the rainbow (red - orange - yellow - green - blue - indigo - violet), and white in the centre. Red, green and blue are primary colours which when added yield white; the are therefore called additive colours. White is obtained by adding diagonally opposed colours (e.g. blue + yellow); diagonally opposed colours are therefore called complementary. Thus white is to be found at the centre of the spectral map. The black line is Planck's curve, used to indicate colour temperature. The table below relates the colour of environmental light to light sources:

sunrise or sunset 2000 K reddish-yellow
tungsten light 100 watt 2800 K orange-yellowish
halogen light 500 watt 3200 K yellowish
halogen film photoflood 500 watt 3400 K yellowish
one hour after sunrise 3500 K yellowish
flash bulbs 4950 K white with minimal yellow cast
daylight at noon 5400 K white
electronic flashlight 5500 K white
unclouded, daytime 6500 K  
unclouded, summertime > 8000 K  

How can a colour cast be handled ?

Another unit often used to express colour temperature is the mired, for “micro reciprocal degrees”. To obtain the mired value for a colour temperature divide 1 million by the colour temperature. Thus 5500 K is the same as 182 mired, since 1 000 000 / 5500 = 182.
Mireds are commonly used for converting light from one colour temperature to another using a colour conversion filter. The decamired system is often used by continental European manufacturers in lieu of the arbitrarily numbered Wratten system applied to colour conversion filters.

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