Colour and light
Coloured light is additive
If you take a beam of white light and pass it through a prism it splits up into its component parts (red, orange, yellow, green, blue, indigo and violet). If you take all those coloured lights and mix them together you would end up with white light again.
For television we use a good approximation to those seven colours, red, green and blue. White equals all three, black equals none of them, and most other colours can be made up by mixing different amounts of these three coloured lights together.
This is the opposite of painting, which has red, yellow and blue as the primary colours (or more accurately magenta, yellow and cyan). Paint subtracts light, i.e. it prevents colours from being reflected.
Quantity
To put not too fine a point on it, outside is bright, inside is dark. The human eye can cope with a huge range in brightness, but even it sometimes takes a few seconds to get used to how dark a room is when you walk in from bright sunlight.
Colour
The human eye also sorts out one of the other problems with light – the fact that it keeps changing colour. From morning, through noon, to dusk the colour of what we think of as ‘white light’ changes. While the human eye makes natural adjustments, a camera doesn’t and needs to be regularly calibrated to produce white-coloured whites.
Where we notice this most is when we shoot using tungsten (ordinary) lights instead of sunlight. Pictures will come out very yellow if we don’t set the camera correctly or use filters to adjust for the difference. White balancing’ is usually done by holding a piece of white card in front of the camera and pressing an auto balance button.
Colour temperature
You will hear lights being referred to by their ‘colour temperature’, which helps the camera operators set their filtering for balanced whites.
If you heat a piece of metal it start to glow red. Keep heating it, and by the time it is at 3200 Kelvin (K) it glows bright yellow (the same colour light as a normal light bulb). If you were able to heat it to 5600 K it would be glowing the same colour as daylight.
