Colour temperature is a characteristic of a light source. Sunlight has colour temperature of 5000°K - 6500°K, overcast northern daylight can reach 9000°K. Artificial lighting has a low colour temperature of around 2500°K (as well as other things!).

These numbers refer to the colour of light given off by a 'black body' when heated. The colour runs from red-hot through yellow and white to blue at its hottest.

White balancing is the process of measuring a light source's colour temperature and setting of that colour temperature to being neutral. When the eyes are concerned the luminosity is taken into account as well with the brightest part of the view being made to look white.

Can you believe your Eyes?

Human vision in white balanced. Compare the view in a room lit by a tungsten light bulb with a photo of the same room, under the same circumstances, taken on a piece of normal colour film.

What the film shows is the true situation. What our eyes show us is the view after it has been colour corrected. In practice, what happens is that the brain looks at the scene, makes a calculated guess as to what it thinks are certain key colours and applies the change to the view as a whole making the colours appear more as expected. This system works quite well for lighting with a colour temperature between 2500°K and 12000°K, below that and you see an orange/red colour cast, above a blue one.

This is a wonderful system and one that all digital cameras try to emulate with their 'Auto-WB' settings. One of the down sides is that the compensation only works for light sources so a print, a reflective image, which is poorly balanced will still look poorly balanced under normal light. Another problem is that the ambient light will affect one's ability to assess the colours in an image; background lighting has a profound effect on how we perceive colour, both reflective and transmissive.

Women see colour better than men. Men see in low-light - grey - conditions better than women. Men are also subject to much more colour-blindness. Red/green colour-blindness is the most common but it is reckoned that about 10% of caucasian men suffer from colour-blindness of some sort compared to about 0.5% of women.

On the plus side human eyes can see a much wider range of colours than any part of a computer system. The range of colours which an inkjet printer can produce is, at the very best, will only be about 20% of that which our eyes can register.

It remains that eyes are not good absolute judges of colour, especially if you are male.