[robski]

Hi Polo

You may or may no of noticed that the colour of daylight changes throughout the day. In the early morning and late afternoon it has a yellow cast and at midday it has a higher blue content. Digital cameras have an auto white balance mode that examines the image to determine the correct value for whites to compensate for the changes in colour of the lighting.

Basically it assumes that if you mix all the colours up it will give you a grey and adjusts the camera RGB levels to achive this. This is fine if you have a broad selection of colours. If you have a few strong colours then this fools the auto white logic and gives a bad result. Hence the grey card to get around these problems. Your notice that the colour of lighting is referred to as colour temperature. This is based on heating a piece of metal. For example if you heat a piece of steel it changes colour. At a lower temperatures it is blue and changes to red as it gets hotter and then gets to white heat.


Some light reading on the subject

http://www.cambridgeincolour.com/tut...te-balance.htm