Most light sources are not 100% pure white but have a certain "color temperature", expressed in Kelvin. For instance, the midday sunlight will be much closer to white than the more yellow early morning or late afternoon sunlight. This diagram gives rough averages of some typical light sources.
|Type of Light||Color Temperature in K|
|Midday Sun, Flash||
|Bright Sun, Clear Sky||
|Cloudy Sky, Shade||
Normally our eyes compensate for lighting conditions with different color temperatures. A digital camera needs to find a reference point which represents white. It will then calculate all the other colors based on this white point. For instance, if a halogen light illuminates a white wall, the wall will have a yellow cast, while in fact it should be white. So if the camera knows the wall is supposed to be white, it will then compensate all the other colors in the scene accordingly.
Most digital cameras feature automatic white balance whereby the camera looks at the overall color of the image and calculates the best-fit white balance. However these systems are often fooled especially if the scene is dominated by one color, say green, or if there is no natural white present in the scene as show in this example.
|The auto white balance was unable to find a white reference, resulting in dull and artificial colors.||The auto white balance got it right this time in a very similar scene because it could use the clouds as its white reference.|
Most digital cameras also allow you to choose a white balance manually, typically sunlight, cloudy, fluorescent, incandescent etc. Prosumer and SLR digital cameras allow you to define your own white balance reference. Before making the actual shot, you can focus at an area in the scene which should be white or neutral gray, or at a white or gray target card. The camera will then use this reference when making the actual shot.
This article is written by Vincent Bockaert,
author of The 123 of digital imaging Interactive Learning Suite
Click here to visit 123di.com