A Viewfinder Darkly
Phil Northeast is Feature Writer for Photography at 
Stock photography by Phil Northeast is available from Ozimages International
Understanding Digtial Camera White Balance
Philip Northeast
Digital cameras automatically adjust their white balance for different lighting conditions, although sometimes they need a little help.
Using only venue lighting ther is anoticeable yellowish cast
and the camera measured the colour temperature as 3550K
White light is a combination of all the visible colours; normally this is from the midday sun in a clear sky. There are atmospheric conditions that block some colours more than others, producing a different colour daylight. Clouds and shadows produce a bluer light while early morning and late afternoon sunlight has a more yellow tinge.
However, our eyes and brain compensate for colour temperature variations and we have no trouble recognizing basic colours under most lighting conditions.
Colour Temperature Scale
The colour temperature scale shows variation in the overall color of light in degrees Kelvin. Similarly, artificial light sources produce light with a characteristic color cast. Here is an approximation of where various light sources sit on the Kelvin scale, from a low or red value to the high or blue end:
• 2000 Candlelight
• 3000 Tungsten and Halogen
• 3500 Morning and Afternoon Sun
• 4200 Fluorescent (White)
• 5000 Daylight, Fluorescent (Neutral) and Flash
• 6000 Cloudy
• 8000 Shade
With film, there were two choices, use a special film for each light condition or use a filter to block the predominant colour cast. Digital cameras offer greater flexibility and convenience as the camera can automatically measure the colour temperature and or the white balance of the image for use as a reference in normal digital image processing.
Digital Camera
Most digital cameras offer the photographer a choice of common preset white balance values as an option to the automatic system. More advanced cameras allow the white balance to be set manually, usually by taking a photo of a white or gray reference object.
The same shot with the colour temperature corrected in
post processing showing the table cloth as white.
The digital camera processes the image based on the current white balance value and then saves it as a jpeg file. Generally, this process is effective and digital cameras produce acceptable images under a wide range of different lighting conditions, with little or no intervention from the photographer. There are situations where the exact colour is important or the lighting is a complex mix of sources.
For example, an indoor shot with sunlight dominating lighting in half the shot while electric lamps light the other part. There may be a tendency for the digital camera's automatic system to average the different colour temperatures over the whole image and decide on a white balance somewhere between sunlight and the electric lamps. Another difficult situation is where there is a large area with a strong dominant colour in the shot that may fool the digital camera resulting in an incorrect AWB, a red object may fool it into thinking the scene is too warm and try to cool it off with a bluer colour temperature, giving washed out reds.
A mixture of fill in flash and artificial light and the colour temperature is 4900K.
These complex photographic situations are where advanced cameras with the capability of saving images as RAW image data come into their own. They allow the photographer to delay setting the white balance until they view the image on their computer and experiment with different white balance values.
Not only is this useful for producing correct colours, it is also a valuable creative tool. Varying the colour temperature for the a RAW image can produce a warm and cosy feeling for a yellow or candle light look, or give a crisp and clear whiter appearance with a bluer cast.
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