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13. Notes and Tips on Digital Photography White Balance 4
Florescent Lights We’ve spoken to the nature of lighting sources that heat particles in order to generate light. They share similar properties with a heated body, and, therefore, are relatively easy to correlate to degrees of Kelvin. All lights created in this fashion spread themselves over the full visible spectrum; they are just biased toward one end of the spectrum or the other. This means our white balance adjustments can push the tonality toward the blue or red side to even out the light across the visible spectrum. However, there are many artificial light sources that do not create light by heating particles; for example, florescent lights. Because they create light in a different fashion, they do not spread their light across the entire spectrum. In fact, they typically spike in particular portions of the spectrum. Florescent lights have the largest spike in the green portion of the spectrum. This explains why unadjusted pictures under florescent lighting come out green or yellowish in tone. For these light sources, accounting for the red and blue tones in our scene to compensate for the color temperature is not enough. There is another component to the depiction of light that we are typically unconcerned with in light sources that spread across the entire spectrum. That component is known as the green-magenta shift. You may be familiar with this component as represented in photo editing software in the “Tint” adjustment. In order to correct florescent lighting it is often necessary to adjust the green-magenta shift, or “tint” of the image.
Auto White Balance Settings When I first began shooting digital images many articles I read suggested that I forego Auto White Balance all together. Auto White Balance is your camera’s best attempt at defining the white balance setting necessary for the shot. In brief, the senor takes a look at the scene’s tone and determines what color it is and how to adjust it accordingly. It actually works fairly well as long as significant white or neutral grey objects are in the scene. Just as we discussed in the “Pre-processing White Balance Adjustments” section of this article, the white objects provide a reference point from which the sensor software may ascertain what white should be, and subsequently, how to adjust the color to make white look white. However, without a sufficient reference for comparison, the auto white balance algorithm may fail.
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