October 2004 HelpLine

In photography, the terminology we use for the color of light (such as “warm” or “cool”) is just the opposite of its meaning in physics. In Kelvin terms, the higher the temperature, the bluer it becomes, and the lower the color temperature, the warmer it looks. In photography, we consider blue to be a cool light, however. In other words, when you’re hot, you’re not!

As you might expect, different light sources have different color temperatures. What’s surprising is that the color temperature of a single light source can fluctuate dramatically as well. Consider sunlight, for example. Its color varies as the sun changes position in the sky because the light traveling from the sun passes through several thicknesses of atmosphere that scatter light rays differently, depending on the wavelength. At noon, when the sun is directly overhead, the sun’s rays travel through the least amount of atmosphere. When the sun is setting, its rays travel at a sharp angle and are affected by more particles in the atmosphere. The blue is scattered, so we see more of the warmer wavelengths.

Artificial light sources vary in their color temperature, too. Depending on the process used to create the light, the color temperature could be on the lower end of the scale (warm), as with incandescent light bulbs. Halogen lamps, on the other hand, become more blue as the color temperature moves higher.

Color Temperature In Digital Photography
We all can make the comparison between the iris in our eye and the iris in a camera, but the comparison can go further. Just like the human eye, a digital camera has red, green and blue photoreceptors and a brain (or CPU) to control the signals coming from those photoreceptors.

A digital camera, which is essentially trying to mimic the human vision system, also tries to copy our approximate color consistency adjustment. Modern-day image processing gives us the ability to correct for color temperature differences without the use of lens filters. The processor in the digital camera takes the output of the sensor and attempts to make adjustments to the RGB values in order to make white look white.



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