We know about RGB and CMY, and we know that if you mix equal amounts of red, green and blue light, you get white light. Color photography, film, and printing, as well as Photoshop, are based upon R + G + B = W. However, there is no such thing as white light. Great—all that effort trying to understand what white light is, and now you say there’s no such thing?

Understanding Kelvin Temperature
All light has color. We measure the color temperature of light in degrees Kelvin, much like we measure heat in degrees Fahrenheit. For example, the color temperature of daylight is 5500°K, which is blue; tungsten floodlights are 3200°K, which is orange (see Figure 1). The color of light ranges from cool to warm, or bluish to reddish. It’s very important to understand that when we talk about blue light, we’re not referring to the actual color of blue (as in the additive system), but to the coolness of the light source.

The higher the number on the Kelvin scale, the cooler the light; the lower the number, the warmer the light. The scale is not proportional, so a change of 1000° at the high (or blue) end is a small difference. However, at the low (red) end, a change of 100° is noticeable. The color of an overcast sky varies from 6000–8000°K, and open shade or skylight varies from 8000–12000°K. Most color films record the 6000° difference as slightly cool, not as almost neutral to drastically blue. But at the warm end, the 400° difference from tungsten (3200°K) to a 100-watt bulb (2800°K), creates a perceptible color shift on film. Use a color temperature meter to measure the actual Kelvin degree of any lighting situation.

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