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Hi Rachel, The color of the output of lamps, especially photo lamps, is customarily measured using a temperature scale proposed by the British scientist Lord Kelvin. This scale has appeal as it starts at absolute zero, the coldest possible. Thus the Kelvin scale never used negative numbers. This avoids the possibility of confusing ten below with ten above. The Kelvin scale starts at – 273 Celsius and calls this low temperature zero. For this reason the Kelvin scale is also called the Absolute scale. Now the metallurgy industry took to this scale. Steel making was centered in Sheffield and they had great respect for Lord Kelvin. Anyway, heated iron glows red-hot. Continue to heat and its color changes eventually reaching blue-white hot. The temperature of the metal corresponds to the color of its light output. Ordinary electric lamps used metal filaments. The metal, usually tungsten, is heated as electric power surges through it. The filament temperature is what we are talking about. Ordinary electric bulbs = 2800°K
These are designed to operate at a specific voltage. In the U.S. this is 120V in Europe 220V. If you apply more than the design voltage the lamp gets much hotter, brighter, and more bluish however, the life of the lamp is severely shortened. Photographers, in their studios and the movie industry, routinely over-volted to get more light. Two arrangements evolved one delivered 3200°K the other 3400°K. As higher voltage is applied the temperature goes up, the lamp gets brighter, its life shortens, the light shifts towards the blue. Soon lamps were made for both applications. Film was also marketed, labeled Tungsten Type B for 3200°K and Tungsten Type A for 3400°K. We carried a gadget bag of conversion filters so we could be ready for anything. Electronic flash replicates daylight 5500°K. Daylight is not constant; at dawn and sunset it is quite ruddy, close to ordinary electric bulbs. North skylight is 6000°K. Modern digitals have automatic white balance, chip logic that measures and adjusts for shifting hues of light. Additionally you can manually set your digital for a specific type of light. Color temperature measurements fail for florescent lamps and most gaseous discharge lamps. However, temperatures are routinely assigned these lamps, often the value is invalid because these sources produce light that has an interpreted spectrum. How about mixing and matching?
Photography is both an art and a science. You are free to mix for imaginative reasons. The logic in the digital camera can’t handle and correct for mismatches. You should strive to keep your lighting uniform. Electronic flash is preferred for a variety of reasons; this is just one of them. If you are forced to mix and match, you can buy colored filters for your lights. Filters are installed over the lamps to alter their color. This method has merit but is costly and requires some exterminating. The bottom line is: Better to buy lamps that are uniform in color output. Alan Marcus (marginal technical gobbledygook)
alanmaxiemarcus@att.net
November 12, 2008
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