Color Temperature

Investigate the apparent "color" of a virtual radiator (in this case, a black metal pot) as it is slowly heated through a wide temperature range by external energy. The concept of color temperature is based on the relationship between the temperature and radiation emitted by a theoretical standardized material, termed a black body radiator, cooled down to a state in which all molecular motion has ceased. Hypothetically, at cessation of all molecular motion, the temperature is described as being at absolute zero or 0 Kelvin, which is equal to -273 degrees Celsius.

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The tutorial initializes with a black cooking pot appearing in the window in a cool state. By translating the Color Temperature slider beneath the pot, visitors can adjust the temperature of the pot and monitor its color changes as they relate to absolute temperature on the Kelvin scale (changing values are indicated to the right of the pot). The warmest part of the pot is always the bottom, with a increasing temperature gradient occurring from bottom to top. Color changes are first apparent as the pot begins to glow a dull red when heated to a temperature above 900 K. As the temperature is increased to a range between 1,500 K and 2,000 K, the pot turns from yellowish to brighter red in color. Increasing the temperature even further, to a range above 3,000 K, transforms the color to a yellow-white (the color temperature of a tungsten filament), and at 5,000 K and above, a bluish-white color appears at the base of the pot (the color temperature of daylight). Progressive temperature increases of the black body shift a greater proportion of emitted light into the higher frequency regions (shorter wavelengths).

The absolute temperature of the black body radiator is expressed in degrees Kelvin, which is equivalent to degrees Celsius plus 273 degrees. The term Kelvin is usually abbreviated simply as K, but with no reference to degrees (by convention), although the Kelvin is equivalent in magnitude to the Celsius degree. For example, 1,000 degrees Kelvin (or K) equals 727 degrees Celsius. Therefore, we can define the color temperature of a light source as the value of the absolute temperature of a black body radiator when the radiator color spectrum, or chromaticity, matches that of the light source. In the case of fluorescent lamps, which can only approximate the chromaticity of a black body, the corrected term correlated color temperature is applied through a calculated chromaticity value.

There are two important points to consider when examining color temperature phenomena. The color temperature value of a light source refers only to the visual appearance of the source, but does not necessarily describe the effect this source will have on photographs or digital images. Also, color temperature does not take into consideration the spectral distribution of a visible light source. In cases where a light source, such as a fluorescent lamp, arc-discharge burner, laser, or gas lamp, does not have a spectral distribution similar to that of a black body radiator, its color temperature alone is not a reliable means of selecting suitable filters or creating look-up tables for color balance corrections. Therefore, although two different light sources may be described as having the same color temperature, exposed photographic emulsions or digital images lacking proper white balance baseline adjustments may respond differently to the sources. When using fluorescent lamps or similar light sources, a per-wavelength comparison of sensitivity and spectral output is often necessary in order to determine the correct filters for color temperature balance.

Contributing Authors

Mortimer Abramowitz - Olympus America, Inc., Two Corporate Center Drive., Melville, New York, 11747.

Matthew J. Parry-Hill and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.