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Kelvin-Scale
Image 2
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Simply Put...
Used to measure color temperature of light sources. A Kelvin rating can be used in the purchasing of light bulbs to anticipate the color output that will be produced. The attached chart shows an example of the Kelvin scale.
History...
The Kelvin scale and the kelvin are named after the British physicist and engineer William Thomson, 1st Baron Kelvin (1824–1907), who wrote of the need for an "absolute thermometric scale". Unlike the degree Fahrenheit and degree Celsius, the kelvin is not referred to as a "degree", nor is it typeset with a degree symbol; that is, it is written K and not °K.
A Thorough Explanation...
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 being at absolute zero or 0 Kelvin, which is equal to -273 degrees Centigrade.
Imagine now that the black body radiator is heated to the temperature of freezing water, equal to 273 Kelvin, 0 degrees Centigrade, or 32 degrees Fahrenheit. The black body is then further heated to the temperature of boiling water: 373 Kelvin, 100 degrees Centigrade, or 212 degrees Fahrenheit. If the black body continues to be heated, it will begin to glow and give off its own light. When the temperature reaches 3200 Kelvin, the radiator will be emitting light with a visible spectrum equal to the color temperature of light produced by a tungsten filament, typical of those found in microscope lamps.
The absolute temperature of a black body radiator is always expressed in Kelvin, which is equivalent (the degree increments are identical) to degrees Centigrade (°C) plus 273 degrees. For example, 1000 K equals 727 °C. 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 chromaticity matches that of the light source. In the case of fluorescent lamps and other sources that can only approximate the chromaticity of a black body, the corrected term correlated color temperature is applied through a calculated chromaticity.
Although this black body radiator does not actually exist, many metals behave in a similar manner, so we can employ a black metal horseshoe as an example for this discussion. In Image 2, the horseshoe is first heated to a temperature of about 900 K (a), where it begins to glow a dull red. As the temperature is increased to between 1500 K and 2000 K, the horseshoe (b) turns a yellowish to brighter red color. Increasing the temperature still further to just over 3000 K produces a yellow-to-white color transition (the color temperature of a tungsten filament, (c), and at 5000 K and above (the color temperature of daylight, (d), a bluish-white color appears.
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