All objects will emit light if they are heated to a sufficiently high temperature. Also, as an object is raised in temperature, the color of the light emitted from it will change. An iron bar, for example, appears dull red when first heated then red-orange, then white, and finally blue-white as it is heated hotter and hotter. In the same way, a tungsten filament in an incandescent lamp changes color when different voltages are applied. This phenomenon was studied by Max Plank in 1900 and is the basis for his law of blackbody radiators.

This law can be used to designate the relative color temperature of any heated object. A color temperature designation, applied to a light source, refers to the absolute temperature in degrees Kelvin of a theoretical blackbody or full radiator whose color apperance matches that of the source in question.

The figure below shows the Planckian locus on the 1931 CIE diagram. The numbers along the center line indicate the color temperature (in degrees Kelvin) for blackbody light sources.

Such a body is black at room temperature, red at 800K, yellow at 3000K, white at 5000K, pale blue at 8000K, and brilliant blue at 60,000K. Tungsten filament lamps used for general lighting have color temperatures in the 2600K to 3000K range. Low wattage lamps used where luminance is not too important operate at about 2000K. Such lamps as TV and studio floods operate in the 3100 - 3400K range, just short of the tungsten melting point of 3500K. In most cases, actual filament temperture is slightly lower than the apparent color temperature.

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