The color temperature or chromaticity of a lamp is usually described in terms
of its color temperature or, more accurately, by its "X" and "Y" coordinates on the CIE Chromaticity Diagram. Color temperature refers to the chromaticity of a theoretical object (called
a black body), that, when heated, emits light. Although no such objects
exists, many metals behave similarly to a black body so that the concept can be described. For example, if a piece of iron were heated to 900°K, it would glow a dull red. When heated to 1366°K, it would change to a brighter red, and at 3000°K it would become yellow-white. At 5000°K it would appear bluish white. Absolute temperature is expressed in degrees
Kelvin (centigrade plus 273°). Thus, color temperature of a light source is defined as the value of the absolute temperature of a black body where chromaticity matches that
of the light source.
Correlated color temperature is the temperature of a black body whose chromaticity most nearly matches that of the light source. Because the chromaticity of a "white" fluorescent lamp only approximates that of a black body, correlated color temperature (CCT) is used. Approximate color appearance can be estimated by plotting the X and Y coordinates on the CIE Chromaticity Diagram. Note that color appearance or chromaticity of a lamp has no relation to its color rendering ability.
