The Correlated Color Temperature Scale (CCT)
Correlated Color Temperature measures the temperature at which a lamp burns.
Color temperature is a method of quantifying the color of a light source. This method of color comparison assigns a Kelvin temperature to a range of hues for a “blackbody radiator” at the same temperature. The blackbody radiator is a theoretical object capable of absorbing all energies that hit it and then perfectly re-radiating this energy. At room temperature the blackbody appears black, at 800K dulled red, at 3,000K it appears yellow, white at 5,000K, pale blue at 8,000K, and bright blue at 50,000K. An example of this would be an ingot of iron placed in a furnace. Initially the bar is dull gray. As it begins to heat it appears dull red. Slowly as the heat increases the bar glows bright red. Gradually, with increased heat, the bar’s appearance will shift to orange, yellow, and finally a bluish white hue. This type of color correlation is the least accurate measure of an object’s color. Technically, this method is only applicable for incandescent sources. When applied to fluorescent fixtures or high-intensity discharge lights, such as metal halide fixtures, the terms “apparent color temperature” or “correlated color temperature” are applied.
Color temperature is a measure of the color appearance of light, not the actual temperature. Light sources that appear bright white to blue-white have a high color temperature (3600K and above) and are described as “cool”. Sources that appear yellow-white to red-white have a low color temperature (3400K and below) and are described as “warm”. Daylight is considered to be around 5500K and plants grown under this light do quite well. The cost of the lamp increases as the color temperature goes up because of the rare materials required to fill the arc tube.
A color temperature of 5,000 to 6,500 Kelvin is required to be considered full-spectrum.
Spectral Power Distribution
This method of color evaluation for a light source depends upon direct examination of its spectral power distribution. The spectral power distribution plots a curve of the radiant power of a light source versus the wavelengths of the visible spectrum. A light source will be able to render colors well for wavelengths where its radiant power is high. Rendering will be poor for wavelengths where the radiant power is substantially lower. Spectral power data can be obtained from the specific lamp manufacturer.