Color temperature, which is also referred to as CCT (Correlated Color Temperature), is the chromaticity observed when a black body radiator is heated to a pre-determined temperature. The units are stated in degrees Kelvin, which indicates the temperature of the black body radiator.
Well, as much as that is a great definition and will make you appear smart at your next cocktail party, it really does not explain what it actually means. Let’s try to unravel the science speak and define color temperature in more relatable terms.
First, some history and an explanation of what we mean by “black body radiator.” In 1862, a German physicist named Gustav Kirchhoff coined the term “Black Body” radiation during his research into spectroscopy. This research was expanded upon by scientists Lord Rayleigh and Max Plank (among others) in the early 20th century. They theorized, and later proved, that an opaque object will emit thermal radiation when heated to a particular temperature. In the correct conditions, the wavelengths of light (regardless if it is visible or invisible to the human eye) can be measured in a repeatable experiment. This was termed a “black body,” because at room temperature, the object appeared to be black since the emitted wavelengths were in the Infra-red region and not visible to the human eye.
The concept evolved beyond the theoretical and tested in real-world experiments. First, a box is constructed of thermally-conductive material, such as metal. The box is built in such a way that light is not able to enter the interior of the box. A small hole is then created in the side of the box. When the box is heated, the resulting temperature of the interior air will reach a point that will emit visible light. Using the formulas created by scientists like Max Plank and others, the wavelength of the emitted light can be correlated to the temperature of the heated air.
Simply put, the fancy definition means the following: If you take your theoretical black body and heat it up, the resulting color of the object will match the color of the light of the same CCT.
This is where we get the colloquial terms like “red hot” and “white hot.” These terms seem to have originated because they just make sense, but in reality are based upon the science of black bodies and color temperature.
In general, lower temperatures are described as warm and tend toward the colors of yellow and orange. In lighting, these are CCT of 2,700K, 3,000K, and 3,500K.
As the temperature rises, the color moves into the white and blue spectrums. We see this in lamp types of 4,000K, and 5,000K.
In the next post, we’ll look at lighting selection as it relates to color temperature.