A black body is a theoretical solid that, when cold, absorbs all electromagnetic radiation (including light) that falls on it. If you heat up this body, it begins to radiate electromagnetic waves, or it begins to shine. Think of a piece of cold black iron, a pretty good approximation of a black body. Now begin heating the iron up by pumping energy into the material with a flame or an electric current. Pretty soon the iron begins to glow red-hot emitting visible light having an abundance of the longer, lazier wavelengths that give the glow a red color. Keep on adding heat and the iron soon gets very excited and turns white hot as it emits more of the energetic blue wavelengths. We could keep on going making the material hotter and the radiated light whiter, but unfortunately the iron soon melts and begins to evaporate. Of course the theoretical black body won’t melt at extremely high temperatures like our piece of iron.

The color temperature of a light source is therefore directly related to the temperature of a theoretical black body radiator heated to a certain temperature. This black body temperature is usually stated I degree Kelvin or simply Kelvin’s. (Remember that 0 degrees Kelvin is absolute zero, the temperature at which all molecular motion stops and no more heat is in the material.) For example, a 20 watt light bulb has a filament that is heated to a relatively low temperature around 2000K. It emits a dim reddish yellow light. But it takes a gas like xenon, and gasses can be good black bodies too, and heat it up to the temperature of the sun with an electric arc and it emits snow-white light with a color temperature or 6000K. Our ubiquitous tungsten halogen light source that is so commonly used in theatrical fixtures has a maximum color temperature of 3200K that is, incidentally, the melting point of tungsten.