Color temperature refers to a “black-body radiator.” This is a theoretical object (as in, it doesn’t really exist but we pretend it does because we find that useful) that will absorb all electromagnetic radiation cast upon it; it will reflect or transfer none. Visible light is a small portion of electromagnetic radiation, which you can think of as the stuff coming from the sun that provides light and heat.
When a black body radiator is heated it begins to glow. There’s a cool java applet on the Olympus Microscopy Resource Center website that demonstrates this idea with a picture of a horseshoe. At the lower range it begins by glowing red. Then as it gets hotter it goes through yellow and white phases, eventually shifting into the very bright blue-white range. At any particular color, we refer to the temperature of the blackbody as the “color temperature.” (A note on that java tutorial, the temperature indicated on the scale refers to the tips of the horseshoe. The other colors move down the horseshoe to show that the tips are being heated and the bottom is cooler than the tip. The temperature does not relate to all the colors shown in the image.) The temperature is on the Kelvin scale, which is the same as celsius plus 273. That is, -273˚C is 0 K, 0˚C is 273 K, and 100˚C is 373 K.
For lighting, it is easier to refer to the temperature generating a color than trying to describe the color itself, which changes slowly and is not really part of our language of color. If you are looking at that java applet from Olympus, try to describe the difference between the color of the horseshoe tips at 2700K, 3000K, and 3200K. You’ll quickly realize why referring to a color temperature is easier.
A bit of confusing terminology, however, is that we refer to red the red side as “warm” and the blue side as “cool.” This is based on our traditional color associations of red and blue. It can be confusing because the bluer tint comes as you increase the temperature. Hence, you may hear people refer to “raising” the color temperature to “cool” the light source.
The filament of a regular incandescent lamp is not a true black-body, but it does go through the same color process as it is heated. That’s why when you put a regular lightbulb on a dimmer it shifts toward the red as you dim it down: it’s the horseshoe in reverse. Therefore, if you’re looking at incandescent lightbulbs color temperature isn’t a very useful metric. Regular and halogen lamps at full power usually end up around 2700K to 3050K, and that can be changed by adjusting the voltage. The real purpose of discussing color temperature is for fluorescent lamps, which I’ll get to tomorrow.
Tags: color, color temperature, color theory, incandescents, light bulbs
