Color temperature is the temperature (using the Kelvin Scale) to which a black body radiator would need to be heated in order to emit a specific color.
If you heat metals to a high enough temperature they begin to emit light (glow). Initially the color will be reddy/orange. But heat it even hotter and the light coming from the metal will contain more yellow. Hotter still and more blue light will appear in the mix.
So a low color temperature will be towards the red/orange end of the spectrum. A high color temperature will contain more blue.
Some metals can be heated so hot that they emit most of the colors of the rainbow. Although they won’t emit them all equally—there will always be a lot more yellow then blue.
A tungsten light bulb is an example of this: electricity flowing through the thin strand of tungsten metal heats it up and makes it glow.
Physicists talk of a theoretical object (a black body radiator) that can become so hot that it can emit a lot of blue light.
The concept of color temperature is important in photography because the color ‘white’ isn’t actually a color at all.
White is a mixture of many colors. It’s our brains that take this mixture and perceive it as white.
Our brain can adjust what we see as white depending on our surroundings. So if we were in a room where the light source emits a lot more yellow light than blue (that is, has a low color temperature), our brains may be able to adjust for this and the objects in the room could appear to be normal to us.
Many cameras have ‘auto white balance’ settings. These attempt to judge the color temperature of the light falling on our subject. However, as cameras and software do not exactly mimic the way our brains perceive color, the image may look differently to the way we saw them.
If you look carefully at the white painted ceiling behind these three light bulbs, you’ll see that all three emit light with different hues or casts.
Normally (without a comparison) we would see them all as white.
The 60 W incandescent bulb looks more yellow because it produces light by heating a tungsten filament.
The two 13 W fluorescent bulbs have been manufactured to produce light of different color temperatures. The 5500 K bulb appears to have a lot more blue light.
If we neglect to adjust the color balance setting on our our cameras or software (or use color correcting filers and the correct film) all of the objects in an image could appear to be the wrong color. This is known as a color cast.
- Glowing metal rod by Alex Lines from Brooklyn, USA (Practicing technique), Accessed 2016-10-01, CC BY-SA 2.0, Source: Wikimedia Commons
- Glowing light bulb, Accessed 2016-10-01, CC0 Public Domain, Source: Pixabay
- Visual comparison of the colors emitted by three light bulbs By Yerocus (Own work), Accessed 2016-10-01, Public domain, Source: Wikimedi