The term 'color temperature' is a specialized way of describing the color of a light source, especially relevant to photography. Its scientific basis may seem a little removed from the practicalities of photography, but it can clarify the process of choosing the appropriate filters and films.
Color temperature is measured in degrees Kelvin (K), which is similar to the Celsius scale, but starts at absolute zero. The color temperature scale covers most of the light sources encountered in photography. The key value to remember for color temperature in photography is that the color white equals 5400K and is known as "mean noon sunlight".
The color of the sun is different from that of, say, a tungsten light bulb or a fluorescent lamp because of the difference in their energy wavelength. A wavelength determines the color temperature of any light source. Most light sources emit not just one single wavelength, but many. They have what is called a continuous spectrum. Thus, if they radiate more energy at some wavelengths than others, then they have a specifically defined color.
Some artificial light sources, on the other hand, radiate energy in just a few narrow lines of the spectrum. Sodium and mercury-vapor lamps are of this type and have a discontinuous spectra. These light sources do have a definite color, but it may be impossible to change it by adding filters.
