Colour temperature is a term used by photographers to describe the balance of colours in visible light. The colour temperature of the various commonly used light sources is quite different. In the case of daylight, it varies enormously with the time of day.

Colour temperature is usually measured in degrees Kelvin (K), the international scientific unit of temperature, although colour temperature actually has little to do with physical temperature. To understand why the Kelvin scale is used, it is necessary to consider how the appearance of an iron bar (a practical approximation to a theoretical black body) changes as it is heated.

Iron radiates energy at ever-shorter wavelengths as its physical temperature is raised. At 300K (room temperature) it appears black, but above 1,000K (727C) its radiated energy moves into the visible spectrum and the metal begins to glow red. At about 3,000K (2,727C) the glow becomes pale yellow, and at about 5,500K (5,227C) most of the radiated energy has a visible wavelength. The eye then sees white heat. At much higher physical temperatures the iron eventually vaporizes and emissions shift to the blue and ultraviolet wavelengths.

It has proved convenient to describe light with a comparable spectrum using a colour temperature measured on the same scale (usually about 2,000K - 20,000K). However, the comparable colours of light are a consequence of the scattering of light rather than the changing wavelengths of radiated energy.

Light emitted by tungsten-filament bulbs, such as domestic light bulbs, has a low colour temperature of about 2,800K. Daylight is at its warmest at sunset, typically about 3,000K. The summer sunlight we see as white normally has a colour temperature of about 5,500 (the point on the scale for which daylight film is balanced). A clear blue sky may, in some circumstances, have a colour temperature as high as 20,000K.

Colour temperature meters are available, and can be useful when assessing interiors where light sources are mixed. However, such instruments are quite expensive.