The colour of the light from a source, or its spectral quality may vary widely, depending on the distribution of energy at each wavelength in the spectrum. Most of the sources used for photographic purposes give what is usually described as 'White' light. This is a loose term that doesn't imply any definite light colour quality.

White light sources vary considerably among themselves and from daylight.

Colour constancy of visual perception

Because of the phenomenon of colour constancy of visual perception our eyes see little difference in everyday life, but the variations can be very important in photography affecting the speed of black and white films and the colour obtained from colour negative and slide films.

The usual method of defining the quality of light is with reference to an incandescent source (glowing with heat) as its colour temperature changes. This is defined in terms of a 'black-body' radiator which is a light source emitting radiation whose distribution only depends on the temperature and not on the material and on the nature of the source.

Colour temperatures are measured on the thermodynamic, or Kelvin scale (°K), which has a unit of temperature interval equal to °C (Celsius or centigrade) but with its zero at -273.15 °C.

Effect of changing light conditions