Kelvin is a unit of measurement on the absolute temperature scale, named after the Scottish physicist William Thomson, 1st Baron Kelvin, who developed the concept of absolute temperature in the nineteenth century. In photography, the Kelvin scale is used as a standardised means of describing and quantifying the colour content - or colour temperature - of continuous spectrum light sources, providing photographers with a consistent and objective reference for understanding and managing the colour quality of different light sources encountered in both natural and artificial lighting conditions.
The concept of colour temperature is based on the theoretical behaviour of a perfect black body radiator - an idealised object that absorbs all light falling on it and emits light purely as a function of its temperature. As such an object is heated, it progressively changes colour, glowing red at lower temperatures, shifting through orange and yellow, and eventually reaching blue-white at very high temperatures. This predictable relationship between temperature and emitted colour forms the basis of the Kelvin colour temperature scale used in photography, even though most practical light sources do not behave exactly as a perfect black body radiator.
On the Kelvin scale used in photography, lower colour temperatures represent warmer, more orange and red toned light, while higher colour temperatures represent cooler, bluer light. Candlelight and domestic tungsten bulbs sit at the warm end of the scale at around 1800K to 3200K respectively, daylight on an overcast day falls in the middle range at around 6000K to 7000K, and open shade under a clear blue sky can reach 8000K or higher, producing a noticeably cool and blue cast.
Understanding colour temperature in Kelvin is fundamental to achieving accurate colour reproduction in photography, as the human visual system adapts automatically to different light sources in a way that camera sensors do not. The white balance control on digital cameras allows photographers to set or adjust the camera's colour response to match the Kelvin value of the prevailing light source, ensuring that neutral tones are rendered accurately and that the overall colour balance of the image reflects the scene as it appeared to the eye. Many cameras allow white balance to be set directly in Kelvin values for precise manual control, while automatic white balance systems analyse the scene and attempt to determine the colour temperature independently.