Colour Temperature

The colour of daylight can change throughout the day having a major impact on an image. It would be useful to define these colours by a fixed unit of measurement, so accurate descriptions can be used when adjusting a light source to simulate a given lighting condition.

Such a measurement exists called the Kelvin scale. The Kelvin scale uses a measurement of degrees Kelvin (K) to describe the apparent colour of a light source relative to heating a black body source such as a piece of metal. Imagine applying a heat source to an ordinary metal key. As the key heats up it first starts to glow red. As the heat source is maintained the temperature increases and the key changes colour accordingly to orange, then yellow, white and finally a blue colour. So the heat at any given point can be directly correlated to the colour of the light emitted and denoted a numeric value.

The diagram displays typical Kelvin values for a range of common lighting conditions. Lower values are considered warm colours, higher values are cold colours. It should be noted that the colour temperature is not influenced by the intensity of the light, but only the colour.

With the Kelvin unit of measure as a base reference it is now very easy to adjust different types of light sources so they match and produce a uniform light colour. For instance if halogen bulbs are being used to supplement natural daylight, a colour correction gel could be used to change the halogen bulbs 3000 K temperature to something closer to the common daylight temperature of about 5000 K.

Next - Colour Emotion & Response