Notes on Colour

colour temperature

The colour temperature of a light source

If you heat something, it radiates. It might be the filament in a light globe, a speck of carbon in a candle flame or helium gas on the surface of the sun. The dominant wavelength of the radiation depends on the temperature of the source.

Warm objects radiate infra-red waves (heat rays) like a radiator. Hot objects radiate visible light as well, like a candle flame or an electric bar heater. This is called incandescence. The hotter the object, the more blue light it radiates and the less red it appears.

The colour of the light from an incandescent source can be expressed as the temperature of a theoretically perfect black body radiator which radiates light of that colour. This temperature is always expressed in kelvins. To find the temperature in kelvins, add 273.15 to the temperature in degrees Celsius. For example, an ordinary domestic 100 W light globe runs at about 2 580 °C, which is roughly 2 850 K.

The human eye rapidly adjusts to changes in the colour of light, so an object will appear to retain its colour even when it’s illuminated with light of a different colour temperature. Candlelight is actually very red compared with daylight, yet you may notice the difference only as a subtle romantic glow in the face of your companion. On the other hand, you will immediately notice the difference as a strong orange-red caste if you take a photograph with a camera’s white balance set to 5 500 K daylight instead of auto.

colour temperature gradient