Colour temperature

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Glossary Terms

Colour temperature is a way of describing a colour quality of light. It is a measure of how much light of different colours is present. High colour temperatures have a lot of blue light and low temperatures have more red light.

Unfortunately, in everyday language, light with a lot of red has become known as warm lighting (the colour of fire), whereas blueish lighting is described as cold (that of icy water). This is the reverse of colour temperature; the longer, less energetic - and so "cooler" - wavelengths of visible light are at the red end of the spectrum, whereas shorter, higher temperature wavelengths are at the blue end.

In film photography, not matching the colour temperature of the film and process to the lighting of the subject can result in pictures looking, for example, overly red or yellow (daylight film used in tungsten lighting), or too blue (film for tungsten lighting used outside). Similar problems result from digital cameras when the white balance and other colour balance settings are preset for the "wrong" type of lighting - although most digital cameras avoid this problem by having some automatic adjustment of colour.

Typical values for colour temperature are approximately

Blue sky up to 18,000K
Overcast ~10,000K
Sunlight 6,500K
Electronic flash or blue flashbulbs 6,000K
"Daylight" fluorescent lamps 4,500K
Clear flashbulbs 3,800K
"warm" fluorescent lighting
(including some low-energy bulbs)
Tungsten filament domestic lighting 2,800K
Candle 1,950K

As is well known, heating a metal object will cause it to glow, first red, then orange, through to bright bluish-white. Technically, the colour temperature is that temperature, measured on the absolute scale in degrees Kelvin, to which a black object would have to be heated to produce light of similar colour - or Spectral Power Density (or Spectral Power Distribution) - SPD.

Fluorescent light

Sunlight and light from heated glowing elements such as filament lamps have a continuous spectrum; in other words they produce light of all visible colours. This means that all colours will reflect some light, giving a natural appearance.

Fluorescent light, produced by tubes (including low energy lamps) and LEDs is very different, having a narrow band of light energy at a particular frequency. This explains why colours look wrong under such lighting and also why it is harder to see properly unless the light is very intense. LED torches have very bright output but it is difficult to see by their light because colours do not reflect properly. So called HID (high intensity discharge) car headlamps are similar.

Tube lights use phosphorous coating inside the glass that is excited by the fluorescent light and emits some continuous spectrum light. Although much lower energy than the fluorescent output, it is just enough for our eyes to make out colour, albeit obviously artificial.

Quoting colour temperature for fluorescent sources is very approximate. A value, called Color rendering index (CRI), it often used as a merit number used to quantify how good such source approximates continuous spectrum.

Electronic photographic flash units use tubes which is why synthetic fabrics sometimes look very different in flash lighting.