What Temperature is a Colour

What Temperature is a Colour?

White light comes in a wide variety of shades, from warm, golden candle light to harsh, blue Antarctic day light. Colour Temperature is a means of identifying, measuring and comparing all of these shades of white. The central concept of colour temperature is the well-observed phenomenon that when materials are heated-up, as long as they don't melt, vaporize or burn - they glow. The amount of glowing and the colour of the glow are related to how far the material has been heated.

White light is really quite a slippery idea to grasp, in that the description really applies to almost any light from any source, so long as it contains enough bits of the colour spectrum to convince our brains that we can see most colours. Some "white" light, notably the output of some discharge sources, has big chunks of the spectrum missing and can cause all kinds of havoc with our ability to see colours accurately. It's always fun to watch a scenic artist struggling to match a colour at night under Mercury vapour worklights: as most of the red part of the spectrum is missing from Mercury vapour light, the scenic artist has almost no means of knowing whether they have added the right amount of red to the mix. The white light from other discharge sources such as HMI, MSR and CSI may contain the whole of the spectrum, but in different proportions from the light that we are accustomed to seeing from incandescent sources such as candles, the sun or a filament lamp. We will look at incandescent sources for now and tackle the peculiarities of discharge light sources in another article.

We have all observed the process of progressively heating a piece of metal, whether it was a poker in a fire, the hot-plate on an over-stoked barbecue or the little piece of Tungsten wire that is the filament of an incandescent lamp. At first the metal gets too hot to touch but not hot enough to glow: there is heat (infra-red radiation) being emitted but as yet no visible light. Next the metal starts to glow a deep red colour: the metal has more energy and is now starting to emit visible light at the lowest energy end of the spectrum[1]. As the temperature of the metal increases further, the energy of the emitted light also increases, progressively adding the other colours of the spectrum and changing the colour of the metal from orange, through yellow, to increasingly cooler-coloured (bluer) white light. If we were to continue heating the metal we would be able to measure Ultra Violet energy being emitted; sufficient energy that some low-voltage Tungsten Halogen desk lamps have been associated with increases in UV-related skin cancers.

The colour of the light is clearly related to the temperature of the source, but different materials will glow at different temperatures and all materials will change in some way if we heat them up sufficiently. In order to create a meaningful comparison of the light emitted by different glowing bodies Colour Temperature refers to the properties of the Black Body Radiator: an imaginary object with a totally non-reflective, matt-black surface, which can be heated to any temperature without melting, boiling or incinerating. The Colour Temperature of a light source is the temperature, in Kelvin[2] degrees, which the mythical black body would have to be heated to in order to produce light of a matching colour[3]. As colour temperature is a measure of the comparative colour of a light source, not its actual temperature, measurements are stated in Kelvin (K) not degrees. Eg. a film/TV studio light source is identified as being 3,200K, indicating that its light matches that of the black body at 3,200 degrees Kelvin, not that the filament is at that temperature.

The approximate colour temperatures of some common light sources are: candle 1,900K, domestic Tungsten filament lamp 2,700K, PAR64 120v 2,000hr life (standard Parcan lamp) 2,850K, stage Tungsten-Halogen lamp 3,000K, Film/TV studio Tungsten-Halogen lamp 3,200K, PAR64 240v 750hr life 3,200K, sunlight 5,600K, overcast sky 7,000K, clear blue sky (no direct sun) 16,000K. Colour temperature correction filters are designed to change the balance of colours from a light source of one colour temperature to match another colour temperature. Everybody's favourite cool blue, Lee 201 or Cinegel[4] 3202(otherwise known as Full CTB -Colour Temperature Blue) is designed to remove the red and orange parts of the spectrum from a Studio (3,200K) lamp so that it will match daylight from the sun (5700K). Full CTO (Colour Temperature Orange), Lee 205 or Cinegel 3401 is designed to perform the opposite task, that of converting daylight (5,700K) to match studio Tungsten Halogen (3,200K).

Perhaps the most confusing aspect of colour temperature has to do with language and terminology. We have been taught the terminology of the artist's colour wheel, when we describe the nature of colours, we talk of orange as a WARM colour and blue as a COLD colour, yet the bluer a light source the HOTTER its colour temperature.

[1] For those whose memory doesn't readily (or willingly) reach back to their high school science days, the light spectrum, in increasing order of energy is : (Infra-Red,) Red, Orange, Yellow, Green, Blue, Indigo, Violet, (Ultra-Violet). Does the name Roy G Biv ring any bells?

[2] The Kelvin scale uses Celsius degrees but has a starting point of Absolute Zero (-273°C), the (as yet unattained) theoretical minimum temperature to which matter may be cooled. A room temperature of 20°C is 296°K.

[3] As discharge light sources don't involve a material being heated up until it glows they cannot actually have a colour temperature, although if they produce the entire colour spectrum they may be assigned a Correlated Colour Temperature.

[4] Rosco's range of colour temperature filters and diffusion materials.

by Andy Ciddor
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