The apparent colour of a beam of light is dependent upon the colour temperature of the light source. Colour temperature is measured in degrees Kelvin (K), where zero degrees Kelvin is equal to -273°C. The higher the temperature, the whiter (or colder) the light appears.
Photographic Daylight colour temperature is usually classed as 5600K, whilst tungsten-halogen lighting is 3200K. Different types of light source (such as tungsten, CID & CSI) all have different colour temperatures. The table below inicates some approximate colour temperature values of common light sources.
More details on lamp types and
The Primary Colours
There are 2 sets of primary colours - additive and subtractive - depending whether we are talking about transmitted or reflected light respectively. This can be the subject of some confusion and the cause of many an argument! The definitions and uses are described below.
Just to add a bit of confusion to the terminology, the secondary colours of emitted light are the primary colours of reflected light. This can often confuse people into thinking that CMY are just secondaries, when in fact they are primaries.
Primary Additive Colours (RGB)
Additive primary colours are used when we are dealing with mixing emitted light. The additive primary colours are red, green and blue. All colours (well not really all, but for the sake of argument) can be produced by mixing these three colours. An example of their use is on a computer monitor, where varying intensities of red, green and blue light are used to create the colours we see. More importantly, this is what happens when we mix light from theatrical spotlights.
If full intensity red, green, and blue are mixed we get white light.
Primary Subtractive Colours (CMY)
Subtractive primary colours are used when we are dealing with reflected light. The subtractive primary colours are cyan, magenta and yellow. Because we use them when we are dealing with reflected light, these primary colours are used when mixing paints or inks. A printer, for example, mixes these three colours to produce different colours on the paper. These are also the colours we use with dichroic colour filters, as these reflect the specified colour.
If we mix full intensity cyan, magenta and yellow, we get black.
The colour of a beam of light from a theatre luminaire can be changed by using a colour filter, or gel. The filter is a coloured, but translucent, material which absorbs certain wavelengths of light whilst allowing others to pass through. For example, a primary blue filter will absorb the wavelengths of light corresponding to red and yellow, but allow blue light to continue, thus making the light beam blue.
Absorbing the light in this way makes the colour filter hot, and will cause it to lose its rigidity. For this reason, filters are placed in a colour frame at the front of the luminaire, which prevents the filter from falling out. The deeper - or more pure - the colour produced by the filter, the larger the number of wavelengths of light absorbed becomes, making the filter hotter. This is the reason that strong, deeper colours 'burn out' before pale, light ones.
Some filters have been designed as 'colour correctors'. These will modify the colour temperature of the beam so that the light appears to have been generated by a different source. This can be useful when you wish colour filter to appear the same in different types of luminaire. It is especially useful in television location filming, permitting a tungsten-halogen light source (3200K) to be converted to 5600K (daylight).
As opposed to normal colour filters, there is another class known as dichroics. These consist of a piece of glass treated with a specially coated surface. The surface reflects certain wavelengths of the colour spectrum, whilst allowing other parts to pass through unaffected. For example, a dichroic filter could reflect blue light but allow red and yellow to continue, thus making the light beam orange.
The advantage of the dichroic filter is in its ability to reflect, rather than absorb, light. This keeps the filter cool, allowing it to last longer, and also reduces the heat of the transmitted light. The use of dichroic filters has been extended by coating the reflecting surface of Par16 lamps. The dichroic used allows the majority of the light to be reflected forward through the front of the lamp, but permits infra-red heat to pass through the back of the lamp and escape. This generates a very cool beam of light, ideal for display work.
When a light source is dimmed, the colour temperature of the light it produces changes, making it hard to balance colours. To overcome this, a fine metal mesh is placed in front of the luminaire to reduce light output. This mesh is called a scrim, and does not affect the temperature of the light, as the source remains at the same level. Scrim is often used within the television industry, but is rare in theatre. (Theatre tends to use one of the available neutral density filters, which do the same job.)
The Visible Spectrum
The visible spectrum of light can be broken down in to definite colour bands. Each perceived colour has a differend wavelength or frequency as summarised in the table below. Wavelengths are measured in nanometers (0.000000001 of a metre, or one-hundred thousandth of a millimetre).