3 Introduction to Colour Temperature
Light has two main characteristics which have an impact on the work of the videographer, namely – ‘Luminance’ (intensity or brightness) and ‘Colour Temperature’. Whilst variations in luminance of light can be discerned by the naked eye, the human brain compensates for variations in colour temperature to ensure that we see the colour of objects consistently. Video cameras, however, will reproduce colours differently depending on the colour temperature of the predominant light source.
Colour temperature is measured in degrees Kelvin – with a lighted match producing around 1700°K, through to a CRT (Cathode Ray Tube) TV screen producing around 9300°K. The hues of colour temperature range from red through to blue – the lower Kelvin values produce red hues and the higher produce blue hues.
3.1 Colour Temperature Examples
Here are some typical examples of approximate colour temperature. The sunlight temperatures are based on readings taken in the UK – temperatures will vary at different latitudes.
10,000 to 18,000°K: Sun-less blue sky
8000 to 10,000°K: Cloudy sky
6000 to 8000°K: Hazy sky, through slightly overcast, to shaded areas
5500 to 6000°K: Normal sunlight with clear sky (5600°K is a standard camera pre-set)
5000 to 5500°K: Midday sunlight with clear sky
4100 to 4500°K: Moonlight
3200 to 7000°K: Fluorescent Light (note the wide variation)
3000 to 3400°K: Studio Tungsten Lamp (3200°K is a standard camera pre-set)
2500 to 3000°K: Household Tungsten Bulbs
2000 to 3000°K: Sunrise or Sunset
1850 to 1930°K: Candle Light
1700-1800°K: Match flame
3.2 White Balance
In order to reproduce colours accurately video cameras have to be adjusted, which is referred to as ‘colour adjustment’ or, more often ‘White Balance’. If the light source is at the red end of the Kelvin scale the camera will need to compensate for this, so the hue has to be adjusted in order to reproduce pictures with an accurate balance – likewise with light sources at the blue end of the scale. Failure to do so will result in a picture that is too ‘cold’ (a blue hue) or too ‘warm’ (a red hue).
Settling on the correct hue balance is achieved by adjusting the camera settings with reference to a white object (White Balance). There are three main methods by which an accurate white balance can be obtained. However, it is worth mentioning that under some lighting conditions it will be virtually impossible to obtain a correct white balance. For instance, when filming under low-pressure Sodium lighting, the resulting picture will have an orangey-yellow cast.
3.3 Automatic White Balance
Most cameras have what is referred to as ‘Automatic White Balance’ or ‘Automatic White Tracing’ (AWT). On this setting the camera will continuously measure the average colour temperature of a scene and adjust itself accordingly
The benefit of AWT is that it requires no intervention from the videographer and can be useful in situations where getting the shot is far more important than obtaining a perfectly white- balanced shot. However, a drawback of this is that the camera will only deliver an estimated (though, usually close) white balance. Another drawback is that the camera will continually adjust its white balance throughout a shot – which in extreme situations can manifest in the colour changing as the shot progresses, especially in situations where the colour temperature of the light source varies.
3.4 Pre-Set White Balance
The second method is by the selection of predefined colour temperature settings – the two main variations being ‘Internal’ (or ‘tungsten bulb’) at 3200°K, and ‘Daylight’ at 5600°K.
The benefit of the Pre-Set White Balance setting is that it will give you a constant setting. The drawback is that colour temperature can change incrementally, especially in situations of varying light conditions. To compensate for this, some professional level cameras allow for the pre-set settings to be adjusted in small steps either way. For example, in most studio situations where lighting is controlled through dimmer circuits, the lamps are usually run at no more than 70% of their maximum output; for this reason most studio-style cameras will have a tungsten pre- set white balance of 2900°K.
3.5 Manual White Balance
The final method is ‘Manual White Balance’ adjustment, whereby the videographer sets the camera to a specific Kelvin measurement by pointing the camera at a white balance card (a calibrated white or sometimes grey card) and then pressing a button that adjusts the camera’s white balance to that reference and stores that setting.
The benefit of this method is that you will get an accurate white balance setting in any given situation. The drawback is that it takes a little time to set the white balance. As general good working practice, Manual White Balance should be re-set about once every hour when working outdoors because the colour temperature will change as the sun moves across the sky during the day. Although not necessarily perceived by the human eye, it is something the camera will register. Also, be sure that the white is in same vicinity as the subject matter of the shot and that you zoom in so that the white surface fills the screen.
3.6 General Colour Temperature Guidance
Always attempt to obtain the correct white balance for two main reasons. Firstly, an incorrect white balance will not be complimentary to your subject and can degrade the picture quality; and secondly, colour hues have an effect on the viewer’s interpretation of the mood the videographer is trying to convey.
For example, a blue hue will make an outside summer scene look cold and appear as if shot in winter. Also, shots used in the same scene which have different white balance settings will lack continuity when edited together, making it difficult and uncomfortable to watch.
Conversely, an incorrect white balance might be a desirable effect – especially when you are trying to recreate a specific mood or tonal quality to your pictures. So, if you use an off-white object as the camera’s point of reference for the white balance you will achieve a different balance to the colour; using a slightly blue object will result in a red (warm) hue, and vice versa.
The manual method of adjusting white balance will also enable a better white balance in mixed lighting situations, such as when you are filming in a room with tungsten lights that also has windows with natural daylight falling on the subject. In this mixed light situation, the white reference card should be positioned to catch the mix of light that falls on the subject. For example, if the subject is a person speaking, then the card should be held immediately in front of their face, perpendicular to the lens axis.
Furthermore, the best method of obtaining perfect white balance in mixed light situations is for the videographer to control or adapt the light source. This is not always easy to achieve, but in the mixed-light scenario you can control this by adding dichroic filters or CTB gels which filter out specific light colours of the tungsten light sources (see section 4.3 Filters). These are attached to either your own lamps or those in situ, correcting them to match the daylight coming through the window.
On a final note, each method of white or colour balance adjustment has its benefits. However, to achieve the best results, use a correctly set-up colour monitor to preview the camera’s output. This will allow you to see precisely what effect your lighting and white balance adjustments are having on the picture.