Colour Temperature of Light
The fact that light has a colour temperature is not something most people are aware of. But if you don’t pay attention to this the light in which you are shooting it can reduce the quality of your pictures.
All light has a colour temperature which is measured in Kelvin or K. For example a general everyday light bulb has a colour temperature of around 2,800K, which gives it a warm red tone. Generally you are not aware of this as your brain compensates automatically and lets you see white light. You only become aware of the “colour shift” of the light when you take a photograph. An example of the effect is shown below.
The image on the left was taken under a halogen light and the image on the right shows how this would have looked had it been taken in daylight. Notice how the image on the left has a warm red tone to it.
Light Colour Temperature
Having established that light can have different colour temperatures and that this can give rise to changes in the colour tone of your images, let’s have a look at some of the common lighting situations you might come across.
On a clear sunny summers day (no clouds in the sky) at 12:00 midday the colour temperature of light is likely to be around 5,500K. If we introduce some cloud and make the sky overcast the colour temperature might rise to around 7,000K. If we move ourselves into a shaded area, perhaps under a tree, the colour temperature could rise to 8,000K or even higher. The effect this gives is to make the light turn blue and images start to look cool or cold.
Only at 5,500K does white actually look white without our brains compensating. Generally speaking the lower the colour temperature the warmer the light and the higher the colour temperature the colder the light. Light therefore moves from being a warm red colour through white to a cool blue.
Once you understand how colour temperature affects the colour of light you need to understand the factors that can affect it:
- Time of day – Only at midday is light going to be around 5,500K. Before and after midday the colour temperature is lower and the light is warmer in tone. The effect is however only really noticeable around sunrise and sunset when the colour temperature is quite low.
- Weather Conditions – Cloudy overcast skies cause the colour temperature to rise and the light takes on a blue cast Shade – Similar to an overcast sky, pictures taken in the shade can appear cold and take on a blue cast.
- Altitude – When taking pictures at altitude the colour temperature can increase significantly and objects take on a strong blue cast.
The following provides basic information about typical light conditions or sources and the colour temperature you might expect to encounter.
Example Light Sources and their Colour Temperature
- Shade (8,000K)
- Overcast Sky (7,000K)
- Lightly Overcast Sky (6,000K)
- Flash (5,800K)
- Daylight (5,500K)
- Early Morning / Late Afternoon (3,500K)
- Domestic Lights (2,800K)
- Sunset (2,000K)
- Candle (1,850K)
Correcting Colour Cast
Having determined that different lighting situations can create a colour cast in your images you might be wondering how to correct the problem. The solution differs depending on whether you are using film or digital.
Images on Film
Where you are shooting on film you only really have one option unless you are willing to scan your shots and correct them in Photoshop. This is to use a colour correction filter to balance the light colour. To compensate for a high colour temperature (that give a blue colour cast) use one of the 81 series of filters. These have an orange appearance and come in a number of strengths from A to EF. The weakest filter is the 81A. In order of increasing strength the filters are 81A, 81B, 81C, 81D and 81EF.
Typically with the colour temperature around 6,000K you might use the 81A or 81B. As it rises to 7,000K you might select an 81C or 81D and at 8,000K the 81EF is likely to be useful. This however is only a rough guide and you are best experimenting with a little with the type of film you will be using (taking notes of course). If the colour temperature is low and you want to counter the warm orange colour cast you could select one of the blue filters in the 80 range. Use an 80A to counter very low temperatures or an 80B if the temperature is nearer that of daylight. Again the best advice I can give is to experiment, take notes and learn for the film.
One final point when using filters is to remember that filters absorb light. This is fine if you are using your cameras built in meter, as the camera will adjust the settings accordingly. If however you are using a hand held light meter you will need to compensate for this absorption by increasing the exposure you use. By how much will depend on the filter but it will typically be between 1/3 and 2/3 of a stop.
Managing colour temperature is much easier with a digital camera and the solution is also much more flexible. The first option is to place a filter on the front of your camera as described above. This solution won't work as well as you would expect however due to the white balance setting of the camera.
The white balance setting is the correct way to manage colour temperature when shooting digitally. Most digital cameras have a number of white balance settings including AWB (Auto White Balance), indoors, sun, cloudy and flash. When you select one of these you are setting the camera to compensate for the expected colour temperature of the light.
On more advanced cameras (usually digital SLR’s) there is a custom white balance setting. This allows you to take a picture of a white object and then have the camera determine the correct white balance for the object. This can then be used as the white balance for other shots.
The option that I prefer for general shooting is also the simplest option is AWB. In this mode the camera attempts to guess the correct white balance and adjust the colour temperature accordingly. Sometimes the camera will get this wrong but you can correct the problem as I will explain in a moment.
The AWB mode of most cameras today is generally quite good but it's still advisable to make further adjustments in Post Capture processing.