Sunday, 15 March 2009

Colour temperature and White Balance

Before I started this trip back into photography, I always wondered why some of my indoor shots would have a strange orange-red cast across the picture. Not knowing any better, I just lived with it.

However, after discovering artificial light the reasons for strange colour shifts is a little easier to digest. Previously, I had just thought that all light was just the same - whether it came from the sun, the lights indoors or the lights in an office building etc.

But this is not the case. All digital cameras tend to have a 'white balance' which tends to be left in auto mode. What this means is, the camera will assess the scene and then decide what type of light it sees and then adjust accordingly... ok, but adjusting to what?

If we back up a little, we've just said that light is not all the same - in fact, light temperature is measured in Kelvins and clearly distinguisable, even though the human eye may not notice.

Daylight is normally in the 5500-6000k range and appears white.
Tungsten (indoor bulbs) is around the 3000k range and appears orange.
Flourescent (office tube lights) is around the 3200k range and appears green.

The colour temperature is based on the idea of heating a black body and observing the colour of the body as it reaches certain temperatures - in real terms, simply remember that a red flame and NOT as hot as a blue flame.

Note however that not all tungsten and flourescent (or incadescent) light sources are the same.

So what does this mean for us in photography terms? Well, remember that the camera can do auto white balancing for us, but this isnt always accurate or you may want to apply a specific cast to your image. So we need to understand what the camera is doing and its quite simply - whenever it is required to adjust the colour temperature to anything other than daylight, the camera simply throws its own colour cast over the image such that it will balance out the environments colour balance.

Daylight : no colour correction applied
Tungsten: blue colour correction applied
Flourescent: red colour correction applied

A colour wheel summarises the colour correction applied:

1 comment:

andrew said...

The other problem with fluorescent light is that it's not a constant spectrum like other (hot) light sources - the spectrum is actually a set of lines. This can make it non-trivial to balance a scene shot in fluorescent light.

You should get yourself a Mini Colour Checker chart:
and write a Gimp script to balance your shot to the known values of the chart.

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