Understanding colour temperature

by Peter Ryan

Colour temperature or the colour of light is measured on the Kelvin scale on a range of 0 – 10,000.

It is easy to rattle off the old mantra of incandescent light is at the red end of the spectrum and a welding torch is so hot it creates a blue flame – the other end of the colour temperature range. But tell me where have you ever seen a scene lit by blue light from a welder’s torch.

This is my personal explanation of white balance as it relates to the light sources used in photography.

Try to think of this NOT as a hot/cold temperature (there is no such word as ‘degrees’ in the Kelvin scale) but as a colour guide for direct, indirect and man-made light sources.

The Sun, the hottest object we have in our universe generates light at around 5,500 on the Kelvin table. This is a pure white light with no colour cast.

If I asked you “what is the colour of a camp fire blaze?” You would answer RED/ORANGE.

If I asked you what colour light a fire gave off you often hear people say ‘there was a warm glow around the fire’. That’s right the light has a colour that is a reddish/orange to yellow.

So it is that light has a colour. Below is a broad range of light sources, their colour and where they appear on the Kelvin scale.

Now let’s look at how that fits the RGB colour space that we all use in photography.

NOTE: You will note that the Kelvin table does not include Green or Magenta, the other part of the colour spectrum. White balance adjustments provide a separate Tint slider that alters the Green/ Magenta axis. You do not have to use this often and if you do a little adjustment goes a long way, so be careful.

The solid arch on the right hand diagram maps the linear line of Kelvin colour chart below it. It starts at Red at the top of the colour spectrum chart, flows through yellow, across the centre (White) and on through Cyan and Blue.

If the sun is a measure of pure white light with no colour cast i.e. items depicted under sunlight reflect their true colour then either side of white we have degrees of colour.

We have discussed that light can be either direct or indirect.

It is important to note we do not have a ‘blow torch setting’ on our white balance commands in-camera but we do have a setting for indirect light sources on a cloudy day or sunny day with the subject in the shade.

The human brain/eye combination automatically adjusts to the colour of light and no matter the light source. If you are reading a book the pages will always appear white no matter the light source.

Your camera’s white balance meter is designed to try an emulate this and bring all subjects, no matter what light source, to the comparable neutral colour, as if they were illuminated by white light.

The Auto White Balance does a pretty good job where there is one light source but struggles with mixed light sources. If you feel you are getting a colour cast – either slightly warm or cool tones in an image or a magenta cast, it is more than likely the white balance is slightly off (your auto setting hasn’t fully compensated).

For light sources left of the centre (or white light) that are on the red side of the ledger you need to add Blue to bring it back toward neutral white.

Conversely for light sources on the higher end you need to add red to bring them back to the neutral white position.

You can try and re-shoot using one of the pre-set controls.  You can fine tune these controls or set your own custom white balance setting on higher end cameras. If you can shoot in RAW mode on auto white balance, fix any errors in post processing. This is your greatest get-out-jail free card in digital photography.

If you shoot jpg all is not lost, as many higher end post production programs allow you to colour balance using the eye dropper tools in the Levels dialogue box (or similar). Photoshop CS5 and later also allows you to open a jpeg in the RAW converter where you can then adjust white balance. Alternatively, Raw therapee (www.rawtherapee.com) has a great freebie RAW converter that you can also use to alter the white balance setting for jpegs.

One time when I deliberately fool the camera’s white balance setting is with sunsets where I shoot them with the white balance on Cloudy.

Sunsets are by their nature red so the camera tends to add blue to neutralise the colour that can wash out your sunsets.

If you move to Cloudy you are telling the camera that what you are pointing at is on the blue side of the ledger and it must add red to bring it back to a neutral setting. This will add the red back to your sunsets.

DON’T FORGET TO RESET YOUR CAMERA IMMEDIATELY AFTER OR ALL YOUR FUTURE SHOTS WILL HAVE A COLOUR CAST.

Article courtesy of 4WD Exposure which offers workshops and tutorials on basic photography and image processing – www.4wdexposure.com.au

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