White Balance & Colour Temperature

Background
Have you ever wondered why the photos you take indoors sometimes come out looking very yellow? It's a common mistake to make, and this tutorial should help you understand why it happens, and how to get it right!

Theory
The human eye is very good at adjusting to different light sources, and the brain compensates without you noticing, when going from outdoors to inside. If you look at a piece of white paper under both sorts of light, you won't see any difference - but a camera will.

This effect is due to the colour temperature of the light. Here are some examples:

• Cadle flame - noticeably orange/yellow [1850K]
• Tungsten lightbulb - slightly yellow [2800K]
• Average daylight - white light [5500K]

You don't need to worry about the degrees Kelvin values of the light, just that they appear to be different "colours" to film or a CCD sensor. It's this difference which affects the white balance of a picture - what the camera records as being white. Most films were biased to record white properly in daylight (daylight balanced film). Others were designed for use indoors with tungsten bulbs - tungsten balanced film.

In the digital world, we have the same issues, but most cameras will let you select the appropriate white balance (as well as guessing it with Auto WB). These are common symbols and what they mean:

[daylight] [shade] [cloudy]
[tungsten] [fluorescent] [custom]
[auto white balance]

Most times, AWB makes a good guess, but if you find the picture has a colour cast, try selecting the correct white balance for the lighting. Custom lets you calibrate the camera to the lighting (for instance, if you don't know what sort the light source is). Point the camera at a piece of white paper and hit the calibrate button - then the camera will "know" what true white is supposed to look like and adjust accordingly.

Examples
So, what happens when we get it wrong? Here are some comparisons.

Tungsten Lighting
Yellow cast because "daylight" was selected under tungsten lighting:

[Tungsten light incorrectly balanced for daylight shows a yellow colour cast]

[Tungsten balance shows much more natural colours (although there's quite a lot of yellow in the picture anyway!)]

Fluorescent Lighting
These three were taken in my office (under fluorescent lighting), but deliberately selecting three different white balance settings. Using Photoshop to sample the proportions of Red, Green and Blue of the "white" paper (highlighted with the red box in each picture, just to the right of the monitor), shows which colour is difficient, causing the shift in colour. Peak white, under neutral lighting, should have a value of R: 255, G: 255, B: 255. Numbers less that that (but still pretty equal) show shades of grey - ie neutral hues.

[Daylight setting gives R:236 G:227 B:182]

Blue is much lower, hence the picture is a bit too yellow (blue and yellow are opposite each other in the colour spectrum). Although this isn't nearly as bad as the previous example, tungsten light balanced for daylight.

[Tungsten setting - gives R:220 G:228 B:246]

Now there is too much blue in the image, and the whites do have a blueish cast.

[Fluorescent setting - gives R:229 G:228 B:228]

Now we've got it right and the white paper is neutral in tones. Notice the picture on the monitor now has "blue" highlights instead of white - this is because the LCD emits light at a different colour temperature to the fluorescent lights overhead - and why we have to be careful when we take pictures with multiple light sources of differing colour temperatures. You can never get them all right in the same picture!

Even outdoors we can run into similar problems when some areas of the photo are in bright sun and others are in the shade.

The image on the left shows a player in the shady part of the pitch. The whites in the background are neutral, but his "white" shirt has a disctinct blue tint to it. Shadow areas will always show a blue hue compared to the full sun conditions (that's why many cameras have a separate setting for it). So if you're taking a portrait in the shade and want natural skin tones, switch to the Shade setting for best results.

Why not experiment with the White Balance settings on your camera. Afterwards, it's very easy to forget and leave tungsten selected, so next time you take pictures in daylight, they will come out too blue!