White Balance Demystified

You probably already know that white balance has something to do with sunny versus cloudy lighting conditions, and warmth versus coolness in a photo. But did you know that you, as a digital photographer, can use white balance to capture colours the way you see them, or more creatively, to enhance the image?

The term “white balance” originates from the world of video imaging where a device (waveform monitor) was used to match or “balance” the signals from the camera’s red, green, and blue channels to make accurate whites under various lighting conditions, thus balancing your white. In this article, we’ll use “white balance” for digital cameras in a similar sense: the process of measuring your light source’s colour temperature accurately, based on your lighting conditions, and using that information to correctly balance your whites and colours.

Symptoms of poorly set white balance

If your camera’s white balance is set incorrectly, or if your camera chose the wrong algorithm for measuring colour temperature, then you will observe a colour cast on your image: it will either look slightly blue, slightly orange, or slightly green. A low colour temperature shifts light toward the red; a high colour temperature shifts light toward the blue. Different light sources emit light at different colour temperatures, and thus the colour cast.

For example, if you shone a halogen light source on a white wall, and photographed it using auto white balance, it would most likely turn out with a slightly yellow hue, instead of white. But if daylight were streaming in through a window at noon, you'd be more likely to see a true white wall.

What is colour temperature and how is it measured?

First, understand that the spectrum of visible light is the range of visible colours and their respective wavelengths, as seen in a typical rainbow (purple is the shortest wavelength at 400nm, and red is the longest at 700nm, with blue, green, yellow, and orange in between).

Second, understand that the concept of colour temperature is dependent on your understanding of a black-body, which is a black object that absorbs all incident light, neither reflecting nor allowing it to pass through. When a black body’s surface is heated to various temperatures (typically measured in degrees Kelvin), it emits different intensities of spectrum light.

For example, when a black-body is heated to 3000K - equivalent to a 200 W bulb - its spectrum has more intensity in the orange/red end, and shows purples and blues with very little intensity. This makes your photo appear “warm”. When a black-body is heated to 5000K - equivalent to daylight - its spectrum has equivalent intensity across the whole spectrum, so you see purples and blues with the same intensity as oranges and reds. But when a black-body is heated to 9000K - equivalent to shade or a heavily overcast sky - the intensity is highest in the blue/purple end, so your oranges and reds will have very little intensity. This makes your photo appear “cool”.

Here are some examples of colour temperatures from common light sources:

1500 K: candle light
2680 K: 40 W incandescent lamp
2800 K: 60 W bulb
3000 K: 200 W incandescent lamp
3200 K: studio lamps, photofloods
3200 K: sunrise and sunset (will be affected by smog)
3400 K: tungsten lamp (ordinary household bulb)
3400 K: 1 hour from dusk/dawn
4000-5000 K: cool white fluorescent bulbs
5000 K: A designation of D50 stands for "Daylight 5000K" and is the most
common standard for professional light booths for photography, graphic arts, and other purposes
5200 K: bright midday sun
5500 K: sunny daylight at noon
5600 K: electronic photo flash.
6000 K: lightly overcast sky
6500 K: heavily overcast sky
8000 K: hazy sky
10000-15000 K: deep blue clear sky

Newer light sources, such as fluorescent and other artificial lighting, require further white balance adjustments since they can make your photos appear either green or magenta.

How does a digital camera auto-detect white balance?

As far as your camera is concerned, colour temperature is based on the ratio of the amount of blue light to the amount of red light; the green light is ignored. Your camera searches for a reference point in your scene that represents white. It will then calculate all the other colours based on this white point and the known colour spectrum. The data measured from its R G B sensors is then run through a whole lot of numbers and predetermined equations to figure out which white balance setting is most likely to be correct. Remember, white balance is the automatic adjustment that makes sure the white colour humans observe will also appear white in the image.

Setting your camera’s white balance to AWB will provide colour accuracy under many conditions. Your camera will adjust the white balance between 4000K – 7000K using a best guess algorithm. Auto white balance is a good choice for situations where the light changes over time and speed is an issue (e.g. animal photography, sports photography). However, you should avoid using auto white balance settings in the following situations:

1) Your scene is heavily dominated by one colour
2) Colour accuracy is absolutely imperative
3) You are photographing particularly warm or cool scenes (e.g. a sunset)

More advanced cameras generally have three types of white balancing: 1) automatic, 2) preset, and 3) custom. When all else fails, photographers that use the raw format can set the white balance manually. For now, let’s look at white balance presets.

White Balance Presets

Most digital cameras come with multiple white balance preset options. These presets work well when:

1) The light source matches one of the preset white balance options
2) Your scene is heavily dominated by one colour

Let’s review the most common preset options:

Tungsten - "Tungsten" is the name of the metal out of which the bulb's filament is made. The color temperature of this setting is fixed at 3,000K. Best Use: indoors at night. Otherwise, your exposure will turn out too blue. Creative Use: Set your exposure compensation to -1 or -2 and use this setting in daylight to simulate night.

Fluorescent - The color temperature of this setting is fixed at 4,200K. Best Use: Fluorescent, mercury, HMI and metal halide lights used in your garage, sports stadiums and parking lots. Otherwise, your exposure will turn out too purple.

Daylight - The color temperature of this setting is fixed at 5,200K. Best Use: studio strobe lights. Otherwise, your exposure may have a slight bluish tinge.

Cloudy - The color temperature of this setting is fixed at 6,000K. Best Use: direct sunlight and overcast light. This setting will warm your photo by giving it an orange tinge, which is often desirable in landscapes and portraits. Creative Use: sunsets.

Shade - The color temperature of this setting ranges from 7,000K - 8,000K. Best Use: shooting in shade, no direct sunlight (cloudy), backlit subjects. Otherwise, your exposure will turn out too orange. Creative Use: direct sunlight – it will warm up your photos even more!

Flash - The color temperature of this setting is fixed at 5,400K. This is almost identical to Cloudy but sometimes redder depending on the camera. Best Use: overcast skies. Otherwise, your exposure will turn out too red.

The descriptions and symbols for the above white balances are just rough estimates for the actual lighting they work best under. In fact, cloudy could be used in place of daylight depending on the time of day, elevation, or degree of haziness. In general, if your image appears too cool on your LCD screen preview (regardless of the setting), you can quickly increase the color temperature by selecting a symbol further down on the list above.

TIP: Once you've selected a white balance setting, remember to reset it when you change your venue, or you may end up with some strange, out of this world, colours!