The color of "white" light is a critical issue in lighting for film or video.
There really isn't any such thing as white light in normal circumstances.
Instead, our energetic and flexible brains recognize a range of light frequencies as white that actually have a predominant tint of yellow, green, or blue. The reason the issue is so important is because the camera (or film stock) isn't as flexible as the human brain--your camera can only recognize a single color, or frequency, as white. Unlike our agile brains, that camera can't blend together a range of tints and say, "Yes, that's white."
Lord Kelvin's bright idea
The color of light is measured on a temperature scale that was proposed way back in 1848 by British mathematician and physicist William Thomson, Lord Kelvin (Figure 1). It wasn't initially conceived as a way for videographers to identify light temperatures; 1848 was the infancy of black-and-white photography, the era of the daguerreotype. The precise color of the light didn't matter for photography in those days, and presumably Lord Kelvin couldn't have foreseen television, the digital CCD camera, or white balance. The Kelvin scale quantified a physics problem, giving a natural, or absolute, scale for measuring temperatures. The scale ranges from absolute zero (-273.15C) upward using units that are equal to the Celsius scale. So a lot of the Kelvin scale is below the levels where any light is emitted. Water freezes at 273.15K, and water boils at 373.15K.
The Kelvin scale helped quantify the color of light emitted by materials as they were heated. If you've ever watched a blacksmith at work, you'll see him heating the iron in the forge until it begins to glow. He can tell whether the iron is hot enough for the specific operation by the color. A dull red isn't hot enough to be malleable, but a glowing yellow is. When iron or steel is tempered, it will get heated until it seems white hot, almost beginning to emit a bluish light. The Kelvin scale quantifies these changes for a theoretical "black body," an object with no reflectivity or other properties and no impurities to alter the color when heated.
Color temperature and camera presets
We are concerned with the part of the Kelvin scale that starts in the upper region of 2,000 K to 3,000 K, a range of yellow-orange light. Ordinary household frosted bulbs generally put out light that is around 2,800 K, definitely beginning to be orange. Quartz and tungsten bulbs, such as those designed for film and TV use, put out light at 3,200 K, a yellow tint that is generally accepted as interior or incandescent light. Your camera will have a white-balance preset for this temperature. Consumer cameras will generally indicate this preset with a symbol that looks like a light bulb. Film stocks that are designed for interior or tungsten light are balanced for this temperature.
The other part of the Kelvin scale that we pay attention to is in the 5,000 K to 6,000 K range, a blue tint that is accepted as daylight. Because the business needs a specific number to peg to film stock and white balance presets, daylight is generally defined as 5,600 K (Figure 2). Again, your camera will have a preset for this color temperature. Consumer cameras will indicate this preset with a symbol that looks like the sun. Film stocks designed for exterior use will be designed to represent this tint as white.
Figure 1. This chart shows the relationship between temperature on the Kelvin scale and approximate tint of light. (From the author's book Lighting for Digital Video & Television, 2nd Ed. [CMP Books, 2004].)
Of course, 3,200 K and 5,600 K aren't the only temperatures you will encounter. Most bulbs differ from their manfuacturers' designated temprature, especially as they age. Actual sunlight varies depending on the weather conditions and time of day. It can go way over 6,000 K and of course can turn golden and even pink-red at sunset. Then there's the evil green range, around 4,000 K, in between that's common with older or cheaper fluorescent tubes. This is generally not the best range for video cameras.
Now, why are we so concerned with these specific light frequencies? Because, as I mentioned, your camera can only represent one frequency (color temperature) as white, and will render all other frequencies as green or orange or blue. Your camera is dumb; you must tell it what color to home in on as white. If you make a mistake and shoot at the wrong setting, your video will be too blue (outdoors while on 3,200 K preset) or very yellow (quartz light while on 5,600 K preset). Recognizing that lamps are sometimes off one way or another, pros will generally do a manual white balance.
Manual white balance
Although the exact procedure for a manual white balance varies somewhat from camera to camera (check your manual!), the basic procedure is similar. A white card (which can be white typing paper, or poster board, or a commercially available card with white on one side and a test pattern on the other) is held up in such a way as to catch the key light for the scene--the light we want to be reproduced as white. The camera operator zooms in on the white card until it fills the screen, and then presses and holds the proper button to register a manual white balance until the indicator shows that the camera has set the temperature. Pro cameras will show the precise color temperature of the manual white balance in the viewfinder.
So what is actually happening when you execute a manual white balance? You are feeding the camera a signal that has a specific color temperature and telling the camera to use it as white. The camera then adjusts the amplifiers on the red, green, and blue CCDs so the combined output meets the spec for white--that is, equal signal level outputs from R, G, and B channels. It then remembers these amplifier settings until it is told to reset to a new temperature.
This operation explains a phenomenon I've often been asked about in workshops. Digital cameras will occasionally seem to add a tint (most noticeable when it's a bit of magenta) to hot highlight areas. This usually shows up when the rest of the scene is fairly dark. This happens when the boosted signal of the highlight areas maxes out but the less-amplified signal corresponding to the rest of the frame still has some room before it clips.
Incidentally, our man of the month, Lord Kelvin, was actually knighted by Queen Victoria for one of his other engineering accomplishments--the laying of the first transatlantic telegraph cable in 1866. Perhaps she liked his great whiskers, too.
The Rev. John Jackman has been shooting video since Jimmy Carter was president. He's the author of Lighting for Digital Video & Television (CMP Books, 2004) and is moderator of The Craft of Lighting forum and column at DV.com. You can reach him there with your questions.