Measuring Color Temperature: The Basics of 'Kelvin'
Most video cameras flash a four or five digit number in the viewfinder whenever you white-balance or change filters. This number, often followed by the letter "K," represents the camera's evaluation of the color temperature of the light under which the white-balance was achieved. The "K" stands for Kelvin, Lord Kelvin actually, who pioneered the science of thermodynamics, and is used by videographers to quantify the relative amount of red or blue in a light source.
Standard incandescent light bulbs, such as those found in most homes and stage-lighting fixtures, burn at temperatures around 2,800K and even lower if they are dimmed. Quartz lamps, the standard for tungsten illumination, radiate at 3,200K. Fluorescent lamps commonly used in offices will generally measure between 3,400 and 3,800. The Kelvin value of the noontime sun is approximately 5,500 while the color temperature of direct shade or an overcast sky can run to 9,000K and even higher. The critical thing to remember is that lower Kelvin values indicate a greater red component to the light source; higher numbers mean less red and more blue.
It is also important to note that a small change of value at the low end of the Kelvin scale, such as from 3,000K to 3,200K, has a far greater effect on color balance than a similar change at the blue end of the scale; a change from 9,000K to 9,200K will go undetected while a shift from 3,200K to 3,000K will noticeably warm up
Now that you are able to correlate the Kelvin values expressed in the camera viewfinder with real-life lighting situations, you can begin to use them to make judgments about whether you have indeed achieved a proper white-balance and to make creative decisions about how to handle mixed-lighting situations. More about that in the next Sharpshooters' Tips.
(For a complete explanation of the Kelvin scale as it pertains to television, check out Randy Hoffner's column "What is White?"