Kelvin, like Fahrenheit and Centigrade, is a scale for measuring temperature. Zero degrees Kelvin (this is defined as absolute zero where there is no molecular movement) corresponds to -459.67 degrees Fahrenheit. The relationship between color and Kelvin temperature is derived from heating a "blackbody radiator" (think of this as a piece of black metal) until it glows. The particular color seen at a specific temperature is the color temperature. When the blackbody is hot enough and begins to emit light, it is dull red. As more heat is applied, it glows yellow, and then white, and ultimately blue.
Color Temperature and Photography
These numbers are used when purchasing photographic strobe equipment and film. For example, the color of the light emitted by a flash is rated at 5500 degrees; it is designed to imitate noon daylight. If the flash produces light that is 6000 degrees Kelvin, it has a slight bluish tinge. If it is rated at 4800 degrees, it is slightly warmer, or more yellowish, than white light.
Similarly, film is manufactured to give you accurate colors indoors with tungsten illumination balanced for 3200 degrees Kelvin. Examples include Fujichrome 64T and Ektachrome 50. Both of these films are designed to be used in the yellow-white light of photofloods that are specifically balanced for 3200 degrees. Household lamps may vary slightly from this color temperature, especially if they are old. If a lamp is emitting light at 2800 degrees, a subject thus illuminated would be slightly yellowish.
Overcast Conditions and Twilight
During the middle of the day when a cloud cover has obscured the sun, some of the red and yellow wave lengths of light are absorbed by the minute water droplets of the clouds. The colder end of the spectrum, the bluish wave lengths, pass through unimpeded.
This is why daylight film produces scenics and outdoor portraits with a bluish cast even during midday. Sometimes this can be very interesting artistically. If the cool tonality is unappealing to you, place a warming filter, such as an 81A, over the lens. and the color balance will shift back toward a more acceptable value.
Twilight appears almost blue-purple on daylight film due to its extremely high Kelvin temperature. When cityscapes are photographed at twilight, the contrast between the lights of buildings and the cobalt blue sky is very dramatic. I actually prefer to shoot city skylines at twilight rather than at night when the sky is black.
Digital technology uses these same traditional concepts but with a new twist. You can simply adjust your white point to change the color balance. For example, if you lower the white point to, say, 3200, you are telling the camera that you want yellowish light to be shown as white noon-type daylight. This means that daylight and flash (5500K) will be bluish, and overcast conditions and shade (about 7500K) will be exceptionally blue.
Crossing Films and Light Sources
When you shoot a film in lighting conditions that it was not designed for, interesting results await you.
Tungsten-balanced films can be used with strobe units or during midday sunlight, but the color balance will shift decidedly toward the blue end of the spectrum. At twilight, the heavy blue shift is even more pronounced. In some situations, this deep, saturated blue can be very beautiful. At sunrise and sunset, when the ambient light is golden yellow, tungsten film brings the color balance back to a more natural, middle-of-the-day look.
A few years ago, I photographed the marble lobby of the opera house in Vienna. I used both daylight and tungsten film to capture the ornate interior, and I thought the daylight film rendition was better. The exaggerated yellow-orange color warmed up the entire lobby and made it more inviting.
An understanding of color temperature helps you maintain greater control over your work. The more creative tools you have at your disposal and the greater your ability to pre-visualize the results, the better your photography will be.
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Article by Jim Zuckerman. To learn more about photography, explore the many online photography and Photoshop classes offered here at BetterPhoto.com.