What Is Color Temperature?
The concept of color temperature is an integral part of photography, and yet many photographers are not really sure what it means.
Color and temperature don't seem to have a direct relationship with each other, but light sources are often defined in terms of their color temperature and we speak of using the correct film with a particular type of light or setting the "white point". In addition, the measurement of color temperature is in Kelvin degrees. What does all this really mean? 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. The colors radiating from the blackbody are correlated to colors we are familiar with in our daily lives. The color emitted from a tungsten lamp in your living room is identical to the yellow-white glow when the blackbody radiator temperature is approximately 3200 degrees Kelvin. When the temperature rises to 5500 degrees, the quality of white light is identical to the color of the sun at midday. The bluish quality of twilight just before dark is similar to the color of the blackbody at about 12,000 degrees Kelvin.
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.
Daylight films, such as Ektachrome E-100, Fujichrome Velvia and Provia, and Agfachrome 200, are balanced for 5500 degrees Kelvin. This means that they produce accurate colors during the middle of the day when the sun is overhead. Before the sun reaches its zenith – say, from sunrise to early morning - the yellowish quality of the sunlight is less than 5500 degrees. The same is true from late afternoon to sunset. During these times, daylight film reproduces a warmer, or more yellow, image.
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, passes through unimpeded. This is why daylight film produces scenic images 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.
Questions
- Explain how colour temperature is measured.
- Why do you think it is not beneficial to use artificial light in some photography?
- When is the optimal time (what type of light) to take specific photos?
