DV101: Perception and Reflection: How to Achieve the Correct Color Temperature7/29/2013 11:15 AM Eastern
Our world is filled with color—deep reds, cool blues, vibrant yellows—all of which we see because the light from the sun contains all of the colors of the rainbow, so the individual objects in our world can reflect those colors to our eyes.
Human vision is incredibly flexible with regard to color. Our eyes and our brains work together to interpret many different lighting situations so that we see them as natural and normal.
The color of natural light is measured on the Kelvin scale, originated by British physicist William Thompson Kelvin in 1848. Kelvin theorized that if you took a perfect black body radiator—a theoretical material that neither reflects nor refracts light—and started to apply heat, the radiator would begin to glow. It would first glow red, then orange, then yellow and then blue. In his scale—which begins at absolute zero (-273.15° C), defined as 0° K—he was able to define the colors of the visible spectrum by specific temperatures.
When color film emulsion was invented—and then the CCD and CMOS chips much later—it was not as flexible as the human brain at seeing “white” light. Film and digital sensors have a very small window of color temperatures that will be rendered as white.
To combat this tendency, the creators of imaging media picked two target points of color temperature that would display correctly. These were:
- The average of measured daylight color temperature at high noon, which is about 5,600° K
- The color temperature of tungsten incandescent fixtures, which burn at about 3,200° K
|Daylight captured by a tungsten-balanced camera looks quite blue.||Tungsten recorded by a daylight-balanced camera is too orange.||Correct lighting renders “neutral” whites in an image.|
Neither of those numbers is infallible, unfortunately. Daylight is rarely exactly 5,600° K and incandescent lamps are rarely 3,200° K. But the numbers represent a “close enough” approximation that allows for slight manipulations in color correction later on to create the right look.
Generally, the “right look” is depicting the world as it appears to the human eye.
|LEE Filters CTB spectral energy curve and specs|
Although we may be able to stand in a room lit with tungsten light (3,200° K) and see everything as though it is lit with white light, the camera cannot do the same automatic translation. If the camera is set to see daylight (5,600° K) as “white,” then the camera will render everything in the tungsten-lit room with a deep orange cast. This is because the camera, with an “outdoor” setting, is programmed to see the higher Kelvin temperature as “white,” and when the actual light captured is of a longer, warmer wavelength, it will appear a lot redder in the captured image.
Inversely, if you set your camera for tungsten (3,200° K, or “indoor” color balance) and you have natural daylight (5,600° K) in the picture, the natural daylight is going to appear very blue.
In Mixed Company
|Rosco CTB spectral energy curve and specs|
It’s in situations with mixed color temperatures that things get tricky. If you have natural daylight mixed with tungsten lights, you’re going to have to pick your approach:
A. Filter one of the sources of light to match the other.
B. Pick one to balance to, knowing the other will be greatly out of “neutral.”
C. Compromise on a middle ground.
D. Replace the tungsten lighting with daylight-balanced lighting fixtures.
If you let the camera decide, it will go with option C and white balance somewhere between the 3,200° and 5,600° K marks. This solution turns your indoor lights slightly warm and your daylight slightly cool, which is rarely the best solution. In this scenario, nothing is rendered “correctly,” as your eye sees it; rather, the entire image is compromised.
If you select option B, then you will set the white balance either to daylight, which will render the indoor lights very orange, or to tungsten, which will render the daylight very blue. For some applications, this may be preferable.
|Color temperatures (in Kelvin)|
Option D isn’t always possible as daylight-balanced lighting fixtures are often too expensive or simply unavailable. You can incorporate fluorescent fixtures with daylight color balance and high color rendering index (CRI), or incorporate LED fixtures, HMI gas-discharge fixtures or tungsten with dichroic (color corrected glass) lenses. These are specialty fixtures, though. They won’t always be available.
The least expensive solution is A, although “least expensive” is relative because color correction gel isn’t cheap, but if you take care of it, it will last a long time. In this case, decide which light will be your primary source and filter the other light to match its color temperature. In this scenario, the best solution is to filter the natural daylight to match the tungsten. Do this by adding CTO gels to the windows to filter out the blue wavelengths and correct the 5,600° K color to 3,200° K.
The reason it’s better to filter the windows than the lights themselves is that any filters (gels) you use will block certain wavelengths of light and reduce the overall intensity of light passing through the filter. CTO gel (“correct to orange” is a good way to remember, or “color temperature orange”) blocks out about two-thirds of a stop of light, or about 33 percent of the light passing through the filter. As daylight is most often the brightest thing around you, this isn’t too much of a compromise, and it’s often desirable to reduce the intensity of the daylight to a level nearer the intensity of the artificial lights.
Inversely, if you filter the tungsten lights to match the daylight, you will use CTB (“correct to blue”) filters, which block out the red, orange and yellow portions of the spectrum. CTB gels are much more saturated than CTO and reduce the light by as much as 75 percent (two stops). Typically artificial lights are already much dimmer than natural daylight; cutting them down by 75 percent makes it more likely they will be outshined by the light of the sun.
A fifth option, which I did not include above, is to shut off all the indoor lights and use bounce cards and reflectors to light with daylight exclusively.