Tuesday, November 18, 2008
Color Temperature: When "white" isn't just white.
If you've ever heard someone talk about color temperature, you may have wondered what all the fuss is about. What does temperature have to do with color? Isn't "white" simple enough to understand on it's own?
The short answer is no. Now, here's the longer answer:
You may remember from elementary school that white is defined as the presence of all color. This was usually demonstrated by taking a white light and shining it through a prism to create a lovely little rainbow. This is the same reason rainbows often appear on sunny days through mists of water hanging in the air.
In order for an object to appear to be white, it must reflect back all of the colors contained within the light that is reflecting off it's surface. If one or more of the colors of the spectrum are missing from either the light source or the reflected light, the object will appear to have a color. The color of an object is determined mainly by the light source illuminating the object.
This is where color temperature comes into the equation. Marketing departments like to describe color temperatures using terms like "cool white" or "warm white", but in film and video, these terms are just not specific enough for our purpose. Instead, we describe the temperature of a light in degrees Kelvin.
The Kelvin Scale
If you've taken a high school chemistry or astronomy class, you may already be familiar with the kelvin scale. Technically speaking, it is a thermodynamic temperature scale, and one of the SI base units from which all other units of measurements can be derived. (For the technically curious, the other SI base units are meter, kilogram, second, ampere, mole and candela.)
The most commonly known temperature on the kelvin scale (noted with the symbol "K") is zero degrees kelvin, also known as Absolute Zero. This is the theoretical temperature at which there is no thermal energy and all life ceases to exist. But from a lighting standpoint, the low end of the kelvin scale doesn't concern us. We are interested in much higher temperatures... mostly those in the 1800K to 5600K range.
Temperature = Color
Looking up into the sky on a clear starry night you will notice that no two stars are exactly the same color. This is because no two stars burn at exactly the same temperature. Our own star Sol (aka. The Sun) is estimated to have a surface temperature of approximately 5,600K. Sirius, the brightest star in the Orion constellation has a temperature of approximately 9,800K and appears blue while Antares, the brightest star in Scorpio, appears red and has a temperature of 3,400K.
Rather than going through mathematical formulas and extensive data, let's just simplify the issue and say that lower temperatures will give the light a red hue, while higher temperatures will appear more blue. The hotter something burns, the more blue it appears. The cooler it burns the more red it will appear.
Keeping with this concept, lighting manufacturers rate their lights based on color temperature. A light with a color temperature of 5,600K is said to be a "cool white" or "sunlight balanced", whereas a tungsten light bulb has a temperature of 3,200K. Fluorescent lights, which often appear with a greenish tint have an approximate color temperature of 4,600K.
It's important to note that these are color temperatures not actual temperatures. In other words, the color of the light approximates the color of a star burning at a certain temperature Kelvin. Obviously, the light bulbs in your house are not as hot as the star Antares...
Determining What "White" Looks Like
Taking into account the light source shining on an object, we can see that changes in the color of the light will affect the color of a "white" object. A "white" piece of paper in direct sunlight will appear more "blue" than the same piece of paper lit by candlelight.
Fortunately (or unfortunately depending on how you look at the problem) the human brain has an automatic compensation device built in to eliminate these inconsistencies. If you look at a white piece of paper outside, it looks... well... white. Take the same piece of paper inside and look at it under a tungsten light bulb and it still looks white. Your brain continuously adjusts itself to take lighting conditions into account when determining color.
If you're bored one night and want to see this effect in action, try the following: Turn off all the lights in your house and turn on a TV set in a room with a window. Now, go outside and wait a few minutes for your eyes to adjust. If you look back at your house, you'll see an eerie blue glow coming from the room with the TV set. Go back inside and everything looks normal.
Cameras and White Balance
Since film and video cameras don't have the advantage of containing a human brain, we need to take these issues into account and tell the camera what it should see as white. We do this by choosing an appropriate film stock (daylight or tungsten) or white balancing the video camera for a certain light temperature. Then, make sure that all of the light sources that are being used match the camera white balance settings. Pick one color temperature and make sure everything is matched.
To help keep things balanced, there are also a number of different filters and gels available to help convert one color temperature of light to another. For example, if you are shooting in an apartment with one window, you can light the scene with tungsten (3200K) and can cover the window with a #85 gel to convert the sunlight (5,600K) to match the tungsten lights. Alternatively, you can put some CTB gels on your tungsten lights to bring their color temperature closer to that of sunlight. It all depends on your goals for lighting the scene.
If we don't give the camera a specific color temperature reference, or worse yet, use different color temperatures of light you will get some rather unexpected results. Your brain may be able to deal with multiple color temperatures of light, but cameras can not.