Color Temperature

Color temperature is a way of measuring (in units derived from the Kelvin temperature scale) and describing the color quality of white light by comparing it to a theoretical black body heated to a specified temperature on the Kelvin scale. It's important in the design and use of computer monitors, solid-state displays and digital cameras.

What color is white? From a physics point of view, white light is what our eyes see as a composite mixture of the full spectrum of visible light. As the sum of all the other colors, white can appear in myriad subtle hues, ranging from red to ivory or cream to yellow to blue. Yet our eyes tend to see any naturally bright light source that’s not filtered as just white, with little sense that it includes more or less of certain parts of the spectrum.

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In this day of relatively smart digital cameras and Photoshop image manipulation, it’s easy to forget how much our brains depend on whites looking white to make all the other colors appear correct. Yet color matching, or color accuracy, is important for visual comfort. Every time we use a computer monitor, take a picture or look at a photograph, our brains adjust the colors we see so that they look the way we think they ought to. Color temperature is a useful way to describe the whiteness of white light, especially when comparing one light source to another.

Here’s how two such seemingly disparate and unrelated concepts as color and temperature unite in a single descriptor: Let’s create a very special light bulb by imagining a theoretical black object that’s really cold: it’s sitting at a temperature of absolute zero, 273 degrees below zero Celsius, -459.6 Fahrenheit. Because absolute zero is our primary reference, we’ll use the Kelvin scale, which has the same intervals as Celsius but begins with 0 at absolute zero.

It doesn’t matter what this black thing is made of because it can’t be seen. We call it black because, by definition, black absorbs all light that hits it. If this body is in balance with its surroundings, it will radiate back the same amount and type of energy that strikes it. This state of equilibrium is called black body radiation. When the black body sits there at 0 degrees Kelvin with no energy coming into it, it’s emitting no light, so it’s not possible to see anything.

If we heat up this black body, we see it beginning to glow with a dim, reddish appearance. As the heat increases, the color changes, appearing first dark red then yellow, moving through the visible colors of the spectrum until it reaches blue and violet. As the object changes color, it also appears brighter, since more heat is being pumped in and a greater amount of energy is being radiated out while its color is getting bluer; we interpret this energy increase as brightness. At the middle and higher ends of the visible light scale, the body appears white to us; our eyes and brain can’t easily distinguish subtle differences at such light levels.

When our black body reaches 2,800 degrees K, it looks like a normal incandescent lamp. At 5,000 degrees, the quality of its light is akin to a sunlit summer day. Using the color temperature scale, we characterize a typical tungsten-filament light bulb as having a color temperature of 2,800 Kelvin (for simplicity’s sake, we drop the word degrees when talking color temperature). Note a real distinction here: We aren’t saying the filament is operating at a temperature of 2,800 degrees K; we’re merely describing the color quality of the light. Similarly, we’ve set the standard reference for daylight at 5,000 Kelvin, regardless of the sun’s actual temperature.

Balance the White

That would be sufficient if all our light came from heating something that emits the entire visible spectrum, such as a burning match or a standard light bulb. But many light sources don’t emit the entire visible spectrum. Some frequencies (and colors) are missing entirely, while other frequencies show large spikes.

Such nonblack body sources include fluorescent tubes, LEDs, and the sodium vapor lamps used outdoors.

We can’t directly or accurately use color temperature to describe the light from such sources. Instead, we measure the relative amounts of red, green and blue light these sources emit and calculate a correlated color temperature.

The problem with this becomes apparent when we mix two dissimilar light sources. For example, take a room that is lit by “cool daylight” fluorescents (correlated color temperature 6,200K) and has sunlight streaming in through an open window (say this light has a real color temperature of 6,200K). To the eye, both light sources seem to have the same color quality, which is what the numbers imply. But when we take a photograph, we see that the parts of the room lit by fluorescent light look strangely greenish.

Why don’t our eyes see this difference in the room itself? Because, like today’s smart digital cameras, we have an “automatic white balance” feature that automatically compensates for such differences.

Source: Adapted from www.sizes.com/units/color_temperature.htm

Source: Adapted from www.sizes.com/units/color_temperature.htm



Kay is a Computerworld contributing writer in Worcester, Mass. You can contact him at russkay@charter.net.

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