Color temperature is a term generally used to describe the color of a light source or the maximum white that a monitor can produce. For accurate color matching between an image viewed on a monitor and a print or transparency, the color temperature (white point) of the monitor must be adjusted to match the color temperature of the light used to view the print or transparency.
Color temperatures are normally measured in degrees based on the black body radiation curve which describes the color change of objects when they are heated. Low temperature light sources light incandescent and quartz halogen lamps (around 3000 degrees) are somewhat orange. Standard daylight is around 5500 degrees. A typical uncorrected color monitor is rather blue (around 9300 degrees).
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.
By Russell Kay
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
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.
A measure of the distribution of power in the spectrum of white, or colorless, light, stated in terms of the kelvin temperature scale. The human visual system is incredibly adept at quickly correcting for changes in the color temperature of light; many different kinds of light all seem “white” to us. Photographic film is not so forgiving; daylight film is made to be exposed by 5500 K light, while “indoor” film requires light with a color temperature of 3400 K (or 3200 K, for professional film). Photographs taken indoors under incandescent light on ordinary daylight film will come out orange. Photographs on indoor film taken in sunlight will be blue, as will photographs taken outdoors in shade illuminated by blue sky.
To understand why a characteristic of light can be described by temperature, imagine a clear incandescent lamp connected to a dimmer. As the dimmer is turned up, the voltage increases and the lamp's filament becomes warmer and warmer until it begins to glow cherry red. As the voltage continues to increase the filament gets hotter and hotter, glows more and more brightly, and is less and less red. The nature of the light is changing with the temperature of the filament.
To formulate mathematical descriptions of the relationship between the temperature of the filament and the light being emitted, physicists use the concept of a blackbody, an imaginary perfect emitter and absorber of radiation.
Most light sources emit light that is a mixture of light with different wavelengths, although some wavelengths are missing from certain kinds of light sources, such as fluorescents. The light from a blackbody, however, is a mixture of light with a continuous range of wavelengths. The light is strongest at some particular wavelength. On each side of this peak, the amount of power gets progressively smaller. The farther a wavelength is from the peak, the less power the light with that wavelength will have. As a blackbody gets hotter, the peak shifts toward shorter wavelengths.
At this point you should stop reading and play around with one of the excellent interactive diagrams which show the change in the spectrum of the emitted radiation as you vary the temperature. See Resources, below.
An incandescent lamp is very nearly a blackbody radiator, and the distribution of power in the wavelengths it produces can be described by the temperature of a blackbody radiator whose light would appear to the human eye to be of the same color. So a typical 100-watt incandescent has a color temperature of 2870 K. Photoflood lights, however, are either 3200 K or 3400 K.
With light sources like fluorescents the situation is more complicated, because power-against-wavelength graph for their light would show many sharp peaks, not just one smooth curve. It does not resemble blackbody radiation. However, a measure called correlated color temperature is assigned to such light sources by visually matching their light with light from a blackbody source.