MadSci Network: Physics

Re: Temperature color and lamp

Date: Sat Jun 24 10:00:25 2000
Posted By: Todd Jamison, Staff, Image Science, Observera, Inc.
Area of science: Physics
ID: 959722773.Ph

Color temperature is the temperature at which a black body radiates a given spectrum of color. A black body is an ideal physical "thermodynamic" object. It radiates and absorbs, but does not reflect electromagnetic energy (e.g., light). All objects have some black body characteristics. If you have ever seen a glowing element from an electric stove, then you have seen a black body that is radiating in the red/orange range of the spectrum. In fact, the radiation spectrum is a curve, where the spectral peak of the curve at a given tempearture is the primary color for that temperature. Basically, color temperature is used to vary the color tone of lighting of computer screens. Lower temperatures go more to the red/orange and higher temperatures go more to the blue/violet. Incandescent light bulbs are simple examples of black body radiation and color temperature. An electric current heats the tungsten wire until it's black body temperature is high enough so that the peak color in the spectrum is yellow. If you have ever used halogen lamps, you have probably noticed that they are hotter and give off a spectrum peak that is more toward the blue.

Now how does this affect the colors that we see? The color of an object comes from the fact that each object absorbs some colors and reflects others. A tree, for example, is green because the chlorophyll in the leaves absorbs all colors except those in the green and reflects mostly the green. Similarly, a white wall is white because the white paint reflects all colors (the sum of all colors is white) and absorbs none. A black wall absorbs all colors, so there is no color of light reflected. If a white light (a light will all colors) is used to illuminate an object, then the object is said to have its "natural" color. If you illuminate an object with a different spectrum, it will still absorb and reflect the same way it did with the white light. But, if less of the colors it reflects are present, it will look very different. Consider the white wall. If we illuminate it with a green light it will look green because that is the only light available to reflect. If we use the same light to illuminate a black wall, it will still look black, because it is absorbing all colors, regardless. Suppose we put the green leaf in front of the white wall and illuminate it with green light? The leaf and the wall look similarly green because they both reflect green. What if we now illuminate them with a light that has all colors except green? The wall will look sort of pinkish and the leaf will look --- black. Why? Because all of the colors available in the light are absorbed by the leaf. Now I wish it was all quite this simple in the real world, but actually real-world objects absorb and reflect different amounts of light from many different parts of the spectrum. Did you know, for example, that leaves reflect a lot of light in the infrared? The color of objects is made up of how we perceive the combination of colors that hit our color retinal cells in our eyes. Our brains build color from cells that respond to three different bands of color - Red, Green, and Blue, which is why TV's and computer monitors use RGB color. Since the color temperature of the light source affects the spectrum of the light, it also affects how the colors look to our eyes.

Here are a few sites you can check out. I did a quick search on HotBot ( for the phrase "color temperature" and got a lot of interesting hits. (link defunct, 8/23/2006) (link defunct, 8/23/2006)

Good luck with your project.

Regards, Todd Jamison, CEO Observera, Inc.

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