The 19th Century Scotch-Irish physicist William Thomson (later Lord Kelvin) first proposed the idea that absolute zero should be defined as the point where all molecular movement ceases, so an object at absolute zero becomes a "black body radiator," and gives off no light. As the temperature rises so does molecular vibration speed and each speed range has a characteristic color. At around 700º, for instance, an object begins to give off a dull red glow. Kelvin's scale directly measured temperature as we normally think of it, not light colors, but he did note that at each temperature his test object (a piece of carbon) had an associated color. The Kelvin scale has since come to be called "color temperature."
Color and light
Although we think of heat as one physical category and light as another, they are viewed in physics as different properties of one thing. The temperature of an object depends upon the wavelength of the light it emits. We experience the wavelength of light as color. The color of an object depends upon its temperature, as Kelvin originally noted.
How Much is a Kelvin?
A difference of one kelvin equals a difference of one degree Celsius, or about 1.8 degrees Fahrenheit. Because the Kelvin scale starts at absolute zero and the Celsius scale starts at the freezing temperature of water, the two scales are offset by 273.15 degrees: 0 degrees Celsius is equal to 273.15 degrees Kelvin.
The Kelvin Range
Absolute zero in kelvins equals -273º Fahrenheit. Sunlight equals around 5000K. Physicists estimate that the temperature at the moment of the Big Bang--the proposed beginning of the universe--was 10 to the 30th power in degrees Kelvin.
Kelvins in Lighting
Photographers and lighting designers speak of color temperatures in "degrees kelvin." For example, 3200K represents a typical indoor color temperature and 5500K represents typical daylight color temperature. In the context of lighting, a specific kelvin temperature expresses the color temperature (dull red, bright red, white, blue) corresponding to the physical temperature (warm, hot, extremely hot) of an object.