color temperature

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Planck radiation curves for various color temperaturesclick for a larger image
Planck radiation curves for various color temperatures

color temperature

Symbol: T c. The surface temperature of a star expressed as the temperature of a black body (i.e. a perfect radiator) whose energy distribution over a range of wavelengths corresponds to that of the star. It can thus be found by matching the energy distribution in the star's continuous spectrum to that of a black body (given by Planck's radiation law). With increasing temperature the star emits a higher proportion of blue and ultraviolet radiation and the position (wavelength) of maximum radiated energy on the energy distribution curve shifts accordingly; the basic shape of the curve remains unchanged (see illustration). Color temperature is related to color index (B V) by an approximation of Planck's radiation law:
T c = 7200/[(B V) + 0.64] kelvin

As a star's spectrum is not precisely that of a black body, the color temperature and effective temperature are not equal: the difference is greatest for hot (O and B) stars. Although the color temperature is not as closely related to the star's surface temperature as is the effective temperature, it has the advantage of being found by measurements of the star's color index. The Sun's color temperature is 5700 K.

Color temperature

The temperature to which one would have to heat a “black body” to produce light of similar spectral characteristics. Low color temperature implies warmer (yellow-red) light, and high color temperature implies a colder (blue) light. Daylight has a rather low color temperature near dawn, and a higher one during the day. Therefore an electrical lighting system should supply cooler light to supplement daylight when needed and fill in with warmer light at night. The standard unit for color temperature is Kelvin (K).

Color Temperature


(Tc), a parameter that characterizes the variation of the intensity I(λ) of the radiation from some source with wavelength λ in the optical region of the continuous spectrum. The color temperature of a source is equal to the temperature of a blackbody that has the same relative distribution of intensity as the source in the wavelength range in question (seePLANCK’S RADIATION LAW). The color temperature characterizes the relative contribution of radiation of a given color to the radiation of the source; that is, it characterizes the apparent color of the source. The concept of color temperature is widely used in astrophysics, mainly in the study of the spectral energy distribution of stars (seeTEMPERATURE).

color temperature

[¦kəl·ər ¦tem·prə·chər]
(statistical mechanics)
Of a solid surface, that temperature of a blackbody from which the radiant energy has essentially the same spectral distribution as that from the surface.

color temperature

Of a light source, the absolute temperature at which a blackbody, 1 radiator must be operated to have a chromacity equal to that of the light source.

color temperature

The color of light, expressed in Kelvins (K). The sun is about 5800 Kelvins. This is called a "temperature," because the hotter fire gets, the more it changes from a warm yellow glow to white.

The lower the Kelvin rating, such as the common incandescent/tungsten bulb, the warmer, softer and more yellow the light. The higher the K number, the cooler, whiter and bluer the light. See white balance.

              Approximate TemperatureLight             In Degrees ofSource             Kelvin (K)

  Match/Candle     1,700-1,800   Warm/More
  Incandescent     2,600-3,000   Yellow|
  Halogen          2,800-3,400     ||
  Fluorescent      3,000-4,300  Daylight|
  Outdoor sun      5,000-6,000     ||
  Outdoor shade    6,000-7,000   Cool/Whiter/
  North sky           10,000+    Bluer

  LED lights range approximately
  from 2,600 to 5,200 K

From Warm to Cool
Increasingly, light bulb packages show the color temperature as a measure from warm to cool. This is a halogen bulb label.
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