Incandescence

As embers or glow is called the visible radiation that emit heated body. The state of such a body is called incandescence. With increasing temperature, the spectrum of the emitted light is shifted toward shorter wavelengths, the light is " blue " (see also Wien's displacement law ).

The radiation behavior of hot metals approximates a black body. From the color of the radiation can be concluded that the temperature ( principle of a pyrometer ). In the following classification of iron smelting Glühfarben is common:

  • Below 400 ° C: invisible infrared radiation
  • 400 ° C: colorless through night vision Grauglut, only perceptible in the dark
  • 525 ° C: Incipient red heat
  • 700 ° C: Dark red heat (about 1.5 cd / m²)
  • 800 ° C a bright red heat (about 14 cd / m )
  • 1100 ° C: yellow heat (about 1,700 cd / m²)
  • 1300 ° C: incipient white heat (16,000 cd / m²)
  • 1500 ° C: full, dazzling white heat ( close to 100,000 cd / m²)
  • 2000 ° C: ( 2,000,000 cd / m )
  • 3000 ° C: ( 60,000,000 cd / m )
  • 4000 ° C: ( 375 000 000 cd / m²)
  • 5000 ° C: ( 1,200,000,000 cd / m²)
  • 5400 ° C: Neutral White ( 1,700,000,000 cd / m²)
  • 6000 ° C: ( 2600000000 cd / m )
  • 8000 ° C: ( 7300000000 cd / m )

The names of the Glutfarben are not to be taken too literally. In fact, the light is also at 1500 ° C black body temperature, no uniform mixing of visible light of all wavelengths. However, the color perception of the human eye is deceived, since the high intensity light stimulates all irradiated color receptors of the retina and thus produces a white light impression through the blinding brightness. The reflection on a white surface appears reddish.

Actual incandescence, or about equal proportions in the red and visible in the blue region, is achieved only at temperatures above about 7000 ° C. The light of the sun at 5500 ° C ( 5778 K), for example, is almost white with a slightly smaller proportion of blue and thus a yellowish impression when it is greatly reduced in its brightness.

Trivia

  • Although, according to law of radiation overall radiated energy increases with the fourth power of the temperature, the visual brightness of extremely hot radiator rises (several thousand Kelvin) only linearly with the temperature, as a large part of the radiation is emitted as an invisible UV and X-rays.
  • Furthermore, this light is then independent of the temperature of bluish color.

Examples

  • The glass bulb of a halogen lamp reaches temperatures of over 500 ° C, but it can not radiate in the visible range. (see black body radiator ).
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