Draper point

The Draper point is the approximate temperature above which almost all solid materials visibly glow as a result of blackbody radiation. It was established at 977˚ F (525˚ C, 798 K) by John William Draper in 1847.^[1][2][3]

Bodies at temperatures just below the Draper point radiate primarily in the infrared band and at longer wavelengths, and emit negligible visible light. The value of the Draper point can be calculated using Wien's displacement law: the peak frequency $\nu_{peak}$ (in hertz) emitted by a blackbody relates to temperature as follows:^[4]

$\nu_{peak} = { 2.821 \over h} kT$

where

k is Boltzmann's constant.

h is Planck's constant.

T is temperature (in kelvins).

Substituting the Draper point into this equation produces a frequency of 83 terahertz, or a wavelength of 3.6 µm—which is well into the infrared, and completely invisible to the human eye. However, the leading edge of the blackbody radiation curve extends, at a small fraction of peak intensity, to the near-infrared and far-red (approximately the range 0.7-1 µm) which are weakly visible as a dull red.^[5]

According to the Stefan–Boltzmann law, a black body at the Draper point will emit 23 kilowatts of radiation per square metre, mostly infrared.

References

1. Draper, John William (1847). "On the Production of Light by Heat". The London, Edinburgh and Dublin philosophical magazine and journal of science (Taylor & Francis): 345–359.
2. Draper, John William (Oct 26, 1878). "Science: Draper's Memoirs". The Academy (London: Robert Scott Walker) XIV (338): 408.
3. Mahan, J. Robert (2002). Radiation heat transfer: a statistical approach (3rd ed.). Wiley-IEEE. p. 58. ISBN 978-0-471-21270-6.
4. Wannier, Gregory H. (1987) [1966]. "Chapter 10-2". Statistical Physics. New York: Dover Publications. ISBN 978-0-486-65401-0. OCLC 15520414.
5. Starr, Cecie (2005). Biology: Concepts and Applications. Thomson Brooks/Cole. ISBN 0-534-46226-X.