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.

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:

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

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.