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
John William Draper (May 5, 1811 – January 4, 1882) was an English-born American scientist, philosopher, physician, chemist, historian and photographer. He is credited with producing the first clear photograph of a female face (1839–40) and ...
in 1847.
Bodies at temperatures just below the Draper point radiate primarily in the
infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of Light, visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from ...
range and emit negligible visible light. The value of the Draper point can be calculated using
Wien's displacement law: the peak
frequency
Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
(in
hertz
The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that o ...
) emitted by a blackbody relates to temperature as follows:
where
* is
Boltzmann's constant,
* is
Planck's constant,
* is temperature (in
kelvin
The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow-based engineer and ...
s).
Substituting the Draper point into this equation produces a frequency of 83 THz, or a wavelength of 3.6
μm
The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer ( American spelling), also commonly known as a micron, is a unit of length in the International System of Uni ...
, which is well into the
infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of Light, visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from ...
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.
According to the
Stefan–Boltzmann law, a black body at the Draper point emits 23 kW of radiation per square metre, almost exclusively infrared.
See also
*
Incandescence
Incandescence is the emission of electromagnetic radiation (including visible light) from a hot body as a result of its high temperature. The term derives from the Latin verb ''incandescere,'' to glow white. A common use of incandescence is ...
References
{{reflist
Heat transfer
Thermodynamics
Electromagnetic radiation