Radiant energy – Radiant energy generation and sources
Patent
1991-11-21
1993-06-15
Anderson, Bruce C.
Radiant energy
Radiant energy generation and sources
2504941, 2504951, 250504R, 2505051, G01J 100
Patent
active
052201732
DESCRIPTION:
BRIEF SUMMARY
This invention relates to pulsating infrared radiation sources, particularly concerning their applications in infrared spectral analysis and in thermal printers. By designing the radiation source as given in this specification, one obtains larger temperature contrasts and smaller time constants than from similar sources presently known. This simplifies the making and improves the performance of equipment encompassing such sources.
All bodies and objects having a certain temperature emit thermal, electromagnetic radiation. For ideal black bodies, the emitted power per unit area within a wavelength interval .delta..lambda. at wavelength .lambda. is given by Planck's radiation law, ##EQU1## in which T is the body's temperature, h is Planck's constant, k is Boltzmann's constant and c the velocity of light; W(.lambda.,T) is termed the spectral radiant excitance of the body. The spectral distribution of such thermal radiation has a pronounced maximum at a wavelength .lambda..sub.max, which to good approximation is determined by the body's temperature through Wien's displacement law, becomes displaced towards shorter wavelengths according to (2). At either side of the maximum, the spectral distribution falls off strongly, very rapidly for decreasing .lambda. and more slowly for increasing .lambda.'s.
Integration of (1) across all wavelengths .lambda. gives Stefan-Boltzmann's law for the total radiant excitance of the body, Stefan-Boltzmann constant. For a 1000K radiator this corresponds to approximately 5 [W/cm.sup.2 ]. Bodies not ideally black are most conveniently described by introducing a function .epsilon.<1 on the right hand side of eqs. (1) and (3); .epsilon. is termed the emissivity of the body. Materials whose .epsilon. is independent of .lambda. are called grey emitters.
When a body at temperature T is subjected to temperature variations of magnitude .delta.T, corresponding variations are produced in the body's radiant excitance. At constant wavelength .lambda., W(.lambda.,T) always increases with rising temperature. Such spectral radiant contrast is largest in a range near .lambda..sub.max. At the same time, the total radiant excitance W of a grey body varies by an amount
In infrared spectroscopy as well as in thermal printers, large and rapid variations in radiative intensity from the thermal source are desireable. The classical infrared radiation sources, however, like Nernst and Globar radiators, operate at constant temperatures. That is also the case with more modern radiation sources, in which thin and electrically conducting films have been deposited onto thermally insulating substrates, cf. British patent 1.174.515, U.S. Pat. Nos. 3,694,624 and 3,875,413, and German Auslegeschrift 24.42.892. Variations in radiative intensity are then afforded by means of mechanically moveable shutters (choppers) interrupting the radiation. This results in large contrast in radiation between the hot source and the cold chopper blade. But it also constrains temperature variations to a fixed frequency, introduces complicating mechanically moveable parts, and obstructs electronic control of the radiation source contrary to other circuit components.
Norwegian patent 149.679 describes a pulsating infrared radiation source, comprising an electrically insulating substrate onto which has been deposited an electrically conducting film, where the thermal time constant of the source--given by the time required for thermal diffusion through the substrate--has been adjusted to suit the pulse frequencies at which the source is to operate. The source should then be made so thick as to thermally insulate, for the duration of the current pulse, the rear side of the substrate from the electrically conducting film on the front side. At the same time the source must be sufficiently thin to support heat diffusion through the substrate between pulses. This gives sources with typical substrate thicknesses of 0.1-1 mm.
However, radiation sources whose thicknesses are as given in the mentioned Norwegian patent, have thermal responses
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Anderson Bruce C.
Kanstad Teknologi a.s.
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