Radiant energy – Radiant energy generation and sources
Patent
1983-09-28
1986-10-28
Anderson, Bruce C.
Radiant energy
Radiant energy generation and sources
250504R, 2504951, 250352, G01J 310, H05B 326
Patent
active
046201041
DESCRIPTION:
BRIEF SUMMARY
This invention is concerned with infrared radiation sources, and is intended to bring forth improvements in various applications where such radiation sources are used, particularly as regards infrared spectral analysis.
All bodies and objects of non-zero absolute temperature emit thermal radiation. This is electromagnetic radiation, the spectral energy density u(.upsilon.) of which is given by Planck's law of radiation: ##EQU1## which is strictly valid for a blackbody. Here, T is the absolute temperature of the body, k is Boltzmann's constant, .upsilon. is the radiation frequency, h is Planck's constant, and c is the speed of light. For bodies at room temperature (T.perspectiveto.300K), this yields a spectrum with a maximum intensity at approximately 10 micrometers (.mu.m) wavelength in the middle infrared spectral range. If the temperature is increased, the spectral distribution will change according to (1), and the wavelength at maximum intensity (.lambda..sub.max) will be displaced towards shorter wavelengths. For T>4000K, .lambda..sub.max is close to or within the visible spectral range. This displacement is described to a good approximation by Wien's displacement law frequencies, one finds Stefan-Boltzmann's law of total radiant excitance from a body: constant. By integration over the emitting surface of the body, the total power radiated from the body is derived; it is seen to increase as the fourth power of the absolute temperature of the source. Total radiation from a body that is not ideally black is given, to a good approximation, through a simple modification of Stefan-Boltzmann's law:
In principle, conventional infrared spectrometers consist of a hot radiation source, an optical filter that selects a restricted spectral region from the continuum-radiation emitted by the source, a chamber containing a sample which is trans-illuminated, and a detector that measures the radiation that has passed through the sample. Usually, the radiation sources operate at a constant temperature T.sub.h, which is much higher than the background-(ambient)temperature T.sub.o. In practical instruments, therefore, it is customary to insert a rotating wheel (a chopper) furnished with equidistant apertures along the rim, into the radiation path to make the radiation pulsed, since many types of infrared detectors only respond to changes in radiation level. This is specitically the case with pyroelectric detectors, and in applications of photoacoustic spectroscopy and related techniques. Pulsating radiation is also advantageous regarding electronic amplification and discrimination against noise.
Commonly used radiation sources, e.g. in general purpose laboratory spectrometers, are incandescent rods with coatings of relatively high and constant emissivity throughout a major portion of the near and medium infrared spectral region (gray-body radiation). Examples of the latter are the so-called Globar and Nernst-radiators. Radiation sources of the Globar and Nernst type are afflicted by the following shortcomings, which make them poorly suited in apparatus that must be robust, compact, cheap and possibly mobile: alternatively Yttria, Zirconia, etc.), radiation,
However, such sources afford the significant advantage that, by chopped pulsing, they yield a train of radiation pulses with large on-off variations in radiation intensity, resulting in a correspondingly large signal S from the infrared detector. This is important in order to overcome the inherent detector noise and, thereby, to optimize the signal-to-noise ratio. Thus, the detector signal S will be proportional to the difference in radiant excitance between the hot body, W.sub.h, and the cold chopper blade, W.sub.o (assuming that the optical system guiding radiation from the source to the detector has been designed to make the source cover the entire field of view of the detector): S.varies.W.sub.h -W.sub.o. Provided the chopper is at ambient temperature T.sub.o, substitution from (4) into this expression yields: .sigma.T.sub.o.sup.4 for T.sub.h >3T.sub.o .congr
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Kanstad Svein O.
Nordal Per-Erik
Anderson Bruce C.
Guss Paul A.
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