Radiant energy – Radiant energy generation and sources – With radiation modifying member
Patent
1997-06-09
1999-01-26
Berman, Jack I.
Radiant energy
Radiant energy generation and sources
With radiation modifying member
2504431, 2504951, H05B 320
Patent
active
058641440
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to an infra-red (IR) radiation emitting device, particularly, but not exclusively, a device adapted for producing pulsating IR radiation.
IR radiation is used in a wide variety of applications and there are a number of known IR emitting devices. Such known devices include lasers, semiconductor IR emitting diodes and electrically heated elements which emit IR radiation when hot.
For certain applications, such as infra-red spectroscopy, for which pulsating IR radiation over a wide band of wavelengths (e.g. 1 .mu.m to 10 .mu.m) is desirable, a number of pulse emitting devices have been developed. Such devices can broadly be separated into two general types: firstly devices in which a constant temperature source is used and variations in the radiated IR intensity are obtained by means of mechanically interrupting the radiation emitted from the source, such as Nernst and Globar radiators; and secondly devices that can produce pulsating radiation directly such as filament lamps (e.g. tungsten, Ni/Cr or Pt filaments which can operate at around 5 Hz to 10 Hz but have output wavelengths below a value of about 5 mm by the filament enclosure transmission characteristics) and devices which comprise a thin conductive film supported on a substrate and which produce pulsed IR radiation by pulsating the heating current supplied to the film.
Examples of the latter type of device are described in PCT Application Nos. WO83/03001 and WO90/14580. In particular WO90/14580 describes a device for emitting pulsed IR radiation which includes a source comprising an electrically conducting plate like thin film (typically less than 4 .mu.m tick) supported on a thin insulating substrate. The source is described as being able to produce larger temperature contrasts and smaller time constants (i.e. faster pulse times) than had previously been known with existing thin film sources. This is achieved through radiative cooling of the source which is much more rapid than conductive cooling as had been relied upon in previous tin film sources. That is, the films proposed are so thin that the majority of the heat energy generated in the film during current ON-pulse time is lost through IR radiation so that the heat retained by the film is significantly less than the heat energy put into the film.
A number of alternative materials which could be used for construction of the conductive film are proposed in WO90/14580, including combinations of nickel, chromium and iron and the higher emissivity oxides of their alloys. However, in all cases the sources disclosed in WO90/14580 suffer disadvantages as a result of the thin conducting film being deposited on a substrate. That is, differences in the thermal characteristics of the thin conductive film material and substrate material can lead to mechanical failure under the thermal stresses resulting from repeated heating and cooling.
The paper entitled "Experiment on Optical Pumping of a Carbon Dioxide Molecular Laser" by P. A. Rokhan (Optics and Spectroscopy, April 1972, Volume 32, page 435) discloses the use of a molybdenum foil (not deposited on a substrate) to produce pulses of infra-red radiation for the specific purpose of exciting a CO.sub.2 laser. A strip 7 m in length, 40 mm in width and 0.02 mm in thickness, is wound in a helix around a quartz-laser tube. However, the foil source requires a large electrical supply current (derived from a bank of capacitors) to heat it to the required temperature. The device is not particularly adapted to produce rapid pulsing as for example in the thin film sources discussed above that can be operated at frequencies of the order of 100 Hz).
In addition, the arrangement is clearly unsuitable for applications in which much smaller centralised sources are required. For instance, even if a relatively small section of the foil is used, for example of the order of 5 mm square, the heating current must still be relatively large (as compared with the currents required for driving the thin film sources discussed above) and could not readil
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Berman Jack I.
Keele University
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