Electric resistance heating devices – Heating devices – Radiant heater
Patent
1998-07-01
2000-02-29
Jeffery, John A.
Electric resistance heating devices
Heating devices
Radiant heater
219553, 250504R, H05B 300
Patent
active
060319704
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention describes a class of infrared radiation emitters suitable for applications in spectroscopic devices and instruments as well as in thermal printers, etc.
BACKGROUND OF THE INVENTION
There are known emitters where the infrared radiation emanates from a surface which is directly or indirectly heated by an electrical current passing through a conducting layer in or below the surface. The elevated temperature leads to increased emission of radiation, with the intensity and spatial as well as spectral distribution of the emitted radiation depending on the element temperature, as well as on its emissivity and surface topography.
For a greybody with emissivity .epsilon., the emitted power per unit area, within a wavelength interval d.lambda. at wavelength .lambda. is given by: ##EQU1## where T is the temperature, h is Planck's constant, k is Boltzmann's constant and c is the velocity of light.
In spectroscopic analysis, parts of the emitted power within restricted wavelength regions are selected by means of optical filters, and the radiation source is sought optimized by having a high emissivity .epsilon. and high temperature T.
In order to achieve compact, efficient and low-cost device solutions, it is desirable to switch or modulate the emitted radiation by rapidly varying the temperature T of the emitter, rather than by mechanical motion of shutters, filters etc. Electrically switched infrared emitters are also of great interest in many non-spectroscopic applications such as thermal printers, etc.
Two well-known types of pulsed thermal emitters in current use are: current, causing heating/cooling and associated variations in emitted infrared radiation. These sources are used in infrared sensors for gas monitoring, etc, and are relatively cheap. Unfortunately, their operative life is short, requiring frequent replacement. Also, they are bulky and consume much electrical power compared to the useful infrared radiation emitted. electrical current. The surface may be a flat, insulating substrate which is coated by a conducting film or layer, or it may be a thin, freely suspended membrane of a material which is itself electrically conducting. U.S. Pat. No. 3,961,155. Drawbacks of these types of emitters include: Poor efficiency (i.e. much electrical power needed compared to the useful radiation emitted), which is in large part due to heat conduction through the relatively thick (typically 0.5 mm) substrate. Also, mechanical mounting and bonding of electrical connections is critical and labor intensive. of a silicone membrane which is etched thin and doped to high conductivity in a central region where infrared emission takes place. These sources have proven very robust and long-lived, and are generally more efficient than the emitters referred above, although the heat loss through the membrane to the mounting fixtures is still quite high. This is due to the thicker portions of the membrane, which must have a certain mechanical strength to permit an acceptable yield during manufacturing operations, as well as robustness in practical use. A serious drawback of these emitters is their high manufacturing cost.
The ideal thermal emitter should convert 100% of the dissipated electrical energy into infrared radiation at the desired wavelengths. Membrane emitters described above are a far cry from this; typically the ratio between total radiated power and supplied electrical power is 10% or less. The pulsating part of the radiated power is a fraction of the total emitted power, and the infrared emission within specific spectral bands is a fraction of this.
As shown schematically in FIG. 1, heat is generated in the thin membrane and can take several different paths:
Radiation is the heat loss mechanism which is sought maximized at the expense of the following:
Each of these can be quantified, subject to defined conditions such as materials, dimensions and operating temperature. Consider, e.g. the membrane shown in FIG. 2, which is freely suspended at opposite ends and surrounded by p
REFERENCES:
patent: 3961155 (1976-06-01), Weldon et al.
patent: 5128514 (1992-07-01), Lehmann et al.
patent: 5352493 (1994-10-01), Dorfman et al.
patent: 5488350 (1996-01-01), Aslam et al.
patent: 5705272 (1998-01-01), Taniguchi
Nordal Per-Erik
Skotheim Terje
Jeffery John A.
Patinor A/S
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