Infrared radiation source

Abstract

A source of infrared radiation is provided which includes a thin film resistive heater of high emissivity evaporated onto a substrate. The thin film heater is positioned between a pair of thin metal elements on the substrate. The resulting structure provides a well-defined, mechanically stable source. In one embodiment of the invention, the resistive element is coated with an antireflecting layer to enhance its emissivity.

Description

BACKGROUND OF THE INVENTION

This invention is concerned generally with radiation sources, and more particularly with a new planar source of infrared radiation.

As more small, portable instruments using infrared sources are being designed, it is becoming of importance to have infrared sources of high overall efficiency. Various infrared sources are presently known. One common source is a Nernst glower, which uses a silicon carbide filament. These glowers have been found to be difficult to start-up and operate, while the typical physical configuration does not provide a good point source for applications requiring such.

Tungsten in the shape of a coil filament is also used to provide infrared radiation. For use in systems which require good imaging and low noise, however, tungsten coil filaments are not adequate. Because of the physical configuration of the coil, the image does not consist of a solid area of radiation, but rather consists of areas of radiation intermingled with areas in which not radiation is present. The image is said to have a poor "fill factor." Furthermore, a system employing a coil filament is very susceptible to errors induced by mechanical movement or jarring of the system. When jarred, the coil filament tends to jiggle resulting in spurious noise in the detected radiation. Also, the tungsten material typically used in coil filaments has a comparatively low emissivity (about 0.15) in the infrared region, and is therefore not well suited for use as an infrared source.

In the prior art, some of the problems of a coil filament have been avoided by using a ribbon of tungsten as a source. A ribbon, however, does not eliminate the problems of using low emissivity tungsten as an infrared source. Furthermore, because of the low resistivity of tungsten, it is difficult to provide a small well-defined source with a tungsten ribbon, there being no discontinuity between source and leads. If, however, only a small section of tungsten is used as a source in conjunction with thin wire current leads, most of the electrical power is then dissipated in the leads rather than in the low resistance source, so that the source is very inefficient.

Another radiation source which is of current interest is the light emitting diode (LED). Present LED's are very low power devices which are not suitable for all uses.

SUMMARY OF THE INVENTION

According to the illustrated preferred embodiment, the present invention provides a small, well-defined source of infrared radiation which can be easily and efficiently imaged through an optical system. The invention includes a thin film resistive heater of a high emissivity substance such as Cr.sub.3 Si evaporated onto a substrate. The resistive heater is confined to a small area between a pair of metal elements on the substrate. The high emissivity of the thin film heater and the low thermal conductivity of the substrate material each contribute to providing a highly efficient source.

The resistive heater of high emissivity is positioned between, and immediately adjacent to, a pair of metal elements of low resistivity and emissivity to provide a well-defined source which is particularly suited to imaging with mirror optics. The source is planar, and may therefore be easily imaged. Furthermore the device is mechanically stable, so that the source will not jiggle in response to a shock.

In accordance with one of the illustrated preferred embodiments, the resistive element is coated with an antireflecting layer to enhance its emissivity. Sources built in accordance with this embodiment have yielded an efficiency of about 0.2% with a bandwidth of about 5% in the infrared region.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a radiation source in accordance with an embodiment of the invention.

FIG. 2 shows a portion of a light source in accordance with another embodiment of the invention including an antireflecting layer.

FIG. 3 illustrates a light source in a package including a reflecting surface.

DESCRIPTION OF THE INVENTION

In FIG. 1 there is illustrated a substrate 11 of low thermal conductivity, e.g. conductivity in the range 0.02 to 0.08 watt/cm.degree.K. Several suitable materials are thin sapphire, Y.sub.2 O.sub.3 and quartz. The dimensions of substrate 11 may be chosen in accordance with a desired size of the light source; e.g. operating devices have been built in which the dimensions of substrate 11 are about 0.150 by 0.020 by 0.002 inch. Positioned centrally with respect to the long dimensions of substrate 11 is a radiation source area 13 which comprises a region of a highly resistive, highly emissive material, emissivity greater than about 0.5 being preferred. Preferably, source area 13 consists of Cr.sub.3 Si evaporated onto substrate 11 to a depth of about 1-2.mu.. In the illustrated embodiment, source area 13 is a 0.02 .times. 0.02 inch square having a resistance of about 100.OMEGA.. In the infrared radiation region of interest, about 4.mu. wavelength, the emissivity of Cr.sub.3 Si is about 0.5. Immediately adjacent to both sides of source area 13 are a pair of metallic conductors 15 which are preferably of a low emissivity relative to that of the Cr.sub.3 Si in the infrared region. Platinum, which has an emissivity in the infrared of about 0.1 is suitable, but other metals such as gold may also be used. Each metallic conductor 15 includes a portion 17 which overlaps a small area of source 13. This configuration helps to provide a well defined source region. A pair of leads 19, of a material such as gold, are bonded to metal layers 15 to serve as input and output leads supplying electrical power to source area 13. As will be explained further below, it is desirable that spurious radiation not be emitted from the device, as from the bottom of substrate 11. To prevent spurious radiation from being emitted, an additional metal layer 21 is deposited onto the bottom of substrate 11.

In operation, current leads 19 are connected to a source which provides sufficient current to heat thin film resistive heater 13 to a temperature of about 700.degree.C. For the 100.OMEGA. square of Cr.sub.3 Si described above, a current in the range of about 50-70 ma has been found to provide adequate heating. Spurious radiation from metal layer 15 is kept to a minimum by using a substrate which is of very low thermal conductivity, thereby ensuring that only small amounts of heat are conducted away from source 13 to metal plates 15 via substrate 11. Additionally, the use of low emissivity metals such as platinum for metal plates 15 adjacent to resistive heater 13 further reduces emission from layers 15, so that the region from which radiation is emitted is spatially well defined.

FIG. 2 again shows a portion of substrate 11, and a portion of both metal layers 15 including raised portions 17. Also shown is thin film resistive heater 13 positioned adjacent to metal layers 15. There is also illustrated an antireflecting layer 23 of a material such as TiO.sub.2, thickness of about 0.44.mu., which is in contact with resistive heater 13. Antireflecting layer 23 serves to effectively increase the emissivity of the infrared source. Preferably, the material of antireflecting layer 23 is selected so that its index of refraction is approximately equal to the square root of the index of refraction of the material of heater 13.

In FIG. 3, there is illustrated source strip 11 encapsulated in an optical package 25. Package 25 includes a base 27 on which is mounted a reflecting surface 29. Reflecting surface 29 is preferably an eliptical or parabolic reflector of a solid piece of metal such as aluminum. A pair of metal posts 31 extend through base 27 and reflector 29 and are connected to leads 19 to provide electrical power to strip source 10. Infrared radiation emitted from source area 13 is directed to reflecting surface 29, from which it is reflected out of package 25 to form a magnified image (not shown). It may be seen from this illustration that light emitted from the side of source 13 away from reflector 29 would be directed toward the right in the figure, and thereby degrade the image produced by rays reflected from reflector 29. As was described above in connection with FIG. 1, a metallic surface is deposited on the back (right side in the figure) of substrate 11 to prevent the occurrence of such spurious radiation. Preferably, package 25 is hermetically sealed to enclose an inert atmosphere such as nitrogen or argon, to prolong the life of the filament.

Claims

I claim:

1. An infrared radiation source comprising:

a substrate;

a pair of metal strips of emissivity less than about 0.2 in the infrared region positioned on one side of the substrate;

a thin film resistive heater of emissivity greater than about 0.5 in the infrared region positioned on the substrate in between the pair of metal strips to serve as a radiation source area bounded by the metal strips; and

input and output leads electrically interconnected with the pair of metal strips for conducting an electrical current through the source.

2. An infrared radiation source as in claim 1 wherein the thin film resistive heater comprises a layer of Cr.sub.3 Si deposited onto the substrate.

3. An infrared radiation source as in claim 2 wherein the substrate material is selected from the group consisting of sapphire and Y.sub.2 O.sub.3 and quartz.

4. An infrared radiation source as in claim 2 including an antireflecting layer on the Cr.sub.3 Si layer for increasing the effective emissivity of the heater.

5. An infrared radiation source as in claim 4 wherein the antireflecting layer is of TiO.sub.2.

6. An infrared radiation source as in claim 5 including another metal strip on another side of the substrate opposite the side on which the resistive heater is deposited, for preventing spurious radiation from said other side of the substrate.

7. An infrared radiation source as in claim 6 wherein the substrate is mounted in a housing including a base and a reflecting surface mounted on the base, the reflecting surface for reflecting and focusing infrared radiation from the thin film resistive heater.

8. An infrared radiation source as in claim 7 wherein:

the housing is of a circular cross section;

the reflecting surface is an eliptical mirror; and

the substrate and thin film resistive heater are mounted in spaced relation with the reflecting surface.

9. An infrared radiation source as in claim 7 wherein:

the housing is of a circular cross section;

the reflecting surface is a parabolic mirror; and

the substrate and the thin film resistive heater are mounted in spaced relation with the reflecting surface.