Infrared source

An infrared source comprising an electrically powered, hollow cylindrical rod which operates at lower temperatures and yet has no decrease in output power is configured by increasing the outside diameter of the rod and inserting grooves therein which provide cavities increasing the apparent surface emissivity of the rod.

BACKGROUND OF THE INVENTION 
This invention relates to infrared sources and more particularly infrared 
sources having increased electrical resistance. 
There are certain equipments which use electrically heated graphite source 
to generate large amounts of infrared energy. These equipments include a 
graphite source enclosed within a transparent envelope such as a quartz 
window. These sources are often used in equipment where the voltage 
available to power the sources is fixed. Presently such sources are 
configured as cylindrical hollow rods. Because of the high temperatures at 
which such rods operate, usually on the order of 2000.degree. K., the 
lifetime of such sources are limited. Generally deterioration of the 
sources occur by having the graphite evaporate at the very high operating 
temperatures and deposit on the surfaces of the enclosing transparent 
window, thereby decreasing the transmissivity thereof. 
It is desirable to have these sources operate at lower temperatures while 
radiating at the same or greater power levels. This source temperature 
reduction can be accomplished in two ways - improving the emission 
characteristics of the radiating surface and increasing the radiating 
surface area. 
This source material is not a perfect radiator, and therefore, some 
emission improvement is realizable. 
The increase in radiating surface area may be obtained by increasing either 
the length or the outer diameter of the cylindrical rod. Increasing the 
length of the rod is impractical because if the rods are configured to fit 
within certain equipment having a defined size, increasing the length of 
the rod will cause a like increase in the size of the equipment. 
Therefore, such rods could not be used in currently configured equipment. 
Increasing the outer diameter of the rod requires a compensating increase 
of the resistance. This, of course, can be done by either increasing the 
resistivity, increasing the length or decreasing the cross-sectional area. 
It is impractical to increase the resistivity since this requires a change 
in materials and there are very few materials which operate at the desired 
temperatures on the order of 2000.degree. K. As stated before, increasing 
the length is impractical. 
The third alternative is to decrease the cross-sectional area, by making 
the walls of the hollow cylindrical rods thinner. This is also undesirable 
since the structural integrity of the source will be severely affected. 
Accordingly, it is an object of this invention to provide an improved 
infrared source. 
It is a further object of this invention to provide an improved infrared 
source having higher emissivity. 
SUMMARY OF THE INVENTION 
Briefly, a hollow cylindrical rod of graphite which is employed as a source 
of infrared energy by passing an electrical current along its length is 
improved by increasing the outside diameter thereof (and sometimes the 
inside diameter) and providing macroscopic grooves in the surface thereof. 
These grooves reduce the cross-sectional area of the increased outside 
diameter rod and thereby maintain constant electrical resistance without 
substantially reducing structural integrity. These macroscopic grooves, in 
addition to reducing cross-sectional area, form cavities, effectively 
increasing the surface emissivity.

DESCRIPTION OF A PREFERRED EMBODIMENT 
Referring now to FIG. 1 of the drawings, there is illustrated thereby an 
infrared source as currently being employed. This infrared source 
comprises a cylindrical rod 10 having a hole 12 therein. The rod is 
typically made of graphite since it is desired that it survive operating 
temperatures on the order of 2000.degree. K. when heated electrically by 
passing an electrical current therethrough. The rod when heated emits 
infrared radiation. The emissivity of such a rod is approximately 0.9. As 
mentioned above, this rod does present problems in that at the operating 
temperatures it evaporates graphite onto a transparent tube enclosing the 
rod decreasing the transmissivity thereof. 
Referring now to FIG. 2 of the drawings, there is illustrated thereby an 
improved infrared source having an increased outer diameter. This source 
includes a rod 14 also having a hole 16 therein, however, further having 
macroscopic grooves 18 in its exterior surface. For clarity the grooves 
have been shown as relatively deep, however, in actual practice they will 
be extremely small on the order of 0.01 to 0.02 inches so as to minimize 
any decrease in structural integrity of the rod. The macroscopic grooves 
can be spirally threading the rod or drawing it through a broaching die, 
for example. Because of the grooving of the source, material is removed 
thereby maintaining the electrical resistance notwithstanding the 
increased diameter of the rod. With the increased outer diameter the rod 
will operate at decreased temperatures, therefore, decreasing the amount 
of evaporation of graphite from the rod. Furthermore, the grooving of the 
rod provides cavities increasing the apparent surface emissivity of the 
rods. It has been found that the emissivity can be increased on the order 
of 10%. 
The fact that the grooved rod has improved emissivity is a result of the 
creation of multiple cavities. It can best be explained if the rod is 
thought of as a collector rather than as an emitter since it is known that 
elements that have higher collection capability will have a like improved 
emission capability. It is readily apparent that if the rod of FIG. 1 is 
considered as a collector, any radiation impinging on the smooth surface 
thereof will either be absorbed or bounce off the rod with only a single 
bounce. In the rod of FIG. 2, it is apparent that some of the radiation 
which would hit the rod and not be absorbed will bounce and hit the sides 
of the grooves and, thus, further provide additional bounces some of which 
will be collected by the rod thereby improving its collection capability. 
In a typical example, a prior art 4.5 inch rod having an outside diameter 
(OD) of 0.163 inches and an inside diameter (ID) of 0.125 inches when 
operating from a 28 volt source radiated approximately 1360 watts at 
2000.degree. K. Rods configured in accordance with FIG. 2 also radiated 
1360 watts but at reduced operating temperatures. Typical configured rods 
are illustrated in the table below: 
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ID OD Groove Temperature 
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.125 .183 .12 1940.degree. K. 
.150 .198 .015 1900.degree. K. 
.158 .210 .012 1875.degree. K. 
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While the grooves shown for the improved infrared source assume a sawtooth 
configuration, this is exemplary only, and other configurations and other 
grooving can be inserted such as substantially square grooves, etc. Thus, 
it is to be understood that the embodiment shown is to be regarded as 
illustrative only and that many variations and modifications can be made 
without departing from the principles of the invention herein disclosed 
and defined by the appended claims.