Abstract:
A sharp edged aperture for use with radiation sensing apparatus to permit radiation to pass through a relatively thick layer of radiation absorbing material with a minimum of reflection off the edges of the opening by utilizing a thin layer of radiation blocking material overlaying the layer of radiation absorbing material.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to radiation sensing devices which include a radiation absorbing layer having an opening therethrough of a thickness which allows reflection of the radiation off the edges of the material as it passes therethrough. 
     2. Description of the Prior Art 
     In the copending application of B.W. Denley, W.J. White and myself, Ser. No. 127,077, filed Dec. 1, 1987 and assigned to the assignee of the present invention, I show a cold shield for use in an array of radiation detectors. In this disclosure, a block of radiation transparent material has an upper surface coating formed of a plurality of parallel rows of apertures and a bottom layer coating including a plurality of apertures shaped generally like the detectors located therebeneath used to sense the radiation passing through the apertures. In this disclosure, the coating may comprise a plurality of infrared absorbing layers and, to be an effective radiation absorber, the coating is relatively thick. It has been found that radiation passing through the apertures in such coatings is able to reflect off the edges of the aperture and thus travel in an undesirable direction to impinge on detectors other than those intended. Accordingly, it is desired to minimize the amount of reflection off the edges of the aperture. One possible solution proposed has been to make the radiation absorbing material thinner so that there is less edge for reflections to strike, but this solution decreases the performance of the radiation absorber. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes this problem by supplying, around the edge of the aperture, a thin layer of infrared radiation blocking material as, for example, a reflecting metal layer which overlaps the thicker absorbing material by a small amount and which defines the edge of the aperture through which infrared radiation may now pass without striking the edge of the absorbing material. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a prior art device comprising a block of radiation transparent material with a thick layer of radiation absorbing material on the bottom thereof defining an aperture; and 
     FIG. 2 shows the present invention comprising the apparatus of FIG. 1 with a thin layer of radiation blocking material overlapping the thick layer of absorbing material. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, radiation from a remote source travels toward an infrared radiation detector 10 along paths shown by arrows 12 through a block of radiation transparent material 14 which has a cover of radiation absorbing material 16 on the bottom thereof. Radiation absorbing cover 16 has an aperture 18 therethrough which permits the passage of the radiation 12 to the detector 10. The block of radiation transparent material 14 may be germanium or, preferably, zinc sulfide so that it is also transparent to visible light for purposes of ease in alignment. The radiation absorbing cover 16 may be a plurality of layers of absorptive material often referred to as a dark mirror coating. If desired, and as shown in the above-referred to co-pending application Ser. No. 127,077, an additional cover of light blocking material may be applied to the upper surface of transparent block 14 and would also have apertures therethrough so as to permit the radiation traveling along paths 12 to reach detector 10. 
     FIG. 1 shows some of the rays 20 being reflected from the edges of aperture 18 and emerging from the aperture in various undesirable directions. These reflections may strike detector 10 or other detectors (not shown) adjacent detector 10 and, because they may represent off axis or background radiation, can reduce the sensitivity of the detector in undesirable fashion or produce an erroneous reading on adjacent detectors. Accordingly, it is desired to eliminate the reflections off of the edges of the aperture through the light absorbing material 16 as much as possible. 
     FIG. 2 shows the appartus of FIG. 1, but in FIG. 2 the aperture 18 is formed by a thin layer of light blocking material 22, for example a metal film, so as to form the same size aperture as was the case in FIG. 1. It is seen, in FIG. 2, that the light absorbing material 16 has been cut back from its position in FIG. 1 and may partially overlap the ends of the thin light blocking material 22. The cut back distance is just large enough so that a ray of radiation, such as ray 24, approaching the aperture 18 at angles encountered in operation will not strike the edges of the light absorbing material 16. It is seen that any reflection off the edge of light blocking material 22, such as shown by ray 26, is greatly reduced from the amount of reflection found in FIG. 1 and accordingly the undesirable radiation received by detector 10 and others will be minimized. Some reflection from the surface of the thin layer 22 as shown by ray 28 may occur. This radiation is reflected back up through the transparent material 14 and will most likely pass out of block 14 or be absorbed by the radiation absorbing material (not shown) along the top of block 14. Nevertheless, since reflections from the upper surface of light blocking material 22, as shown by ray 28, are undesirable, it is better to minimize the area of the light blocking material 22. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.