Infrared Detector Device With A Mosaic Of Oppositely-poled Adjacent Elements

Abstract

An infrared detector device incorporating a checkerboard arrangement of photoconductor detector elements is disclosed. Adjacent elements are oppositely poled, resulting in the cancellation of background signals on any two adjacent elements, thereby providing background discrimination.

Description

This invention relates to improvements in infrared detectors and more particularly to an improved infrared detector employing the principle of cancellation to break up the image plane into a mosaic of point size detector elements polarized in such a manner that any signal induced on any two adjacent elements cancel each other out and thereby provide background discrimination.

As will be understood by those skilled in the art, the detection sensitivity of infrared surveillance systems employing point detectors for mechanically scanning the scenery are limited by the spurious noise induced by background. sources. These sources commonly referred to as scanning or sky noise, are in the form of clouds and intensity gradients as might be found in a sky or terrain background. A differential in energy between two consecutive points along the line of scan constitutes a target in the employment of these systems. Since the existence of energy differential is not unique to real targets, but applies also to background sources, subsequent techniques for discriminating energy differentials is also required.

A number of techniques in general use are all characterized by one or more disadvantages or limitations. One technique in use in the prior art depends upon spatial differences between the target and background sources. This is possible because the target appears as a point source, for all intents and purposes, in the image plane of the optical system. Background sources of point size do not emit enough energy to be discernible because they are usually materially cooler than the target. Therefore, background sources must have an extended area in order to emit sufficient energy to be detectable.

This spatial difference is exploited in prior art circuits in two general ways, one of which is called pulse width discrimination and the other is called space filtering. Pulse width discrimination takes advantage of the difference in the frequency spectra produced by the area difference between the target and background sources when swept over by the scanning aperture. Space filtering utilizes a reticle of alternate opaque and transparent apertures which are essentially of point size. A chopped signal is generated by the radiation from a point source target as it is swept across this reticle disposed in the image plane during scanning. The electronic system is made selective to this chopped signal. Extended area sources do not generate this signal because of an averaging out of the chopping due to an equal number of transparent and opaque apertures in the scan. Thus extended area induced signals are filtered out.

Although these techniques may at least partially suppress background noise, they do not eliminate it and thus leave much to be desired. In the contemporary system the reticle is used to break up the image plane into a checkerboard pattern of point size detector elements, the alternate ones of which are opaque and transparent. With extended area sources of radiation background, the averaging out of the chopping due to the alternate opaque and transparent apertures provide a low amplitude noise type signal at gives no particular difficulty. However, in such system the edges of such a background area gives rise to transient signals which can be readily mistaken for real target signals and such false signals cannot be readily eliminated. Such a typical contemporary system is illustrated in the drawings and is compared with the operation of the system in accordance with the present invention.

As contrasted to the prior art systems using the reticle type system, the present invention employs a cancellation detector having an array of elemental detector elements arranged along sets of parallel and horizontal orthogonal axes, adjacent elements along each axis being alternately oppositely polarized so as to give responses of opposite polarity when scanned by radiation from a point source. Since any extended area source of radiation can be considered as having an infinite number of point sources, signal cancellation is effective at the edges of an extended source as well as it is throughout the body of the extended source and therefore no false target signals will be generated as the beam sweeps across an extended area source.

Accordingly, a primary object of the present invention is to provide a new and improved infrared radiation detector.

Another object is to provide a new and improved infrared detector employing cancellation to eliminate signals due to background radiation.

Another object is to provide a new and improved infrared detector cell for use in the system in accordance with the present invention.

The above and other objects will become apparent when considered in connection with the accompanying drawings, in which:

FIG. 1 is a schematic representation of a typical apparatus for scanning an infrared optical aperture over a surveillance area;

FIG. 2 is a schematic representation of a contemporary system where the aperture is scanned over a checkerboard reticle;

FIG. 3 is a schematic representation of the spatial relation between an extended background, a target and a detector cell together with the associated signals and circuitry;

FIGS. 4 and 5 are views of one embodiment of the invention, FIG. 5 showing the optical portion of the detector apparatus removed from the photoconductive assembly for clarity of illustration;

FIG. 6 is a simplified electrical circuit diagram illustrating the operation of the invention;

FIGS. 7A, 7B, 7C and 7D are views of a second embodiment of the invention in various stages of assembly;

FIG. 8 is a sectional view of FIG. 7D on line VIII--VIII and looking in the direction of the arrows; and,

FIG. 9 is a representation of a greatly enlarged photograph of a third embodiment.

The general schematic arrangement of the present invention is illustrated in FIG. 1 wherein any suitable optical system 1 is adapted to direct infrared radiation on a detector cell 2 which is located in the focal plane of the optical system. The exact type of optical system is immaterial. It is illustrated for convenience as being a single positive lens although as very well understood in the art a reflective type of system could be used. The optical system 1 and the detector cell 2 are mounted in such a manner that they can be moved in unison. The optical system 1 may be mounted in any suitable means so that it can oscillate about orthogonal axes for the purpose of providing the necessary scanning motion so that the aperture of the optical system is scanned over the surveillance area. In this connection, it should be understood that the optical system would be mounted in the necessary manner so that it could oscillate about the vertical axis, the oscillation being indicated by the double pointed arrow 3 in proper time relation with slower oscillation movement about the center of the optical axis indicated at 4. In this manner, the aperture is scanned from side to side as the optical axis is tilted progressively up and down. The operation of the scanning system would be in accordance with conventional practice and constitutes no part of the present invention. The above is mentioned primarily to indicate that the scanning operation of the optical system has the effect of producing the pulsating signal in the present system much in the manner as a light chopper produces a pulsating signal in other systems using the contemporary checkerboard pattern reticle system.

FIG. 2 schematically illustrates the disadvantage of contemporary systems using the checkerboard pattern reticle system wherein the radiation from the surveillance area would be scanned across the reticle, which corresponds to the detector cell shown in FIG. 1. It will be seen that when a large area background, such as a cloud, is scanned, a noise-like signal illustrated at 6 will be produced at the output of the single detector cell used with the reticle. The body of a cloud may be considered to include an infinite number of reflectors spaced very close together so that the amount of energy reflected from the cloud is comparatively high and substantially constant as the cell is swept across the area. On the other hand, at the edges of the clouds there is a differential in energy between two consecutive points along the line of scan and this causes the generation of transient signals 7 and 8 which cannot readily be distinguished from a signal from a real target such as that indicated at 9.

The advantage of the present invention over the system schematically symbolized in FIG. 2 will be readily apparent from a comparison of the latter figure with FIG. 3. The scanning system may be the same as that schematically represented in FIG. 1 and the detector cell 2 will be made in accordance with one of the embodiments of the present invention. Because of the cancellation technique, hereinafter pointed out in greater detail, the point size detector elements of the detector cell 2 are polarized in such a manner that any signal induced on any two adjacent elements cancel each other and therefore only the noise-like signal of low amplitude, such as that indicated at 10, will be developed in the output circuit of the detector cell 2. This is to be compared with the signal 6 of FIG. 2. On the other hand, a point size target 11 will develop an oscillating signal 12 which can be clearly recognized as that coming from a point size target.

In the first embodiment, shown in FIGS. 4 and 5, the detector cell 2 has associated therewith a plurality of light pipes 13 having entrance pupils 14 and exit pupils 16 adjacent to a supporting structure 17 on which are disposed a plurality of radiation sensitive elements 18' and 18". The areas and dimensions of these elements correspond approximately to the areas and dimensions of the exit pupils 16 of the light pipes. The use of light pipes in infrared detectors is more fully explained in a copending application by Francis J. Keisler and Richard F. Higby entitled "High Resolution Radiation Detector", Ser. No. 57,177, filed Sept. 20, 1960 now U.S. Pat. No. 3,110,816, dated Nov. 12, 1963 and assigned to the assignee of the instant application.

In that copending patent application the advantages to be obtained by the use of light pipes having exit pupils smaller than the entrance pupils is described in detail. Such construction provides concentrations of radiation on the detector elements and also provides space to make the electrical connections. As will appear from the subsequent description, the light pipes shown in FIGS. 4 and 5 are associated with the first and second embodiments of the invention while the third embodiment, illustrated in FIG. 9, does not require the use of these light pipes.

Referring to FIG. 6, it will be noted that the output signals are delivered by the leads 19 and 21. Lead 19 is connected to one side of detector elements 18' and lead 21 is connected to the opposite sides of alternate detector elements 18" of the upper row and to the same corresponding sides of the elements of each of the other transverse rows. In other words, lead 19 is connected to alternate sides of adjacent cells arranged in a checkerboard pattern while lead 21 is connected to the other sides of alternate cells in the checkerboard pattern. Lead 22 is connected to the other sides of the remaining alternate elements. As shown in FIG. 6, a source of direct current biasing voltage, such as the battery 23 has its positive terminal connected to lead 22 and its negative terminal connected to the lead 21, which is also connected to ground at 24. In other words, lead 19 is common to one side of all of the detector elements and alternate detector elements disposed in vertical and horizontal rows are oppositely polarized. The positively polarized detector elements are designated 18' and the negatively polarized detector elements are designated 18".

It will be readily understood that if the elements 18', 18", which are identical but oppositely polarized photoconductive elements, receive no illumination, or are illuminated uniformly with electromagnetic radiation such as infrared light, the output lead 19 will be at a constant potential which will be substantially one-half the potential of the battery 23. If the total illumination on detector elements 18' becomes greater than the total illumination on detector elements 18" so that their conductivity increases, the potential of lead 19 rises toward the positive potential of lead 22 whereas, if the total illumination on detector elements 18" is greater than that on the elements 18' the potential difference between leads 21 and 19 is decreased and, in effect, lead 19 becomes less positive. In this way, as the elements 18' and 18" are alternately illuminated, such as when being scanned by infrared radiation through a scanning aperture, the amplitude of the voltage on lead 19 alternately varies in positive and negative directions about some average value to provide an output analog signal.

In manufacturing the detector of the present, the electrode pattern may be laid down on the detector substrate 17 by a photoetching process. The detector elements 18' and 18" may then be laid down in small squares either by photoetching, scribing or some other suitable process. One significant aspect of the apparatus of FIG. 5 is the interleaving of the electrodes such that no crossovers occur and the desired polarity reversals occur at every element. The sensitivity of the detector of FIGS. 4 and 5 is augmented by the integrated light pipes 13 which, as previously mentioned, provides space between the exit pupils 16 for interconnecting the signal electrodes.

Particular reference is made now to FIGS. 7A, 7B, 7C and 7D, which together illustrate a second embodiment of the invention. Upon a substrate supporting member 17 of suitable insulating material there is supported two sets of interleaved bias electrodes, the electrodes of one group being electrically connected together and designated 26, 27 and 28 and having a common terminal strip 29 and the electrodes of the other group being designated 31, 32, 33 and 34 and having a common terminal strip 36. The electrodes and terminal strips may be in the form of a gold film plated upon the substrate 17. An insulating film 37 (FIGS. 7B and 7D) is then deposited over the gold film electrodes. The insulating film 37 covers substantially the entire surface of the substrate 17. A common electrode member 38 somewhat in the form of an egg carton separator and having spaced square recesses 39 therein, is then secured in any convenient manner over the insulating film 37. The common electrode member 38 has a terminal strip 41. As will be noted the square recesses 39 are spaced in a plurality of parallel horizontal and vertical rows in such a manner that substantially the center of each of the square recesses 39 lies over one of the electrodes 26, 27, 28, 31, 32, 33 or 34. A plurality of small holes 42, one in each recess 39, are then etched through the insulating film 37 exposing the common electrode 38 as shown in FIGS. 7B, 7C and 7D. A photosensitive film, shown at 43 in FIG. 7D is then laid over the entire assemblage in block fashion. The photoconductive film 43 flows into the recesses 39 and through the holes 42 into contact with the finger electrodes 26, 27, 28, 41, 32, 33 and 34, thus forming photoconductive cells, in general, similar to those of FIG. 1. The terminal strip 41 of the common electrode 38 corresponds to the output lead 19 of FIG. 6. The terminal strips 29 and 36 are connected, respectively, to terminals corresponding to leads 22 and 21, respectively, of FIG. 6 to the positive and negative terminals of a suitable direct current source (not shown) to give a circuit configuration similar to that shown in FIG. 6 and providing a checkerboard pattern of adjacent alternately oppositely biased detector cells similar to that shown in FIG. 5.

As the detector apparatus is scanned and the target image moves across member 43 in either a horizontal or a vertical direction, areas of the sensitive member 43 opposite the holes 42 which are alternately biased at different polarities, as previously described, are successively illuminated by the relatively high intensity radiation of the target source. Accordingly, a signal which varies alternately from positive to negative is generated in the output circuit as infrared radiation from the target is scanned across the member 43.

Particular reference is made now to FIG. 9 in which a third embodiment of the invention is shown. The detector of FIG. 9 is seen to consist of sets of interleaving hairlike electrodes in contact with a photoconductive film 44. One set of electrodes 46a serves as common biasing electrodes and is connected to a terminal strip 46, corresponding electrically to the lead 19 of FIG. 6. A second set of electrodes 47a are connected to a terminal strip 47, which might correspond electrically to the lead 22 of FIG. 6, while a third set of electrodes 48a are connected to a terminal strip 48. This terminal strip corresponds electrically to lead 21 of FIG. 6. Similar to the previous embodiments, the terminal strip 46 serves as the signal output terminal while the terminal strips 47 and 48 may be connected to the positive and negative terminal of a direct current source, not shown. It will be seen that between each positive and negative electrode there is a biasing electrode 46a. Because of the hairlike size of the electrodes 46a, 47a and 48a and the manner in which they are connected to their terminal strips it is not necessary to use the light pipe arrangement as is used with the two previous embodiments. It should be understood that the electrodes 47a and 48a, which must pass either under or over the terminal strip 46, are insulated therefrom in any suitable manner. The photoconductive film 44 is laid over the electrodes 46a, 47a and 48a in conductive relation therewith thereby forming strip-like detector cells. It will be seen that as these strip-like cells are scanned as the infrared radiation from a point target moves across the cells the alternately positively and negatively biased electrodes will be successively illuminated with the result that an alternating current signal, having a frequency determined by the rate of movement, is produced. On the other hand, since the spacing between adjacent positive and negative electrodes is substantially equal to that of the diameter of the blur circle of the optical system, background radiation such as that from a cloud, will illuminate two or more cells simultaneously thereby producing potentials which tend to cancel out and thereby produce no substantial output signal.

The invention contemplates the use of filtering circuits, if desired, to filter out undesired extraneous alternating current components of the detector output signal caused either by targets or by background environment.

In FIG. 9 preferably the bias electrode span, that is, the distance between the positive and the next adjacent negative electrode is substantially equal to the blur circle of the imaging system.

Whereas the invention has been shown and described with respect to some embodiments thereof which give satisfactory results it should be understood that changes may be made and the equivalent substituted without departing from the spirit and scope of the invention.

Claims

We claim as our invention:

1. An infrared detector comprising, in combination, an insulating supporting plate member, a plurality of photoconductive detector elements arranged in a plurality of parallel vertical and horizontal rows disposed orthogonally with respect to each other, means for biasing said detector elements said elements being arranged in checkerboard pattern and each being poled so as to develop potential drops of opposite polarity at common output terminals in response to infrared radiation, a plurality of elongated spaced electrodes on said supporting plate member, all of said first plurality of spaced electrodes being electrically connected together to a first terminal, a second plurality of spaced elongated electrodes mounted on said supporting plate member, all of said second plurality of electrodes being electrically connected to a second terminal, said first and second plurality of electrodes being in interleaved relation, all of said electrodes being disposed at an angle of 45.degree. with respect to said horizontal and vertical rows, means forming an insulating film disposed over all of said electrodes and said supporting late, said insulating film having a plurality of small apertures therein at spaced intervals corresponding to the centers of said detector elements, each of said apertures lying over an electrode, and a photoconductive film overlying all of said electrodes and apertures and in contact with said electrodes.