Abstract:
An improved detector assembly  10  having decreased sensitivity both to Narcissism and to stray light ghosting is disclosed herein. The improved detector assembly  10  of the present invention includes a housing  70  having an input aperture  142  in communication with a chamber within said housing. A detector  130  for sensing electromagnetic energy passing through the input aperture  142  within a first field of view is mounted within the chamber. Also mounted within the chamber is a detector mirror  100  for reflecting energy passing through the input aperture  142  within a second field of view outside of the first field of view. The improved assembly  10  of the present invention further includes a second mirror  110  mounted within the chamber for reflecting energy reflected by the first mirror  100  through the input aperture  142.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to imaging systems. More specifically, this invention relates to apparatus used to detect electromagnetic radiation irradiating such systems. 
     While the present invention is described herein with reference to a particular embodiment, it is understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional embodiments within the scope thereof. 
     2. Description of the Related Art 
     Forward Looking Infrared (FLIR) thermal imaging systems are generally used to view scenes by using the infrared energy emitted by the scene. FLIR systems commonly include a telescope, imaging optics and a dewar. The dewar contains a detector which emits a signal in response to infrared energy emitted by the scene. Typically, the detector is cooled to less than one hundred degrees Kelvin to reduce internal thermal noise and thereby improve sensitivity. A cooled coldshield is also typically installed within the dewar. This coldshield is configured so -as to not vignette (obscure) radiation focused on the detector by the telescope and imaging optics. Further, the coldshield enhances system performance by minimizing the amount of radiation striking the detector from sources other than the scene. 
     The performance of FLIR systems may be degraded by several optical phenomena. Included among these potential difficulties are ghosting (a form of stray light) and narcissism. Ghosting occurs when radiation either inside or ouside of the instantaneous field of view (IFOV) is partially reflected off (typically two) surfaces and thereby laterally displaced from the original radiation path. This errant radiation then strikes the detector at a location different from that at which it would had the radiation not been laterally displaced. Hence, the detector sees multiple images of the radiation source. 
     Narcissus, as implied by the term, occurs when the detector sees an image of the cold portion of itself (or of cold structures within the dewar) superimposed on the image of the scene. For this “cold image” to be detrimental to system performance it must be time-varying, as is the case in scanned systems. In addition, the narcissus generating partially reflecting surface(s) must lie beyond the scanner from the detector. Unfortunately, the detector sees a very cold narcissistic image when the scanner mirror is “on axis” and looking back into the cold dewar, but sees a relatively constant warm image when the scanner mirror moves slightly off axis. This generates a “cold spike” background as the scanner mirror moves through the “on axis” position. Moreover, the cold spike is generally located in the center of the resultant image—typically that part of the scene in which the viewer is most interested. 
     At least two characteristics of a given FLIR system directly impact the severity of potential narcissus problems. The first pertains to the reflectance of the narcissistic surface(s). A higher reflectance will result, proportionally, in a colder spike. Second, the curvature of the reflecting surface defines the degree of focus of the narcissistic image. If the curvature is such that the surface is normal to all incident rays from the detector, then the detector will see a sharply focused image of itself. 
     Several schemes have been employed to mitigate narcissism. In one such scheme the system is designed so as to minimize “detector to detector” imagery. That is, optical surfaces within the system are adjusted so that reflections onto the detector are “defocused.” Unfortunately, this correction of narcissism by design, although favorable, is sometimes limited in efficacy by other constraints. 
     A second partial remedy for narcissism is known to those skilled in the art as ARC-NARC (Automatic Responsivity Correction Narcissus). This approach involves the superposition of an image of a warm source over the narcissistic image on the detector. At the point in time when the scanning mirror is positioned such that the detector “sees” itself most fully, the detector will also see a superimposed image of the warm source. By adjusting the temperature of the warm source, the narcissistic cold spike in the resultant scene image is ostensibly masked by the image of the warm source. However, difficulties in “matching” the (blackbody) radiation curves associated with the warm and cold sources to generate a resultant radiation profile indistinguishable from that of the ambient environment have limited the efficacy of this technique. 
     A third technique employed for reducing narcissism involves filtering to a relatively narrow spectral band radiation from the scene that is seen by the detector. This technique, however, cannot be expected to remedy narcissism to the extent desired in certain FLIR applications. 
     In a fourth method of narcissus reduction, a “detector mirror” is placed within the dewar immediately in front of the detector. The mirror has an aperture so as to not vignette the field of view of the detector. The center of curvature of the mirror is typically located on the optical axis at the center of the coldshield aperture. Any ray passing through the aperture and striking the detector mirror is reflected back out the aperture at a conjugate height from the optical axis. Thus, if one were looking into the dewar of this configuration, the detector would be the only cold appearing object. The detector mirror would appear warm because the viewer would be looking back at an inversed image of the viewer (warm). The coldshield outer surface is typically gold plated for thermal considerations, so the viewer would see the warm “outside” world in reflection off this surface. That leaves only the detector itself to appear cold. The detector mirror in effect optically transforms certain “physically cold” objects within the field of view of the detector into appearing warm. The detector mirror provides the added benefit of reducing the thermal load on the dewar by reflecting radiation which would normally be absorbed thereby. Although this approach may result in a reduction in narcissism, the efficacy of this approach is limited when the detector image is sharply focused onto the detector. 
     Further, although detector mirrors reduce narcissism while leaving most other aspects of optical performance essentially unchanged, detector mirrors tend to contribute to ghosting. In particular, radiation entering the aperture in the coldshield which is not focused on the detector (i.e. stray light) may be reflected by the detector mirror, This reflected stray light may again be reflected by other surfaces onto the detector, thus generating image ghosts. It follows that the addition of the detector mirror may increase the susceptibility of the system to stray light ghosting. 
     Hence a need in the art exists for an infrared detection apparatus having decreased sensitivity both to narcissism and to stray light ghosting. 
     SUMMARY OF THE INVENTION 
     The need in the art for an infrared detection apparatus having decreased sensitivity both to narcissism and to stray light ghosting is addressed by the improved detector assembly of the present invention. The improved detector assembly of the present invention includes a housing having an input aperture (coldshield) in communication with a chamber within the housing. A detector for sensing electromagnetic energy passing through the input aperture within a first field of view is mounted within the chamber. Also mounted within the chamber is a first mirror for reflecting energy passing through the input aperture within a second field of view outside of the first field of view. The improved assembly of the present invention further includes a second mirror mounted within the chamber for reflecting energy reflected by the first mirror through the input aperture. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top plan view of a scanning imaging system which includes a conventional infrared detection apparatus. 
     FIG. 2 is,a magnified top sectional view of the conventional detection apparatus of FIG.  1 . 
     FIG. 3 is a top sectional view of the improved detector assembly of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a top plan view of a conventional infrared dewar (detection apparatus)  10 ′. In FIG. 1 the dewar  10 ′ is included within a scanning imaging system  20 . The imaging system  20  includes an optical telescope  60  for collecting infrared energy from a scene  145 . The collected infrared energy from the scene  145  is reflected by a scanning mirror  30  to an imaging/relay optical lens  40 . 
     As discussed below, the conventional dewar  10 ′ includes an outer housing  70 ′, a dewar window  80 ′, a coldshield  90 ′, a detector mirror  100 ′, and a radiation detector  130 ′. As mentioned in the Background of the Invention, the detector mirror  100 ′ is included within the conventional dewar  10 ′ to reduce narcissism. The spectral transmission characteristics of the window  80 ′ are typically controlled by the application of a suitably reflective coating known to those skilled in the art. An aperture  140 ′ is included in the detector mirror  100 ′ so as not to vignette (obscure) the field of view of the detector  130 ′. Similarly, an aperture  142 ′ is defined by the coldshield  90 ′ to allow radiation reflected by the scanning mirror  30  to be collected by the detector  130 ′. 
     As shown in FIG. 1, infrared radiation R o  emitted by objects within the scene  145  to be imaged is collected by the telescope  60  and typically collimated before being reflected by the mirror  30 . After being reflected by the mirror  30 , the radiation is focused by the imaging optics  40  passed through the window  80 ′ and aperture  142 ′, and is incident on the detector  130 . The detector  130 ′ modulates an electrical signal in response to the energy received thereby R o . Processing electronics (not shown) associate the modulated signal with the instantaneous orientation of the scanning mirror  30  to construct an image of the scene  145 . 
     FIG. 2 is a magnified top sectional view of the conventional dewar  10 ′. As shown in FIG. 2, the housing  70 ′ forms an evacuated chamber. Typically, the window  80 ′ is substantially optically transparent and forms a vacuum seal with the housing  70 ′. The detector  130 ′ generally comprises a conventional infrared photon detector and has a field of view defined by the aperture  142 ′ in the coldshield  90 ′. The detector  130 ′ may also be fabricated from materials familiar to those skilled in the art (e.g. lead sulfide or lead telluride). In the embodiment of FIG. 2 the detector  130 ′ is cooled to reduce the amount of noise (undesired thermally generated radiation) sensed thereby. 
     The coldshield  90 ′ surrounds the detector  130 ′ and the detector mirror.  100 ′. The coldshield  90 ′ is fabricated from thermally conductive material (e.g. metal) and is typically thermally coupled to a heat sink or other suitable cooling apparatus. Interior surfaces of the coldshield  90 ′ are generally painted black. 
     In the embodiment of FIG. 2 the detector mirror  100 ′ is a concave reflecting mirror. An aperture  140 ′ located at the center of curvature of the detector mirror  100 ′ is included so as not to vignette (obscure) the field of view of the detector  130 ′. 
     Although the detector mirror  100 ′ reduces narcissism, as mentioned in the Background of the Invention the detector mirror  100 ′ typically increases the sensitivity of the apparatus  10 ′ to ghosting. As shown in FIG. 2, the stray light ray R 1 ′ (from outside the field of view of the detector  130 ′) is incident on the lens  40 . After passing through the window  80 ′ and aperture  142 ′, the ray R 1 ′ is reflected by the detector mirror  100 ′ back out of the dewar  10 ′. The ray R 1 ′ is reflected out at a conjugate height with a probability that it will strike a surface, such as the lens  40  at such an angle so as to be reflected back to the detector  130 ′. In this manner the detector mirror  10 ′ contributes to ghosting within the conventional dewar  10 ′ by reflecting stray light from outside of the field of view of the detector  130 ′. 
     The improved detector assembly  10  of the present invention substantially inhibits the stray light induced ghosting experienced by the conventional dewar  10 ′ above. FIG. 3 shows a top sectional view of the improved detector assembly  10  of the present invention. As described more fully below, the assembly  10  includes an outer housing  70 , a dewar window  80 , a coldshield  90 , a detector mirror  100 , an anti-ghosting mirror  110 , a radiation shield  120  and a radiation detector  130 . Again, the window  80  is typically coated with a spectral bandpass filter to limit its spectral transmission characteristics. An aperture  142  is formed by the coldshield opening limiting the radiation to the detector  130 . 
     The detector mirror  100  is a concave reflective surface mirror. For simplicity, in the embodiment of FIG. 3 the detector mirror  100  is secured to the coldshield  90  by conventional means. The mirror  100  is not limited to such an attachment configuration since it can be physically warm or cold if adequate precautions are taken to reduce stray light therefrom. An aperture  140  is located at the center of curvature of the detector mirror  100  so as not to vignette (obscure) the field of view of the detector  130 . The radius of curvature, conic constant and higher order coefficients of the mirrors  100  and  110  are such that radiation passing through the aperture  142  and striking the mirror  100  is reflected to the anti-ghosting mirror  110  and then reflected to the light trap  160 . 
     For example, as shown in FIG. 3 a stray light ray R 1  from outside of the field of view of the detector  130  strikes the detector mirror  100  after passing through the lens  40 , the window  80  and the coldshield aperture  142 . The ray R 1  is reflected by the detector mirror  100  to the anti-ghosting mirror  110 . The anti-ghosting mirror  110  redirects the ray R 1  through the coldshield aperture  142  to a light trap  160 . The light trap  160  may be comprised of a variety of optically absorptive materials known to those skilled in the art. In this manner the assembly  10  of the present invention has prevented the detector  130  from seeing the stray light R 1  via reflection from the detector mirror  100 , while still retaining desirable anti-narcissus characteristics. 
     In the preferred embodiment, the anti-ghosting mirror  110  is annular in shape and circumscribes the field of view of the detector  130 . Those skilled in the art will recognize other shapes for the anti-ghosting mirror within the scope of the invention. In the embodiment of FIG. 3, the anti-ghosting mirror  110  is contiguous and “specularly” reflective. That is, radiation is reflected by the anti-ghosting mirror  110  in primarily a controlled, linear (as opposed to diffuse) manner. 
     The specularly reflective nature of the anti-ghosting mirror  110  allows determination of appropriate locations for placement of stray light collection devices (e.g. the light trap  160 ) outside of the housing  70 . For example, with knowledge of the radius of curvature of the detector mirror  100  and the reflective properties of the anti-ghosting mirror  110 , one skilled in the art may appropriately place the light trap  160  for collection of the ray R 1 . 
     In alternative embodiments of the present invention the position of the anti-ghosting mirror  110  may vary from that shown in FIG.  3 . Of course, translation of the anti-ghosting mirror  110  would require appropriate adjustment of the radius of curvature of the detector mirror  100  and relocation of the light trap  160 . 
     Given the relative positions of the mirrors  100  and  110  shown in FIG. 3, computer programs known to those skilled in the art (such as “Code- 5 ” by optical Research Associates of Pasadena, Calif.) may be utilized to determine precise relationships between the optical parameters of the mirrors  100  and  110  such that stray light incident upon the detector mirror  100  follows an optical path similar to that of the ray R 1 . Specifically, parameters of the detector mirror  100  may be selected such that for substantially all angles of incidence of the ray R 1  upon the detector mirror  100  the ray R 1  will be reflected to the anti-ghosting mirror  110 . Similarly, parameters of the anti-ghosting mirror  110  may be chosen so that the anti-ghosting mirror  110  redirects substantially all reflections of stray light from the detector mirror  100  out of the assembly  10  through the aperture  142 . These parameters may typically include the radius of curvature, conic constant and other optical surface characteristics of the detector mirror  100  and the anti-ghosting mirror  110 . 
     A comparison of the ray paths R 1 ′ of FIG. 2, and R 1  of FIG. 3 makes it apparent that the anti-ghosting mirror  110  reduces the stray light susceptibility of the system  10 . As shown in FIG. 3, the radius of curvature of the detector mirror  100  and placement of the anti-ghosting mirror  110  are chosen such that the ray R 1  is collected by the light trap  160 . In contrast, as shown in FIG. 2 the radius of curvature chosen for the detector mirror  100 ′ in the conventional apparatus  10 ′ may result in a reflection of the ray R 1 ′ by the surface of the lens  40 ′ and thereby cause ghosting. 
     As shown in FIG. 3, the radiation shield  120  is attached to the anti-ghosting mirror  110 . In the embodiment of FIG. 3 the shield is annular in shape and comprised of an optically absorptive material. the shield  120  is secured to the anti-ghosting mirror  110  by conventional means (e.g. glue, epoxy). The shield  120  is positioned between the anti-ghosting mirror  110  and the detector  130  such that radiation reflected by the anti-ghosting mirror  110  does not directly illuminate the detector  130 . In an alternative embodiment the shield  120  is positioned between the anti-ghosting mirror  110  and detector  130  as described above but is secured directly to the coldshield  90 . 
     An example of the utility of the shield  120  in decreasing the stray light susceptibility of the assembly  10  may be appreciated by considering the stray light ray R 2  shown in FIG.  3 . The ray R 2  is emitted by a surface  170  outside of the field of view of the detector  130 . As shown in FIG. 3, the ray R 2  passes through both the window  80  and the aperture  142  and is then reflected by the anti-ghosting mirror  110 . Next, the shield  120  absorbs the ray R 2  following reflection by the anti-ghosting mirror  110 . In this manner the shield  120  prevents the reflected ray R 2  from being collected by the detector  130 . Further, the shield  120  prevents the detector  130  from “seeing” reflections (e.g. such as from the mirror  110 ) from outside of the intended field of view. The surface of  120  facing the detector is typically painted black. 
     Thus the present invention has been described with reference to a particular embodiment in connection with a particular application. Those having ordinary skill in the art and access to the teachings of the present invention will recognize additional modifications and applications within the scope thereof. For example, mirrors of shapes and orientations differing from those of the anti-ghosting mirror  110  of the illustrative embodiment may be utilized to redirect optical energy reflected by the detector mirror  100  to regions external to the assembly  10  without departing from the scope of the present invention. Similarly, with access to the teachings of the present invention, one skilled in the art may chose other appropriate locations for the placement of additional mirrors to further reduce the stray light susceptibility of the assembly  10 . The invention is similarly not limited to the particular shape or placement of the radiation shield  120  disclosed herein. As mentioned above, a securing of the shield  120  directly to the coldshield  90  may be appropriate in alternative embodiments of the present invention. Additionally, more than one radiation shield may be employed without departing from the scope of the present invention. 
     It is therefore contemplated by the appended claims to cover any and all such applications, modifications and embodiments.