Forward looking infrared imaging system

Abstract: Apparatus and methods for modifying existing forward looking infrared systems and for forming new forward looking infrared systems. Generally, the system comprises a Galilean afocal system (26) in combination with a reimaging afocal system (22). A reimaging afocal lens system (22) is formed with at least first and second lenses (40,42,44) and a plane (50) for forming an image therebetween. A thermal reference source (58) is positioned in the imaging plane (50) of the reimaging system (22). The method for modifying an existing forward looking infrared imaging system includes positioning the reimaging system (22) along the optical path of the system between the Galilian lens combination (26) and the scanning device (20). According to a method for forming a forward looking infrared system an imaging lens system (18) is arranged along an optical path in combination with a detector array to focus collimated radiation upon the detector array. The detector array subtends the field of view along a first direction. A reimaging afocal system (22) is incorporated along the optical path to provide collimated radiation to the imaging lens system (18) and a scanning device (20) is positioned between the reimaging afocal system and the imaging lens system to vary the field of view along a second direction in order to provide a two dimensional image. A galilean afocal lens system (26) is positioned to receive radiation from a scene and transmit the radiation to the reimaging afocal system (22).

Description: FIELD OF THE INVENTION This invention relates to forward looking infrared imaging systems and, more particularly, to components and methods for forming and modifying such systems. BACKGROUND Forward looking infrared (FLIR) imaging systems incorporate photodetector arrays of photosensitive semiconductor materials such as Hg 1-X Cd X Te which can be formulated to respond to wavelengths in the 3 - 5 and 8 - 12 micrometer atmospheric windows. Both p-n junction photodiode arrays and MIS photocapacitor arrays of Hg 1-X Cd X Te provide satisfactory focal plane imaging in the infrared regime. These systems produce standard video pictures wherein the thermal temperatures of the viewed scene are displayed as various shades of gray. Continued efforts to improve the performance of FLIR systems frequently result in new advances for photodetector arrays as well as modifications to image processing circuitry and introduction of additional system components. Field implementation of these improvements often presents design difficulties. This is because FLIR systems are complex and costly electro-optical/mechanical assemblies that must be individually configured to meet the physical constraints of each application. For example, although different systems may be formed of many common modules, the systems may be incompatible because they are arranged for different space geometries or because they each require optical inputs from different directions. In addition, performance requirements, e.g., field of view, magnification and resolution, which affect the overall size of a system, will generally differ. Implementation of FLIR system improvements may occur when a complete system is designed for a new application, or, as part of an effort to upgrade an older system. When cost effective it is desirable to upgrade or retrofit older systems rather than to replace them. However, the redesign necessary to incorporate an improvement into an older system may require substitution of more costly components which are, at most, only indirectly related to the improvement. In addition to incorporation of more advanced detector arrays, recent improvements in the performance of FLIR systems have required the provision of thermal reference sources as well as circuitry to perform DC restoration and detector electronic gain balance functions. When upgrades to scanning FLIR systems include these improvements it is necessary to modify the optical path so that the detector array can view the thermal source. This can be accomplished by replacing either the imaging optics or the afocal optics with a lens system that includes an additional image plane such that the reference source can be introduced during inactive scan time, i.e., when the scene is not being viewed. In the past each of these approaches has required substantial optical or mechanical modification to the system. Most FLIR systems do not include reimaging imagers and additional space would be required to replace an imager with a reimaging imager. Furthermore, a thermal reference source cannot simply be positioned in the intermediate image plane of a reimaging imager. This is because FLIR detectors are not designed to scan such image planes. Thus, in order to view the thermal source apart from the radiation which forms an image of the scene, it is necessary to introduce an optical/mechanical chopper wheel within the optical path. For example, in order to periodically provide an image of the thermal source to the detector array a rotating mirror could be synchronized and phased with the scanner. With this arrangement an image of the thermal source is provided to the detector array during periods when the scanner is not providing image data to the detector array. An additional mirror may be required to complete the optical path, particularly if an existing system is being upgraded to include the thermal reference source. Introduction of the mirrors, as well as an electromechanical system to effect synchronous rotation, can be a difficult and costly task, especially in existing systems, because of severe space constraints and the requirements for moving parts and circuitry for effecting synconous chopping. Alternately, a reimaging afocal may be introduced in lieu of a Galilean afocal to provide an image plane for the introduction of thermal sources in front of the scanner. By replacing the Galilean afocal with a reimaging afocal lens system the thermal sources can be scanned along with the field of view. However, different FLIR systems often require different fields of view or different afocal levels of magnification. Thus introduction of the thermal reference source would require redesign for each distinct application. Furthermore, reimaging afocals are considerably more expensive and require more space than Galilean afocals. In summary, known methods for introducing thermal reference sources into FLIR systems are costly and require custom modifications for dissimilar systems. It would be advantageous to provide a means for incorporating thermal reference sources into both new and existing FLIR systems which is simpler and less costly than known techniques. SUMMARY OF THE INVENTION The present invention provides a method of modifying an existing forward looking infrared imaging system wherein the system is of the type which processes radiation received from a scene to form an image of the scene, wherein the system includes a Galilean afocal lens combination, sensor components including a detector element array of the type which subtends the field of view along a first direction, an imaging lens system arranged to focus collimated radiation upon the detector, and a scanning device for varying the field of view along a second direction in order to provide a two dimensional image, the method comprising the steps of: forming a reimaging afocal lens system comprising at least first and second lenses and a plane on which is formed an image of the scene therebetween; positioning a thermal reference source in the reimaging system; and positioning the reimaging system along the optical path of the existing system between the Galilean lens combination and the scanning device. The present invention also provides a forward looking infrared system for processing radiation received from a scene to form an image thereof comprising: an infrared detector element array of the type which subtends the field of view along a first direction; an imaging lens system for focusing collimated radiation upon the detector; a scanning mirror for varying the field of view along a second direction in order to provide a two dimensional image of the scene; a Galilean afocal lens system positioned to receive radiation from the scene; and a reimaging afocal lens system positioned between the Galilean afocal lens system and the scanning mirror, said reimaging system including at least first and second lenses for forming an image of the scene at a reimaging plane therebetween and for providing collimated radiation to the scanning mirror; and a thermal reference source positioned in the reimaging system. The present invention further provides a method of processing radiation received from a scene in which reference thermal radiation is taken into account, comprising: receiving radiation from the scene using a Galilean afocal lens combination and directing the radiation along an optical path, producing an intermediate image of the scene on a plane on the optical path, supplying the reference thermal radiation at or near the plane of the intermediate image, focusing the radiation from the scene upon and directing the reference thermal radiation on to a detector which extends along a first direction so as to subtend a field of view, and so scanning the field of view along a second direction that the detector is responsive to a two dimensional area. BRIEF DESCRIPTION OF THE DRAWING The invention may best be understood by reference to the following detailed description when read in conjunction with the Drawing wherein: Figure 1 which illustrates in partial schematic form an exemplary FLIR imaging system according to the invention; and Figure 2 illustrates an optical arrangement for removing thermal reference source nonuniformities which would otherwise be viewed by individual detector elements. Certain preferred embodiments of the invention are disclosed herein. However, it should be appreciated that the specific optical arrangements are merely ilustrative and do not delimit the scope of the claims. DESCRIPTION OF THE PREFERRED EMBODIMENTS Heretofore, parallel scan FLIR systems which incorporate thermal reference sources have not been produced in large numbers. In those instances when reimaging afocal lens systems have been used to incorporate the thermal reference sources, the afocal systems have been individually designed to provide the resolution and field of view required by the particular system. The present invention provides a simpler and more cost effective method for incorporating thermal reference sources into FLIR systems. The FLIR system arrangement disclosed herein makes possible the upgrade of existing FLIR systems as well as the production of new systems with greater economies of scale. Figure 1 illustrates in partial schematic form an exemplary parallel scan forward looking infrared imaging system 10 constructed according to the invention. The system includes a Hg 1-X Cd X Te linear array infrared detector 12 contained in a dewar 13 and coupled to a cryogenic cooler 14. The detector comprises a vertical array of elements which are photosensitive to radiation in either or both the 3 - 5 and 8 - 12 micrometer range. By way of example, the detector 12 may comprise a column of 240 or 480 elements, or may include multiple parallel columns of elements. The detector 12 is coupled to processing electronics 15 which, in the preferred embodiments, includes circuitry 16 for performing DC restoration and/or detector electronic gain balance functions. The detector array subtends the field of view in a first, e.g., vertical, direction. Incoming radiation is scanned with mirror optics along a second direction and reflected upon the array to provide information for generating a two dimensional image. Limitations in system resolution due to separation distances between detector elements may require interlacing between successive fields to complete a frame. The cooler may be of the split cycle variety with the expander portion of the cooler in the dewar. The compressor portion of the cooler (not illustrated) is located in a convenient position near the dewar and is connected to the expander portion by a tube. Electronics utilized with the complete system is not integral to the invention and is therefore not described with detail. The detector 12 receives radiation from a scene along an optical path which includes an imager 18, a mirror scanner 20, a reimaging afocal lens system 22 and a Galilean afocal lens system 26. The detector 12 in combination with the imager 18 and the scanner 20 form the optical sensor portion of the system 10. The imager 18, positioned to focus collimated radiation upon the detector 12, comprises at least a pair of lenses 30 and 32 and may include a fold mirror 34 positioned in between the lenses to provide a turn in the optical path. The scanner 20 is mechanically driven to subtend the field of view in a direction perpendicular to the direction subtended by the detector array. A position sensor (not illustrated) is coupled to the scanner 20 to provide electrical pulses at various positions of the mirror scanner for displaying the sensor output. The reimaging afocal lens system 22 is positioned in front of the scanner 20 to receive collimated radiation from the Galilean afocal system 26. As a minimum the reimaging system 22 requires two lenses. In the preferred embodiment it comprises three lenses 40, 42 and 44, forming a color correcting combination, and two fold mirrors 46 and 48 positioned between lenses 40 and 44. An intermediate image plane 50 is formed between the lenses 40 and 44. The region 54 of the image plane 50 corresponds to that portion in which an image of the scene is formed when the system is adjusted to provide a maximum field of view. Preferably the reimaging system includes range and thermal focal adjustments which are indicated schematically by an arrow 52 positioned along side lens 44. Mirror 46 may be used to perform interlace. In a simple arrangement one or more thermal reference sources, e.g., a pair of sources 58, such as, for example, thermo-electric coolers, may be fixed in the image plane 50. The reference sources 58 are positioned outside the region 54. Only one reference source is required to perform the DC restoration function. A pair of sources is required for gain normalization. More preferably, each thermal source 58 is positioned out of focus so as to remove nonuniformities and provide the same temperature reading to each detector element. This can be effected by positioning a pair of mirrors 55 in the image plane 50 as illustrated in Figure 2. Pairs, each comprising a focusing lens 59 and a reference source 58, are cooperatively positioned to condense radiation from a reference source onto each mirror 55. Each reference source is located at a pupil or aperture plane so as not to be focused on the mirror 55. This arrangement removes nonuniformities which would otherwise be viewed by individual detector elements. Prior art FLIR systems are not known to include a reimaging afocal lens system 22 in combination with a Galilean afocal system. A function of the reimaging system 22 is to facilitate the use of existing Galilean afocals in FLIR systems which are upgraded to include DC restoration or detector electronic gain balance circuitry which circuitry requires inclusion of a thermal reference source. Otherwise modifications in system resolution and field of view would have to be performed by the reimaging afocal at much greater expense. In preferred embodiments of the invention the reimaging system 22 is of a standard size and optical design such that it may be incorporated in a wide variety of existing, e.g., common module, and new systems which also comprise a Galilean afocal system --not-withstanding differing geometric and physical constraints as well as differences in optical performance parameters among the variety of systems. Formulation of a standard reimaging system allows for more economical upgrading and fabrication of FLIR systems. For example, a benefit of using a standard size reimaging afocal is that existing systems can be updated without discarding Galilean afocals and their gimbal pointing and stabilization turrets. One of the most expensive components of the FLIR system, the afocal lens switching assembly, could thus be retained. According to the invention, in both new and existing systems, modifications in system resolution and field of view are primarily performed by the Galilean afocal lens system 26. Obtaining similar performance features without the Galilean system would be considerably less desirable because the reimaging afocal would then have to be greater in length and contain more optical elements. In addition to being more costly, this alternative arrangement would also be more difficult to package. Furthermore, if resolution and field of view were to be provided by the reimaging afocal, the thermal reference sources 58 would have to be reconfigured for each system application or field of view. There has been illustrated a FLIR imaging system comprising a Galilean afocal lens system in combination with a reimaging afocal lens system. The combination provides an economical method for modifying existing FLIR systems and for designing new FLIR systems to include thermal reference sources. More specifically, the invention includes a method for replacing a component of the optical sensor, e.g., the detector 12, or an electronic component in an existing first FLIR system with a component which requires a thermal reference source. The electronic component may, for example, include circuitry for performing DC restoration and/or detector electronic gain balance functions. Certain preferred embodiments of the invention have been described. Various other arrangements and configurations of the disclosed embodiments and various alternative components thereof will be apparent. For example, the sensor portion may be of the type which does not require cooling and the lens systems illustrated may be modified to include reflective lenses instead of refractive lenses. Accordingly, the scope of the invention is only to be limited by the claims which follow.

Claims: A method of modifying an existing forward looking infrared imaging system (10) wherein the system is of the type which processes radiation received from a scene to form an image of the scene, wherein the system includes a Galilean afocal lens combination (26), sensor components including a detector element array (12) of the type which subtends the field of view along a first direction, an imaging lens system (18) arranged to focus collimated radiation upon the detector (12), and a scanning device (20) for varying the field of view along a second direction in order to provide a two dimensional image, the method comprising the steps of: forming a reimaging afocal lens system (22) comprising at least first and second lenses and a plane (50) on which is formed an image of the scene therebetween; positioning a thermal reference source (58) in the reimaging system; and positioning the reimaging system (22) along the optical path of the existing system (10) between the Galilean lens combination (26) and the scanning device (20). A method according to claim 1 further including the step of replacing the detector array (12) of the existing system (10) with a different detector array. A method according to claim 1 or 2 further including the step of incorporating into the existing system (10) circuitry for performing detector electronic gain balance functions. A method according to claim 1, 2 or 3 further including the step of incorporating into the existing system (10) circuitry for performing DC restoration. A method according to claim 1 wherein a first and a second thermal reference source (58) are so positioned in the reimaging afocal lens system (22) as to provide more uniform temperature readings to each detector element of the array (12), said method further including the steps of: positioning a first and a second mirror (55) in the image plane (50) of the reimaging afocal system (22); cooperatively positioning a first focusing lens (59) and the first reference source (58) to condense radiation from the reference source (58) onto the first mirror (55); and cooperatively positioning a second focusing lens (59) and the second reference source (58) to condense radiation from the reference source (58) onto the second mirror (55). A method according to claim 1 further including the step of replacing an optical or electronic component in the system (10) with a component which enables use of the thermal reference source (58). A forward looking infrared system for processing radiation received from a scene to form an image thereof comprising: an infrared detector element array (12) of the type which subtends the field of view along a first direction; an imaging lens system (18) for focusing collimated radiation upon the detector; a scanning mirror (20) for varying the field of view along a second direction in order to provide a two dimensional image of the scene; a Galilean afocal lens system (26) positioned to receive radiation from the scene; and a reimaging afocal lens system (22) positioned between the Galilean afocal lens system and the scanning mirror, said reimaging system including at least first and second lenses for forming an image of the scene at a reimaging plane (50) therebetween and for providing collimated radiation to the scanning mirror (20); and a thermal reference source (58) positioned in the reimaging system (22). A forward looking infrared system according to claim 7 wherein the thermal reference source (58) is positioned in the reimaging plane (50) of the reimaging afocal lens system (22). A forward looking infrared system according to claim 7 wherein the relative positions of the lenses of the reimaging afocal lens system (22) are adjustable. A forward looking infrared system according to claim 7 wherein a first and second thermal reference source (58) are so positioned in the reimaging afocal lens system (22) as to provide more uniform temperature readings to each detector element, said system further comprising: a first and second mirror (55) in the image plane (50) of the reimaging afocal system (22); a first focusing lens (59) positioned to condense radiation from the first reference source (58) onto the first mirror (55); and a second focusing lens (59) positioned to condense radiation from the second reference source (58) onto the second mirror (55). A forward looking infrared system according to claim 7 wherein the Galilean afocal lens system (26) is adjustable to modify focal length and field of view of the forward looking infrared system. A forward looking infrared system according to claim 7 wherein the reimaging afocal lens system (22) comprises at least a third lens with the first and second lenses. A forward looking infrared system according to claim 7 wherein the reimaging afocal lens system (22) comprises at least one fold mirror (46,48) with the first and second lenses to direct the optical path onto the scanning device (20). A method of processing radiation received from a scene in which reference thermal radiation is taken into account, comprising: receiving radiation from the scene using a Galilean afocal lens combination (26) and directing the radiation along an optical path, producing an intermediate image of the scene on a plane (50) on the optical path, supplying the reference thermal radiation at or near the plane (50) of the intermediate image, focusing the radiation from the scene upon and directing the reference thermal radiation on to a detector (12) which extends along a first direction so as to subtend a field of view, and so scanning the field of view along a second direction that the detector (12) is responsive to a two dimensional area. A method according to claim 14 wherein the reference thermal radiation is supplied by one or more thermal reference sources (58) at or near the plane (50) of the intermediate image. A method according to claim 14 wherein the reference thermal radiation is supplied by focusing one or more thermal reference sources (58) onto one or more mirrors (55) at or near the plane (50) of the intermediate image.

Inventors: COOPER ERWIN E, DAVIS FRANKLIN L, SAGAN STEPHEN F, COOPER, ERWIN E., DAVIS, FRANKLIN L., SAGAN, STEPHEN F.
Applicants: TEXAS INSTRUMENTS INC, TEXAS INSTRUMENTS INCORPORATED
Application year: 1989
Main classification: G02B23
IPC codes: H04N3