Compact Second Generation FLIR Kit

1. Field of the Invention

The present invention generally relates to a Forward Looking Infrared imaging system (FLIR). More Particularly it involves a space saving set of optical elements that are flexible in form and function.

2. Description of Prior Art

In the patent entitled "UNIVERSAL VIEWER FOR FAR INFRARED", to Patrick J. Daly, issued 18 Sep. 1973, there is described a modular FLIR which the Army now refers to as a First Generation or GEN I FLIR. A Second Generation or GEN II FLIR has now been proposed in which the field units are kits rather than modules. Modules are used in the kits, but are no longer required to have common interfaces as long as the external interfaces of the kit remains unchanged. To test operate each kit requires only a standard power supply, control panel, and display as found in an Army vehicle, and there will be a such a vehicle or other system for every type of kit. Ground vehicles such as tanks and weapons carriers have limited spaces originally designed for GEN I FLIR's, which are now used to house GEN II FLIR's. Since the scanning systems now available for GEN II systems have larger apertures and provide greater resolutions, it follows that the optical elements such as lenses and reflectors will be larger also. There is thus a problem presented in how to fit the GEN II kits into the same spaces which hold Gen I modular systems.

According to the invention, a compact GEN II FLIR kit is provided by using a close-coupled axially folded set of optical lenses that adapt to various off,axis mounting platforms associated with the head mirrors mounted in most vehicles. These mirrors raise and extend the field of view while isolating the sensor from missiles directed toward the vehicle.

One object of the invention is to provide an afocal and imaging lens system to match the high resolution detector used in GEN II FLIR's.

A further object of the invention is to provide the above lens systems in a compact folded arrangement that is applicable to all existing ground vehicles used by the Army and some civilian agencies as well.

The foregoing and other objects, aspects and advantages will be better understood from the following detailed description of a preferred embodiment of the invention with reference to the drawings, in which:

FIG. 1 shows a side view of the infrared lens assembly including fixed lenses, reflecting mirrors, removable wide field of view lenses and planar window elements of the above kit in proper sequence along the optical axis;

FIG. 2 shows an isometric view of a GEN II Kit according to the present invention;

FIG. 3 shows the kit mounted in the Gunner's Primary Sight (GPS) in an M1 Bradley tank;

FIG. 4 shows the kit mounted in the Commanders Independent Thermal Viewer (CITV) in an M! Abrams tank;

FIG. 5 shows the kit mounted in the Integrated Sight Unit (ISU) of the M1 Bradley tank;

FIG. 6 shows a graph of the Modulation Transfer Functions for the wide field of view lens assembly of FIG. 1;

FIG. 7 shows a side view of the lens assembly in FIG. 1 converted to a narrow field of view and having a reflector added, to represent the true three dimensional aspect of the optical path;

FIG. 8 shows a graph of the Modulation Transfer Function of the narrow field of view assembly in FIG. 7.

Referring now to the drawings, and more particularly to FIG. 1, there is shown an optical schematic view of a compact GEN II FLIR Kit. This kit includes a complete assembly of infrared transparent or reflecting elements 1 through 16. The complete specification of these elements and critical spacings therebetween, if required, is given in Table I. Elements 15 and 16 are plane windows that are included in the opposed walls of a supercooled vacuum insulated detector-dewar assembly 18 shown in FIG. 2, one version of which is described in patent application Ser. No. 08/176,866 entitled "Interface Design and Technique for Detector-Cooler Assembly", filed 30 Dec. 1993, by Neil D. Supula, et al. The lens compositions are defined generally as Ge or ZnSe, however, as is well known the art, these lenses are specially formulated and

As shown in FIG. 2 the afocal lenses and housing comprise a module 23 and the remaining reflectors and lens elements 8-14 are housed in a tubular right angle elbow housing 24 having a pair of straight arms of uniform circular cross-section the axes of which coincide with the optical axis and which axes intersect to define a common point. The elbow housing further includes a flat wall 24C with a reflecting surface including the common point. The walls of the arms intersect at the corner to form a combined arm structure. As is well understood in the optical art, the flat wall 24c must be perpendicular to the common plane of the arm axes and a normal to the wall in the common plane must bisect the angle between the arm axes. For a 90.degree. corner this produces 45.degree. angles, but this is not critical in the present invention. The resulting corner may be a complex structure defined by the intersection of the walls on the combined arm and the flat wall or some simpler structure. The arms have an inner diameter to accommodate lenses with a little more than a 11/4 inch diameter aperture. The input end of this housing is adapted to receive a plug-in motorized scanning mirror module 25 that includes at least one mirror element 8. Mating threads, grooves, detent holes and the like can be provided in elbow housing and the scanning module for this purpose. A scanning motor is included in the scanner housing with an external power lead 25A sealed through the scanner housing wall. Decollimating lenses 9 and 10 are mounted in the input arm 24A of the housing and imaging lenses 12, 13 and 14 are mounted in the output arm 24B. The fixed reflector element 11 is mounted on the inside surface of the wall 24C and steers the image beam from the input arm into the output arm. This places the optical axis of the output arm parallel to that axis in the afocal housing but spaced therefrom by the length of that axis in the input arm.

Each lens provides a critical function in the operation of the FLIR. The objective lens 1 gathers a large sample of radiation from the distant object. Relay lens 4 focusses this light to fit the smaller aperture of the subsequent lenses and corrects for frequency aberration by the objective, owing to its different dielectric constant. Lens 4 Also provides a quality focus for inserting planar thermal reference devices or protective devices for the sensitive detector elements used in FLIR's. Lenses 2 and 3 increase the field of view, the use of two lenses permits an increase in light gathering power while maintaining the required exit aperture. The three collimating lenses 5, 6, and 7 prepare the beam for substantially undistorted reflection by the scanning mirror 8. Again the combination of germanium and zinc-selenide lenses compensates for the frequency aberration of either type used alone. The collimated image beam at the scanner 25 allows the image to be shifted normal to the optical axis without any other distortion. It also compensates for variations in spacing between the afocal and scanner housings. The same is true for the spacing between the housings of the scanner and decollimating lenses 9 and 10. This spacing can thus be chosen to meet mounting space geometry in the vehicle to which the FLIR is assigned by using a housing 24 with the required length of input arm 24A. The use of two decollimating lenses reduces spherical aberration and provides an intermediate focus such that the dewar aperture can be the optical stop for the whole system. In the final imaging lenses, the use of three lenses with differing dielectric constants provides an image well corrected for spherical and frequency aberration. The judicious use of aspherical lens surfaces increases the useable areas of those lenses reducing the required size and weight of the overall optics and the amount of special germanium or zinc-selenide material required, these materials being expensive and not particularly abundant.

Also as shown in FIG. 2, the preferred embodiment of the scanner unit has two mirrors set at preselected folding angles to one another. The first is the scanning mirror 8 which folds the optical axis between the input and output optical axes of the scanner, which axes can be defined as the respective Z and Y casrtesian coordinate axes of the FLIR. The and the second is a fixed mirror 25B that folds the optic axis at the scanner output, where the light remains collimated, directing it normal to both the input and output optical axes of the scanner mirror or along the X coordiate axis of FLIR. This folding of the optics into a more compact package adds another degree of adjustment to the environment in which the FLIR is used. The L-shaped elbow housing 24 slips snugly over a cylindrical pedestal 18A on the housing 18 of the detector-dewar assembly (SADA II). The optical axis of the housing is thus firmly aligned with the center of the detector array, the focal plane is aligned parallel with the plane of the detector array and the image can be rotated with respect to the array within the constraints set by the mounting environment. Rubber O-rings, set screws or clamping bands (not shown) may be added to,prevent relative movement and seal out contaminants, if needed.

The entire NV-80 FLIR kit consists of the afocal lens assembly, the L-shaped imaging optics, the scanner unit, the SADA II dewar unit, a one watt Sterling cycle cooler 26 and, in some cases two or three mounting adapters. Image processing and control circuit cards can be mounted to the kit, as desired. Besides the mechanical coupling described above, the dewar, cooler, scanner and afocal assembly are electrically coupled through leads 20A, 25A, 18B and 26A to a con, non Gable or harness (not shown) which in turn is coupled to the power supply of the vehicle involved. Plugs and sockets may be used at the ends of the leads and/or harness to facilitate packing and shipping the kit, but the kit is otherwise complete and ready to operate as shipped.

The electro-mechanical cooler 26 is connected to the dewar by a flexible pressure resistant coolant conduit 26B. Detachable couplings, similar to Schraeder valves, might be designed to make these units separable, but the low density cryogenic fluid used, e.g. helium, makes it preferable to use permanent connections. This further makes it desirable to provide a bracket 26C on the cooler with mounting holes and screws 26D aligned with the holes on the base of the dewar. The holes in either can be threaded or a nut or other fastener used to retain a pin or screw in each pair of holes. Thus only the dewar unit and the afocal assembly need a separate mounting adapters in the vehicle or system requiring the FLIR. If there is a vertical space limitation, however, the cooler can be removed and mounted on a larger version of the same adapter or on a separate adapter. In special cases some angular adjustment of the afocal and/or dewar assemblies can be provided to improve the fit or to improve viewing. A preferred method is to move the afocal platform so that the optical exit axis of the afocal always intersects the center of the scanner mirror axis and remains normal to this axis.

FIG. 3 shows an NV-80 kit mounted in an M1 Bradley tank using the sight platform 30 provided for the Gunner's Primary Sight. The platform is a generally rectangular chamber that opens into an external nacelle 30A containing a head mirror 30B. Near the bottom of the chamber an L-shaped metal shelf 30C is placed where the dewar unit is to be mounted. The vertical portion of the shelf is removeably attached or welded to the platform, while the horizontal portion has holes that match the comer holes in the baseplate of the dewar unit. As explained, the cooler is held on by screws or pins that pass through the baseplate and the cooler's mounting bracket. These screws are removed allowing the cooler to be placed below the shelf and the screws reinserted through both the baseplate and shelf into the cooler bracket. Thermal insulation, e.g. glass cloth or foamed plastic, is preferably placed between the bracket and the shelf to prevent heat transfer to the dewar. The afocal assembly is rigidly mounted above the scanner unit facing the head mirror by means of an adaptor plate 30D, for example, extending from the mounting flange on the afocal module over the upper flange on the top of the sight housing.

FIG. 4 shows the same kit mounted in an Abrams tank on the platform provided for the Commander's Independent Thermal Viewer (CITV). This platform 40 is a generally cylindrical chamber 40A topped by a small nacelle 40B containing a small head mirror 40C. The dewar unit is mounted off center near the bottom of CITV chamber on a thermally insulated shelf as in FIG. 3, but adapted to the cylindrical wall. The afocal assembly is again mounted above the scanner facing the head mirror by a mounting plate that engages the mounting flanges at the top of the cylindrical chamber.

FIG. 5 shows the same kit mounted in an M2 Bradley tank on a platform 50 provided for an Integrated Sight Unit (ISU). The arrangement here is similar to the FIG. 3 sight platform with a dewar 50A mounted on a shelf 50B and the afocal mounted on an upper flange 50C.

FIG. 6 shows the Modulation Transfer Function of the wide FOV FLIR optics shown in FIG. 1 at a wavelength of about 10 microns. The x and y components for 0.00.degree.-curves 61 & 62, 1.80.degree.-curves 63 & 64 and 3.74.degree.-curves 65 & 66 follow one another closely and also closely approach the diffraction limited case.

FIG. 7 shows the narrow Field of View version of the system from FIG. 1. The folding mirror after the scanner is shown and demonstrates how to make the system more compact and easier to conform with a more limited space environment. The wide angle lenses 2 and 3 in FIG. 1 are omitted, but the remaining lenses are equivalent to lenses found in FIG. 1. Table II gives the Specification of the lens surfaces and spacings, if critical.

FIG. 8 shows the Modulation Transfer Functions of the FLIR optics shown in FIG. 7. Note I that these functions do not differ greatly from the wide FOV functions of FIG. 6.

While this invention has been described in terms of preferred embodiment consisting of a standardized kit, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims.