Abstract:
A vehicle including a scanning imaging system includes a vehicle body having an outer surface, a propulsion source, and an optical window secured to the outer surface of the vehicle positioned on an optical axis for transmitting electromagnetic radiation received from a portion of an area of interest to the scanning imaging system. The scanning imaging system includes a first achromatic prism pair having prisms with different materials that have different refractive properties, and a second achromatic prism pair having prisms with different materials that have different refractive properties, both positioned on the optical axis. A camera fixed in location is optically coupled to form images from the electromagnetic radiation after being bent by the achromatic prism pairs. A motor including a controller independently rotates the first and second achromatic prism pairs about the optical axis for scanning within the area of interest.

Description:
FIELD 
       [0001]    Disclosed embodiments relate to scanning optical systems that include imaging devices, and more particularly to scanning optical systems that include optical arrangements comprising prisms that eliminate the need to move the imaging device to image an area of interest. 
       BACKGROUND 
       [0002]    Imaging systems used for monitoring an area typically include mechanical components for moving the imaging device, such as a camera, to direct the photodetectors associated with the camera towards an area of interest. In order to search over a given field-of-regard, the optical system must either be gimbaled or have its field of view otherwise directable. 
         [0003]    Conventional beam-steering arrangements for missiles include an optically transmissive dome and an optical arrangement behind the dome, and a gimbal that rotates the entire optical arrangement. A disadvantage of conventional gimbaled optical arrangements for certain applications is that conventional gimbaled optical arrangements need ample amounts of sway space in order to sweep through a field-of-regard and therefore can impose expensive packaging constraints on other system attributes. Other disadvantages of gimbaled optical arrangements include significant weight and cost, as well as poor performance. 
         [0004]    For example, conventional electro-optical missile technologies employ a dome-gimbal configuration where the optical system is placed in a gimbal behind an electro-optically transmissive dome. The domes are typically rotationally symmetric, placed at the tip of the missile, are spherical or conformal in shape, and are selected with aerodynamic performance as the primary design consideration. 
         [0005]    In some applications, it is not practical to move the lens assembly and camera. For those instances, it would be desirable for the optical system to provide pan and tilt functionality without requiring physical movement of the lens assembly and camera. 
       SUMMARY 
       [0006]    Disclosed embodiments include a vehicle comprising a scanning imaging system that includes a vehicle body having an outer surface, and a propulsion source. An optical window is secured to the outer surface of the vehicle, positioned on an optical axis for transmitting or receiving electromagnetic radiation to or from a portion of an area of interest to the scanning imaging system. The scanning imaging system includes a first achromatic prism pair comprising prisms each with different materials that have different refractive properties and at least a second achromatic prism pair comprising prisms with different materials that have different refractive properties, both positioned on the optical axis. 
         [0007]    The respective prisms in each prism pair are paired in opposite directions to eliminate color dispersion in polychromatic light. Being achromatic as used herein means there is no rainbow effect normally associated with prisms and polychromatic light and thus no measurable chromatic aberration (i.e., the line-of-sight caused by the prism does not change with wavelength across the spectral band of the optical system). As used herein, the prism pairs being “paired in opposite directions” means that for a zero net angular deviation or tilt, one prism pair directs the light rays in identical but opposite directions with respect to the other prism pair. Use of two (or more) prism pairs comprising different materials that have different refractive properties as disclosed herein thus allows the prism pairs to be achromatic prism pairs and the scanning imaging system comprising at least two prism pairs to be achromatic. The respective first and second materials for the first achromatic prism pair can be the same or different as compared to the first and second materials for the second achromatic prism pair. 
         [0008]    As known in optics, a Risley prism is a high resolution beam-steering device comprising a pair of independently rotatable prisms that redirects a radiation beam by refraction. Risley prisms are also sometimes referred to in the art as Herschel or Crété prisms. The achromatic prism pairs disclosed herein are generally Risley prism pairs, except the respective prisms in the prism pairs are rotated together as prism pairs. 
         [0009]    A camera fixed in location is optically coupled to form images from the electromagnetic radiation after the line-of-sight is bent by the achromatic prism pairs. As used herein, a “camera” includes one or more lenses and a light sensitive device (e.g., CCD, or photodiode array) at the focal plane. At least one motor including a controller independently rotates the first and second achromatic prism pairs about the optical axis for scanning within the area of interest. The first and second achromatic prism pairs can also be used to scan a beam that is transmitted to an area of interest (e.g., used for laser designation). 
         [0010]    In one disclosed embodiment the vehicle can be a military vehicle, such as a missile, a bomb, a torpedo, an airplane, a helicopter, a tank, or a truck. In such embodiments, the vehicle can utilize a disclosed scanning imaging system for scanning an area of interest. The scanning imaging system can be used for a seeker for laser guided munitions, where the scanning imaging system receives light, and wherein the images provide a target seeking function for the laser guided munition (e.g., laser guided bomb, missile, torpedo, or a precision artillery munition). The vehicle can further comprise a laser, wherein the laser is part of a laser designator that directs a laser beam from the laser using an optical arrangement comprising the motor driven first and second achromatic prism pairs as disclosed herein for directing the laser beam to a selected target within the area of interest based on an identification of the target using the images. The laser designator can use the same optical arrangement and aperture that is used for imaging, or use a separate optical arrangement and aperture. In other embodiments the laser designator is off the munition. The military vehicle can also include a radar system (e.g., for long-range seeking/targeting). 
         [0011]    In one embodiment the military vehicle comprises a missile having a head and a missile axis. As used herein, the “head” of the missile includes the tip and extends to include front ½ of the length of the missile. In this embodiment the optical window can comprise a flat optical window that is secured to the head, that in one embodiment is a monolithic window. The optical axis of the scanning imaging system is typically tilted at least 20 degrees relative to the missile axis. 
         [0012]    In one embodiment, the achromatic prism pairs are both athermal as well. An “athermal” prism pair is defined herein as a prism pair that for a maximum uplook condition linearly deviates (i.e. displaces) a ray traced along the optical axis for the imaging system ≦0.0005 radians of line-of-sight change due to temperature, over a temperature range of 0.0 to 100° C. It is noted that 70° C. represents the approximate maximum temperature within a missile (where the prism pairs and electronics are located) for a missile travelling at supersonic speeds that is heated by air friction. This embodiment compensates for degraded scanning imaging system performance recognized by the Inventors due to refractive index changes of the prisms over temperature, which can be significant for missiles due to frictional heating at supersonic speeds (e.g., 600° F. (about 315° C.) to 800° F. (about 425° C.). As described herein, athermal prism pair performance as described herein can be realized passively by selection of appropriate material combinations. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1A  is a depiction of an exemplary scanning imaging system comprising first and second achromatic prism pair and a fixed camera, according to a disclosed embodiment. 
           [0014]      FIG. 1B  is a depiction of an exemplary scanning imaging system comprising first, second and third prism pairs and a fixed camera, according to another disclosed embodiment. 
           [0015]      FIG. 2A  is a depiction of a vehicle shown as a tank including a scanning imaging system according to a disclosed embodiment. 
           [0016]      FIG. 2B  is a longitudinal section depiction through the missile nose of a missile including a scanning imaging system for missile seeking, according to a disclosed embodiment. 
           [0017]      FIG. 2C  is a depiction through the missile nose of a missile including a scanning imaging system for missile seeking depicted in  FIG. 2B  that is shown providing an elevation field-of-regard from 0° to 100°, according to a disclosed embodiment. 
           [0018]      FIG. 3  shows a prism pair along with angular measures and equations for obtaining design parameters for implementing achromatic and athermal prism pairs, according to a disclosed embodiment. 
           [0019]      FIG. 4  provides modulation transfer function (MTF) data as a function of spatial frequency for pair of achromatic prism pairs, according to a disclosed embodiment, in a temperature range from −40° C. to 70° C. 
           [0020]      FIGS. 5A-C  provide MTF data showing athermal performance for an exemplary scanning imaging system for missile seeking, according to a disclosed embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    Disclosed embodiments are described with reference to the attached figures, wherein like reference numerals, are used throughout the figures to designate similar or equivalent elements. The figures are not drawn to scale and they are provided merely to illustrate aspects disclosed herein. Several disclosed aspects are described below with reference to example applications for illustration. It should be understood that numerous specific details, relationships, and methods are set forth to provide a full understanding of the embodiments disclosed herein. One having ordinary skill in the relevant art, however, will readily recognize that the disclosed embodiments can be practiced without one or more of the specific details or with other methods. In other instances, well-known structures or operations are not shown in detail to avoid obscuring aspects disclosed herein. Disclosed embodiments are not limited by the illustrated ordering of acts or events, as some acts may occur in different orders and/or concurrently with other acts or events. Furthermore, not all illustrated acts or events are required to implement a methodology in accordance with this Disclosure. 
         [0022]      FIG. 1  is a depiction of an exemplary scanning imaging system  100  having a fixed lens assembly  116  and fixed imaging device  112 , according to a disclosed embodiment. Imaging device  112  can be embodied as a camera that comprises an image capture element  114 , for example, a charge coupled device (CCD) array or a photodiode or phototransistor array image sensor. A lens assembly  116  is mounted on, or adjacent to, the camera  112 . A beam-steering optical assembly  118  is positioned in front of the lens assembly  116 . 
         [0023]    The beam-steering optical assembly  118  includes a pair of achromatic prism pairs  120  and  122  that are mounted and include motors to allow their rotation about a central optical axis  124 . Prism pair  120  includes prisms  126  and  128 , and prism pair  122  includes prisms  130  and  132 . Motors  131  and  133  (e.g., stepper motors) are for rotating the prism pairs  120  and  122 , respectively. Since the prism pairs  120  and  122  are rotatable, there is no need for a conventional gimbal to rotate the entire scanning imaging system  100  (e.g., lens assembly  116  and camera  112  can remain fixed). 
         [0024]    The four-prism chromatically-corrected beam-steering optical assembly  118  shown in  FIG. 1A  can be compared to conventional two-prism Risley modules. Each set of prism wedges in a conventional Risley-prism module must be equivalent in wedge angle and optical material compensation in order to function properly. Thus, in a conventional two-prism Risley module, both wedges are made of the same material and therefore cannot correct for chromatic aberration. In contrast, in the four-prism  126 ,  128 ,  130  and  132  beam-steering optical assembly  118  shown in  FIG. 1A , each prism pair  120  and  122  includes two different optical materials, and is individually achromatized over the beam-steering optical assembly&#39;s  118  designed field-of-regard. 
         [0025]    In the first achromatic prism pair  120 , prism  126  includes a first surface  134  that lies in a plane that is substantially perpendicular to the optical axis  124 . A second surface  136  of the prism  126  is inclined with respect to the optical axis  124 . Prism  128  includes a first surface  138  that lies in a plane that is substantially parallel to the surface  136  of prism  126 . A second surface  140  of the prism  128  is inclined with respect to the optical axis  124 . 
         [0026]    In the second prism pair  122 , prism  130  includes a first surface  142  that lies in a plane that is substantially perpendicular to the optical axis  124 . A second surface  144  of the prism  130  is inclined with respect to the optical axis  124 . Prism  132  includes a first surface  146  that lies in a plane that is substantially parallel to the surface  144  of prism  132 . A second surface  148  of the prism  132  is inclined with respect to the optical axis  124 . The prisms of each prism pair  120  and  122  are shown positioned adjacent to each other, but can be separated from one another. 
         [0027]    Each achromatic prism pair  120  and  122  is rotated as a set. The prism pairs  120  and  122  having separate motors and motor controllers can be rotated independently of each other. By rotating the prism pairs  120  and  122 , light (ultraviolet, visible or infrared) received from different portions of an area of interest are directed onto the image capture element  114 . Thus, the direction of the field-of-view (FOV) of the camera  112  is effectively changed by rotating the prism pairs  120  and  122  without the need to move the camera  112 . Rotation of the prism pairs  120  and  122  thus provides effective pan (θ) and tilt (φ) functionality. 
         [0028]    The material selection for the prisms in the prism pairs  120 ,  122  will generally depend on, or be limited by, the wavelength band for which chromatic correction is desired and the level of chromatic correction needed. The surface tilts on the respective prisms are largely a function of the desired level of refraction (i.e. angular deviation/steering). In one embodiment, the materials for the first achromatic prism pair and second achromatic prism pair are all infrared transmissive in a wavelength band from 3 to 5 μm or 8 to 12 μm. 
         [0029]    As described above, the prism pairs  120  and  122  and the beam-steering optical assembly  118  as a result can be both achromatic and athermal. The line-of-sight and performance (e.g., MTF, wavefront error and image quality) affected by Risley prisms is a function of the wedge angle and the refractive index. Since the index of refraction of the prism materials change with temperature, the line-of-sight and performance provided by the prism pairs  120  and  122  absent athermalization as described herein would also all significantly change with temperature. As described above, one embodiment of the invention provides simultaneous achromatic and athermal (i.e. line-of-sight and performance unaltered by changes in temperature) correction in the design of the prism pairs. Since the beam-steering optical assembly  118  comprises two (or more) pairs of prisms, each prism pair in this embodiment is independently corrected in its design to be athermal. 
         [0030]    Scanning imaging system  100  is also shown including a processor (e.g., microprocessor or ASIC)  145  and a control circuit  147 . The control circuit  147  is coupled to drive the motors  131  and  133  which as described above, independently rotate the first and second prism pairs  120  and  122  to achieve the desired pan and tilt angles. The pan and tilt angles can be set by a user via input  149  to the processor  145 . In other embodiments, the output from the camera  112  is coupled to input  149 , such as for target tracking in target seeking applications. 
         [0031]      FIG. 1B  is a depiction of an exemplary scanning imaging system  150  comprising a six-prism chromatically-corrected beam-steering optical assembly  168  comprising first prism pair  120 , second prism pair  122  and third prism pair  162 , according to another disclosed embodiment. System includes the components in system  100  shown in  FIG. 1A , and adds a third prism pair  162  and a third motor  163 . In one embodiment, the third prism pair  162  can comprise a Jackson prism. 
         [0032]    There are advantages for certain applications that can be obtained by including more than two prism pairs in a beam-steering optical assembly. For example, prism pairs  120  and  122  can be used for beam-steering as described above, and the third prism pair  162  can be used for correction. In one example, the third prism pair  162  can comprises a Zernike prism pair (Risley&#39;s with Zernike surfaces) that can be used for correction in a domed application where there is a dome along the optical axis of the beam-steering optical assembly, such as when the head of the missile includes a dome for aerodynamic performance that inherently functions as the outermost optical element in the beam-steering optical assembly. Disadvantages of including more than two prism pairs in the beam-steering optical assembly can include reduced optical transmittance, increased volume consumption (i.e., more space required), greater weight, and an increase in algorithmic complexity to steer three (3) prism pairs. It may also be possible to achieve advantages of scanning imaging systems having three prism pairs by having a first and second prism pair each add a third prism. 
         [0033]      FIG. 2A  is a depiction of a vehicle  200  shown as a tank including a scanning imaging system  220 , according to a disclosed embodiment. Other exemplary vehicles that can benefit from disclosed scanning imaging systems include missiles, bombs, airplanes, helicopters, tanks, and trucks. Vehicle  200  includes a propulsion source  210 , such as an engine for the tank shown. 
         [0034]    As shown in  FIG. 2A , scanning imaging system  220  comprises the components of scanning imaging system  100  shown in  FIG. 1A  along with a flat optical window  215 . As used herein, a “flat optical window” refers to a plane-parallel plate. When the surfaces of flat optical window  215  are parallel to each other (i.e. flat), the Inventors have recognized that there is no angular deviation of light (visible, infrared, or otherwise), which is optically desirable. A flat optical window as used herein includes up to a minor amount of wedge (i.e. non-parallel surfaces) typical for the manufactured item of less than 10 arc seconds. A flat optical window  215  is unlike a conformal dome, which the Inventors have recognized can introduce severe wavefront distortions. In a typical embodiment, the flat optical window  215  is an unsegmented window to ensure that pupil-splitting effects will not be present. The optical window  215  can comprise a variety of different optically transmissive materials, such as sapphire, ALON™, or spinel. 
         [0035]      FIG. 2B  is a longitudinal section depiction through the missile nose/head  251  of a missile seeker  250  including a windowed scanning imaging system  260  for missile seeking, according to a disclosed embodiment. The missile seeker  250  shown is actually a dual missile seeker that includes an electro-optic seeker based scanning imaging system disclosed herein and a radar based seeker comprising a radar transmitting  257  and receiver  259 . The radar based seeker is for long-range targeting. 
         [0036]    The missile seeker  250  shown can be a fighter-launched supersonic missile designed to strike short, medium and long-range air-to-air and air-to-ground targets. Windowed scanning imaging system  260  provides the electro-optical seeker portion for the missile seeker  250 . Windowed scanning imaging system  260  includes the scanning imaging system  100  shown in  FIG. 1A  together with a flat optical window  270  shown as a monolithic window that is tilted relative to the missile axis  285 . 
         [0037]    As shown in  FIG. 2B , missile seeker  250  is embodied as an internal laser designator and comprises laser  246 . Laser  246  is coupled to scanning imaging system  100  and laser radiation emitted can be scanned by scanning imaging system  100  over the field-of-regard provided by scanning imaging system  100  to designate a target. As known in the art, when a target is marked by a laser designator, the beam is invisible and does not shine continuously. Instead, a series of coded pulses of laser-light are fired. These signals bounce off the target (e.g., into the sky), where they are detected by the seeker, which steers itself towards the center of the reflected signal. Alternatively, laser  246  may be excluded and an external laser designator may be used with missile seeker  250 . 
         [0038]    Missile seeker  250  is shown including a propulsion source shown as a rocket motor  255 , and a warhead  256 . Missile seeker  250  is also shown including a guidance control system  258 . Guidance control system  258  including processor  263  directs the missile seeker&#39;s  250  maneuvers and causes the maneuvers to be executed by the control section of guidance control system  258 . Guidance control system  258  implements a radar-based homing guidance system. Embodied as shown as an active homing system, target illumination is supplied by a component carried in the missile  250 , such as the radar transmitter  257  shown. The radar signals transmitted from the missile seeker  250  by radar transmitter  257  are reflected off the target back to the receiver  259  in the missile seeker  250 . These reflected signals give the missile seeker  250  information such as the target&#39;s distance and speed. This information lets the guidance control system  258  compute the correct angle of attack to intercept the target. 
         [0039]    The control section that receives electronic commands from the guidance control system  258  controls the missile&#39;s angle of attack. Mechanically manipulated wings (not shown), intake  267 , or canard control surfaces (not shown) are mounted externally on the body of the missile seeker  250 . As known in the art, the wings, etc., are actuated by hydraulic, electric, or gas generator power, or combinations of these to alter the missile&#39;s course. 
         [0040]    Missile seeker  250  can also be embodied as a semiactive homing system, in which the missile gets its target illumination from an external source, such as a radar transmitter carried in the launching aircraft, so that radar transmitter  257  would not be needed. The receiver  259  in the missile seeker  250  receives the radar signals reflected off the target, computes the information, and sends electronic commands to the control section. The guidance control system  258  functions in the same manner as previously discussed. Missile seeker  250  can also be embodied as a passive homing system, where the directing intelligence is received from the target. Examples of passive homing include homing on a source of infrared rays (such as the hot exhaust of jet aircraft) or radar signals (such as those transmitted by ground radar installations). Like active homing, passive homing is completely independent of the launching aircraft. The missile receiver receives signals generated by the target and then the missile control system  258  functions in the same manner as previously discussed. 
         [0041]    The tilted flat window  270  minimizes the drag for missile seeker  250 , yet still provides the capability of looking forward, sideways and up. However, as shown in  FIG. 2B , the missile axis and optical axis for missile seeker  250  are shown tilted about 50 degrees, and are more generally at least 20 degrees relative to one another, with the optical axis  124  facing upward. 
         [0042]    The tilting of the flat window  270  on the missile minimizes vignetting while maximizing the field-of-regard for the scanning imaging system  260 . Embodied as a monolithic flat window prevents both window-splitting effects and dome-induced optical aberrations. Tilted flat window  270  should be of sufficient thickness to prevent bowing due to aerodynamic forces on its exterior. For certain applications for missile seeker  250 , electro-optic seeker operation will be in the IR bands from 3 to 5 μm or 8 to 12 μm. Accordingly, the materials for the respective prism pairs  120  and  122  can be selected for operation from 3 to 5 μm or 8 to 12 μm. 
         [0043]      FIG. 2C  is a depiction through the missile nose of a missile including a scanning imaging system for the missile seeker depicted in  FIG. 2B  that shows providing an elevation field-of-regard from 0° to 100° relative to the missile axis  285  with a fixed camera, according to a disclosed embodiment. Although not shown, the azimuthal field-of-regard provided is generally wide as well, being from. This wide field-of-regard provided by scanning imaging system  260  with a fixed camera eliminates the space required and field-of-regard limitations imposed by a conventional gimbaled optics configuration. Therefore, a significant advantage provided by missile seekers disclosed herein over conventional gimbaled missile seekers is that conventional gimbaled systems require ample amounts of sway space in order to sweep the entire optical system including the camera through a field-of-regard and therefore can impose expensive packaging constraints on other system attributes. 
         [0044]    A significant performance improvement is also provided. When compared to conventional dome-gimbal missile configurations, disclosed missile seekers provides a wider field-of-regard and consistent aberration correction. While spherical dome-gimbal configurations can provide high-quality images, their fields of regard are comparatively limited relative to those provided by disclosed embodiments because a missile is essentially a long cylinder with a short diameter, so that the field-of-regard for a fully gimbaled system is limited by the missile&#39;s diameter and by the length of its optical system. 
         [0045]    Missile seeker  250  provides cost reductions as compared to conventional gimbaled systems that require ample amounts of sway space in order to sweep through a field-of-regard and therefore can impose expensive packaging constraints on other system attributes. Missile seeker  250  effectively eliminates the space required and field-of-regard limitations imposed by a gimbaled optics configuration. Used in other applications, the invention could replace all gimbal components and thereby free up packaging volume. 
         [0046]    Other performance improvements are also provided. When compared to conventional dome-gimbal missile configurations, the missile seeker provides a wider field-of-regard and consistent aberration correction. While spherical dome-gimbal configurations can provide high-quality images, their fields-of-regard are comparatively limited relative to those provided by disclosed embodiments. Similarly, with conformal dome-gimbal configurations, image quality is variable and highly aberrated as a function of field-of-regard. Disclosed embodiments also provide a reduced window size as compared to a conventional gimbaled configurations. 
         [0047]    Although scanning imaging systems have been described above for moving applications, stationary embodiments can also benefit, such as for monitoring on a raised fixed surface, such as on a pole. Generally, disclosed scanning imaging systems can be used for one or more of monitoring, target seeking, or laser designating. 
       EXAMPLES 
       [0048]    Disclosed embodiments of the invention are further illustrated by the following specific Examples, which should not be construed as limiting the scope or content of this Disclosure in any way. 
         [0049]    As described above, disclosed scanning imaging systems can be athermal besides being achromatic by making the respective prism pairs both achromatic and athermal. Referring to  FIG. 3 , a prism pair  310  is shown comprising a first prism shown as prism  1  and a second prism shown as prism  2 . It is well known that, for typically under 11 degrees, to achieve achromatic correction for a prism pair, conditions 1 and 2 shown below must be satisfied: 
         [0000]      β=( n   1 −1) α 1 +( n   2 −1) α 2    Condition 1
 
         [0000]      α 1   /V   1 +α 2   /V   2 =0   Condition 2
 
         [0000]    Where: β=Line-of-Sight Deviation due to the prism pair
       where: n 1 =index of refraction of Prism No.  1 , n 2 =index of refraction of Prism No.  2 , α 1 =wedge angle of Prism No.  1 , α 2 =wedge angle of prism No.  2 , V 1 =V−number of prism No.  1  material and V 2 =V−number of prism No.  2  material.       
 
         [0051]    The Inventors have recognized that athermalization of the prism pair imposes a third constraint upon the design of the prism pair. The constraint imposed on the prism pair is as follows: 
         [0000]      α 1 ( dn   1   /dT )+α 2 ( dn   2   /dT )=0   Condition 3
 
         [0000]    Where: dn 1 /dT=the change in index of refraction with temperature of prism No.  1  material, and dn 2 /dT=the change in index of refraction with temperature of prism No.  2  material. 
         [0052]    The Inventors have also recognized that for simultaneous chromatic aberration correction and athermalization, Conditions 2 and 3 can be combined into condition 4 shown below: 
         [0000]        V   2   /V   1 =( dn   2   /dT )/( dn   1   /dT )   Condition 4
 
         [0053]    Condition 4 can be used to select optical materials such that the respective prisms can be designed such that the prism pairs are corrected for chromatic aberration and are corrected to be athermal such that the line-of-sight deviation is independent of temperature, as noted above is defined herein to be ≦0.0005 radians of line-of-sight change due to temperature, over a temperature range of 0.0 to 100° C. Condition 1 as described above can be used to establish initial wedge angles for the prisms to provide the required line-of-sight deviation. Modern optical design codes can be used to optimize the angles to provide the required line-of-sight deviation to larger angles where small angle approximations are no longer valid, typically greater than 11 degrees, such that the design is both achromatic and athermal over a broader range of angles. 
         [0054]    One particular exemplary achromatic and athermal prism pair implementation is provided below. This embodiment uses silicon for prism No.  1  and germanium for prism No.  2  for operation in the mid-wave IR band from about 3.5 to 5.2 μms. The data for these materials is as follows: 
         [0055]    V 1 =523 
       V 2 =221 
       [0056]    dn 1 /dT=160×10 −6 /° C.
 
dn 2 /dT=400×10 −6 /° C.
 
Performing the analysis of Condition 4:
 
         [0000]        V   2   /V   1 =2.4≈( dn   2   /dT )/( dn   1   /dT )=2.5
 
         [0057]    These ratios are sufficiently close that modern commercially available optical design software codes ZEMAX™, CODE V™, and Optics Software for Layout and Optimization (OSLO™) can be used to determine the wedge angles of these materials to achieve the desired line-of-sight deviation with both high levels of chromatic aberration correction and athermalization. 
         [0058]      FIG. 4  provides modulation transfer function (MTF) data as a function of spatial frequency for a pair of achromatic prism pairs, such as prism pairs  120  and  122  described above. While the chromatic aberration can be corrected with the prism pairs as described above, there is an artifact whenever the beam “looks” (or directs radiation in laser designator applications) off of the optical axis. The center beam in the  FIG. 3  depiction shows the beam size and shape (spherical) for the optical system and for the prism pairs in their null (i.e. angularly non-deviating-on axis) orientation. As the scan direction look angle increases (either looking up or down), the area viewed by the optical system decreases and the beam shape becomes asymmetric, resulting in some performance degradation in the scan direction (up/down; noted as “Tangential” in the MTF plots). This effect also occurs in the azimuth direction (side-to-side; noted as “Sagittal” in the MTF plots). Thus, whenever the prism pairs are out of their null orientation, the performance will degrade to some degree. However, algorithms can alter receive imagery to improve performance in this case. 
         [0059]      FIGS. 5A-C  provide MTF data as a function of spatial frequency for pair of achromatic prism pairs, such as prism pairs  120  and  122  described above using silicon for prism No.  1  and germanium for prism No.  2  for a beam traced at the maximum elevation downlook (about 50°) with the prism pairs housed in aluminum and thermally soaked at various temperatures. The MTF (i.e. the optical performance) can be seen to not change appreciably as a function of temperature in the temperature range from −40° C. to 70° C., thus evidencing athermal performance. 
         [0060]    While various disclosed embodiments have been described above, it should be understood that they have been presented by way of example only, and not as a limitation. Numerous changes to the disclosed embodiments can be made in accordance with the Disclosure herein without departing from the spirit or scope of this Disclosure. Thus, the breadth and scope of this Disclosure should not be limited by any of the above-described embodiments. Rather, the scope of this Disclosure should be defined in accordance with the following claims and their equivalents. 
         [0061]    Although disclosed embodiments have been illustrated and described with respect to one or more implementations, equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. While a particular feature may have been disclosed with respect to only one of several implementations, such a feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application. 
         [0062]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting to this Disclosure. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. Furthermore, to the extent that the terms “including,” “includes,” “having,” “has,” “with,” or variants thereof are used in either the detailed description and/or the claims, such terms are intended to be inclusive in a manner similar to the term “comprising.” 
         [0063]    Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this Disclosure belongs. It will be further understood that terms, such as those defined in commonly-used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.