Abstract:
An infrared imaging optical system for focusing infrared radiation on an infrared detector, including: a front lens group having a negative optical power to receive infrared radiation and including a first front lens and a second front lens each with at least one aspherical surface; an intermediate lens group that receives the infrared radiation from the front lens group and includes a first intermediate lens, a second intermediate lens, and a third intermediate lens each with at least one aspherical surface; and a rear lens group having positive optical power, wherein the rear lens group receives the infrared radiation from the intermediate lens group and includes a first rear lens and a second rear lens each with at least one aspherical surface, and a third rear lens, wherein the imaging optical system has a stop between the rear lens group and a focal plane at said infrared detector.

Description:
FIELD 
       [0001]    The present invention is related to an infrared imaging optical system, and more particularly, to an inverse telephoto optical system that is highly compact. 
       BACKGROUND 
       [0002]    In certain airborne wide area surveillance missions, particularly on board a small unmanned aerial vehicle (“UAV”), there are sensors positioned on the UAV that include infrared imaging optical systems used to image light in the infrared optical spectrum (from about 2 micrometers to about 7 micrometers), which is produced by hot objects such as engines, the human body, or missiles fired at the UAV, onto detectors. The detectors then convert the light into electrical signals that can be further analyzed. 
         [0003]    These sensors may use an inverse-telephoto lens system, also known as a fisheye lens, because it allows the field of view to be very large. Further, the detectors in these systems must be cryogenically cooled, while the optical system is not. Therefore, these optical systems have external pupils (or stops) for the location of cold shield that surrounds only the cryogenically cooled detectors. 
         [0004]    Previously, inverse telephoto optical systems have had physical length to effective focal length ratios in the 8× to 10× range. These are too large to fit the anticipated 10 inch to 12 inch diameter gimbals that can be employed on small UAV platforms. Further, these known systems employ large diameter lenses that are expensive to manufacture. 
         [0005]    The size of the optical system is as important as the field of view (“FOV”), aperture (or stop), and focal length that the optical system is capable of providing. The availability of large two-dimension staring arrays with pixel counts up to 8,000 by 8,000 further emphasizes the need for very compact optical systems. 
       SUMMARY 
       [0006]    An embodiment of the present invention provides a compact inverse-telephoto infrared imaging optical system having a wide field of view, a fast optical speed, and a small ratio of the physical length to the effective focal length (“EFL”). Aspects of an embodiment of the present invention are related to an inverse telephoto optical system with a physical length to effective focal length ratio that is about 2.74, and provides FOVs of about 90 degrees and optical speeds of about F/1.61 while operating in the mid-wavelength infrared (“MWIR”) spectral band. The present inverse-telephoto infrared imaging optical system is therefore much smaller and lighter than previous systems. 
         [0007]    Aspects of another embodiment of the present invention are related to an inverse telephoto optical system with a physical length to effective focal length ratio that is about 1.94, and provides FOVs of about 80 degrees and optical speeds of about F/3.0 while operating in the mid-wavelength infrared (“MWIR”) spectral band. 
         [0008]    An embodiment of the present invention provides an infrared imaging optical system for focusing infrared radiation on an infrared detector. The optical system includes: a front lens group having a negative optical power, wherein the front lens group receives an infrared radiation and includes a first front lens with at least one aspherical surface and a second front lens with at least one aspherical surface. The optical system also includes an intermediate lens group that receives the infrared radiation from the front lens group, wherein the intermediate lens group includes a first intermediate lens with at least one aspherical surface, a second intermediate lens with at least one aspherical surface, and a third intermediate lens with at least one aspherical surface. The optical system also includes a rear lens group having positive optical power, wherein the rear lens group receives the infrared radiation from the intermediate lens group, and wherein the rear lens group includes a first rear lens with at least one aspherical surface, a second rear lens with at least one aspherical surface, and a third rear lens. Further, the imaging optical system has a stop between the rear lens group and a focal plane located at said infrared detector, and a ratio of the physical length to the effective focal length may be less than about 3.0. 
         [0009]    The optical system may further include a corrector plate between the third rear lens and the stop. The corrector plate may be a Schmidt corrector plate. The corrector plate may be formed of germanium. 
         [0010]    The optical system may be optimized for operation in the 3.4 micron to 4.8 micron wavelength range. The first rear lens may be formed of barium fluoride. The third rear lens may be formed of gallium arsenide. The field of view may be about 90 degrees. The F/# may be about 1.6. The ratio of the physical length to effective focal length may be about 2.7. The field of view may be about 80 degrees. 
         [0011]    The F/# may be about 3.0. The ratio of the physical length to the effective focal length may be about 1.94. 
         [0012]    The optical system may further include a cryo-vac window between the third rear lens and the stop. The cryo-vac window may be formed of germanium. A ratio of the diameter of the stop to the diameter of an entrance pupil may be about 0.96. A ratio of the diameter of the stop to the diameter of an entrance pupil may be about 0.74. A ratio of the diameter of the stop to the diameter of an entrance pupil is less than about 1.0. 
         [0013]    It should be appreciated that while certain embodiments can be optimized for performance in the MWIR, the principles and techniques utilized in this invention can be equally applied to embodiments optimized for performance in the LWIR spectrum, provided certain refractive materials changes appropriate to the LWIR spectral band are made. These materials changes are generally well known, such as the use of Si as the crown material of choice in the MWIR being replace by Ge as the crown material of choice in the LWIR. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is schematic view of an optical system according to an embodiment of the present invention. 
           [0015]      FIG. 2  is an optical prescription for an embodiment of the present invention as shown in  FIG. 1 . 
           [0016]      FIG. 3  is a schematic view of the optical system of  FIG. 1  and two other prior art optical systems, all in the same scale. 
           [0017]      FIG. 4  is a schematic view of an optical system according to another embodiment of the present invention. 
           [0018]      FIG. 5  is an optical prescription for an embodiment of the present invention as shown in  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    An embodiment of the present invention provides an inverse telephoto optical system with a length-to-focal-length ratio that is about 2.7, and provides a FOV of about 90 degrees and an optical speed of about F/1.6 while operating in the mid-wavelength infrared (“MWIR”) spectral band. Here, the choice of material, number and location of various lenses, the use of aspheric lenses, and the employment of relatively low pupil magnifications (e.g., values in the region of 1.0×) make the short length of the optical system possible. The present invention may be utilized with respect to ground intelligence surveillance and reconnaissance systems (“GISR”) and other wide area surveillance efforts from airborne or UAV platforms (e.g., with about a twelve foot wingspan) to detect infrared radiation. 
         [0020]      FIG. 1  shows an infrared imaging optical system  50  according to an embodiment of the present invention.  FIG. 2  is an optical prescription for an embodiment of the present invention as shown in  FIG. 1 . It is generally known that certain wide field of view optical systems are best and routinely raytraced backwards, from the image plane out into object space. As the light path is completely reversible, this presents no physical problem, and it routinely avoids certain mathematical ray aiming convergence problems that most raytrace codes are prone to in wide field of view situations. Thus, the optical prescriptions of  FIGS. 2 and 5  are in the reverse order of he actual propagation of light. Here, the optical system  50  includes a front lens group  51 , an intermediate lens group  52 , and a rear lens group  53 . The front lens group  51  receives infrared radiation and directs it onto the intermediate lens group  52 . The intermediate lens group receives the infrared radiation from the front lens group  51  and directs it onto the rear lens group  53 . The rear lens group  53  receives the infrared radiation from the intermediate lens group  52  and directs it onto an infrared detector, which is located at the focus  1  (or focal plane) of the lens system  50 . 
         [0021]    The infrared detector may be a focal plane array (“FPA”) that detects infrared radiation with wavelengths in a range from about 3.4 microns to about 4.8 microns, which are known in the art. The infrared radiation detector converts the incident infrared radiation into a signal to be analyzed by image analysis electronics, which are also known in the art. However, this type of detector typically operates most effectively at cryogenic temperatures of about 77K, so the detector would be located in a vacuum dewar, which serves as a cold shield. An opening of the cold shield is located at a cryo-vac window  55 . A stop  2  is located just beyond the cryo-vac window  55  so that the stop is inside the vacuum dewar to reduce the likelihood that the detector will detect energy from the lens surfaces or other components. 
         [0022]    The front lens group  51  includes a first front lens  51   a  and a second front lens  51   b.  Infrared radiation is incident on a first surface  22  of the first front lens  51   a  and passes out through a second surface  21 , which is aspheric, of the first front lens  51   a  to a first surface  20  of the second front lens  51   b.  The infrared radiation then passes out of the second front lens  51   b  through a second surface  19  of the second front lens  51   b  to the intermediate lens group  52 . 
         [0023]    The intermediate lens group  52  includes a first intermediate lens  52   a,  a second intermediate lens  52   b,  and a third intermediate lens  52   c.  The infrared radiation from the second surface  19 , which is aspheric, of the second front lens  51   b  is incident on a first surface  18 , which is aspheric, of the first intermediate lens  52   a  and passes out through a second surface  17  of the first intermediate lens  52   a  to a first surface  16  of the second intermediate lens  52   b.  The infrared radiation then passes out of the second intermediate lens  52   b  through a second surface  15 , which is aspheric, of the second intermediate lens  52   b  to a first surface  14 , which is aspheric, of a third intermediate lens  52   c.  The infrared radiation then passes out of the third intermediate lens  52   c  through a second surface  13  of the third intermediate lens  52   c  to the rear lens group  53 . 
         [0024]    The rear lens group  53  includes a first rear lens  53   a,  a second rear lens  53   b,  and a third rear lens  53   c.  The infrared radiation from the second surface  13  of the third intermediate lens  52   c  is incident on a first surface  12 , which is aspheric, of the first rear lens  53   a  and passes out through a second surface  11  of the first rear lens  53   a  to a first surface  10 , which is aspheric, of the second rear lens  53   b.  The infrared radiation then passes out of the second rear lens  53   b  through a second surface  95  of the second rear lens  53   b  to a first surface  8  of a third rear lens  53   c.  The infrared radiation then passes out of the third rear lens  53   c  through a second surface  8  of the third rear lens  53   c  to a first surface  6  of a corrector plate  54 . 
         [0025]    The infrared radiation passes through the corrector plate  54 , the cryo-vac window  55 , a and a stop  2 , before the infrared radiation is incident on the detector located at a focus  1 . Here, the infrared radiation then passes out of the corrector plate  54  through a second surface  5  of the corrector plate  54  to a first surface  4  of the cryo-vac window  55 . 
         [0026]    In an embodiment of the present invention  54 , the corrector plate  54  may be a Schmidt corrector plate to correct for spherical aberration. For example, a Schmidt corrector plate is an aspheric lens which is designed to correct spherical aberration. Here, the second surface  5  of the corrector plate  54  has an aspheric surface. 
         [0027]    The infrared radiation then passes out of the cryo-vac window  55  through a second surface  3  of the cryo-vac window  55  and through a stop  2 . The stop  2  allows some of the infrared radiation to pass through an open central region of the stop  2 , and this infrared radiation is incident on the detector located at the focus  1 . 
         [0028]    The detector detects the incident infrared radiation, and information based on this detected infrared radiation is converted to an electrical signal which may be further processed by other electronics. 
         [0029]    The optical system  50  is an “inverse-telephoto lens group” such that the front lens group  51  has a negative optical power and the rear lens group  53  has a positive optical power. This combination of optical powers allows the optical system  50  to function in the “fisheye” manner, with a very wide field of view in both the azimuth and elevation. Here, there is some distortion of the image on the infrared detector, but that distortion is acceptable for the applications of interest in return for the very wide angle of view. 
         [0030]    In an embodiment according to the optical prescription shown in  FIG. 2 , the stop diameter is about 4.296 cm, the optical speed is about F/1.61, the diagonal dimension of the focal plane array (“FPA”) of the detector is about 11.576 cm, the FOV average effective focal length (“EFL”) is about 7.48 cm, the entrance pupil is about 4.466 cm, the spectral band of operation is about 3.4 microns to about 4.8 microns, the FOV diagonal is about 89.4 degrees, the total length from the first surface  22  of the first front lens  51   a  to the focus  1  is about 20.5 cm, the ratio of the stop diameter to the entrance pupil diameter is about 0.96, and the ratio of the physical length P L  to the EFL is about 2.74. Here, the physical length P L  is the distance from the outwardly facing surface of the front lens to the image plane. 
         [0031]      FIG. 3  is a schematic view of the optical system  50  of  FIG. 1  and two previous optical systems  60  and  70 , all in the same scale and with a common EFL. As shown in  FIG. 3 , the optical system of the present invention is much shorter in physical length than previous optical systems. 
         [0032]    Optical system  60 , described in U.S. Pat. No. 6,989,537 to Cook, the entire content of which is incorporated by reference, has an FOV of 120 degrees, F/1.0 while operating in the long-wavelength infrared spectral band, and a ratio of physical length to EFL of 9.9. Optical system  70 , described in U.S. Pat. No. 5,446,581 to Jamieson, has an FOV of 120 degrees with F/1.0 while operating in the long-wavelength infrared spectral band, and a ratio of physical length to EFL of 24.9. 
         [0033]    Embodiments of the optical system of the present invention utilize more lenses, including more aspheric lenses, than previous optical systems, and the lenses of the present optical system are formed of different materials than the previous optical systems. The present approach yielded surprising and unexpected results, since the present optical system achieves a small ratio of physical length to effective focal length as compared to previous optical systems. Further, the ratio of the size of entrance pupil to the size of aperture stop (i.e., the pupil magnification) is about 1.0 in the optical system of the present invention, as compared to much smaller ratios in previous optical systems. 
         [0034]    However, because the pupil magnification is about unity or 1.0, the angle of incidence of the infrared radiation on the focus in the present optical system may be much larger (0 to about 45 degrees) than in previous optical systems. Therefore, different anti-reflection coatings on detector may be required. 
         [0035]    As can be seen in  FIG. 3 , the diameters of the lenses in the present optical system  50  are smaller than those of the previous optical systems  60  and  70 . Therefore, the length of the required support structure is smaller, leading to reduced weight and size, which are important advantages for UAV applications. 
         [0036]    The reduced weight and size are achieved without a significant loss in optical performance. While the total transmission or throughput of the optical system is slightly reduced due to the greater number of lenses used, the quality of the wavefront, as measure by the RMS wavefront error or departure, is comparable to that of optical designs in the prior art. 
         [0037]    The advantages of the present optical system primarily result from the use of alternative optical materials for the construction of the lenses, and from the selective use of aspheric surfaces for some of the lenses. 
         [0038]    The lenses of the optical system  50  are preferably formed of infrared radiation transparent material. In an embodiment, the first front lens  51   a,  the second front lens  51   b,  the first intermediate lens  52   a,  the second intermediate lens  52   b,  the third intermediate lens  52   c,  and the second rear lens  53   b  are formed of silicon. 
         [0039]    In an embodiment, the first rear lens  53   a  is formed of barium fluoride. In an embodiment, the third rear lens  53   c  is formed of gallium arsenide. 
         [0040]    In an embodiment, the corrector plate  54  and the cryo-vac window are formed of germanium. 
         [0041]    In another embodiment of the present invention shown in  FIG. 4 , an inverse telephoto optical system  150  has a physical length to EFL ratio that is about 1.94, and provides FOVs of about 80 degrees and optical speeds of about F/3.0 while operating in the MWIR spectral bands.  FIG. 5  is an optical prescription for an embodiment of the present invention as shown in  FIG. 4 . The optical system  150  includes a front lens group  151 , an intermediate lens group  152 , and a rear lens group  153 . The front lens group  151  receives infrared light and directs it onto the intermediate lens group  152 . The intermediate lens group receives the infrared light from the front lens group  151  and directs it onto the rear lens group  153 . The rear lens group  153  receives the infrared light from the intermediate lens group  152  and directs it onto an infrared detector, which is located at the focus  1  (or focal plane) of the lens system  150 . The lenses of the optical system  150  are preferably formed of infrared transparent material. 
         [0042]    The front lens group  151  includes a first front lens  151   a  and a second front lens  151   b.  Infrared radiation is incident on a first surface  122  of the first front lens  151   a  and passes out through a second surface  121 , which is aspheric, of the first front lens  151   a  to a first surface  120  of the second front lens  151   b.  The infrared radiation then passes out of the second front lens  151   b  through a second surface  119 , which is aspheric, of the second front lens  151   b  to the intermediate lens group  152 . 
         [0043]    The intermediate lens group  152  includes a first intermediate lens  152   a,  a second intermediate lens  152   b,  and a third intermediate lens  152   c.  The infrared radiation from the second surface  119  of the second front lens  151   b  is incident on a first surface  118 , which is aspheric, of the first intermediate lens  152   a  and passes out through a second surface  117  of the first intermediate lens  152   a  to a first surface  116  of the second intermediate lens  152   b.  The infrared radiation then passes out of the second intermediate lens  152   b  through a second surface  115 , which is aspheric, of the second intermediate lens  152   b  to a first surface  114 , which is aspheric, of a third intermediate lens  152   c.  The infrared radiation then passes out of the third intermediate lens  152   c  through a second surface  113  of the third intermediate lens  152   c  to the rear lens group  153 . 
         [0044]    The rear lens group  153  includes a first rear lens  153   a,  a second rear lens  153   b,  and a third rear lens  153   c.  The infrared radiation from the second surface  113  of the third intermediate lens  152   c  is incident on a first surface  112 , which is aspheric, of the first rear lens  153   a  and passes out through a second surface  111  of the first rear lens  153   a  to a first surface  110 , which is aspheric, of the second rear lens  153   b.  The infrared radiation then passes out of the second rear lens  153   b  through a second surface  109  of the second rear lens  153   b  to a first surface  107  of a third rear lens  153   c.  The infrared radiation then passes out of the third rear lens  53   c  through a second surface  108  of the third rear lens  153   c  to a first surface  106  of a corrector plate  154 . 
         [0045]    The infrared radiation passes through the corrector plate  154 , the cryo-vac window  155 , and a stop  102 , before the infrared radiation is incident on the detector located at a focus  101 . Here, the infrared radiation then passes out of the corrector plate  154  through a second surface  105 , which is aspheric, of the corrector plate  154  to a first surface  104  of the cryo-vac window  155 . The infrared radiation then passes out of the cryo-vac window  155  through a second surface  103  of the cryo-vac window  155  and through a stop  102 . The stop  102  allows some of the infrared radiation to pass through an open central region, and this infrared radiation is incident on the detector located at the focus  101 . 
         [0046]    In an embodiment according to the optical prescription shown in  FIG. 2 , the stop diameter is about 1.932 cm, the optical speed is about F/3.0, the diagonal dimension of the focal plane array (“FPA”) of the detector is about 11.314 cm, the FOV average effective focal length (“EFL”) is about 8.00 cm, the entrance pupil is about 2.60 cm, the spectral band of operation is about 3.4 microns to about 4.8 microns, the FOV diagonal is about 80.0 degrees, the total length from the first surface  122  of the first front lens  151   a  to the focus  101  is about 15.5 cm, the ratio of the physical length P 1 , to the EFL is about 1.94, and the ratio of the stop diameter to the entrance pupil diameter is about 0.74. 
         [0047]    Although the present invention has been described and illustrated in respect to exemplary embodiments, it is to be understood that it is not to be so limited, and changes and modifications may be made therein which are within the full intended scope of this invention as hereinafter claimed.