Abstract:
An image forming system and lens system including at least one molded chalcogenide lens element and configured to simultaneously image light at the medium wave infrared region (MWIR) and the long wave infrared region (LWIR) at a common focal plane.

Description:
TECHNICAL FIELD 
   The present application generally relates to imaging devices and, more specifically, a dual band lens system incorporating molded chalcogenide. 
   BACKGROUND 
   Many imaging applications, such as military, biomedical, telescopes, reconnaissance planes, satellites, forward-looking infrared, staring sensor systems, night-vision goggles, and other optic and/or electro-optic detection systems demand simultaneous detection in separate wavelength bands, e.g. in the mid-wave infrared and the long-wave infrared spectral ranges. Traditionally, such dual band imaging has required separate refractive lens systems with separate associated detectors, or reflective systems including multiple mirror reflectors. Reflective systems have suffered from a narrow field of view, thus leading to a preference for refractive systems. 
   More recently, dual band refractive lens systems configured to simultaneously image light from first and second wavelength bands onto a common focal plane, e.g. at a multi-band detector, have emerged. Such systems, however, tend to include lenses with aspheric surfaces. Manufacturing of aspheric surfaces may require expensive and time consuming diamond point turning (DPT) manufacturing processes and/or additional post polishing. 
   Therefore, there is a need for a dual band lens system incorporating components that may be efficiently manufactured. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     So the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention may be had by reference to the embodiments, some of which are illustrated in the appended drawings. It is to be noted; however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. Moreover, it should be appreciated that the specific materials, lens formulations and systems applications disclosed herein are merely illustrative and do not delimit the scope of the invention. 
       FIG. 1  is a schematic diagram of an exemplary embodiment of an image forming system consistent with the present invention; and 
       FIG. 2  is a schematic diagram of one exemplary embodiment of a lens system consistent with the present invention. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a simplified block diagram of one exemplary embodiment of an image forming system  100  consistent with the present invention. The illustrated exemplary system  100  includes a lens system  110 , a detector  120 , a signal processing unit  125 , and a display unit  130 . Collimated light  105  imparted on the lens system  110  may be detected by detector  120 . In general the detector  120  converts the light imparted thereon to electrical impulses. A variety of detector configurations are well-known to those of ordinary skill in the art. In one embodiment, for example, the detector  210  may be configured as a microbolometer. 
   The electric impulses provided by the detector  120  may be coupled to the signal processing unit  125 . In a known manner, the signal processing unit  125  may translate the electrical impulses into data representative of an image(s) of objects that generated or reflected the light received at the lens  110 . The data may be coupled to the display unit, which displays the image(s). 
   In one embodiment, the system  100  may be configured for simultaneously imaging light from the MWIR and LWIR bands onto a common focal plane. As used herein the “MWIR” band or spectrum shall refer to wavelengths from about 3 to about 5 micrometers, and the “LWIR” spectrum or band shall refer to wavelengths from about 8 to about 12 micrometers. Imaging multiple wavelength bands onto a common focal plane allows use of a single detector and avoids the need to refocus when imaging light from one band to the other. Such a system may be useful, for example, in providing combined thermal and night vision imaging. 
   Consistent with the present invention, dual band achromatic imaging may be achieved in a system incorporating one or more molded chalcogenide lens elements. The molded chalcogenide lens elements may be molded with one or more aspheric surfaces, thus avoiding expensive DPT and/or post polishing processes. Moreover, the characteristics of molded chalcogenide lens elements in the MWIR and LWIR bands allow an achromatic condition that is believed to be superior to other lens materials. 
   Molded chalcogenide lens elements are commercially available from Umicore Optics of Quapaw, Okla. (herein referred to as “Umicore”), e.g. under the tradename GASIR1. GASIR1 is a composition according to the formula Ge 22 As 20 Se 58 . Molded GASR1 lens elements are characterized as exhibiting a refractive index at 8 micrometers N 8 =2.5015, and an Abbe number V 3-13 =(N 8 −1)/(N 3 −N 13 )≈47.8, where N 3  is the refractive index at 3 micrometers and N 13  is the refractive index at 13 micrometers. According to the Umicore data sheet, the indices of refraction for GASIR1 vary between lots by less than 0.0006. Thus, GASIR1 lens elements may exhibit an index of refraction N 8  of about 2.5009 to 2.5021, and Abbe number V 3-13  about 47.799 to 47.837. Although the invention may be described herein with respect to Umicore GASIR1 elements, it is to be understood that molded elements having comparable characteristics but produced by other manufacturers may be used. 
     FIG. 2  illustrates one exemplary embodiment  200  of at least a portion of a lens system  110  consistent with the present invention. The illustrated exemplary embodiment  200  includes first through sixth lens elements  202 ,  204 ,  206 ,  208 ,  210  and  212  disposed along a common optical axis O, and may be used to simultaneously image light in the MWIR and LWIR bands onto a common focal or imaging plane  214 , as shown. 
   The first  202 , second  204  and third  206  lens element may form an objective lens  218  of the system. The first lens element  202  may be a positive aspheric molded GASIR1 chalcogenide element, and the second lens element  204  may be a negative element made of germanium. The third lens element  206  may be negative element made of zinc sulfide, and may be positioned adjacent an aperture stop  216  established by the objective lens  218 . 
   A positive triplet  220  including the fourth  208 , fifth  210 , and sixth  212  lens elements may follow the aperture stop  216  and may focus light from the apertures stop onto the common focal plane  214 . The fourth lens element  208  may be a positive aspheric molded GASIR1 chalcogenide element. The fifth lens element  210  may be an element made of germanium, and the sixth lens element  212  may be an element made of zinc sulfide. 
   The lens system  200  provides a super-achromatic condition in the MWTR and LWIR bands using aspheric molded chalcogenide lens elements, which may be produced without use of expensive DPT and/or post polishing processes. The prescription for the exemplary embodiment  200  is complied in Table 1 below, with reference to the surface numbers shown in  FIG. 2 . All of the surfaces are spherical except as otherwise indicated. Aspheric surfaces are defined herein by the sag, Z, given by 
                 Z   =         cr   2       1   +       1   -       (     1   +   k     )     ⁢     c   2     ⁢     r   2               +     Ar   4     +     Br   6     +     Cr   8     +     Dr   10               (     Equation   ⁢           ⁢   1     )               
wherein c is the base curvature of at the vertex, k is a conic constant, r is the radial coordinate measured perpendicularly from the optical axis, and A, B, C and D are higher order aspheric constants.
 
   Also, in the example of Table 1, the effective focal length is 1.72 inches, the f-number (F/#) is 1.25, and the field of view half angle is 22.5 degrees. The index of refraction for molded chalcogenide lens elements ranges from about 2.489000 to 2.516400 for wavelengths from 12 to 3 micrometers. The refractive index of the germanium lens elements ranges from about 4.003900 to 4.044600 and the refractive index of the zinc sulfide lens element ranges from about 2.170071 to 2.257187 for wavelengths from 12 to 3 micrometers. Germanium and zinc sulfide lens elements exhibiting such characteristics are commercially available from Janos Technology Inc. of Keene, N.H. 
   
     
       
             
             
             
             
             
             
             
             
             
           
             
             
             
             
             
             
             
             
             
           
         
             
               TABLE 1 
             
             
                 
             
             
                 
               Radius 
               Thickness 
               Lens 
                 
                 
                 
                 
                 
             
             
               Surface 
               (Inches) 
               (Inches) 
               Material 
               K 
               A 
               B 
               C 
               D 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               252 
               1.63416 
               0.25 
               GASIR1 
               0 
               −.00944601 
               −.00872125 
               .00399527 
               −.0067282 
             
             
               254 
               3.3716 
               0.005 
             
             
               256 
               1.13328 
               0.257233 
               Germanium 
             
             
               258 
               0.84009 
               0.166561 
             
             
               260 
               6.31789 
               0.1 
               Zinc Sulfide 
             
             
               262 
               2.28327 
               0.504144 
             
             
               264 
               1.69138 
               0.562585 
               GASIR1 
               0 
               −.0555303 
               −.0478887 
               −0.221533 
               0.128622 
             
             
               266 
               −2.70443 
               0.005 
             
             
               268 
               −4.08629 
               0.980281 
               Germanium 
               0 
               0.101368 
               −0.142656 
               0.275986 
               −0.150388 
             
             
               270 
               −6.78355 
               0.117148 
                 
               0 
               .184561 
               −1.65657 
               .086152 
               .00545905 
             
             
               272 
               −2.41873 
               0.15 
               Zinc Sulfide 
             
             
               274 
               26.51385 
               0.281927 
                 
               0 
               −0.181055 
               .0790944 
               1.36345 
               −1.743600 
             
             
                 
             
           
        
       
     
   
   There is thus provided, according to one aspect of the present invention, a lens system including a plurality of lens elements including at least one molded aspheric chalcogenide lens element having an associated Abbe number V 3-13  in the range from about 47.799 to 47.837. The lens elements may be configured to simultaneously image light in a wavelength range from about 3 to about 5 micrometers and light in a wavelength range from about 8 to about 12 micrometers at a common focal plane. 
   According to another aspect of the invention, there is provided a system including: a plurality of lens elements including at least one molded aspheric chalcogenide lens element having an associated Abbe number V 3-13  in the range from about 47.799 to 47.837, the plurality of lens elements being configured to simultaneously image light in a wavelength range from about 3 to about 5 micrometers and light in a wavelength range from about 8 to about 12 micrometers at a common focal plane; a detector positioned at the common focal plane and configured to create electrical impulses in response to the light in the wavelength range from about 3 to about 5 micrometers and the light in the wavelength range from about 8 to about 12 micrometers; and a signal processing unit configured to process the electrical impulses for displaying an image. 
   According to another aspect of the invention, there is provided a method including: imaging light in a wavelength range from about 3 to about 5 micrometers and light in a wavelength range from about 8 to about 12 micrometers at a common focal plane using a plurality of lenses including at least one molded aspheric chalcogenide lens element having an associated Abbe number V 3-13  in the range from about 47.799 to 47.837; and displaying an image in response to the light at the common focal plane. 
   While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope. Various other configurations and arrangements of the disclosed embodiments will be apparent to those of ordinary skill in the art. Accordingly, the scope of the invention is determined by the claims that follow.