Abstract:
An infrared refractive lens triplet having color correction properties for radiation within the 3 to 12 micrometer spectral band for use with a quantum well detector for simultaneous dual band imagery. In certain embodiments of the invention, two widely spaced triplets form a Petzval-type objective lens. Each lens triplet is made up of a negative zinc sulfide lens, a positive zinc selenide lens, and a negative gallium arsenide lens coaxially positioned along a chief ray of the lens system.

Description:
BACKGROUND 
     1) Field of the Invention 
     The present invention relates to an objective lens system and, in particular, a Petzval objective, capable of simultaneous dual band imagery in both the 3-5 and 8-12 micrometer spectral ranges, for use with a dual spectral band detector, such as a quantum well detector. 
     2) Discussion of Related Art 
     Recent advancements in quantum well detector technologies permit the simultaneous processing and display of dual infrared band imagery with a single detector at a single focal plane. There are advantages of imaging each spectral band and the possibility of focusing both bands simultaneously may yield greater information for surveillance and/or target discrimination by imaging devices. 
     Imaging devices, search and track sensors such as forward looking infrared systems, and other types of optical devices which operate in the 3-12 micrometer spectral region use either reflective or refractive objective lens systems. An advantage of reflective optics is that they operate within a wide spectral band. However, reflective optics systems have several undesirable features, such as, for example, a more limited field of view and a large physical size. In addition, central obstructions in reflective optics limit the modulation transfer function (MTF), or sine wave response. 
     Additionally, efficient cold shielding is important with any cooled detector array. Reflective optics are not appropriate for wide field of view applications, particularly when efficient cold shielding is necessary. 
     In the past, refractive lens systems have been designed for limited ranges, e.g., either the 3-5 or the 8-12 micrometer bands. This is because the refractive indices of the component lens materials vary significantly between the two spectral bands. For example, germanium behaves like a crown (a low dispersion material) in the 8-12 micrometer wavelengths and like a flint (a high dispersion material) in the 3-5 micrometer region. Systems designed to operate in one band exhibit intolerable chromatic aberrations in the other band. The design for either long wave (8-12 micrometers) or short wave (3-5 micrometers) lenses are commonplace, but the need for simultaneous dual band operation is a relatively new requirement. 
     The number of optical materials that can transmit wavelengths in the spectral range of interest for this application is limited, and many infrared lens materials are soft or water soluble, which make them unsuitable for tactical systems. 
     Several studies have been carried out to isolate optical material combinations that yield refractive lens designs that are color corrected over the entire 3-12 micron spectral region. The following publications describe such studies: Thomas H. Jamieson, “Ultrawide Waveband Optics”,  Optical Engineering , Vol. 23, No. 2, April 1984, pages 111-116; M. O. Lidwell, “Achromatism of Lenses for Thermal IR”,  Proceedings SPIE , Vol. 518, 1984, pages 73-80; and, M. Roberts and P. Rogers, “Wide Waveband Infrared Optics”,  Proceedings SPIE , Vol. 1013, 1988, pages 84-91. 
     These publications describe optical design studies that employ many different infrared lens material combinations. The solutions described in these studies yield excellent color correction over the spectral range of 3-12 microns, but the material combinations are poor choices from the standpoint of durability, manufacturability and reliability. 
     SUMMARY 
     The present invention overcomes the problems of the prior art by providing a combination of lens materials that yield excellent broad band color correction in the 3-12 micron spectral range; that are durable, easily produced and highly reliable; and therefore that are suitable for tactical infrared imaging systems. 
     Specifically, the present invention is an optical lens system made up of at least one lens triplet of a negative zinc sulfide lens, a positive zinc selenide lens, and a negative gallium arsenide lens, each of the lenses being positioned along a chief ray for simultaneous dual band imagery in both the 3-5 and 8-12 micrometer spectral bands. Alternatively, the lens system can include two widely spaced, net positive triplets, forming a Petzval-type lens. A quantum well detector may be placed at the focal plane for dual band image detection. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be best understood by reference to the following detailed description when read in conjunction with the following drawings, wherein: 
     FIG. 1 illustrates a YZ profile of a wide band Petzval objective lens employing two widely spaced triplets in accordance with the present invention; 
     FIGS. 2 a - 2   c  are spot diagrams illustrating the performance achieved in the spectral band of 3-5 micrometers; and 
     FIGS. 3 a - 3   c  are spot diagrams illustrating the performance achieved in the spectral band of 8-12 micrometers. 
    
    
     DETAILED DESCRIPTION 
     The index of refraction of optical materials varies with wavelength. The refractive index is generally higher at shorter wavelengths so that light converges faster (the focal length is shorter) than at longer wavelengths. This spread in the focal position over a spectral range is the primary chromatic aberration called axial color. It is relatively easy to correct the axial color aberration in either the 3-5 or the 8-12 micrometer spectral range by simply combining low and high dispersion elements and dividing the magnification power of the lens system among positive and negative lens elements. Color correction of both bands is much more difficult. 
     The present inventor has discovered a unique combination of lens powers, including an aspheric surface, and materials that yield color correction for both spectral bands simultaneously. The type of lens and the materials are, in order of their appearance along a chief ray of a focal optical lens system, as follows: 
     
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Lens 
                 Material 
               
               
                   
                   
               
             
             
               
                   
                 Negative 
                 Zinc sulfide (ZnS) 
               
               
                   
                 Positive 
                 Zinc selenide (ZnSe) 
               
               
                   
                 Negative 
                 Gallium arsenide (GaAs) 
               
               
                   
                   
               
             
          
         
       
     
     Recent advances in the quantum well detectors, or any present or future detectors capable of dual spectral band imagery, give rise to a need for dual band optics such as disclosed herein so that such detectors can be utilized to their best efficiency. 
     An exemplary set of lens characteristics for a conventional quantum well detector is as follows: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Effective Focal Length (EFL) 
                 7.5″ 
               
               
                   
                 f/# 
                 2.0 
               
               
                   
                 Field of View 
                 4.0 × 4.0 degrees 
               
               
                   
                 Pixel Size 
                 0.002″ 
               
               
                   
                 Format Size 
                 0.5 × 0.5 
               
               
                   
                 Spectral Range 
                 3 to 5 μm and 
               
               
                   
                   
                 8 to 12 μm 
               
               
                   
                 Cold Shield Efficiency 
                 100% 
               
               
                   
                   
               
             
          
         
       
     
     To achieve the above characteristics, the simple lens triplet, while acceptable for some applications, does not provide optimum speed and field of view. Speed refers to the light gathering power of the lens. In commercial camera optics, this property of the lens system is indicated by the f/number. The exposure time required for a given photograph is proportional to the square of the f/number and will be shorter when a lens with a low f/number is used; thus, a lens having a higher speed reduces light gathering time. 
     Two widely spaced triplets in the form of a Petzval-type lens provide an optimum speed and field of view for the above-listed lens characteristics. A classic Petzval lens involves two spaced doublets, but the term is used here by analogy because each triplet is a net positive lens group. 
     The six element combination with two aspheric surfaces can be optimized on the ACCOSV optical design program. A resulting lens design shown in FIG.  1  and listed in Table 1 below. 
     
       
         
               
             
               
               
               
               
               
               
               
             
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
             
             
               
                 BASIC LENS DATA 
               
             
          
           
               
                   
                   
                   
                   
                   
                 Refrac. 
                   
               
               
                 Surf. 
                 Radius 
                 Thickness 
                 Medium 
                   
                 Index 
                 Label 
               
               
                   
               
               
                 0 
                 0.00000000 
                 1.50000000E+20 
                 AIR 
               
               
                 1 
                 0.00000000 
                 −18.27594933 
                 AIR 
               
               
                 2 
                 7.28700000 
                 0.33000000 
                 MATL 
                 C_ZnS 
                 2.200833 
                 10A 
               
               
                 3 
                 4.64064000 
                 0.06544000 
                 AIR 
                   
                   
                 10B 
               
               
                 4 
                 5.14400000 
                 0.98000000 
                 MATL 
                 ZnSe 
                 2.406485 
                 11A 
               
               
                 5 
                 −33.65800000 
                 0.05074000 
                 AIR 
                   
                   
                 11B 
               
               
                 6 
                 −21.27500000 
                 0.28000000 
                 MATL 
                 GaAs 
                 3.277944 
                 12A 
               
               
                 7 
                 33.65800000 
                 4.27000000 
                 AIR 
                   
                   
                 12B 
               
               
                 8 
                 21.27500000 
                 0.30000000 
                 MATL 
                 C_ZnS 
                 2.200833 
                 13A 
               
               
                 9 
                 7.05700000 
                 0.22000000 
                 AIR 
                   
                   
                 13B 
               
               
                 10  
                 5.14400000 
                 0.52500000 
                 MATL 
                 ZnSe 
                 2.406485 
                 14A 
               
               
                 11  
                 0.00000000 
                 0.21700000 
                 AIR 
                   
                   
                 14B 
               
               
                 12  
                 2.20300000 
                 0.22500000 
                 MATL 
                 GaAs 
                 3.277944 
                 15A 
               
               
                 13  
                 1.90900000 
                 1.01703000 
                 AIR 
                   
                   
                 15B 
               
               
                 14  
                 0.00000000 
                 0.08000000 
                 MATL 
                 ZnSe 
                 2.406485 
                 16A 
               
               
                 15  
                 0.00000000 
                 0.10000000 
                 MATL 
                   
                   
                 16B 
               
               
                 16* 
                 0.00000000 
                 2.50000000 
                 AIR 
                   
                   
                 17     
               
               
                 17  
                 0.00000000 
                 0.00000000 
                 AIR 
               
               
                   
               
             
          
           
               
                 CONIC CONSTANT (CC) AND ASPHERIC DATA 
               
             
          
           
               
                 Surf. 
                 CC 
                 AD 
                 AE 
                 AF 
                 AG 
               
               
                   
               
               
                 3 
                 0.00000E+00 
                 −2.5363E−04 
                 −1.59381E−05 
                 −7.91405E−07 
                 −6.71513E−08 
               
               
                 9 
                 0.00000E+00 
                 9.85522E−04 
                  3.35206E−05 
                  7.40680E−06 
                 −2.89437E−06 
               
               
                   
               
               
                 wherein AD, AE, AF and AG represent 4th, 6th, 8th and 10th order deformation coefficients, respectively and the asterisk * signifies where the aperture stop/cold shield is located.  
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
               
             
           
               
                   
               
               
                 REFRACTIVE INDICES 
               
             
          
           
               
                 Surf. 
                 N1 
                 N2 
                 N3 
                 N4 
                 N5 
                 ABBE 
               
               
                   
               
               
                 2 
                 2.200833 
                 2.212892 
                 2.186965 
                 1.000000 
                 1.000000 
                  46.316416 
               
               
                 4 
                 2.406485 
                 2.412209 
                 2.400068 
                 1.000000 
                 1.000000 
                 115.848590 
               
               
                 6 
                 3.277944 
                 3.282937 
                 3.272502 
                 1.000000 
                 1.000000 
                 218.286360 
               
               
                 8 
                 2.200833 
                 2.212892 
                 2.186965 
                 1.000000 
                 1.000000 
                  46.316416 
               
               
                 10  
                 2.406485 
                 2.412209 
                 2.400068 
                 1.000000 
                 1.000000 
                 115.848590 
               
               
                 12  
                 3.277944 
                 3.282937 
                 3.272502 
                 1.000000 
                 1.000000 
                 218.286360 
               
               
                 14  
                 2.406485 
                 2.412209 
                 2.400068 
                 1.000000 
                 1.000000 
                 115.848590 
               
               
                   
               
             
          
         
       
     
     These refractive indices correspond to the 8-12 micrometer range and are different for the 3-5 micrometer range when generating the spot diagrams of FIGS. 2 a - 2   c . 
     
       
         
               
             
               
               
               
               
               
               
             
               
             
               
               
               
               
               
             
               
             
               
               
               
               
               
             
               
             
               
               
               
               
               
               
               
             
           
               
                   
               
             
             
               
                 WAVELENGTH DESIGNS 
               
             
          
           
               
                 Wavelength # 
                 1 
                 2 
                 3 
                 4 
                 5 
               
               
                   
               
               
                 Wavelength 
                 10.00000 
                 9.00000 
                 11.00000 
                 0.00000 
                 0.00000 
               
               
                 Spectral Weight 
                 1.0000 
                 1.0000  
                 1.0000 
                 1.0000  
                 1.0000  
               
               
                   
               
             
          
           
               
                 OPERATING CONDITIONS 
               
             
          
           
               
                 Reference Object 
                 Reference 
                 Object 
                 Reference 
                 Image 
               
               
                 Height 
                 Aperture Ht. 
                 Surface 
                 Surface 
                 Surface 
               
               
                   
               
               
                 −0.7336233+19 
                 0.62393 
                 0 
                 16 
                 17 
               
               
                 (2.8000 Deg.) 
               
               
                   
               
             
          
           
               
                 APERTURE STOP AT SURFACE 15 (EN ADJUSTMENT) 
               
             
          
           
               
                 Effective Focal 
                   
                   
                   
                 Gauss Image 
               
               
                 Length (EFL) 
                 Back Focus 
                 f/Number 
                 Length 
                 Height 
               
               
                   
               
               
                 7.4911 
                 2.5000 
                 2.00 
                 8.6602 
                 0.3662 
               
               
                   
               
             
          
           
               
                 STANDARD AND BOOLEAN APERTURE (TM) DATA (CA ON) 
               
             
          
           
               
                 Surf. 
                 Type 
                 CAY 
                 CAX 
                 Y-OFFSET 
                 X-OFFSET 
                 LABEL 
               
               
                   
               
               
                 2 
                 Cir. 
                 2.54959 
                   
                 0.00000 
                 0.00000 
                 10A 
               
               
                 3 
                 Cir. 
                 2.42975 
                   
                 0.00000 
                 0.00000 
                 10B 
               
               
                 4 
                 Cir. 
                 2.43030 
                   
                 0.00000 
                 0.00000 
                 11A 
               
               
                 5 
                 Cir. 
                 2.36548 
                   
                 0.00000 
                 0.00000 
                 11B 
               
               
                 6 
                 Cir. 
                 2.36344 
                   
                 0.00000 
                 0.00000 
                 12A 
               
               
                 7 
                 Cir. 
                 2.30949 
                   
                 0.00000 
                 0.00000 
                 12B 
               
               
                 8 
                 Cir. 
                 1.43501 
                   
                 0.00000 
                 0.00000 
                 13A 
               
               
                 9 
                 Cir. 
                 1.38404 
                   
                 0.00000 
                 0.00000 
                 13B 
               
               
                 10  
                 Cir. 
                 1.37450 
                   
                 0.00000 
                 0.00000 
                 14A 
               
               
                 11  
                 Cir. 
                 1.31524 
                   
                 0.00000 
                 0.00000 
                 14B 
               
               
                 12  
                 Cir. 
                 1.08791 
                   
                 0.00000 
                 0.00000 
                 15A 
               
               
                 13  
                 Cir. 
                 0.97822 
                   
                 0.00000 
                 0.00000 
                 15B 
               
               
                 14  
                 Cir. 
                 0.68097 
                   
                 0.00000 
                 0.00000 
                 16A 
               
               
                 15  
                 Cir. 
                 0.66855 
                   
                 0.00000 
                 0.00000 
                 16B 
               
               
                 17  
                 Cir. 
                 0.37445 
                   
                 0.00000 
                 0.00000 
               
               
                   
               
               
                 Lens units are inches.  
               
               
                 Evaluation mode is focal.  
               
               
                 Control wavelength is 1.  
               
             
          
         
       
     
     Naturally, other lens characteristics can be used within the scope of the present invention. The above data is exemplary and reflects an optimum lens data for a particular quantum well detector chip. 
     In the preferred embodiments of the present invention, in each lens triplet, a first lens  10 ,  13  is negative, a second lens  11 ,  14  is positive, and a third lens  12 ,  15  is negative as taken along the chief ray. Surfaces  10 B and  13 B of the ZNS lens are selected for receiving aspheric surfaces. ZnS is a diamond turnable lens material which greatly eases manufacturing requirements. Also, of the manufacturing methods available for making ZnS lenses, the Cleartran™ method provides a better, more homogenous material (C_ZnS) which is clearer in the 3-5 micrometer range than produced by some other methods. 
     As illustrated in FIG. 1, parallel light impinging on the Petzval lens system is focused on an image plane  18 . A quantum well detector  19  can be placed at the image plane  18 . A cryogenic vessel or Dewar houses the quantum well detector  19  to maintain the detector  19  at a temperature of approximately 77° Kelvin in order to increase detector sensitivity. The cold shield  20  of the Dewar is shown. 
     In these types of systems, optimum performance is achieved by cold shielding the detector array. This is done by introducing a cold diaphragm, or cold shield  20 , with an aperture stop  17  in front of the detector array so that the viewing angle of the detector to the warm background is limited as nearly as possible to only that required for transmission of radiation from the scene. Since the amount of radiation reaching the detector from the cold shield  20  is negligible compared to the amount reaching the detector from the warm background of the field of view, induced noise in the detector is minimized. The cold shield  20  is typically placed within the Dewar. Also included in the optical system may be a Dewar window  16 . 
     The performance achieved in each spectral band is shown by the spot diagrams in FIGS. 2 a - 2   c  and  3   a - 3   c . Rays representative of three wavelengths in each of the 3 to 5 and 8 to 12 spectral bands from an on-axis and two off-axis field points are distributed through the full aperture and are traced through the system, producing FIGS. 2 c ,  2   a  and  2   b , respectively. In FIG. 2 a , the FOB-Y is 1.0000 and the FOB-X is 0.0000; in FIG. 2 b , the FOB-Y is 0.7000 and the FOB-X is 0.0000; in FIG. 2 c , the FOB-Y is 0.0000 and the FOB-X is 0.0000; in FIG. 3 a , the FOB-Y is 1.0000 and the FOB-X is 0.0000; in FIG. 3 b , the FOB-Y is 0.7071 and the FOB-X is 0.0000; and in FIG. 3 c , the FOB-Y is 0.0000 and the FOB-X is 0.0000, wherein FOB stands for fractional object height. FOB refers to the object point (or field angle when the object is at infinity) from which rays are traced. 
     As shown in FIGS. 2 a - 2   c  and  3   a - 3   c , the square in phantom is the pixel size of a typical quantum well detector, e.g., 50.8×50.8 micrometers (0.002 inches). In FIGS. 2 a - 2   c  and  3   a - 3   c , all spot diagram origins are on the chief ray. Since virtually all of the ray intercepts are contained within the pixel, the image resolution will be limited by such a detector. 
     By the above disclosed imaging system, the optimum capability of a quantum well detector can be utilized. A quantum well detector&#39;s capability could go under-utilized without an optical system such as disclosed herein that can simultaneously deliver the image quality required over both the 3 to 5 μm and 8 to 5 μm spectral bands. The lens system in accordance with the present invention yields excellent broadband color correction by a combination of infrared lens material suitable for tactical applications. 
     Certain preferred embodiments of the invention have been described by way of example. Various other embodiments and components thereof will be apparent, particularly in applying the lens triplets to perform various functions. Various parameters may be optimized for different purposes. For example, a beam splitter may be introduced into the lens system to split an image into the 3-5 μm range and the 8-12 μm range, whereupon the lens powers may vary and color correction requirements loosened because of the using more than one focal plane. The lens system can be useful with detectors capable of imaging either the 3 to 5 μm or 8 to 12 μm spectral bands, and an imaging system can be envisioned wherein high and low spectral band detectors can be selectively placed at the focal plane(s), by use of a beam splitter or employing a detector change-over mechanism. Naturally, optimum performance is achieved when employing a single detector capable of detecting both the 3 to 5 μm and 8 to spectral bands. The foregoing description is offered as illustrative and not limitative. Other variations and equivalents thereof are intended to be encompassed by the invention, which is defined by the claims appended hereto.