Patent Publication Number: US-4322124-A

Title: Low cost wide field of view infrared sensor

Description:
BACKGROUND OF THE INVENTION 
     Wide field of view infrared sensors have a number of useful applications. One such application is in an electro-optical helmet sight system, wherein an infrared sensor is mounted in a helmet for detecting infrared radiation impinging on the sensor. Although high image forming quality is generally not required, it is desirable that the sensor have a relatively uniform response over at least a hemisphere. The currently available sensors fall short of this requirement. Some prior art sensors provide a wide field of view in a first axis, but not in a second axis. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved infrared sensor with an increased and more symmetric field of view. It accomplishes this by use of a single element lens and thus also results in a lower cost. The sensor includes a detector element which in the preferred embodiment is a quartz substrate with PbS coating on one side and a long wavelength pass filter coating on the other side. A single element silicon negative meniscus lens, coated on both sides with SiO is mounted over the detector element such that infrared radiation impinging on the lens from any direction within a wide angle is appropriately bent by the lens and directed towards the detector. The lens is designed so as to take advantage of the optical principle known as &#34;pupil distortion&#34;. The result is that the area of the detector as seen through the lens remains essentially constant over a very large angular field; thus the cosine losses normally experienced with planar detectors are effectively eliminated. 
     It is therefore an object of the present invention to provide an infrared sensor with a wide field of view. A further object of the present invention is to provide a wide field of view infrared sensor with axial symmetry and having a single lens element. 
    
    
     These and further objects will become apparent to those skilled in the art upon examination of the following specification, claims, and drawings, in which: 
     FIG. 1 shows a cross-section infrared sensor according to the present invention; and 
     FIG. 2 is a representation of infrared radiation received by the sensor from different angles to the normal. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a cross section of an infrared sensor according to the present invention is shown. Quartz substrate 10 is mounted by a sensor mount 11 on a header element 12. A lead sulfide coating constituting an infrared detector 14 is applied to the lower surface of quartz substrate 10, while a 2.4 to 3.0 micron optical filter 15 is positioned on the upper surface of quartz substrate 10. The lead sulfide coating is sensitive to infrared radiation and generates an electric signal in response thereto. Sensor leads 17 and 18 provide means for electric current conduction through header element 12 and are electrically connected to opposite ends of lower surface of quartz substrate 10 and in contact with detector 14. Sealant material 19 surrounds leads 17 and 18 on the detector side of header element 12 to provide a hermetic seal. 
     A negative meniscus lens 25 is held by a lens holder 20 in a fixed relationship to detector 14, such that detector 14 is positioned substantially normal to and centered on the lens axis. Header element 12, lens holder 20, and lens 25 comprise a hermetically sealed container enclosing the infrared detector/filter assembly. 
     The internal surface of lens 25 nearest the detector/filter assembly has a curvature of a relatively smaller radius as compared to the radius of curvature of the external surface of lens 25. The center of curvature of both lens surfaces lie on the side of the lens nearest detector 14 along the axis intersecting lens 25 and detector 14. The ratio of the two radii is approximately two to one. 
     In the preferred embodiment, the radius of curvature of the internal surface of lens 25 is 4.04 millimeters, while the radius of curvature of the external surface is 7.50 millimeters. Lens 25 is constructed of silicon (index of refraction 3.439 at 2.55 microns) and coated on both the internal and external surfaces with an anti-reflection coating of silicon monoxide (index of refraction 1.78 at 2.55 microns). 
     FIG. 2 illustrates how the light rays impinging on lens 25 from various angles with respect to the lens axis are bent by lens 25 and directed onto detector 14. The apparatus shown in the preferred embodiment of FIG. 1 yielded experimental results shown in Table 1 below. 
     
                       TABLE 1                                                     
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α, Deg                                                              
          Volts       α, Deg                                        
                                  Volts                                   
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 0        1.00         0          1.00                                    
20        1.08        -20         1.10                                    
40        1.20        -40         1.15                                    
60        1.50        -60         1.25                                    
80        1.90        -80         1.45                                    
90        1.30        -90         1.40                                    
100       0.80        -100        0.90                                    
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     Table 1 represents the voltage amplitude of the signal appearing between leads 17 and 18 in response to a constant intensity infrared light source positioned at various angles with respect to the axis of lens 25. It can be seen that the sensor produced a wide field of view response, in accordance with the desired objective of the present invention. 
     A unique and improved wide field of view infrared sensor has been shown and described in the foregoing specification. Various modifications of the inventive concepts will be obvious to those skilled in the art, without departing from the spirit of the invention. It is intended that the scope of the invention be limited only by the following claims.