Abstract:
A detector having a field of view in elevation on the order of one hundred eighty degrees in one plane and three hundred sixty degrees in a perpendicular plane includes a generally hemispherical lens in combination with an optical frustum. The combination directs incident radiant energy within the field of view onto a centrally located sensor.

Description:
FIELD 
       [0001]    This invention pertains to optical sensors. More particularly, the invention pertains to such sensors that have fields of view on the order of one hundred eighty degrees and are responsive to incident radiant energy. 
       BACKGROUND 
       [0002]    The Multiple Integrated Laser Engagement System (MILES) employs a suite of detectors on each target. The target may be an individual or a vehicle. The Individual Weapon Systems (IWS) include a vest employing 8 detectors, and a headband employing 4 detectors. The vehicle systems use one or more belts employing up to 8 detectors in each. In both cases the requirement is that the suite of detectors covers 360 degrees in azimuth. 
         [0003]    The transmitted laser energy is kept as low as possible to minimize the eye-damage hazard. Because of this, the detectors need to be as sensitive as possible, within limits set by cost, size, weight, complexity, and downward compatibility in sensitivity. 
         [0004]    Most of the MILES detectors in use today have evolved little since the inception of the MILES program several decades ago. These detectors employ a silicon active element behind a slightly-curved plastic cover, with an electromagnetic interference (EMI) filter interposed between the active element and the cover. The detector assembly is hermetically sealed to exclude the environment. 
         [0005]    The silicon active element is specified to be 1+/−0.2 square centimeters in area. These detector assemblies are about 1⅝ inches in diameter and ⅝ inches high. They are hermetically sealed in a relatively heavy metal can having a glass window. 
         [0006]    Because of the detector geometry and the optical properties of the cover, the field of view of the detectors is limited. Typical coverage in elevation is on the order of 60 degrees from a line normal to the detector. Detector sensitivity falls to 50 percent at 45 degrees from the normal. This means that at least four detectors are required on the headband to have 360 degrees of coverage with equal sensitivity in all azimuthal directions. 
         [0007]    For the vest, four detectors in a square array are on the front of the individual, and another four detectors are on the individual&#39;s back, leaving azimuthal zones of 90 degrees on the right and left sides of the individual where the sensitivity falls to zero directly right and left. 
         [0008]    There is thus an on-going need for detectors which can provide better coverage to thereby address above noted problems of known detectors. Preferably improved coverage could be achieved while at the same time reducing the total number of detectors that is needed. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a side elevational view sectioned along an axis of symmetry of a detector in accordance with the present invention; 
           [0010]      FIG. 2  is a side view of portions of the detector of  FIG. 1  illustrating deflections of incoming radiant energy from a direction on the order of ninety degrees to a central axis of the detector; 
           [0011]      FIG. 3  is a top plan view of one form of a sensor usable with the detector of  FIG. 1 ; and 
           [0012]      FIG. 4  illustrates a simulation system which incorporates detectors in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    While this invention is susceptible of embodiment in many different forms, there are shown in the drawing and will be described herein in detail specific embodiments thereof with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the specific embodiments illustrated. 
         [0014]    In accordance with the invention, a detector has a 180 degree field of view (FOV) in elevation of effectively equal sensitivity in all azimuthal directions. This structure enables the use of two rather than four detectors on the headband, provides 360 degree, equal-sensitivity, azimuthal coverage by the vest, and reduces the number of detectors required on the vehicle systems. 
         [0015]    Detectors which embody the invention are advantageously low-cost, small, light-weight and simple in design. An EMI shield and hermetic seal are preferably included. 
         [0016]    One embodiment of a wide-FOV detector includes a hypohemispherical lens and an approximately 30 to 40 degree-to-the-symmetry-axis frustum of a cone in close proximity to a silicon detection element. An EMI shield is preferably interposed in the optical path, between the lens and the frustum. Preferably the lens has a thickness of about ¾ of the radius of a hemisphere. 
         [0017]    The combination of a hypohemispherical lens and a frustum causes incident laser light from all angles to be refracted and reflected onto a silicon detector element. The sloping surface of the frustum is silvered or otherwise made highly reflective to the laser light. As a result, in the worst case, light that is adjacent to and parallel to the mounting surface will be refracted and reflected toward the detector element. 
         [0018]    Light incident at smaller angles to the normal is refractively directed onto the detector element. The frustum is added to the lens so that all light rays are as nearly normal to the two (frustum and silicon) air/interface surfaces as possible. Light loss due to reflection at the interfaces is thereby reduced to a minimum. 
         [0019]    The index of refraction of the silicon detection element is about 3.5, and that of low-cost plastic or glass optical materials suitable for use in fabricating the lens and frustum is about 1.5. Because of these large differences in indices of refraction a high percentage (equal to 32 percent at normal incidence and rising to 100 percent at 90 degrees to the normal) of the incident light would be reflected away from entering the detector element. If the indices of refraction of the lens and detector were the same the two elements could be bonded together with an index-matching adhesive, and the light loss due to reflection would be eliminated. However, there are no practical materials having an index of 3.5 that are available for use as the lens and frustum, or as the index-matching adhesive. 
         [0020]    Appropriate anti-reflection (AR) coatings can be applied to the air-spaced facing surfaces of the frustum and silicon, or, if the frustum were to be bonded to the silicon an AR coating could be interposed between the two to reduce the otherwise 16 percent interfacial light loss at normal incidence. AR coatings can reduce the reflectivity to less than a percent, particularly if they are designed for use at a single wavelength, as would be the case where incoming radiant energy was emitted as monochromatic light by a laser source in a MILES-type system. AR coatings provide the best anti-reflective properties at normal incidence. 
         [0021]    In the present application only light collection, and not image-forming is required. Hence, any unequal distribution of the light rays on the light sensing element is not a concern. 
         [0022]    The lens and frustum could be injection-molded of acrylic, polycarbonate or other transparent optical plastics. The two elements can be cemented together using an index-matching adhesive. 
         [0023]    The lens and/or frustum could also be made of glass. Glass lenses would be more resistant to scratches resulting from rough field use than plastic lenses. However, since the MILES detectors are light gatherers rather than image-making devices, the presence of scratches merely creates slight vignetting of the incoming energy, rather than creating undesired image distortion and/or artifacts. 
         [0024]    A thin, open grid, metallic EMI screen can be located between the hypohemispheric lens and the frustum. It would be advantageous to chemically or otherwise deposit the screen on the optical element to minimize its thickness, and thereby minimize vignetting of near ninety degree incoming light. The screen needs to be electrically insulated from the silicon element so that the silicon is not short-circuited. The EMI screen can wrap around the edges of the frustum to complete a Faraday-cage EMI shield to the base. 
         [0025]    A sun-blocking filter can be provided by doping the plastic lens material with an appropriate dye. If glass is used for the optical material, a thin optical filter can be located between the lens and the light sensing element. 
         [0026]    The light sensing element is preferably round to match the geometry of the optics with an active diameter of about 1.13 centimeters. The active diameter of the light sensor must be equal to that of the extreme-angle light rays emerging from the frustum base in order to capture the maximum amount of incoming light at all angles. 
         [0027]    In one aspect of the invention, the diameter of the optics/detector assembly can be determined so that the effective sensitivity is the same as that of known detectors. This will insure downward compatibility with existing MILES systems. Alternatively, the assembly can be enlarged to increase the effective sensitivity, thus reducing the required laser energy and reducing laser eye hazards. 
         [0028]    In another aspect the total assembly can be mechanically captured and retained by means of a thin metal frustum. The frustum fits snugly over, and is cemented to, the optical frustum. This provides the conductive base for the assembly to complete the EMI Faraday-cage enclosure, as well as the hermetic seal. The resultant total assembly will be approximately one inch in diameter, and about one-half inch high. Detectors in accordance with the invention are much smaller, and lighter than most of the known MILES detectors. 
         [0029]    Incident radiant energy is captured by the effective area of the detector optics in response to a given energy density created at the target by a laser transmitter. Because of its geometry a detector in accordance with the invention will present slightly more energy-capture area for incoming laser light at normal incidence than at +/−90-degrees. However, at 90 degrees at least two of the detectors can be expected to be responding simultaneously which minimizes this effect. 
         [0030]      FIG. 1 , a side section view of detector  10  in accordance with the invention (taken along a central axis A) illustrates various elements thereof. The detector  10  incorporates a generally hemispherical lens  12 , which in a preferred embodiment is a hypohemispherical lens. 
         [0031]    The lens  12  terminates at a frustum of a cone  14  which carries a reflective surface  14 ′. Preferably the frustum will be oriented at an angle on the order of 30 to 40 degrees relative to the axis A. 
         [0032]    The frustum  14  is located adjacent to a preferably circular silicon sensing element  16 . The element  16  is preferably symmetrical relative to the axis A and converts incoming radiant energy, for example monochromatic incident laser light, to electrical signals which can be sensed via front and rear electrodes  20   a ,  20   b.    
         [0033]    An EMI emissions filter  22  is preferably located between the lens  12  and sensor  16 . An exterior mechanical frustum of a cone  26  can be electrically coupled to the filter  22  as well as to a metallic base plate  28  to form an EMI excluding chamber for the sensor  16 . The filter or shield  22  can be formed as a metallic screen. 
         [0034]    Circuitry  30 , as would be understood by those of skill in the art, can be coupled to connectors  32   a  and  32   b  which are electrically coupled to the respective front and rear electrodes  20   a,b . The detector  10  can include a housing  36 , indicated in phantom, and can be coupled to other local MILES circuitry via signal wires  38 . The base plate  28  can be attached to the housing  36  via connectors such as screws or rivets through mounting holes  40   a ,  40   b.    
         [0035]    It will be understood that the detector  10  could be used with other types of systems or circuitry without limitation. Those of skill will also understand that the detector  10  could be used with incident radiant energy of a variety of wavelengths and which includes one or more wavelengths without limitation. 
         [0036]    The lens  12  in combination with frustum  14  directs incident radiant energy, such as monochromatic light, from all angles by refraction and reflection onto the sensing element  16 . The lens/frustum combination  12 ,  16  produces a field of view on the order of 180 degrees in elevation. Incident light which is parallel to the metallic base  28  can be refracted and reflected onto the detector element  16  as discussed subsequently relative to  FIG. 2 . 
         [0037]    As illustrated in  FIG. 2 , incident radiant energy R which is generally perpendicular to the axis of symmetry A is refracted by the lens  12  toward the frustum  14  from whence it is reflected onto the sensor  16 . Incident radiant energy parallel to the axis A or at a lesser angle than 90 degrees relative to the axis A may only be refracted by the lens  12  onto the sensing element  16 . 
         [0038]    In the detector  10 , slight losses due to reflection at the interfaces between the lens  12  and frustum  14  can be reduced to a minimum by cementing them together with an index-matching adhesive. Light losses due to reflection at the frustum and silicon air/interface surfaces are reduced to a minimum because the incoming light rays are as nearly normal to the surfaces as possible. If desired, appropriate anti-reflective coatings can also be applied to the air-space facing surfaces of the frustum  14  and the sensor  16 . If the frustum  14  is bonded to the sensor  16  an antireflective coating can be interposed between the two to reduce light losses of the interface at normal incidents. 
         [0039]    Where the lens  12  is formed of plastic, a sun-blocking filter can be provided by incorporating an appropriate dye into the resin. Where glass is used for the optical material for the lens  12  an optical filter can be located between the lens  12  and the sensor  16 . 
         [0040]    As illustrated in  FIG. 2 , the light rays incident from the right of the detector  10  impinge on the left edge of the sensor  16 , see rays R- 1  of  FIG. 2 . The reverse is also true for parallel light beams incident and arriving from the left side of the lens  12 . Rays incident on the top of the lens  12  arrive at a central portion of the sensor  16 . 
         [0041]    The detector  10  can determine the direction from which incident radiant energy, for example laser light, is coming using a multi-element sensor  16 ′ as illustrated in  FIG. 3 . The sensor  16 ′ incorporates a central portion  16 - 1  which receives light rays or radiant energy generally incident on the top of the lens  12 . 
         [0042]    Incoming radiant energy from greater angles relative to the axis A, as illustrated in  FIG. 2 , for example, will be incident upon separate sensor surfaces such as  16 - 2 ,  16 - 3 ,  16 - 4  or  16 - 5 . If desired the elements  16 - 2 ,  16 - 3 ,  16 - 4  and  16 - 5  can all extend accurately across a common angle relative to the axis A. Thus, incident radiant energy R- 1  which impinges upon sensor region  16 - 2  can be recognized by circuitry  30  as incoming generally from the right side of the detector  10 . Similarly radiant energy incident upon the sensor region  16 - 3  can be determined via circuitry  30  as generally incoming from the left side of the detector  10 . Similar comments apply to incident radiant energy impinging upon regions  16 - 4 ,  16 - 5 . 
         [0043]      FIG. 4  illustrates usage of detectors, such as the detector  10 , in a MILES-type simulation system  50 . In the disclosed system  50  individuals I 1 , I 2  are participating in the simulation and wear or carry first and second detectors  52   a,b,    54   a,b  corresponding to the detector  10 . The detectors  52   a,b  each have a field of view in elevation on the order of 180 degrees in all azimuthic directions. As illustrated, the individual I 1  need only carry two detectors. Similar comments to the configuration of detectors  54   a,b  carried by the individual I 2 . Vehicles participating in the simulation indicated generally at  60 - 1 ,  60 - 2 ,  60 - 3 , and  60 - 4  could carry four detectors corresponding to the detector  10  each of which would have a 180 degree field of view in elevation in all asymmetric directions. 
         [0044]    From the foregoing, it will be observed that numerous variations and modifications may be effected without departing from the spirit and scope of the invention. It is, to be understood that no limitation with respect to the specific apparatus illustrated herein is intended or should be inferred. It is, of course, intended to cover by the appended claims all such modifications as fall within the scope of the claims.