Abstract:
An optical system has an optical component with a curved surface and zero optical power. The optical system is configured so that the optical component is the first optical structure encountered by radiation entering the optical system from externally thereof. According to a different aspect, a method is provided for making an optical system that has an optical component with a surface, where the optical component is the first optical structure encountered by radiation entering the optical system from externally thereof. The method includes configuring the surface to be curved, and configuring the optical component to have zero optical power.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates in general to optical systems and, more particularly, to techniques for reducing or adjusting reflections from an optical surface of an optical system. 
       BACKGROUND 
       [0002]    Over the years, various techniques have been developed to help a person accurately aim a weapon such as a rifle. One common approach is to mount a sight or scope on the weapon. A person then uses the sight or scope to view an image of a scene that includes an intended target. 
         [0003]    In many weapon sights, the first optical surface encountered by radiation entering the sight is a flat surface. When a beam of radiation such as sunlight is traveling along a path and encounters this flat surface, some portion of that radiation is typically reflected, and all of the reflected radiation will then travel away from the surface in the same direction. In a hunting situation, this glint may startle an animal being hunted, such as a deer. In a combat situation, the glint may alert an enemy combatant to the presence of the soldier using the weapon. 
         [0004]    Moreover, if a person holding the weapon sight pivots the weapon with the sight thereon, the reflected radiation will be swept through an angle twice that of the angle of weapon movement. This increases the size of a remote region where the glint from the reflection is likely to be noticed. 
         [0005]    In an attempt to reduce reflections of this type, various arrangements have previously been utilized, such as antireflection coatings, shades and baffles. Although these known techniques have been generally adequate for their intended purposes, they have not been satisfactory in all respects. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    A better understanding of the present invention will be realized from the detailed description that follows, taken in conjunction with the accompanying drawings, in which: 
           [0007]      FIG. 1  a diagrammatic perspective view of an apparatus that embodies aspects of the invention, and that includes a tripod, a weapon, and a weapon sight. 
           [0008]      FIG. 2  is a diagrammatic sectional top view of the weapon sight of  FIG. 1 . 
           [0009]      FIG. 3  is a diagram showing two positions of a conventional optical component with a flat outer surface. 
           [0010]      FIG. 4  is a diagram similar in some respects to  FIG. 3 , but showing two positions of a zero-power window that is a component of the weapon sight of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0011]      FIG. 1  a diagrammatic perspective view of an apparatus  10  that embodies aspects of the invention, and that includes a tripod  12 , a weapon  13 , and a weapon sight  14 . In the disclosed embodiment, the tripod  12  and the weapon  13  are devices of a type known in the art, and are therefore discussed here only briefly. More specifically, the illustrated weapon  13  is a firearm, and in particular a rifle, but it could alternatively be some other type of weapon. The rifle  13  is removably mounted on an upper portion of the tripod  12 , and the upper portion of the tripod  12  with weapon  13  thereon can move relative to the lower portion of the tripod, including pivotal movement about an approximately vertical axis  18 . 
         [0012]    The weapon sight  14  is removably mounted on the rifle  13  by a mount  26 . The mount  26  cooperates with the rifle  13  in a manner that is known in the art, and that is therefore not described here in detail. 
         [0013]      FIG. 2  is a diagrammatic sectional top view of the weapon sight  14 . The sight  14  includes a tubular housing  41  that is open at both ends. For simplicity, the housing  41  is depicted in  FIG. 2  with a cylindrical shape, but it could alternatively have any of a variety of other shapes. At the front end, the housing  41  has coupling structure in the form of internal threads  43 . An optical system  46  is mounted within the housing  41 , and is represented diagrammatically in  FIG. 2  by broken lines. Optically, the system  46  functions as a weapon sight that can be used to aim the rifle  13  ( FIG. 1 ). The optical system  46  is an arrangement of a known type, and is therefore not illustrated and described here in detail. The optical system  46  includes some optical components, such as lenses, prisms, mirrors, beam splitters and/or other optical components. Two of these optical components are represented diagrammatically in  FIG. 2  by broken lines  48  and  49 . 
         [0014]    The sight  14  further includes an auxiliary part  56  that is detachably coupled to the front end of the housing  41 . The auxiliary part  56  includes a housing  58  in the form of a cylindrical sleeve. The housing  58  has coupling structure at the rear end, in the form of external threads  59 . The external threads  59  engage the internal threads  43  of the housing  41 , in order to detachably secure the auxiliary part  56  to the housing  41 . 
         [0015]    The auxiliary part  56  includes a window  71  that is fixedly mounted within the housing  58 , at the front end thereof. The window  71  is curved, but is configured so that it has no optical power with respect to radiation of interest that passes through it. The curved window  71  has an outer surface  72  that is curved, and an inner surface  73  that is also curved. In the disclosed embodiment, the outer and inner surfaces  72  and  73  are each approximately spherical, but each could alternatively be aspherical. In the disclosed embodiment, the spherical surface  72  has a radius  74 , and a centerpoint that lies on or near the approximately vertical pivot axis  18  of the tripod  12  ( FIG. 1 ). The surface  73  also has a centerpoint that lies on or near the approximately vertical pivot axis  18  of the tripod  12 . The window  71  may optionally be made from a material that filters out certain wavelengths of radiation. For example, it may be a material that filters out wavelengths corresponding to laser energy, in order to help protect the eye of a user from being damaged by an incoming laser beam. 
         [0016]    In the disclosed embodiment, a thin coating  81  is provided on the outer surface  72  of the window  71 . In  FIG. 2 , the coating  81  is an antireflection (AR) coating of a type known in the art. Alternatively, the coating  81  could be a filter layer of a known type that includes a plurality of sub-layers and that filters out a selected waveband of radiation, such as laser energy. As still another alternative, the coating  81  could include an antireflection layer as well as layers that effect filtering. A similar coating could optionally be provided on the inner surface  73 . 
         [0017]    In operation, when rays  101 - 104  of radiation from the sun or some other source are traveling in approximately the same direction and impinge at various spaced locations on the outer surface  72  of the window  71 , the curvature of surface  72  will cause any reflections  106 - 109  to be scattered, and to travel away from the surface  72  in respective different directions. Consequently, the amount of reflected energy that reaches a person or animal at a remote location is less than would be the case if the surface  72  was flat and directed all reflected radiation in the same direction toward the person or animal. Most of the incoming radiation at  101 - 104  will pass through the outer surface  72  and through the window  71 , and some of it may be reflected by the curved inner surface  73 . The considerations that apply to the outer surface  72  also apply to the inner surface  73 . In particular, the curvature of the inner surface  73  will cause any reflections to be scattered, and to travel away from the surface  73  in respective different directions. Consequently, the amount of reflected energy that reaches a person or animal at a remote location is less than would be the case if the inner surface  73  was flat and directed all reflected radiation in the same direction toward the person or animal. 
         [0018]    A further consideration is that, since the outer and inner surfaces  72  and  73  are each configured to have a centerpoint on or near the axis  18 , if the rifle  13  ( FIG. 1 ) and its weapon sight  14  are pivoted on the tripod about the axis  18 , there will be minimal alteration in the reflection characteristic of the radiation  101 - 104  reflected by the outer surface, or the radiation reflected by the inner surface. Consequently, in contrast to traditional flat surfaces, any glint traveling from either curved surface  72  or  73  to an animal or person at a remote location will be less noticeable than in the case of reflections from a flat surface. 
         [0019]    For example,  FIG. 3  is a diagram showing a conventional optical component  201  with a flat outer surface  203 . When the component  201  is in the position shown in solid lines, a ray of radiation represented by an arrow  211  will be reflected, and will then travel away from the component in a manner represented by arrow  212 . In contrast, if the component  201  is pivoted about a pivot point  206  from the position shown in solid lines to the position shown in broken lines, the same ray of radiation  211  would be reflected in a substantially different direction, as represented by the arrow  214 . In fact, as the component is pivoted from the solid line position to the broken line position, the direction of the reflection will progressively sweep from the direction indicated at  212  to the direction indicated at  214 . As the reflected radiation is sweeping through this angle, if it happens to sweep past a person at a remote location, the change in intensity of the radiation at that remote location will represent a highly noticeable glint that is very likely to attract the attention of the person. 
         [0020]      FIG. 4  is a diagram similar to  FIG. 3 , but showing the zero-power window  71  of  FIG. 2 , which has the approximately spherical outer surface  72 . When the window  71  is in the position shown in solid lines in  FIG. 4 , the ray of radiation represented by arrow  211  will be reflected, and will then travel away from the window  71  in a manner represented by arrow  221 . If the window  71  is pivoted about the pivot axis  18  from the position shown in solid lines to the position shown in broken lines, the same ray of radiation  211  would be reflected in the same direction, or substantially the same direction, as represented diagrammatically by the arrow  222 . The arrow  222  is substantially coincident with the arrow  221 . Thus, as the window  71  is pivoted from the solid line position to the broken line position, the direction of reflection for radiation  211  will under go little or no change. Consequently, as the window  71  is moved, a person at a remote location would see little or no significant change in the intensity of reflected radiation. Consequently, the person would not be exposed to a noticeable glint that would be highly likely to attract the attention of that person. 
         [0021]    The foregoing discussion is directed to an embodiment ( FIG. 1 ) that pivots about a pivot axis  18  of a weapon support such as the tripod  12 . Alternatively, however, in a situation where it is expected that the rifle  13  will be supported manually, rather than by the tripod  12 , pivotal movement of the rifle  13  and the sight  14  could occur about a roughly vertical axis extending approximately through the head and torso of a person using the rifle, rather about than the pivot axis  18 . In that case, the outer surface  72  could have a curvature with a centerpoint located in the region of the head or torso of the user, rather than on or near the pivot axis  18  of the tripod. 
         [0022]    Although one selected embodiment has been illustrated and described in detail, it should be understood that a variety of substitutions and alterations are possible without departing from the spirit and scope of the present invention, as defined by the claims that follow.