Abstract:
A hollow retroreflector includes three mutually perpendicular reflective plates each having at least one hole on a surface thereof and at least three threaded fasteners, such as screws or bolts. One of the threaded fasteners passes though the holes of each adjoining pairs of reflective plates. Beneficially, epoxy is applied to at least one surface of each adjoining pair of reflective plates before passing the threaded fastener through the holes of each adjoining pair of reflective plates. The hollow retroreflector may be incorporated into a hollow retroreflector assembly by passing at least one threaded fastener through a hole in a hollow retroreflector mount and through one of the holes in one of the reflective plates.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001]     This patent application claims priority under 35 U.S.C. § 119(e) from U.S. Provisional patent application 60/702,663 filed Jun. 27, 2005 the entirety of which is hereby incorporated by reference herein for all purposes as if fully set forth herein. 
     
    
     BACKGROUND AND SUMMARY  
       [0002]     1. Field  
         [0003]     This invention pertains to the field of retroreflectors, more particularly the field of hollow retroreflectors, and more specifically a hollow retroreflector capable of tolerating extreme temperatures without catastrophic failure or substantial degradation in optical performance.  
         [0004]     2. Description  
         [0005]     Hollow retroreflectors, consisting of three plates having optically flat reflective surfaces disposed at right angles to each other, are well known in the art. In general, hollow retroreflectors return reflected light along a parallel path to incident light. In insuring this performance, the relative perpendicularity of the reflective surfaces needs to be maintained. Hollow retroreflectors are precision optical devices that are often required to maintain very precise performance tolerances over a wide variety of environmental conditions. Such devices may be sold and delivered as standalone devices, but are often sold and delivered as hollow retroreflector assemblies where a hollow retroreflector is mounted to a retroreflector mount in order to minimize the affect of external stresses that the retroreflector may be subject to in operation.  
         [0006]     Existing hollow retroreflectors and hollow retroreflector assemblies are assembled by gluing three glass mirrors together so that each mirror is perpendicular (at 90 degrees) to the adjoining mirrors. Epoxy is the only feature which holds such a hollow retroreflector together. Such existing hollow retroreflectors and hollow retroreflector assemblies are quite sensitive to temperature variations. Adhesives generally considered acceptable for such hollow retroreflector assemblies are not suitable for high temperature use because the epoxy ingredients begin to breakdown at temperatures between 150 and 200 degrees F. When such a hollow retroreflector is subjected to temperatures of 200 degrees F. or above, the adhesive degrades and optical performance is completely lost. Often, the adhesive will loose all qualities and the mirrors will fall apart causing catastrophic failure.  
         [0007]     Furthermore, hollow retroreflectors made of glass mirrors can break when they experience loads beyond that which the glass and glue can withstand.  
         [0008]     Accordingly, it would be advantageous to provide an improved hollow retroreflector capable of tolerating extreme temperatures without catastrophic failure or substantial degradation in optical performance. It would also be advantageous to provide an improved hollow retroreflector assembly capable of tolerating extreme temperatures without catastrophic failure or substantial degradation in optical performance. It would further be advantageous to provide a method of making such a retroreflector and retroreflector assembly. Other and further objects and advantages will appear hereinafter.  
         [0009]     The present invention comprises a retroreflector, retroreflector assembly, and method of making a retroreflector and retroreflector assembly.  
         [0010]     In one aspect of the invention, a hollow retroreflector comprises three mutually perpendicular reflective plates each having at least one hole in a surface thereof; and at least three threaded fasteners, each one of the threaded fasteners passing though the holes of an adjoining pair of the three mutually perpendicular reflective plates.  
         [0011]     In another aspect of the invention, a hollow retroreflector assembly comprises a hollow retroreflector, including three mutually perpendicular reflective plates each having at least one hole in a surface thereof, and at least three threaded fasteners, each one of the threaded fasteners passing though the holes of an adjoining pair of the three mutually perpendicular reflective plates; and a hollow retroreflector mount having a hole passing therethrough, wherein at least one of the screws or bolts passes through the hole in the mount and through the at least one hole of one of the reflective plates.  
         [0012]     In yet another aspect of the invention, a method of making a hollow retroreflector comprises arranging three reflective plates, each having at least one hole in a surface thereof, to be mutually perpendicular to each other to form three adjoining pairs of reflective plates; and passing a threaded fastener though the holes of each adjoining pair of reflective plates.  
         [0013]     In still another aspect of the invention, a method of making a hollow retroreflector assembly comprises arranging three reflective plates, each having at least one hole in a surface thereof, to be mutually perpendicular to each other to form three adjoining pairs of reflective plates; and passing a threaded fastener though the holes of each adjoining pair of reflective plates, wherein at least one of the threaded fasteners is also passed through a hole in a hollow retroreflector mount.  
         [0014]     In yet a further aspect of the invention, A method of making a hollow retroreflector assembly comprises arranging three reflective plates, each having one or more holes in a surface thereof, to be mutually perpendicular to each other to form three adjoining pairs of reflective plates; passing a first threaded fastener though at least one of the holes of each adjoining pair of reflective plates; and passing a second threaded fastener through at least a second one of the holes in one of reflective plates and through a hole in a hollow retroreflector mount. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  shows a front view of a retroreflector assembly;  
         [0016]      FIG. 2  shows a side view of the retroreflector assembly of  FIG. 1 ;  
         [0017]      FIG. 3  shows a single reflective plate of a hollow retroreflector that may be used in the retroreflector assembly of  FIGS. 1 and 2 ;  
         [0018]      FIG. 4  shows another single reflective plate of a hollow retroreflector that may be used in the retroreflector assembly of  FIGS. 1 and 2 . 
     
    
     DETAILED DESCRIPTION  
       [0019]     Described herein is an improved retroreflector and retroreflector assembly capable of withstanding extreme high and low temperatures, and referred to herein as an Extreme Temperature Retroreflector (ETR) and ETR assembly.  
         [0020]      FIGS. 1 and 2  show front and side views of an ETR assembly  200 . ETR assembly  200  comprises a retroreflector (ETR)  100  having three optically flat mirrors or reflective plates  110 ; each mirror  110  being constructed with long and short sides which are perpendicular to each other. Beneficially, each of the three mirrors or reflective plates  110  is made of metal. Preferably each mirror or reflective plate  110  comprises aluminum, but other metals such as steel, beryllium, etc. can be used.  
         [0021]     As shown in  FIGS. 3 and 4 , a long side  116  of each reflective plate or mirror  110  has one or more (preferably a series of) first holes  120  provided therein at specific locations parallel to, yet in between, the reflective surface  112  and surface  114  (which may be non-reflective) of the reflective plate  110 . Beneficially, these first holes  120  are tapped with a thread to accept a threaded fastener, such as a screw or bolt thread. Beneficially, these first holes  120  do not pass all the way through the reflective plate or mirror, but only reach to a predetermined depth. Meanwhile, each reflective plate or mirror also has one or more (preferably a series of) “through-holes”  130  which pass completely through the reflective plate  110  at specific locations adjacent or near a short side  118 , perpendicular to the reflective (mirror) surface  112 . Beneficially, through-holes  130  may also be tapped with a thread to accept a threaded fastener, such as a screw or bolt thread.  
         [0022]     The first (beneficially, tapped) holes  120  and the through-holes  130  are positioned to properly align with each other when the reflective plates or mirrors  110  are assembled in an over-lapping configuration whereby the long side  116  is positioned underneath the short side  118  of the adjoining reflective plate or mirror  110 . The third reflective plate or mirror  110  is positioned in the same fashion creating an overlapping design. Threaded fasteners, such as screws or bolts,  170  are passed into the holes  120 ,  130  of each pair of adjoining plates  110 —passing through the through holes  130  of one of the reflective plates or mirrors  110  and into the first holes  120  of the other reflective plate or mirror  110 .  
         [0023]     In one embodiment using the reflective plate  110  as shown in  FIG. 4 , an adhesive such as epoxy is applied to one or both (preferable only one) of the adjacent surfaces of each pair of adjoining reflective plates  110 .  
         [0024]     For retroreflector  100  to perform well, each reflective plate or mirror  110  must be aligned so that it is perpendicular (at 90 degrees) to each adjacent reflective plate or mirror  110 . Properly aligned, light will enter retroreflector  100  and make three reflections off each mirror or reflective plate  110 , exiting retroreflector  100  with the exiting beam being perpendicular to the entering beam.  
         [0025]     Two embodiments of methods of assembly and alignment of retroreflector  100  will now be described. The particular embodiment that is selected may depend on the application for retroreflector  100 .  
         [0026]     A first method for assembly and alignment of the ETR is to perform a special machining operation to long side  116  of each reflective plate or mirror  110  whereby the surface of each long side  116  is machined precisely perpendicular (preferably to within a few seconds of arc) to reflective mirror surface  112 . This will insure that when long side  116  of each reflective plate or mirror  110  is attached to short side  118  of the adjoining reflective plate or mirror  110 , the resulting angle between any two neighboring mirrors will be 90 degrees.  
         [0027]     The second method for assembly and alignment incorporates a very thin layer of epoxy filler between long side  116  of each reflective plate or mirror  110  and the portion of the reflective surface  112  of the adjoining reflective plate or mirror which long side  116  abuts. One preferred adhesive is J-B WELDS (manufactured by J-B WELDS Co., Sulphur Springs, Tex.), but other adhesives can be used. Beneficially, the adhesive is not used to hold the ETR together, but rather it is used as a filler. Beneficially, the adhesive is placed on reflective surface  112  in a series of stripes between through-holes  130 . Care is taken so that when reflective plates or mirrors  110  are put together and the glue is pressed out, excess material does not seep into any screw threads in through-holes  130 . Once the adhesive is applied, retroreflector  100  is assembled and then aligned and held in proper alignment until the epoxy fully cures. The adhesive, which is captured between two surfaces, thereby forms a “compensation surface” and compensates for any angular error that exists between the long side and reflective surface of each mirror panel. Once cured, retroreflector  100  threaded fasteners, such as screws or bolts,  170  are passed through aligned pairs of first holes  120  and through-holes  130 , and alignment is verified.  
         [0028]     Turning again to  FIGS. 1 and 2 , in one embodiment, ETR  100  may mounted by affixing a single, rigid-arm mount  180  to retroreflector  100 , using existing holes  120 ,  130  provided in reflective plates or mirrors  110  of retroreflector  100  and a hole  185  in mount  180 . In this case, longer threaded fasteners, such as screws or bolts,  170  are employed to couple mount  180  and two adjoining reflective plates or mirrors  110  of retroreflector  100 . This construction is shown in  FIG. 2 . Although not shown in  FIGS. 1 and 2 , alternatively, a first set of threaded fasteners  170  may attach a first one of the reflective plates or mirrors  110  to mount  180  by means of one or more of the holes  120 ,  130 , and a second set of threaded fasteners  170  may attach the first reflective plate or mirror  110  to an adjoining reflective plate or mirror  110  by means of one or more separate holes  120 ,  130 . In that case, longer threaded fasteners  170  may not be required.  
         [0029]     Beneficially, mount  180  is constructed of the same material as reflective plates or mirrors  110  of retroreflector  100 . Beneficially, this allows for excellent thermal stability since there is not any mismatch of materials, and therefore mount  180  and retroreflector  100  will have the same thermal coefficient of expansion.  
         [0030]     Beneficially, ETR  100  and ETR assembly  200  as described above exhibit substantially improved tolerance of extreme high and low temperatures and will tolerate extreme temperatures well above and below that of other hollow retroreflectors that are assembled by gluing three mirrors together, without suffering loss of optical performance or suffering catastrophic failure. One embodiment of ETR  100  described above has been tested up to 500 degrees F. without loss of optical performance or catastrophic failure.  
         [0031]     Another benefit of ETR  100  as described above is that it is virtually indestructible compared to the hollow retroreflectors which are assembled by gluing three mirrors together. Beneficially, ETR  100  is made of metal (preferably aluminum, but other metals can be used such as steel, beryllium, etc.) which are held together by screws. Metal, being very strong and capable of withstanding very strong forces therefore makes a superior material for the construction of retroreflectors, if properly assembled.  
         [0032]     While preferred embodiments are disclosed herein, many variations are possible which remain within the concept and scope of the invention. Such variations would become clear to one of ordinary skill in the art after inspection of the specification, drawings and claims herein. The invention therefore is not to be restricted except within the spirit and scope of the appended claims.