Right-angle detachable variable-position reflector assembly
BACKGROUND OF THE INVENTION 
     This invention relates to borescopes and endoscopes, which are long, flexible instruments which carry a viewing device at the end of an elongated insertion tube. Borescopes are intended for inspection of concealed machine parts and the like, for example, inspection of vanes of a turbine or jet engine without disassembly thereof. 
     The present invention is more particularly concerned with a detachable right angle reflector assembly that is demountably attached onto the viewing head on the distal end of the borescope insertion tube. 
     For many borescope applications forward viewing is required, but for other applications side viewing is needed. That is, it is sometimes necessary to view a hidden or obstructed target from a direction that is more or less radial with respect to the axis of the borescope. To permit both forward and side viewing with the same instrument, a detachable right angle optics assembly can be employed. 
     One detachable right angle prism assembly is shown and described in U.S. Pat. No. 4,727,859 to Raymond A. Lia. In that arrangement a housing is coupled by a bayonet fitting onto the viewing head, and includes a right angle prism that changes the viewing direction of the image sensor from axial (straight ahead) to radial (sideward). Because the assembly uses a bayonet coupling, alignment of the image sensor and the mirror or prisms is easily achieved, and the right angle viewer can be interchanged among several borescopes of the same type. 
     In many borescopes, particularly those which may have large, oversize illumination fiber bundles and/or wide viewing angles, the large outside diameter of the borescope distal tip presents a design constraint. In large-diameter borescopes, the use of a threaded coupling is required, rather than a bayonet mechanism. However, when the right angle viewer is attached by helical threads to the viewing head of the probe, each viewer and probe have to be made as a matched pair, and interchangeability is difficult or impossible to accomplish, since the alignment of the mirror or prism to the image sensor is determined by the amount of thread engagement. Matching thread engagements for a probe and a sideview mirror assembly are impractical to manufacture, because of variations in thread cutting and polish dimensions on the probe head. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is a general object of this invention to provide a sideview mirror or prism assembly that avoids the drawbacks of the prior art, and permits mounting onto a borescope or similar probe by means of helical threads. 
     It is another object of this invention to provide a sideview mirror or prism arrangement in which the orientation of the reflector relative to the image sensor of the probe is variable to permit alignment of the mirror or prism with the probe. 
     It is a further object to provide a side viewing reflector attachment that is interchangeable from one probe to another. 
     According to one aspect of this invention the right angle detachable mirror or prism assembly has a generally tubular housing or cap that overfits the distal end of the head of the probe. A mirror (or equivalent reflector means) is mounted within the housing along the optic axis of the image sensor to redirect the viewing angle to the side, through a window provided on the side of the housing. A threaded mounting ring is disposed in the housing proximally of the mirror, and has female threads that engage and mount onto male helical threads on the probe head. A helical compression spring is positioned between the ring and retaining structure on the proximal end of the housing. The spring acts to urge the ring distally within the housing, i.e., to urge the housing proximally with respect to the probe head on which the ring is mounted. The ring is provided with a series of detents, for example bores or recesses in the distal face, and these detents are spaced around the ring at regular intervals. A cooperating member, e.g. a proximally facing pin, is mounted on the inside wall of the housing and engages one of the recesses to interlock the housing and the ring against rotation. The pin is held in the recess by the compressive force of the spring. The right angle viewing assembly is attached onto the probe head by engaging the threaded ring and the probe threads and rotating the assembly until the ring is tightly engaged on the probe head. 
     After the assembly is mounted on the head, the operator can grasp the mirror housing and pull it distally, away from the head. This causes the spring to compress and pulls the pin out of engagement with the recess in the ring. Then the operator can rotate the housing to reposition the mirror or prism relative to the image sensor and any associated light guides to obtain an optimal viewing angle. 
     After this, the operator releases the housing, and the pin enters engagement with another recess to lock the mirror into the new orientation. 
     Of course, other detent means besides a pin and recesses on the ring could be used. In one embodiment there are thirty recesses on the ring, spaced at about twelve degrees. This permits accurate alignment within about six degrees. 
     The above and many other objects, features, and advantages of this invention will be more fully understood from the ensuing description of a preferred embodiment, which should be considered in connection with the accompanying Drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a perspective view, partly cut away, of a right angle mirror assembly according to one preferred embodiment of the invention. 
     FIG. 2 shows the assembly of FIG. 1 installed on the viewing head of a borescope or similar probe. 
     FIG. 3 shows the assembly of FIG. 1 with the housing pulled forward to permit alignment of the right angle mirror with the imaging and illumination optics of the probe. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to the Drawing, and initially to FIGS. 1 and 2, the right angle viewing attachment 10 according to one preferred embodiment has a mirror 12 oriented at 45° from the axis of a viewing head 14 that is positioned at a distal tip of a borescope insertion tube 16. The viewing head 14 has an imaging device 18, which can be either a fiber optic imager or a video imager, as well as fiber optic bundles 20 for illumination. As shown in FIG. 1, the attachment 10 has a tubular housing 22 with a cutout or window 24 through which the mirror 12 redirects the viewing direction of the associated imaging device 18. The housing 22 has a distal shoulder 26 positioned just proximal of the window 24 and a proximal shoulder 28 at the proximal end of the housing 22. In this case the shoulder is in the form of a threaded fitting. A fitting ring 30 is disposed within the housing 22 against the distal shoulder 26. The ring is provided with female interior threads 32 and with cylindrical recesses 34 on a distal surface of the ring 30. These recesses are evenly distributed around the ring 30 and in this particular embodiment there are thirty recess 34 spaced at 12 degree intervals. 
     The female threads 32 of the ring 30 couple with male helical threads 36 that are provided on a cylindrical outer surface of the viewing head 14. 
     A helical compression spring 38 is situated against an inner wall of the housing 22 and is compressed between the ring 30 and the proximal shoulder 28. The spring 38 fits within an annular gap between the housing 22 and the outer cylindrical wall of the borescope viewing head 14. 
     A retaining pin 40, which is axially oriented, is implanted in the shoulder 26. The pin 40 enters one of the recesses 34 and thus holds the ring 30 against rotation with respect to the housing 22. 
     Also shown in FIG. 1 is a knurled surface 42 on the proximal portion of the housing 22, and a hexagonal receptacle 44 at the distal end of the housing 22. This receptacle 44 accepts a hex wrench to facilitate tightening of the mirror 12 into housing 22. The hex wrench can also be employed for removal of the mirror. 
     Because of difficulties or impracticalities in forming of identical threads 36 in a series of borescopes the position of the mirror 12, which is affixed in the housing 22, will not always align properly with the imaging device 18 or with the illumination fiber optic bundles 20. The right angle attachment 10 is installed by rotating the housing 22 until the threads 32 are sufficiently tightened onto the threads 36 on the viewing head 14. At this stage, the pin 40 holds the housing 22 and the mirror 12 against rotation with respect to the ring 30. To align the mirror 12 with the imaging device 18, the operator can grasp the housing 22 and pull it distally, i.e., away from the head 14 to the position as shown in FIG. 3. Pulling the housing 22 distally compresses the spring 38 and moves the pin 40 out of engagement from the associated recess 34. This permits rotation of the housing 22 relative to the ring 30. Now the operator rotates the housing 22 to position the angular orientation of the mirror 12 relative to the imaging device 18 and the fiber optic bundles or light guides 20. This obtains an optimum viewing angle. Then the operator releases the housing 22, and the pin 40 engages into another recess 34 at the new position. Thereupon, the borescope is ready for use for side viewing. 
     While a single retaining pin 40 is shown here, more than one pin could be employed. Also, rather than the pin-and-recess configuration, a ball type detent could be employed, as could a crown gear arrangement or any of numerous other releasably interlocking means for releasably securing the ring 30 with respect to the housing 22. 
     Moreover, while a mirror 12 is used here as the reflector, in other possible embodiments one or more prisms could be used. 
     It should be understood that this invention is not limited to the precise embodiments depicted here, but rather, many modifications and variations would present themselves to those of skill in the art without departing from the scope and spirit of this invention, as defined in the appended claims.