Abstract:
A method and an instrument are provided for checking and adjusting the accuracy of an alignment device which transmits a collimated (e.g. laser) light beam that serves as an alignment reference. That beam generator defines a precise reference for purposes of alignment of various other forms of apparatus, often over great distance. The checking and adjusting instrument (which is totally passive) provided by this invention is precisely attached to the alignment device, temporarily, and receives the reference beam of light from the device. The instrument displays an indication of misalignment of the beam, and also aids in correcting same.

Description:
This application claims benefit of Provisional App Ser. No. 60/118,061, filed Feb. 1, 1999. 
    
    
     FIELD OF INVENTION 
     Various optical devices have been designed to align one optical system with another. An autocollimator is one such device. 
     BACKGROUND OF THE INVENTION 
     The autocollimator, as shown in FIG. 1 (Prior Art), projects a highly collimated beam of light  10  at the optical device  12  to be aligned with respect to the autocollimator. The device  12  to be aligned is made to reflect light back towards the autocollimator and light from the beam source also is reflected directly to a view screen through beam splitter  13 , each producing light spots on the view screen  15 . The light reflected back will be deviated from the transmitted beam by twice the angle of misalignment (tilt). The light directed to the device  12 , and reflected back into the autocollimator, travels through the telescope optics  14 , e.g. the eyepiece and objective lenses, and is reflected by beam splitter  13  to the eyepiece or view screen  15 . This means that an image of the received light appears as a spot. 
     Most autocollimators are designed in such a way as to show the incoming light as a spot on a reticle, the reticle having markings that indicate angular displacement. Furthermore, autocollimators are generally designed with costly very high precision diffraction limited optical elements. This is important because the autocollimator serves as a precision collimator to produce a highly parallel transmitted beam that can propagate as far as diffraction will allow. This enables the user to align devices separated by great distances. An example of such an autocollimator in the patent art is found in U.S. Pat. No. 3,836,258. 
     SUMMARY OF THE INVENTION 
     The purpose, of this invention (herein called an “angle finder”) is to develop a relatively simple instrument for aligning a device or devices which transmit (or use) as an alignment feature a collimated (e.g. laser) light beam. Such a device is equipped or includes an adjustable light transmitter such as a laser beam generator, and for purposes of alignment has the instrument of this invention, which is a totally passive instrument, attached to it. This instrument acts as a receiver of the beam of light and displays an indication of misalignment of the beam, and also aids in correcting same. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 (Prior Art) is a schematic view of an autocollimator; 
     FIG. 2 is a side view of an angle finder/alignment apparatus incorporating the features of the invention; 
     FIG. 3 is a view of the top of FIG. 2; 
     FIG. 4 is a schematic view of the apparatus showing an “out of alignment” condition; and 
     FIG. 5 is a view similar to FIG. 4 showing a condition of alignment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     There are at least two methods of using the invention: Method 1 involves the angular alignment of a light transmitter to the angle finder; Method 2 involves alignment of the angle finder with a light transmitter. 
     A typical physical embodiment of the angle finder is illustrated in FIG. 2, comprising a reference mount  20  on an optical instrument (not shown) to which a laser light source  22  is precisely affixed, directing a collimated light beam  25  along an optical axis  26 . It is desired to align this beam precisely, and thus secure proper alignment of the instrument. FIG. 4 illustrates the beam misaligned, and in FIG. 5 the beam, and thus the instrument, is properly aligned. 
     The angle finder can be separated essentially into two functional parts. The first part I is a telescope, of either Gallilean or Keplerian type, used to increase the sensitivity of the second part by magnifying the angle that the input beam deviates from the optical axis (axis of alignment). The telescope comprises a pair of lenses  30 ,  32  which are supported within a tubular housing  35 . 
     The second part II is a beam splitting and re-directing device  40  comprising at least three optical elements, namely a beam splitter  41  and a pair of first surface mirrors  42 ,  43 , all supported in a second housing  45  that is centered and angularly aligned (so its optical elements will be on-axis) with the telescope housing  35  during manufacture. Collar  46 , which may be part of the telescope, is fastened to housing  35  and becomes part thereof. This beam splitting device separates the beam  26  into at least two paths which are then reflected, respectively, by mirrors  42  and  43  and imaged adjacent each other onto a view screen  47 . 
     It should be noted that this view screen  47  can be a simple diffusing screen and contrast enhancement filter, for direct perception, or can be a photo receptor device such as a multi-element photodiode, or a CCD camera or the like, in order to enable the determination of the position and relationship of the images. 
     At the top of housing  45  there is a high precision omni-directional (e.g. bulls eye) bubble level  50 , mounted such that it is level (bubble centered) when the optical axis  26  of the apparatus is true vertical. 
     Considering the angle finder in a typical practical use, namely the alignment of a laser beam output from a scanning beam generator, the telescope housing  35  is attached to the reference mount  20  of the generator and the transmitted laser beam  25  from the laser beam source in that generator enters the angle finder on or nearly aligned along the optical axis  26  (the axis to be aligned to). The light exits the telescope part I with a beam diameter equal to the product of the input beam diameter, and the inverse of the lateral magnification of the telescope, and with a beam divergence equal to the angular magnification and the product of the input beam divergence. This is noted since it affects the relative spot sizes on the viewing screen. If the beam  25  is misaligned from the optical axis, as in FIG. 5, the beam will exit the telescope part I, with an increased angle relative to the optical axis  26  that is equal to the angular magnification of the telescope. For example, if the magnification of the telescope is 10× and the input beam is off-axis by one milliradian, then the beam output from the telescope will have an angle of ten milliradian from the optical axis. Since in the example shown in FIG. 4 the input beam is exactly aligned along the optical axis, the exit beam will also be aligned along the optical axis. This is ensured by the careful alignment of the telescope optics during manufacture. 
     This beam now enters the beam splitting part II of the apparatus and impinges on beam splitter  41  which is supported at a predetermined angle, preferably forty-five degrees, with respect to the optical axis. The beam is reflected as first light towards view screen (or to a multi-element photodiode array or a CCD camera or the like). Some of the light passes through the beam splitter as second light and is reflected back towards the beam splitter by the first mirror  42 . Some of this second light passes through the beam splitter  41  and back towards the input beam. The remainder of the second light is reflected towards a second mirror  43 . 
     This second light is reflected from second mirror  43  and is redirected back toward the beamsplitter perpendicular to the optical axis of the telescope (part I). Then as a consequence of the alignment of the optics the first light will produce an image (spot S 1 ) on the center of the viewing screen. 
     The third light will also produce a spot S 2  on the viewing screen. It will have a larger diameter because of the divergence of the beam and its longer optical path. The spot from third light will also be reduced in apparent brightness relative to the spot from first light because of light lost from the multiple passes through the beam splitter and since the energy is spread over a larger area on the view screen. 
     Considering the situation illustrated in FIG. 4, where the input beam is not aligned along the optical axis, and for convenience choosing an example where an input beam of 1 cm. diameter and 1 milliradian divergence enters the telescope at some angle (e.g. 1 milliradian), if the telescope has a 10× magnification, then the output beam of the telescope exits with a beam diameter of 0.1 cm. and 10 milliradian divergence, and at an angle from the optical axis of 10 milliradian. 
     Furthermore, choosing the distance from the telescope to the beam splitter along the optical axis to be 5 cm. and the distance from the beam splitter along the optical axis to either mirror or to the viewing screen to also be 5 cm., by simple geometry it is seen that the first light produces a 0.3 cm. diameter image on the viewing screen that is displaced laterally from the optical axis at the view screen by 0.1 cm. For the third light, by applying the law of reflection at each of the mirrors, it will be seen that the displacement will be in the same direction as the beam from first light but displaced laterally by 0.3 cm. from the optical axis and with image spot size of 0.3 cm. The difference in displacements is 0.2 cm. Thus, the separation of the images is proportional to changes in angular alignment. 
     To align the instrument (e.g. the scanning generator) with the attached angle finder, the two are adjusted as a unit until the bubble level  50  is centered. This places the angle finder housings in plumb with respect to the ground surface. Then, the laser beam source within the generator is adjusted until the spot images are coincident on the view screen, and this places the generator output beam on plumb. 
     Furthermore, if the incoming beam is at some arbitrary angle (as in FIG. 4) to the optical axis  26 , and if the angle finder is moved laterally with respect to its optical axis, it will be seen that the images maintain their separation on the view screen and are merely displaced together in a direction on the view screen corresponding to the direction of lateral displacement of the angle finder. Therefore it can be said that the device is insensitive or invariant to changes in lateral position of the novel angle finder with respect to the incoming beam. This is a complement of laser targets as disclosed in U.S. Pat. No. 5,710,647 and 5,760,932, issued to the same assignee as this application, where the devices are sensitive to lateral displacement and invariant to changes in input beam angle. 
     Thus, it should be noted that one can incorporate the position sensing features of the laser target and the angle aligning features of the angle finder of the present invention into one unit useful as a “universal laser target” that would simultaneously allow the positioning of the target on the beam centerline and the coaxial alignment of the target to the input beam. 
     While the methods herein described, and the forms of apparatus for carrying these methods into effect, constitute preferred embodiments of this invention, it is to be understood that the invention is not limited to these precise methods and forms of apparatus, and that changes may be made in either without departing from the scope of the invention, which is defined in the appended claims.