Abstract:
A method is taught for measuring magnification of an afocal optical system. The method comprises the steps of directing a collimated light beam through the afocal optical system; intercepting the collimated beam exiting the afocal optical system with a prism; generating two reflected return beams at a first angle therebetween with the prism; passing the two reflected return beams through the afocal optical system; observing an interference pattern created by the two reflected return beams after exiting the afocal optical system; measuring a spacing between at least two fringes of the interference pattern; determining a second angle between the two reflected return beams exiting the afocal optical system using the spacing of the at least two fringes of the interference pattern; and comparing the second angle between the two reflected return beams exiting the afocal optical system to the first angle between the two reflected return beams immediately exiting the prism to thereby measure the magnification of the afocal optical system.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to afocal optical systems and, more particularly, to methods and apparatus for accurately measuring the magnification of afocal optical systems. 
     BACKGROUND OF THE INVENTION 
     An afocal optical system accepts an input beam of collimated light and creates an output beam that is also collimated. Examples include binoculars, spyglasses, rifle scopes and telescopes. An afocal system does not, by itself, form a final image, and by definition, does not have a finite focal length. However, a comparable first order parameter for such a system is its (afocal) magnification. This is essential to know when combining an afocal optical system with other imaging elements. 
     Afocal optical instruments, such as binoculars and telescopes are common devices for making a distant object appear larger. It is also quite common to compute the afocal magnifying power of such instruments without independent experimental measurement. The magnification of a simple afocal system can be theoretically computed with knowledge of the individual components and the design of the system, by either using the ratio of the focal lengths of optical components or the ratio of the angles of the incoming and outgoing beams. This approach is consistent with U.S. Pat. No. 4,678,899 to Baba, et al, which discusses a class of variable magnification afocal lens systems where the magnification is changed by moving optical components with respect to each other. The resulting (afocal) magnification can be computed by knowing the locations of the components. However, no independent test is described to confirm it. In addition, if an afocal system is not perfectly aligned, a small amount of beam convergence or divergence may remain in the system. For many applications this slight departure from true afocal performance is not a problem. This slight convergence or divergence of the beam is commonly referred to as “residual power”, or simply “power”. 
     Independent measurement techniques for measuring afocal magnification are known for telescopes and binoculars based on visually comparing the angular subtense of an object, with and without the aid of the binoculars (or a telescope). No special instrumentation is used for this measurement, and accuracy is limited to a few percent with such methods. 
     More accuracy is attainable by measuring the change in the angle of incoming and outgoing beams, using optical alignment telescopes. Such methods, described hereinafter with reference to FIGS. 1 through 3, may be able to achieve 0.1% accuracy, but are unable to consistently provide the 20 to 50 parts per million accuracy (0.002% to 0.005%) required of certain very high performance afocal systems. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a method for accurately measuring magnification of an afocal optical system. 
     It is a further object of the present invention to provide a method that allows for simultaneous, closely timed sequential, and/or iterative measurement of wavefront error, power, and magnification of an afocal optical system. 
     Another object of the present invention to is provide a method that has the ability to adjust or trim either or both the magnification and the power of an afocal system to great accuracy, reducing or eliminating the risk that a system will incur a residual power or wavefront error when adjusting magnification, or vice-versa. 
     Briefly stated, the foregoing and numerous other features, objects and advantages of the present invention will become readily apparent upon a review of the detailed description, claims and drawings set forth herein. These features, objects and advantages are accomplished by providing a method for measuring magnification of an afocal optical system comprising the steps of directing a collimated light beam through the afocal optical system, intercepting the collimated beam exiting the afocal optical system with an optical beam splitting device such as a prism, generating two return beams at a first angle therebetween with the optical beam splitting device, passing the two return beams through the afocal optical system, observing an interference pattern created by the two return beams after exiting the afocal optical system, measuring a spacing between at least two fringes of the interference pattern, determining an angle between the two return beams exiting the afocal optical system using the spacing of the at least two fringes of the interference pattern, and comparing the angle between the two return beams exiting the afocal optical system to an angle between the two return beams immediately exiting the prism to thereby measure the magnification of the afocal optical system. 
     The method of the present invention allows for greater accuracy in the measurement of the magnification of an afocal system. The method is especially useful for making or testing multiple afocal units. A specific example is described for a metrology station capable of unit-to-unit magnification control of better than ±30 parts/million. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic of a two-lens afocal optical system. 
     FIG. 2 is a schematic of a first step in a prior art method for measuring the magnification of an afocal optical system. 
     FIG. 3 is a schematic of a second step in the prior art method depicted in FIG. 2 for measuring the magnification of an afocal optical system. 
     FIG. 4 is a schematic of the method of the present invention for measuring the magnification of an afocal optical system. 
     FIG. 5 is an exemplary interference light intensity pattern. 
     FIG. 6 is a schematic of one typical prior art alignment test stand for an afocal optical system. 
     FIG. 7 is a schematic of a second typical prior art alignment test stand for measuring power of an afocal optical system. 
     FIG. 8 is a schematic of an alignment test stand that can be used to simultaneously or intermittently measure magnification and power of an afocal optical system. 
     FIG. 9 is an exemplary interference light intensity pattern showing a portion of the aperture being blocked from view by the prism. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning first to FIG. 1 there is a presented a schematic of a two-lens afocal system  10 . There is a first lens  12  having a focal length F 1  and a second lens  14  having a focal length F 2 . While there is an intermediate image plane  16  between the lenses  12 ,  14 , the lenses  12 ,  14  act together to take an input collimated beam  18  having a diameter, D IN , and have it emerge as an exiting collimated beam  20  having a diameter, D OUT . When the input beam  18  enters the first lens  12  at an angle of α IN , the exiting collimated beam  20  emerges at an angle, α OUT . 
     Using the two-lens afocal system  10  of FIG. 1 as an example, there are three ways to define the afocal magnification. First, afocal magnification may be defined as the ratio of focal lengths F 1 /F 2 . Second, afocal magnification may be defined as the ratio of input to output diameters, D IN /D OUT . Third, afocal magnification may be defined as the ratio of output to input beam angles, α OUT /α IN . To a first approximation, these three methods for determining afocal magnification give identical results. However, for real, multi-element optical systems, the internal focal lengths are difficult to determine especially if one only has access to an assembled and sealed afocal system. Further, precise measurement of the diameters D IN , D OUT  of the input and output beams  18 ,  20  may also be difficult, as one or both may be defined by the light beam size rather than any physical aperture accessible or measurable from the outside of the system. The most consistent and accurate method is the third approach, that being the measurement of input and output beam angles α IN , α OUT . 
     Referring next at FIG. 2, using the measurement of input and output beam angles α IN , α OUT , angular magnification can be measured for an afocal system  22 , comprising of two or more lenses or other optical elements  24  having an optical axis  26  passing through the centers of curvature of the optical elements  24  in the afocal system  22 . Per the known method for determining angular magnification, an alignment telescope  28  is placed on this optical axis  26  on one side of the afocal system  22 , and a reflecting flat  30  is placed to intercept the optical axis  26  on the other side of the afocal system  22 . The angular orientation of the reflecting flat  30  is adjusted to be perpendicular to the optical axis  26 . When that is done, the image of cross hair target  32  projected from within the alignment telescope  28  is seen reflected back upon itself. This establishes the “zero” value for the next step of the process. 
     Next (as depicted in FIG. 3) the reflecting flat  30  of FIG. 2 is reoriented to reside at an angle  34  of value, θ/2. The light from the cross hair target  32 , when returned from the reflecting flat  30 , as reflected beam  36  travels at an angle θ with respect to the optical axis  26 . After passing through the afocal system  22 , the reflected beam  36  emerges as beam  38  at an angle θ′ with respect to the optical axis  26 , angle θ and angle θ′ being different from one another. The alignment telescope  28  forms an image  40  of the cross hair  32 , displaced by a value, Y, corresponding to the angle θ′. Angle θ′ can be calculated from Y by knowing the effective focal length (f) of the alignment telescope  28  using the equation 
     
       
           Y= ( f )(θ′)   (eq. 1)  
       
     
     Finally, the ratio of angles is the afocal magnification M as given by Equation 2 below 
     
       
           M=θ′/θ   (eq. 2)  
       
     
     While there are numerous variations on the sequence just described, this and related approaches require the independent measurement of two angles, θ′ and θ each time magnification is determined. 
     The improved approach of the present invention is schematically depicted in FIG.  4 . This method and apparatus reduces both the number and the magnitude of the error sources and improves the consistency when measuring angular magnification of an afocal system  50 . Again, the afocal system  50  comprised of two or more lenses or other optical elements  52  having an optical axis  54  passing through the centers of curvature of the optical elements  52 . A point source of light  56  of wavelength, λ, is placed at the rear focal plane of lens  58  such that it creates a collimated plane wave  59  moving in the direction of arrow  60  toward a beam splitter  62 . Beam splitter  62  then redirects a portion of the light along the optical axis  54  towards the afocal system  50 , as shown by arrow  64 . The plane wave passes through the afocal system  50  and continues along the axis  54  as shown by arrow  65 , towards a wedged glass plate or prism  66 . A portion of the light is reflected from the first surface  68  of the wedged plate  66 , and returns towards the afocal system  50  in the direction of the arrow  70 . Another portion of the light is reflected from the rear surface  72  of the wedged plate  66 , and returns towards the afocal system  50  in the direction of the arrow  74 . The angle between the directions  70  and  74  is 2θ and is fixed by angle β of the wedged plate or prism  66  between the front surface  68  and the rear surface  72 , and the refractive index N of the wedged plate  66 . The beams of light directed along arrows  70  and  74  then pass through the afocal system  50  and emerge as plane waves with new directions as indicated by arrows  76 ,  78 , which form an angle between them of 2θ′. 
     A portion of each of the beams represented by arrows  76 ,  78  passes through the beam splitter,  62  to reach a detector  80 . These beams interfere with each other to create an interference light intensity pattern  82  (see FIG. 5) consisting of a series of high contrast bands of light  84  of pitch, P, on the detector  80 . The relationship between two angles, θ′ and θ and the afocal magnification M is as defined previously in equation 2, 
     
       
         θ′/θ= M= 2θ′/2θ 
       
     
     The prism wedge angle β is a constant of the test set, providing a consistent value of 2θ. With no moving parts, the only measurement to be made is that of 2θ′. This is done by evaluating the interference fringe pattern  82  at the detector plane  80 . As should be evident to those skilled in the art, the common path configuration followed by beams  70  and  74  on one side of the afocal system  50  and their corresponding beams  76  and  78  on the other side of the afocal system  50  creates a very stable interferometnic pattern  82  on detector  80 . The spacing of the fringes of this interferometric pattern yields an accurate measurement of the angle 2θ′ between the beams  76 ,  78 . 
     In addition to providing a method and apparatus for measuring magnification of an afocal optical system, the present invention also allows for simultaneous, closely timed sequential, and/or iterative measurement of wavefront error, residual power, and magnification of the afocal optical system. A schematic of a typical prior art alignment test stand for an afocal optical system  100  is shown in FIG.  6 . An interferometer  102  transmits a collimated test beam  104  through the afocal optical system  100 . A still collimated beam  106  emerges from the afocal optical system  100 . Collimated beam  106  is then intercepted by the optical test flat  108  creating a reflected beam  110  that retraces the path of collimated beam  106  back through the afocal optical system  100 . Reflected beam  110  which remains collimated exits the afocal optical system  100  as return beam  112  to return to the interferometer  102 . Because the complete beam comprising beam segments  104 ,  106 ,  110 ,  112  travels through the afocal optical system  100  twice, this is commonly referred to as a “Double Pass” test. The return beam  112  is compared to a reference beam (not shown) generated in the interferometer  102 , and optical aberrations, including power, are evaluated. Adjustments can be made to the components of the afocal optical system  100  under test at this test stand to evaluate the wavefront quality or to adjust the power to insure collimated input and output beams. 
     The same test described with reference to FIG. 6 can also be performed with an external reference beam, generated by a beam splitter  114 , such as shown in FIG. 7 (prior art). This is also a common approach. An interferometer  116  transmits a collimated test beam  118  that is split by the beam splitter  114  into a transmitted beam  120  and a reference beam  122 . The transmitted beam  120  continues on through the afocal optical system  124 , just as discussed for FIG.  6 . Reference beam  122  is intercepted by a reference mirror at  126  and is bounced back as reflected beam  128 . Transmitted beam  120  exits afocal optical system  124  to become beam  132 . Beam  132  is intercepted by optical test flat  134  thereby generating reflected beam  136 . Reflected beam  136  exits afocal optical system  124  as beam  138 . Beam  138  and reflected reference beam  128  are recombined as they return through the beam splitter  114  to become return beam  140  which is subsequently evaluated in the interferometer  116 . 
     For some afocal optical systems, it is possible to independently change the power and the magnification. In such cases, measuring the wavefront quality (including power) and the magnification at the same time or at the same test location offers a saving of time and an improvement in accuracy. Either of the prior art systems depicted in FIGS. 6 and 7 can be modified to add this new capability by inserting a wedge  150  (such as shown in FIG. 4) therein as a subaperture of the optical test flat (see FIG.  8 ). A collimated beam (as indicated by arrow  152 ) from an interferometer  154  is directed at a magnification test beam splitter  156 . The beam splitter  156  splits the collimated beam  152  into a transmitted beam  158  and a deflected beam  160 . The transmitted beam  158  continues on through the afocal optical system  162 . Deflected beam  160  is absorbed by a beam block  166 . Transmitted beam  158  exits afocal optical system  162  to become beam  168 . A portion of beam  168  is intercepted by a wedged glass plate or prism  150 . A portion of the light is reflected from the first surface  170  of the wedged plate  150 , and returns towards the afocal system  162  as a beam (as indicated by arrow  172 ). Another portion of the light is reflected from the rear surface  174  of the wedged plate  150 , and returns towards the afocal system  162  as a beam (as indicated by arrow  176 ). The angle between the direction of beams  172  and  176  is 2θ and is fixed by angle β of the wedged plate or prism  150  between the front surface  170  and the rear surface  174 , and the refractive index N of the wedged plate  150 . The beams of light directed along arrows  172  and  176  then pass back through the afocal system  162  and emerge as plane waves with new directions as indicated by arrows  178 ,  180 , which form an angle between them of 2θ′. A portion of each of the beams represented by arrows  178 ,  180  is reflected by beam splitter  156  yielding reflected beams as indicated by arrows  186 ,  188  directed toward the detector array  164  (e.g.—a pixelated sensor device such as a linear or two-dimensional CCD array). The other portion of return beams  178 ,  180  pass through the beam splitter as beams indicated by arrows  198 ,  200  toward interferometer  154 . These beams indicated by arrows  198 ,  200  interfere with each other to create an interference light intensity pattern  82  (again as shown in FIG. 5) consisting of a series of high contrast bands of light  84  having a pitch (P). Again, as previously discussed with reference to FIG. 5, the relationship between angles θ′ and θ, and the afocal magnification (M) is as defined previously in Equation 2 
     
       
         θ′/θ= M= 2θ′/2θ 
       
     
     The spacing of the fringes of the interferometric pattern  82  yields an accurate measurement of the angle 2θ′ between beams  186 ,  188  as recorded by detector  164 . 
     In addition, a portion of beam  168  misses prism  150  and is intercepted by optical test flat  192  thereby generating a reflected beam as indicated by arrow  194 . Reflected beam  194  passes through the optical system  162  and the beam splitter  156  to become return beam  196 . The return beam  196  is compared to a reference beam (not shown) generated in the interferometer  154 , and optical aberrations, including power, are evaluated. 
     The two interfering reflected beams indicated by arrows  186 ,  188  which are sensed by the detector array  164 , allow a parallel and simultaneous measurement of magnification while the interfereometer,  154 , evaluates optical aberrations and power. Sequential measurement, if preferred, can be done by inserting the beam splitter  156  intermittently, to acquire the angularly separated returning beams. 
     FIG. 9 illustrates the light intensity pattern generated in the interfereometer,  154 , by the returning beams  196 , 198 , 200 . The concentric circular fringes,  202 , represent the interference pattern associated with residual power in the returning beam  196  (from FIG.  8 ). These fringes can be interpreted by conventional methodology to determine power and other aberrations associated with the afocal system. 
     The small rectangular area,  204 , represents the outline of the prism  150  (from FIG. 8) as projected into the interferometer  154 . Within this outline are a series of very high density fringes,  84  as shown in FIG.  5 . These fringes will be too high in density to be interpreted by the interferometer,  154  but can be evaluated by the auxiliary detector,  164 , previously described. 
     Since, as shown in FIG. 9, the prism  150  will block a portion of the aperture from view for the wavefront/power tests, some thought and planning must go into the subaperture size and placement. The majority of the aperture should remain available for conventional interferometric wavefront analysis. The interferometer will treat it as an “obstruction”, working around it as it would any other obstructed area. Nonetheless, if the obstruction becomes too large, the accuracy of the wavefront characterization will be degraded. 
     There are several advantages to creating a combined afocal test station as described here. As already discussed, it allows simultaneous, closely timed sequential, and/or iterative measurement of wavefront error, power, and magnification. Of particular value is the ability to adjust or trim either or both the magnification and the power of an afocal system to great accuracy, reducing or eliminating the risk that a system will incur a residual power or wavefront error when adjusting magnification, or vice-versa. For example, this can be done by adjusting the relative axial spacing of the components of the afocal system. This also provides a simple method to add magnification metrology to an existing test station without requiring an additional laser or interferometer. 
     In terms of the metrology for the magnification channel, the two beams reflected from the prism return along essentially the same optical path, creating what is known as a “common path” configuration. Thus, when measuring the fringe spacing at the detector created by the beam interference, the pattern will be largely insensitive to laser mode changes, unequal path coherence differences, or air turbulence effects that can cause troublesome errors in conventional interferometry. 
     A prism  66  as discussed above with reference to FIGS. 4 and 8 is the preferred element for generating the two return beams needed in the practice of the method of the present invention. However, those skilled in the art will recognize that other optical bi-angular beam reflecting devices may also be used in place of a prism to generate the two return beams. For example, a diffraction grating or a hologram may be used in place of the prism. Understand, however, that with a prism, the two return beams would be reflected beams whereas, with a diffraction grating or a hologram, technically speaking, the two return beams would be diffracted beams. 
     From the foregoing, it will be seen that this invention is one well adapted to obtain all of the ends and objects hereinabove set forth together with other advantages which are apparent and which are inherent to the apparatus. 
     It will be understood that certain features and sub-combinations are of utility and may be employed with reference to other features and sub-combinations. This is contemplated by and is within the scope of the claims. 
     As many possible embodiments may be made of the invention without departing from the scope thereof, it is to be understood that all matter herein set forth and shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense. 
     
       
         
               
             
               
               
             
           
               
                   
               
               
                 PARTS LIST 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 10 
                 two lens afocal system 
               
               
                 12 
                 first lens 
               
               
                 14 
                 second lens 
               
               
                 16 
                 intermediate image plane 
               
               
                 18 
                 input collimated beam 
               
               
                 20 
                 exiting collimated beam 
               
               
                 22 
                 afocal system 
               
               
                 24 
                 optical elements 
               
               
                 26 
                 optical axis 
               
               
                 28 
                 alignment telescope 
               
               
                 30 
                 reflecting flat 
               
               
                 32 
                 cross hair target 
               
               
                 34 
                 angle 
               
               
                 36 
                 reflected beam 
               
               
                 38 
                 beam 
               
               
                 40 
                 image 
               
               
                 50 
                 afocal system 
               
               
                 52 
                 optical element 
               
               
                 54 
                 optical axis 
               
               
                 56 
                 source of light 
               
               
                 58 
                 lens 
               
               
                 59 
                 collimated plane wave 
               
               
                 60 
                 arrow (showing direction) 
               
               
                 62 
                 beam splitter 
               
               
                 64 
                 arrow (showing direction) 
               
               
                 65 
                 arrow (showing direction) 
               
               
                 66 
                 wedged plate or prism 
               
               
                 68 
                 front surface 
               
               
                 70 
                 arrow (showing direction) 
               
               
                 72 
                 rear surface 
               
               
                 74 
                 arrow (showing direction) 
               
               
                 76 
                 arrow (showing direction) 
               
               
                 78 
                 arrow (showing direction) 
               
               
                 80 
                 detector 
               
               
                 82 
                 light intensity pattern 
               
               
                 84 
                 bands of light 
               
               
                 100 
                 afocal optical system 
               
               
                 102 
                 interferometer 
               
               
                 104 
                 test beam 
               
               
                 106 
                 collimated beam 
               
               
                 108 
                 optical test flat 
               
               
                 110 
                 reflected beam 
               
               
                 112 
                 return beam 
               
               
                 114 
                 beam splitter 
               
               
                 116 
                 interferometer 
               
               
                 118 
                 test beam 
               
               
                 120 
                 transmitted beam 
               
               
                 122 
                 reference beam 
               
               
                 124 
                 afocal optical system 
               
               
                 126 
                 reference mirror 
               
               
                 128 
                 reflected beam 
               
               
                 132 
                 beam 
               
               
                 134 
                 optical test flat 
               
               
                 136 
                 reflected beam 
               
               
                 138 
                 beam 
               
               
                 140 
                 return beam 
               
               
                 150 
                 wedge 
               
               
                 152 
                 collimated beam 
               
               
                 154 
                 interferometer 
               
               
                 156 
                 test beam splitter 
               
               
                 158 
                 transmitted beam 
               
               
                 160 
                 deflected beam 
               
               
                 162 
                 afocal optical system 
               
               
                 164 
                 detector array 
               
               
                 166 
                 beam block 
               
               
                 168 
                 beam 
               
               
                 170 
                 first surface 
               
               
                 172 
                 beam 
               
               
                 174 
                 rear surface 
               
               
                 176 
                 beam 
               
               
                 178 
                 arrow (showing direction) 
               
               
                 180 
                 arrow (showing direction) 
               
               
                 186 
                 arrow (reflected beams) 
               
               
                 188 
                 arrow (reflected beams) 
               
               
                 192 
                 optical test flat 
               
               
                 194 
                 arrow (reflected beam) 
               
               
                 196 
                 return beam 
               
               
                 198 
                 arrow (beam) 
               
               
                 200 
                 arrow (beam) 
               
               
                 202 
                 circular fringes 
               
               
                 204 
                 rectangular area