Abstract:
An assemblage and method for testing a lens having a plurality of field angles employs an improved modulation transfer function (MTF) design system for evaluating image quality produced by the lens being tested. A reflecting surface capable of translational and rotational movements is arranged along a predetermined optical path for receiving a collimated array of light rays and then directing the collimated array of light rays to the lens being tested.

Description:
FIELD OF THE INVENTION 
     The invention relates to a method for measuring performance characteristics of lenses. More particularly, the invention concerns an apparatus and method for testing the image quality of a lens using an improved modulation transfer function (MTF) test system. 
     BACKGROUND OF THE INVENTION 
     Electronic methods have largely replaced visual methods, such as resolving power, for testing lens in the industry. Presently many in the photographic and optical industries custom build electronic systems, typically either analog or digital modulation transfer function (MTF) designs, for testing lenses being produced in high volume. These lenses have thus far been manufactured with fixed, predetermined magnifications. 
     Modulation transfer function (MTF) design systems are characterized by a graphical representation of image contrast relative to the pattern contrast over a range of spatial frequencies, where high frequency in the test pattern corresponds to small detail in an object. As shown in FIG. 1, existing MTF design systems  1  typically include the following major elements: the test pattern arranged at the object plane  2 ; the lens  4  under test; and, the displaceable detector  40  (displacement noted by arrow) at the image plane  8 . An important advantage of MTF design systems is that they provide information about image quality over a range of frequencies rather than just at the limiting frequency as does other conventional methods, such as resolving power. For many fixed magnification lenses, these test patterns are placed at fixed positions in the object plane. Moreover, the long conjugate distance d, i.e., the distance from the object plane  2  to the lens  4  shown FIGS. 1,  3  and  4 , are also predetermined or fixed with fixed magnification lens. 
     Referring to FIG. 2, illustrated is a schematic diagram of a typical optical test pattern  10  imaged at the object plane (not shown). The test pattern  10  is imaged by the particular lens under test on the detector (not shown) at the image plane. The detector ( 40 ), preferably a charge coupled device (CCD), is generally moved in the direction of the optical axis and the image is analyzed in terms of modulation transfer function (MTF) as a measure of image quality. It should be appreciated that for a fixed magnification lens to be tested, such as wide angle and telephoto lenses, test patterns are placed at fixed positions in the object plane. 
     Referring to FIG. 3, a schematic diagram of a prior art wide-angle, fixed magnification lens is illustrated. Wide angle lens typically have a field angle, i.e. the angle between the axis test pattern and the field test pattern, between 15 and 20 degrees. Two things in the test system generally need to change when the lens changes from wide to telephoto. First, the field angle needs to change because the lens will have different fields of view from wide to telephoto. Those skilled in the art will appreciate the desire to want to test the lens at some percentage of the total field. Second, the spatial frequency of the test pattern must change to account for the different magnification when the lens goes from wide to telephoto. 
     Referring to FIG. 4, a schematic diagram of a prior art telephoto, fixed magnification lens with typical field angles of 4 to 6 degrees is illustrated. 
     Skilled artisans will appreciate that for zoom lenses a different set of test criteria must be employed. This is because the more versatile zoom lens has a variable focal length that must be tested at more that one zoom setting. Thus, the position of the test patterns in the MTF design system need to change to accommodate the changing field of view. Further, the test pattern spatial frequency also needs to change to keep the spatial frequency at the image plane appropriate for the lens and the detector. 
     Accordingly, a major shortcoming of current methods for testing image capabilities of lenses, such as zoom lenses that are to be tested at more than one zoom setting, is the position of the test patterns needed to change to accommodate the changing field of view. Moreover, another problem associated with present methods and systems for testing such lenses is that the test pattern spatial frequency also needs to change to keep the spatial frequency at the image plane appropriate for the lens and the detector. 
     Therefore, a need persists for an apparatus and method for testing lenses that incorporate improvements to the digital system to accommodate the testing of lenses having variable focal lengths. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the invention to provide a method for testing a lens that can vary the spatial test frequency and field angle for a lens being tested. 
     It is another object of the invention to provide a method for testing a lens that permits infinitely variable field angles of the lens being tested. 
     Yet another object of the invention is to provide a method for testing a lens that uses a reflecting surface arranged in a predetermined optical path which translates and rotates so the field angle of the lens being tested can change and still use the same test pattern system. 
     It is a feature of the invention that a flexibly mounted reflecting surface and a rotatable support plate containing a plurality of test patterns are arranged in a predetermined optical path contains a plurality of test patterns each having a distinct single spatial frequency for imaging by the lens being tested. 
     The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the present invention, a method of testing a lens having a plurality of field angles characteristic of the magnification of said lens, comprising the steps of: 
     providing a platform for holding at least one test pattern; 
     providing means to illuminate one of said at least one test pattern; 
     providing a reflecting surface capable of rotating and translating movements about an optical path defined by a beam of light having a predetermined optical path, said beam of light having been converted to a collimated array of light rays by a collimating lens arranged in said optical path between said at least one test pattern and said reflecting surface; 
     illuminating one of said at least one test pattern with a beam of light, said beam of light passing through said at least one test pattern and then said collimating lens thereby forming a collimated array of light rays; and, 
     reflecting said collimated array of light rays off said reflecting surface arranged at a first position and then directing said collimated array of light rays through said lens at a first field angle to form a first image at a first image plane; 
     translating and rotating said reflecting surface along said optical path to a second position; and, 
     reflecting said collimated array of light rays off said reflecting surface and then directing said collimated array of light rays through said lens at a second field angle to form a second image at a second image plane. 
     It is an advantageous effect that the apparatus of the invention can perform lens testing at any field angle and many magnifications without the need to build a separate tester for each lens. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features, and advantages of the present invention will become more apparent when taken in conjunction with the following description and drawings wherein identical reference numerals have been used, where possible, to designate identical features that are common to the figures, and wherein: 
     FIG. 1 is a prior art MTF design system; 
     FIG. 2 is a prior art schematic diagram of a test pattern imaged on an image plane; 
     FIG. 3 is prior art schematic diagram of a wide-angle lens; 
     FIG. 4 is a prior art schematic diagram of a telephoto lens; 
     FIG. 5 is schematic diagram of the apparatus of the invention for testing a lens; and, 
     FIG. 6 is an enlarged top plane view of the metallic plate wheel of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning now to the drawings, and particularly to FIG. 5, apparatus  20  for testing a lens  24  having a plurality of field angles according to the principles of the invention is illustrated. The plurality of field angles is achieved by translating and rotating a reflecting surface or mirror  36  (described below) in order to change the angle that a collimated beam of light along predetermined optical path  32  (described below) is presented to the lens being tested. FIG. 5 shows two possible positions for the mirror  36 . As the mirror  36  is translated from position A to position B, it is also rotated so that the collimated beam is reflected to the lens under test  24 . The mirror  36  could be positioned at any of an infinite number of positions so that the combination of the linear position and angle of the mirror  36  directs the collimated light  32  from the collimating lens  34  to the lens under test  24 . For illustrative purposes, only one mirror  36  is shown in FIG.  5 . In a preferred embodiment of the invention, however, at least two mirrors  36  are used to extend the test to multiple field positions. 
     It is important to the invention that apparatus  20  can vary the spatial test frequency for a lens  24  being tested having variable field angles. The spatial test frequency of the test pattern is the number of line pairs (one dark and one light) per some distance. This is normally given as cycles (one line pair) per millimeter. FIG. 6 shows a round metallic plate with test patterns made of chromed glass with the test pattern etched out of the chrome. Each test pattern consists of two sets of parallel lines at right angles to each other. These two orientations are to allow both sagittal and tangential testing of the lens  24  being tested. 
     Referring to FIG. 5, apparatus  20  includes a source of illumination  26  for emitting light and a support means, preferably a rotatable platform  28 , for supporting a plurality of spatially separated test patterns or objects  18  (shown in FIG.  6 ). 
     Further referring to FIG. 5, the rotatable platform  28  is arranged in apparatus  20  so that one of the plurality of test patterns or objects  18  can be illuminated by the light source  26 . Platform  28  is preferably rotated by motor drive  30  although other means may be used with similar results. Importantly, motor drive  30  must be capable of rotating platform  28  so as to position any one of the plurality of test patterns or objects  18  in a predetermined optical path  32  for varying the spatial test frequency of the test patterns or objects  18 . More particularly, preferably platform  28  is a metallic plate wheel mounted on a rotation stage with cut-outs or recesses (FIG. 6) for the chrome-on-glass single frequency test patterns. This arrangement enables the spatial frequency at the displaceable detector  40  be maintained in an appropriate range for the lens  24  being tested at a particular zoom setting. As will be appreciated, the metallic plate wheel has multiple test patterns  18  of different spatial frequencies. Consequently, the appropriate spatial frequency pattern for the current zoom setting can be rotated into the optical path  32 . 
     Referring again to FIG. 5, a collimating lens  34  is arranged in the predetermined optical path  32  for receiving the light transmitted through any one of the plurality of test patterns  18 . Further, collimating lens  34  converts the light to a collimated array of light rays. The collimating lens  34  makes the test pattern appear to be at infinity no matter what the zoom setting happens to be. 
     Instead of having the test patterns  18  at a fixed location in an MTF system, the field angle (shown in FIG. 5) can be varied by using a substantially flat reflecting surface  36 , preferably a polished mirror, that translates (noted by arrows) and rotates about a fixed axis defined by optical path  32 . As shown in FIG. 5, single reflecting surface  36  is shown in two different positions A and B. The translation and rotation of this reflecting surface  36  serves to change the field angle of the lens  24  being tested while keeping the collimated beam of light directed at lens  24  under test. According to this configuration, the selected test pattern  18  would always appear to be at infinity due to the collimating lens  34 . The test pattern assembly, i.e., the rotating test pattern platform  28  and test patterns  18 , are the preferred single frequency chrome-on-glass with heat glass and color correctors. This assembly would allow zoom lenses to be tested anywhere in their zoom range in one test fixture. Advantageously, this allows the lens  24  to be digitally tested at multiple zoom positions on a single test station. 
     Referring still to FIG. 5, substantially flat reflecting surface or mirror  36  is arranged in the predetermined optical path  32  for receiving the collimated array of light rays passing through collimating lens  34 . According to the invention, the mirror  36  is capable of translational and rotational movements about an axis defined by the predetermined optical path  32 . Translational and rotational movements of mirror  36  is accomplished by preferably a motor drive (not shown). These movements enable the mirror  36  to reflect and then direct the collimated array of light rays through the lens  24  being tested. An important advantage of translational and rotational mirror  36  is that it compensates for varying field angles of lens  24  and enables the use of a single test pattern system. 
     Further with reference to FIG. 5, the image (not shown) produced by the lens  24  being tested defines an image plane  38 . This image can be detected by a detector  40 , preferably a charge coupled device (CCD), arranged proximate to the image plane  38  for detecting the image produced by the lens  24  being tested. 
     The invention, therefore, has been described with reference to preferred embodiments thereof. However, it will be appreciated that variations and modifications can be effected by a person of ordinary skill in the art without departing from the scope of the invention. 
     PARTS LIST 
     θ field angle 
     d distance from object plane to lens 
       1  MTF design system 
       2  object (test pattern) plane 
       4  lens 
       8  image plane 
       10  typical optical test pattern 
       18  test patterns or objects 
       20  apparatus 
       24  lens being tested 
       26  source of illumination 
       28  rotatable platform 
       30  motor drive 
       32  optical path 
       34  collimating lens 
       36  reflecting surface or mirror 
       38  image plane 
       40  displaceable detector