Optics: measuring and testing – Lens or reflective image former testing
Reexamination Certificate
1999-08-31
2001-01-23
Font, Frank G. (Department: 2877)
Optics: measuring and testing
Lens or reflective image former testing
C356S125000, C356S126000
Reexamination Certificate
active
06177986
ABSTRACT:
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.
REFERENCES:
patent: 4272190 (1981-06-01), Shapiro
patent: 4274737 (1981-06-01), Howland
patent: 5048000 (1991-09-01), Tsuji et al.
patent: 5500767 (1996-03-01), Maruyama et al.
patent: 5621520 (1997-04-01), Hoffman
Bailey, Sr. Clyde E.
Eastman Kodak Company
Font Frank G.
Ratliff Reginald A.
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