Apparatus and Method for Measuring an Optical Property of an Optical System

Abstract

An apparatus for measuring the MTF or another optical property of an optical system includes an object to be imaged, which has a plurality of structures arranged in a plane and separated from one another, a two-dimensional image sensor, and collecting optics having a focal length f. The image sensor has a distance a from the collecting optics with 0.94.Math.f?a?1.1.Math.f. A holder for the optical system is arranged such that the optical system is located in a beam path between the object and the collecting optics. The image sensor and the collecting optics are configured such that all structures can be imaged by the optical system and the collecting optics onto the image sensor simultaneously.

Claims

1-12. (canceled)

13. An apparatus for measuring an optical property of an optical system, the apparatus comprising: an object to be imaged having a plurality of structures arranged in a plane and separated from each other; collecting optics having a focal length f; a holder configured to hold the optical system, wherein the holder is arranged such that the optical system is located in a beam path between the object and the collecting optics; and a two-dimensional image sensor having a distance a from the collecting optics with 0.9.Math.f?a?1.1.Math.f, wherein the image sensor and the collecting optics are configured such that all structures can be imaged by the optical system and the collecting optics onto the image sensor simultaneously.

14. The apparatus of claim 13, wherein the object is an illuminated reticle.

15. The apparatus of claim 13, wherein the structures are crosshairs.

16. The apparatus of claim 13, wherein the holder is arranged in a diverging beam path.

17. The apparatus of claim 13, wherein a collimator is arranged in a light path between the object and the holder.

18. The apparatus of claim 17, wherein the collimator is a conoscopic lens.

19. The apparatus of claim 17, wherein the object is movable along an optical axis of the apparatus to vary a distance between the collimator and the object.

20. The apparatus of claim 13, wherein the image sensor is movable along an optical axis of the apparatus.

21. A method of measuring at least one optical property of an optical system, the method comprising the following steps: providing an object, a two-dimensional image sensor, and collecting optics having a focal length f, wherein the image sensor has a distance a from the collecting optics with 0.9.Math.f?a?1.1.Math.f; inserting the optical system into a beam path between the object and the collecting optics; simultaneously imaging the object within a field of view of the optical system and the collecting optics onto the image sensor using the optical system and the collecting optics; and determining the at least one optical property by evaluating an image of the object formed on the image sensor.

22. The method of claim 21, wherein the optical system has a variable focal length, and wherein the optical properties of the optical system are measured for at least two different focal lengths.

23. The method of claim 21, wherein the optical system is afocal, and wherein a collimator is disposed in the light path between the object and the optical system.

24. The method of claim 21, wherein at least one optical property is selected from a group consisting of: distortion, image field curvature, field of view, and edge light falloff.

25. An apparatus for measuring an optical property of an optical system, the apparatus comprising: an object to be imaged having a plurality of structures; collecting optics; a holder configured to hold the optical system, wherein the holder is arranged such that the optical system is located in a beam path between the object and the collecting optics; and a two-dimensional image sensor that is movable along an optical axis of the apparatus, wherein the image sensor and the collecting optics are configured such that all structures can be imaged by the optical system and the collecting optics onto the image sensor simultaneously.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In the following, embodiments of the present disclosure are explained in more detail with reference to the drawings. In these show:

[0029] FIG. 1 a schematic meridional section through a measuring apparatus according to the prior art;

[0030] FIG. 2 a schematic meridional section through a measuring apparatus according to a first embodiment;

[0031] FIG. 3 a top view of the reticle used as object in the embodiment according to FIG. 2;

[0032] FIG. 4 the apparatus shown in FIG. 3 with additionally drawn rays and a feeding device for similar optical systems to be measured;

[0033] FIGS. 5a and 5b meridional sections through the apparatus shown in FIG. 2 during the measurement of a zoom lens in different displacement positions of the zoom lens;

[0034] FIG. 6 a schematic meridional section through a measuring apparatus according to a second embodiment for measuring an afocal optical system;

[0035] FIGS. 7a and 7b meridional sections through a measuring apparatus according to a third embodiment, in which the reticle is axially displaceably arranged, in different displacement positions of the reticle;

[0036] FIGS. 8a and 8b meridional sections through a variant of the measuring apparatus shown in FIGS. 7a and 7b, in which the collimator additionally has a variable refractive power, in various displacement positions;

[0037] FIGS. 9a and 9b meridional sections through a measuring apparatus according to a fourth embodiment, in which the image sensor is arranged to be axially displaceable, in different displacement positions.

DESCRIPTION OF EMBODIMENTS

1. Prior Art

[0038] In order to explain the operation of the measuring apparatus according to the disclosure, reference is first made to FIG. 1, in which a measuring apparatus according to the prior art is shown in a schematic meridional section and is designated 10 in its entirety.

[0039] The measuring apparatus 10 is intended to measure the modulation transfer function (MTF) of an optical system, hereinafter referred to as specimen 12. The specimen 12 is indicated here only as a single lens; often it will be an optical system with several refractive and/or reflective optical elements. The specimen 12 is held by a holder 13. The holder 13 can comprise an adjustment device with which the specimen 12 can be positioned axially centered and untilted in the beam path of the device 10.

[0040] The modulation transfer function is an important tool for quantitatively evaluating the imaging quality of optical systems and describes the resolving power of an optical system by the ratio of the relative image contrast to the relative object contrast. When an object is imaged by an optical system, there is inevitably a reduction in quality in the image plane due to aberrations and diffraction phenomena. Manufacturing deviations as well as assembly and alignment errors also weaken the imaging performance of the specimen 12.

[0041] For measuring the modulation transfer function, the specimen 12 images an object; the modulation transfer function of the specimen 12 can be inferred from the image of the object. The object imaged by the specimen 12 is formed by a light pattern generated by a 10 light pattern generating device 14. The light pattern generating device 14 has a reticle 16 which is uniformly illuminated by a light source 20 represented as a bulb using a condenser 22.

[0042] A reticle is a glass sheet that carries a structured coating on one side. The structuring can be produced, for example, by a photolithographically defined etching process. In FIG. 1, several light-transmitting structures in the coating are designated with 18.

[0043] The specimen 12 is arranged in the measuring apparatus 10 so that its optical axis is aligned with a reference axis 24 of the measuring apparatus 10. The reference axis 24 of the apparatus 10 thereby coincides with the optical axis of the condenser 22. In addition, the holder 13 is used to axially position the specimen 12 such that the reticle 16 is arranged in the focal plane 26 of the specimen 12. As a result, the light pattern defined by the structures 18 is imaged to infinity by the specimen 12.

[0044] Two identically constructed cameras 28a, 28b are arranged on a side of the specimen 12 opposite the light pattern generation device 14. The cameras 28a, 28b each contain a lens 30 and a spatially resolving image sensor 32, which is located in a focal plane of the lens 30. A cutout of the light pattern generated by the light pattern generating device 14 is thereby formed on the image sensor 32 in each case. The cutout is thereby determined, among other things, by the arrangement of the cameras 28a, 28b with respect to the reference axis 24 and by the field of view of the cameras. The camera 28a, whose optical axis 34a is aligned with the reference axis 24, captures an image of a structure 18 in the center of the reticle 16. The optical axis 34b of the other camera 28b is inclined to the reference axis 24. As a result, the camera 28b captures an image of one of the outer structures 18.

[0045] Other cameras are usually arranged around the central camera 28, which are not shown in FIG. 1 for reasons of clarity. These other cameras capture the images of the other structures 18. By evaluating the images of the structures 18 formed on the image sensors 32 of the cameras 28a, 28b, the modulation transfer function of the specimen 12 can be determined in a manner known as such.

[0046] The conventional setup shown in FIG. 1 is particularly advantageous when the focal length of the specimen is small and the field of view is correspondingly large. Cameras can then be arranged in such a way that they can pick up light which leaves the specimen 12 at very large angles relative to the reference axis 24.

[0047] With the known measuring apparatus 10 shown in FIG. 1, it is not possible to make a statement about the modulation transfer function at field points that are not covered by the field of view of one of the cameras 28a, 28b. However, it is often desirable to measure the modulation transfer function at as many different field positions as possible. It is clear from FIG. 1 that, because of the limited installation space, the number of cameras cannot be increased arbitrarily.

2. First Embodiment

[0048] FIG. 2 shows a meridional section of an apparatus according to the disclosure and designated 10, in an illustration similar to FIG. 1.

[0049] Components marked with unstroked reference numerals X correspond to components X in FIG. 1 and are only explained again if there are differences worth mentioning.

[0050] The plurality of cameras 28a, 28b are replaced in the apparatus 10 according to the disclosure by a single camera 28, which also has collecting optics 30. In the illustrated embodiment, the distance a between a sensor plane 33 and the collecting optics 30 (or, more precisely, its image-side main plane H) is equal to the focal length f of the collecting optics 30. Measuring light that is incident onto the collecting optics 30 as collimated beam is therefore focused on the image sensor 32.

[0051] The dimensions of the image sensor 32 and the collecting optics 30 may be selected such that the entire field of view of the specimen 12 is captured by the image sensor 32. This means that all structures 18 on the reticle 16, which is shown in a top view in FIG. 3, can be imaged simultaneously by the specimen 12 and the collecting optics 30 on the image sensor 32, as long as the structures 18 are in the field of view of the specimen. If, for example, further field points are to be measured in another measurement at positions which are located between the structures 18, it is only necessary to exchange the reticle 16 for another reticle which contains structures at the desired field positions. The use of structures that extend over the entire reticle is also possible in principle.

[0052] In FIG. 4, the FOV (Field Of View) of the specimen 12 is indicated by dashed lines. The field of view of an imaging optical system is the area in the three-dimensional object space that can be sharply imaged with the optical system. In the rectangular imaging fields typically present, the field of view is an infinite truncated pyramid whose pyramid apex lies in the entrance pupil of the specimen 12. The aperture angles of the truncated pyramid are determined by the dimensions of the image field and the focal length of the specimen 12.

[0053] In FIG. 4, it is assumed that the specimen 12 is measured in the reverse direction of light. In later use, the light passes through the specimen 12 from above as seen in FIG. 4, which is why the field of view FOV is drawn on the side of the image sensor 32. On the image side, the field of view FOV corresponds to the image space, i.e. each point in the field of view corresponds to a point in the image space.

[0054] The apparatus 10 is characterized by the fact that the entire reticle 16 lying in the field of view FOV or image space is simultaneously imaged onto the image sensor 32. The collecting optics 30 and the image sensor 32 are thus designed so that all field points that can be imaged by the specimen 12 are actually imaged on the image sensor 32. In this way, distortion of the specimen 12, for example, can be measured very easily and with a high degree of accuracy, since, unlike conventional apparatuses of this type, no individual images are produced, but the entire field of view/image field is captured. Typically, a reticle 16 whose structures 18 form a regular grid is used to measure the distortion.

[0055] It is also very easy to measure the size of the field of view FOV, since the image sensor 32 is normally larger than the field of view. Furthermore, any edge light fall-off can be easily detected with the apparatus 10.

[0056] Indicated by 38 in FIG. 4 is a feed device with which a large number of similar specimens 12 can be fed to the apparatus 10 in an automated quality inspection process and measured there with respect to their optical properties. The specimens are conveyed step by step along the feed direction indicated by an arrow 40 in such a way that the specimens 12 are positioned one after the other in the beam path of the apparatus 10.

[0057] FIGS. 5a and 5b show meridional sections through the apparatus 10 shown in FIGS. 2 and 4 during the measurement of a zoom lens 112 in different zoom positions. The position of the object-side main plane H1 of the zoom lens 112 is indicated in each case by a dashed line. The advantages of the apparatus 10 according to the present disclosure are particularly obvious when measuring the zoom lens 112. Indeed, when the zoom lens 112 is moved axially by shifting a plurality of lens elements, the images of the structures 18 do not remain stationary, but move in a radial direction across the image plane. In FIGS. 5a and 5b, this can be seen in the position of the off-axis pixel on the image sensor 32. With conventional apparatuses of this type, as shown in FIG. 1, the images of the structures 18 would migrate out of the field of view of the individual cameras 28a, 28b and could no longer be evaluated. With the apparatus 10 according to the disclosure, on the other hand, these images can be captured and evaluated simultaneously in all positions of the zoom lens 112, i.e., regardless of the imaging scale 8, without having to adjust the apparatus 10.

3. Second Embodiment

[0058] FIG. 6 shows a meridional section of an apparatus 210 according to the disclosure in accordance with a second embodiment. The apparatus 210 is configured to measure optical properties of specimens 212 that are afocal. In the illustrated embodiment, the specimen 212 is a bilateral telecentric lens. However, prisms or waveguides, such as those used in AR or VR systems, are also afocal.

[0059] In the apparatus 210, a collimator 42 is arranged between the reticle 16 and the holder 13 for the specimen 212, in whose focal plane 226 the reticle 16 is located. The collimator 42 images the reticle 16 to infinity so that the specimen 212 is located in the collimated beam path. Otherwise, the apparatus 210 does not differ from the apparatus 10 of the first embodiment.

[0060] The collimator 42 can be designed as a conoscopic lens. This allows a virtual aperture to be optically created in the plane of the specimen without having to introduce a physical aperture into the beam path near the specimen.

4. Third Embodiment

[0061] FIGS. 7a and7b show meridional sections of an apparatus 310 according to the disclosure in accordance with a third embodiment. The apparatus 310 is substantially the same as the apparatus 210 shown in FIG. 6, but here the collimator 42 is housed in a common housing 43 together with the light pattern generating device 14. In addition, the reticle 16 of the light pattern generating device 14 is movable along the optical axis 34 by means of an adjusting device 46. If the reticle 16 is located exactly in the focal plane of the collimator 42, as shown in FIG. 7a, the collimator 42 images the structures 18 on the reticle 16 to infinity so that collimated light passes through the specimen 12. If the reticle 16 is moved out of the focal plane of the collimator 42, the light behind the collimator 42 is no longer collimated, but diverged or converged. FIG. 7b shows the case where the specimen 12 is in the convergent beam path.

[0062] If the collimator additionally has a variable focal length, which is indicated in FIGS. 8a and 8b by an adjustment device 44, then beam paths having different beam diameters can be set by simultaneously adjusting the collimator 42 and moving the reticle 16, as shown by a comparison of FIGS. 8a and 8b. The ability to adjust, without loss of light, the size of the area illuminated in the specimen 12 is sometimes advantageous in certain measurement tasks, for example when measuring photometric quantities.

5. Fourth Embodiment

[0063] FIGS. 9a and 9b show meridional sections through an apparatus 410 according to the disclosure in accordance with a fourth embodiment. In this embodiment, the image sensor 32 is movable along the optical axis 34 of the apparatus 410 by means of an adjustment device 48. This is particularly advantageous if the measuring light emanating from the specimen 12 is not exactly collimated. The reason for this may be, for example, that the reticle 16 is not located exactly in the focal plane of the specimen 12, or that in the embodiment shown in FIGS. 6 to 8 the specimen 12 is only approximately afocal.

[0064] FIG. 9a illustrates the case where the light emanating from the specimen 12 is exactly collimated and the distance a between the collecting optics 30 and the image sensor 32 is equal to the focal length f of the collecting optics 30. FIG. 9b illustrates how a sharp image can be obtained by reducing the distance a, even though the light emanating from the specimen 12 converges slightly. Generally, the distance a deviates no more than 10% from the focal length f.

[0065] The apparatus 410 can also be used to measure image field curvature in a simple manner. For this purpose, for example, several images of the reticle 16 can be taken in different axial displacement positions of the image sensor 32 and the image contrast can be measured as a function of the distance a.