Apparatus and method for measuring optical properties of an optical system - Patents.com

Abstract

An apparatus for measuring optical properties of an optical system (12, 112, 212) comprises an object (16) to be imaged having a plurality of structures (18) arranged in a plane and separated from one another, a two-dimensional image sensor (32) and a collecting optics (30) having a focal length f. The image sensor (32) has a distance a from the collecting optics (30) where 0.9·f≦a≦1.1·f. A holder (13) for the optical system (12, 112, 212) is arranged such that the optical system is located in a beam path between the object (16) and the collecting optics (30). According to the invention, the image sensor (30) and the collecting optics (30) are designed in such a way that all structures (18) can be imaged simultaneously on the image sensor (32) by the optical system (12, 112, 212) and the collecting optics (30).

Description

[Technical field]

[0001] The present invention relates to an apparatus and method for measuring the optical properties of an optical system. The optical properties can be, for example, the imaging properties of a refractive or afocal optical system, as can be described by the modulation transfer function (MTF), distortion, or chief ray angle. [Background technology]

[0002] In the prior art, devices are known for measuring the MTF, in which a light pattern is generated in the focal plane of the optical system to be measured, which may consist, for example, of an arrangement of crosshairs, dots or line patterns. To generate the light pattern, for example, a reticle can be used which is illuminated by a light source and has a pattern of apertures.

[0003] On the other side of the optical system, a lens and a number of mutually independent cameras are arranged, each having an image sensor arranged in the focal plane of the lens. The cameras are distributed in such a way that each camera captures an image of exactly one cutout of the light pattern (typically exactly one crosshair) with its image sensor. In this way, the modulation transfer function can be measured independently of each other at several field positions.

[0004] Due to the ever-increasing demands on the imaging properties of optical systems, it is desirable to measure the modulation transfer function or another imaging property of the optical system at as many field positions as possible. Since a separate camera, which requires a certain construction volume, is required for each cutout of the light pattern, it becomes difficult to arrange a corresponding number of cameras with a high density so that the measurement light can enter the corresponding camera from the desired direction with a larger number of field positions.

[0005] From WO 2016 / 180525 A1 (corresponding to US 2018 / 0136079) an apparatus for measuring the MTF is known, which makes it possible to arrange more cameras than in conventional arrangements by means of special beam folding.

[0006] However, even with this method, it is not possible to measure the MTF for all field points. This is particularly problematic when the optical system to be measured has a variable focal length and the MTF is to be measured for different focal lengths. When the focal length is changed, the individual images of the crosshairs are produced in different locations, so the camera must be realigned every time the focal length is changed. This is time-consuming and therefore not economical for high-speed measurements of mass-produced optical systems, such as machine-made optical elements of smartphone cameras. Other devices for measuring optical properties are also known from Taiwan Utility Model Application No. 579270, Taiwan Utility Model Application No. 583048, Taiwan Utility Model Application No. 579270, and China Utility Model Application No. 206638403. Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention aims to provide an apparatus and method for measuring optical properties of an optical system, whereby measurements can be performed at as many different field points as possible using a minimum of equipment. [Means for solving the problem]

[0008] According to the invention, this object is achieved by an apparatus for measuring optical properties of an optical system, comprising an imaged object having a plurality of structures arranged in a plane and separated from one another. The apparatus further comprises a two-dimensional image sensor, a collecting optics with a focal length f, the image sensor having a distance a from the collecting optics with 0.9·f≦a≦1.1·f, and a holder for the optical system. The holder is arranged such that the optical system is in the optical path between the object and the collecting optics. The image sensor and the collecting optics are configured in such a way that all structures can be imaged simultaneously by the optical system and the collecting optics onto the image sensor.

[0009] The invention is based on the idea that, at least in the case of optical systems with a long focal length and a correspondingly small field of view (FOV), the entire object, and therefore several structures separated from one another, can be imaged simultaneously on an image sensor via optical systems and collecting optics. The large high-resolution image sensors required for this, ideally with more than 10 megapixels, are now available at reasonable cost. Due to the overall larger étendue, the collecting optics are also more complex than in the case of camera lenses used up to now. However, since only one image sensor and one collecting optic need to be provided, preferably arranged axially centered, the design and manufacturing effort for the device according to the invention is nevertheless significantly lower than in the case of conventional devices in which multiple cameras are arranged in a dome-like manner above the optical system to be measured. The object can be an illuminated reticle, as known in the prior art. The structures are typically line patterns or single or double aiming lines.

[0010] It is particularly advantageous if the holder is arranged in the diverging beam path. The optical system to be measured has a variable focal length and can in particular be designed as a zoom lens. Whereas in conventional devices, when the focal length of the optical system to be measured is changed, the image of the structure generally moves completely or partially outside the imaging field of the respective camera, in the device according to the invention, when the focal length is changed, the image can always be completely captured by the image sensor. Therefore, no adjustment or readjustment has to be performed when the focal length of the optical system is changed. This means that the device can also be used to check the imaging properties as part of the quality control of zoom lenses intended for use in devices such as smartphones or webcams.

[0011] In another embodiment, a collimator is arranged in the optical path between the object and the holder. In this case, the optical system to be measured is not in the divergent beam path, but in the parallel beam path. Such an arrangement is advantageous when the optical system to be measured itself does not have a refractive power and is therefore afocal. Examples of such optical systems are bilaterally telecentric lenses, prisms, or waveguides. In this case, the optical property to be measured can be a photometric quantity, such as the angle-dependent luminance or color distribution.

[0012] It is particularly advantageous if the collimator is a conoscopic lens. This allows a virtual aperture to be created in a plane parallel to the surface of the test object. This property of the conoscopic lens has the advantage that no physical aperture has to be introduced into the beam path near the test object.

[0013] The object can be arranged to move along the optical axis of the device to vary the distance between the collimator and the object. If the object is positioned exactly in the focal plane of the collimator, the optical system to be measured is in a collimated beam path. At all other axial displacement positions of the object, the beam path is only approximately collimated. In this way, it is possible to determine, for example, at which optical condition the optical system to be measured has the best optical properties. Preferably, the object and the collimator are arranged in a common housing for this purpose. If the collimator also has a variable focal length, the diameter of the illuminated area in the optical system can be changed by changing the focal length of the collimator and the distance to the object.

[0014] The measurement light emerging from the optical system to be measured does not have to be perfectly collimated (i.e. the image of the object is at infinity). Even images at a great distance, for example more than 2 meters, do not require a large sacrifice of measurement accuracy and allow highly accurate measurements of optical properties. If deviations from a collimated beam path are no longer acceptable, this can be accommodated by moving the image sensor along the optical axis. The same effect can be achieved if the collecting optics has a variable focal length. Thus, the distance a between the collecting optics and the image sensor can deviate from the focal length f of the collecting optics by up to 10%, resulting in the condition 0.9·f≦a≦1.1·f. In some embodiments the deviation is up to 5%, in other embodiments no deviation is allowed.

[0015] With regard to the method, the object defined in the introduction is to provide a method for measuring optical properties of an optical system, comprising the steps of: providing a two-dimensional image sensor, a collecting optics having a focal length f, where the image sensor is at a distance a from the collecting optics, where 0.9·f≦a≦1.1·f, and an object having a plurality of structures arranged in a plane and separated from one another; Inserting a measurement target optic in the beam path between the object and the collecting optic; Simultaneous imaging of the test piece and objects within the field of view of the collection optics onto an image sensor using an optical system and a collection optics; evaluating an image of the object formed on the image sensor to determine optical characteristics; The considerations and advantages explained above with respect to the device according to the invention apply accordingly to the method.

[0016] In one embodiment, the optical system has a variable focal length and the optical properties of the optical system are measured for at least two different focal lengths.

[0017] In another embodiment, the optical system is afocal. In this case, a collimator is placed in the optical path between the object and the optical system. The object can be moved along the optical axis of the device between successive measurements of the same optical system or between measurements of different optical systems.

[0018] The method according to the invention can be used to measure at least one optical property selected from the group consisting of distortion, image field curvature, field of view and edge light fall-off. The method according to the invention uses a large image sensor so that objects in the entire field of view of the test object can be imaged, resulting in a high measurement point density. As a result, the above-mentioned optical properties of the test object can be measured with high precision and to a higher order. For example, it is possible to determine high-order distortion polynomials, which was only possible to a limited extent with known measuring devices. In addition, the large image sensor allows structures that extend over the entire image field, for example solid lines, to be imaged. This allows new approaches for evaluation, which are advantageous, among other things, for measuring the field of view. [Brief description of the drawings]

[0019] In the following, embodiments of the invention are explained in more detail with reference to the drawings. [Figure 1] FIG. 1 is a schematic meridional section through a measuring device according to the prior art; [Diagram 2]1 is a schematic meridional section through a measuring device according to a first embodiment; [Diagram 3] FIG. 3 is a top view of a reticle used as an object in the embodiment according to FIG. 2; [Figure 4] 4 is the apparatus shown in FIG. 3 with additionally depicted beam and delivery devices for a similar target optical system. [Figure 5a-5b] 3A-3C are meridional sections through the apparatus shown in FIG. 2 during measurement of the zoom lens at different displacement positions of the zoom lens; [Figure 6] 5 is a schematic meridional section through a measuring device according to a second embodiment for measuring an afocal optical system; FIG. [Figure 7a-7b] 5A to 5C are meridional sections through a measurement apparatus according to a third embodiment, in which a reticle is arranged axially displaceable, at different displacement positions of the reticle; [Figure 8a-8b] 7a and 7b, in which the collimator additionally has a variable optical power, in various displacement positions. [Figure 9a-9b] 5A-5C show meridional sections through a measuring device according to a fourth embodiment, in which an image sensor is arranged so as to be axially displaceable, in different displacement positions; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] 1. Conventional Technology To explain the operation of the measuring apparatus according to the present invention, reference is first made to Figure 1, in which a first prior art measuring apparatus is shown in schematic meridional section and generally designated 10'.

[0021] The measuring device 10' is intended to measure the modulation transfer function (MTF) of an optical system, hereafter referred to as test object 12'. Test object 12' is indicated here only as a single lens, but in many cases it will be an optical system with several refractive and / or reflective optical elements. Test object 12' is held by a holder 13', which may include an adjustment device by which test object 12' can be positioned axially centered and tilt-free in the beam path of the device 10'.

[0022] The modulation transfer function is an important tool for quantitatively evaluating the imaging quality of an optical system, and describes the resolution of the 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 will inevitably be degradation in quality at the image plane due to aberrations and diffraction phenomena. Manufacturing deviations, as well as assembly and alignment errors, will also weaken the imaging performance of the test object 12'.

[0023] To measure the modulation transfer function, the test object 12' is imaged to an object, from which the modulation transfer function of the test object 12' can be inferred. The object imaged by the test object 12' is formed by a light pattern generated by a light pattern generating device 14'. The light pattern generating device 14' has a reticle 16' that is uniformly illuminated by a light source 20', represented as a light bulb with a concentrator 22'.

[0024] A reticle is a glass plate with 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 by 18'.

[0025] The test object 12' is placed in the measurement device 10' such that its optical axis is aligned with a reference axis 24' of the measurement device 10'. This causes the reference axis 24' of the device 10' to coincide with the optical axis of the collector 22'. In addition, the holder 13' is used to axially position the test object 12' such that the reticle 16' is located in the focal plane 26' of the test object 12'. As a result, the light pattern defined by the structure 18' is imaged by the test object 12' to infinity.

[0026] Two identically constructed cameras 28a', 28b' are arranged on the side of the test object 12' opposite the light pattern generating device 14'. The cameras 28a', 28b' each include a lens 30' and a spatially resolved image sensor 32' located in the focal plane of the lens 30'. This results in a cutout of the light pattern generated in each case by the light pattern generating device 14' on the image sensor 32'. The cutout is determined, among other things, by the positioning of the cameras 28a', 28b' relative 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 the structure 18' in the center of the reticle 16'. The optical axis 34b' of the other camera 28b' is tilted relative to the reference axis 24'. As a result, the camera 28b' captures an image of one of the outer structures 18'.

[0027] Other cameras, not shown in FIG. 1 for clarity, are typically positioned around the central camera 28'. These other cameras capture images of 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 test object 12' can be determined, in a manner known as such.

[0028] 1 is particularly advantageous when the focal length of the test object is small and the field of view is correspondingly large. The cameras can then be positioned in such a way that they can capture light exiting the test object 12' at a very large angle relative to the reference axis 24'.

[0029] In the known measuring device 10' shown in Fig. 1, it is not possible to describe the modulation transfer function at field points that are not covered by the field 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. From Fig. 1 it is clear that due to limited installation space the number of cameras cannot be increased arbitrarily.

[0030] 2. First embodiment Figure 2 shows a meridional section of a device according to the invention, designated 10, in a view similar to that of Figure 1. The components labelled with the reference letter X without dashes correspond to the components X' in Figure 1 and will be described again only if there are differences worth mentioning.

[0031] The multiple cameras 28a, 28b are replaced in the device 10 according to the invention by a single camera 28, which also comprises a collecting optics 30. In the embodiment shown, the distance a between the sensor plane 33 and the collecting optics 30 (or, more precisely, its image-side principal plane H) is equal to the focal length f of the collecting optics 30. Thus, the measurement light incident on the collecting optics 30 as a collimated beam is focused onto the image sensor 32.

[0032] The dimensions of the image sensor 32 and the collection optics 30 are preferably selected such that the entire field of view of the test object 12 is captured by the image sensor 32. This means that all structures 18 on the reticle 16, shown in the top view in FIG. 3, can be imaged simultaneously onto the image sensor 32 by the test object 12 and the collection optics 30, as long as the structures 18 are within the field of view of the test object. If, for example, in another measurement, further field points at positions located between the structures 18 are to be measured, it is only necessary to exchange the reticle 16 for another reticle that contains the structures at the desired field positions. The use of structures that extend over the entire reticle is also possible in principle.

[0033] In Fig. 4, the FOV (field of view) of the test object 12 is indicated by a dashed line. The field of view of an imaging optical system is the area in three-dimensional object space that can be sharply imaged with the optical system. For the commonly occurring rectangular image field, the field of view is an infinite truncated pyramid with the pyramid apex in the entrance pupil of the test object 12. The opening angle of the truncated pyramid is determined by the size of the image field and the focal length of the test object 12.

[0034] In Fig. 4, it is assumed that the test object 12 is measured in the reverse direction of light. In later usage, the light passes through the test object 12 from above as viewed in Fig. 4. This is why the field of view FOV is depicted 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.

[0035] The apparatus 10 is characterized by the fact that the entire reticle 16 within the field of view FOV or image space is imaged simultaneously onto the image sensor 32. The collection optics 30 and the image sensor 32 are therefore designed in such a way that all field points that can be imaged by the test object 12 can actually be imaged onto the image sensor 32. In this way, unlike conventional apparatuses of this type, distortions of the test object 12, for example, can be measured very easily and with high precision, since no individual images are produced but the entire field of view / imaging field is captured. Typically, a reticle 16 in which the structures 18 form a regular grid is used to measure the distortions.

[0036] It is also very easy to measure the size of the field of view FOV, since the image sensor 32 is usually larger than the field of view. Furthermore, any vignetting can be easily detected with the apparatus 10. Indicated by 38 in Fig. 4 is a feed device, by which a number of similar test objects 12 can be fed in an automatic quality inspection process to the apparatus 10, where they can be measured for their optical properties. The test objects are transported one by one along a feed direction indicated by arrow 40, in such a way that the test objects 12 are positioned one after the other in the beam path of the apparatus 10.

[0037] 5a and 5b show meridional sections through the device 10 shown in Figs. 2 and 4 during the measurement of the zoom lens 112 in different zoom positions. The position of the object-side principal plane H1 of the zoom lens 112 is indicated in each case by a dashed line. The advantages of the device 10 according to the invention are particularly evident when measuring the zoom lens 112. Indeed, when the zoom lens 112 is moved axially by shifting the lens elements, the images of the structures 18 do not remain stationary but move radially across the image plane. In Figs. 5a and 5b this can be seen in the positions of the off-axis pixels on the image sensor 32. In a conventional device of this kind as shown in Fig. 1, the images of the structures 18 would move outside the field of view of the individual cameras 28a, 28b and would no longer be able to be evaluated. On the other hand, the device 10 according to the invention allows these images to be captured and evaluated simultaneously in all positions of the zoom lens 112, i.e. regardless of the imaging ratio β, without the need to adjust the device 10.

[0038] 3. Second embodiment 6 shows a meridional section of an apparatus 210 according to the invention according to a second embodiment. The apparatus 210 is configured to measure the optical properties of a test object 212 that is afocal. In the embodiment shown, the test object 212 is a double telecentric lens. However, prisms or waveguides, such as those used in AR or VR systems, are also afocal.

[0039] In the device 210, a collimator 42 is arranged between the reticle 16 and a holder 13 for a test object 212 in whose focal plane 226 the reticle 16 is located. The collimator 42 images the reticle 16 to infinity, so that the test object 212 is located in the collimated beam path. In other respects, the device 210 does not differ from the device 10 of the first embodiment. The collimator 42 can be designed as a conoscopic lens. This allows a virtual aperture to be optically created in the plane of the test object without the need to introduce a physical aperture in the beam path near the test object.

[0040] 4. Third embodiment 7a and 7b show meridional sections of an apparatus 310 according to the present invention according to a third embodiment. The apparatus 310 is substantially the same as the apparatus 210 shown in FIG. 6, but here the collimator 42 is accommodated together with the optical pattern generating device 14 in a common housing 43. In addition, the reticle 16 of the optical pattern generating device 14 is movable along the optical axis 34 using an adjustment device 46. As shown in FIG. 7a, when the reticle 16 is exactly located in the focal plane of the collimator 42, the collimator 42 images the structures 18 on the reticle 16 to infinity, so that the collimated light passes through the test object 12. When 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 diverges or converges. FIG. 7b shows the case when the test object 12 is in the convergent beam path.

[0041] If the collimator additionally has a variable focal length, as indicated in Figures 8a and 8b by adjustment device 44, then it is possible to set beam paths with different beam diameters by simultaneously adjusting collimator 42 and moving reticle 16, as shown by a comparison of Figures 8a and 8b. The ability to adjust the size of the illuminated area in test object 12 without loss of light is sometimes advantageous in certain measurement tasks, for example when measuring photometric quantities.

[0042] 5. Fourth embodiment Figures 9a and 9b show meridional sections through an apparatus 410 according to the invention according to 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 measurement light emitted from the test object 12 is not strictly collimated. The reason for this can be, for example, that the reticle 16 is not exactly located in the focal plane of the test object 12 or that, in the embodiment shown in Figures 6 to 8, the test object 12 is only approximately afocal.

[0043] Figure 9a shows the case where the light emanating from the test object 12 is tightly collimated and the distance a between the collection optics 30 and the image sensor 32 is equal to the focal length f of the collection optics 30. Figure 9b shows how by reducing the distance a, a sharp image can be obtained even if the light emanating from the test object 12 is slightly convergent. Typically, the distance a does not deviate from the focal length f by more than 10%.

[0044] The apparatus 410 can also be used to measure the image field curvature in a simple manner. For this purpose, for example, several images of the reticle 16 can be acquired at different axial displacement positions of the image sensor 32 and the image contrast can be measured as a function of the distance a.

Claims

1. An apparatus for measuring optical characteristics of an optical system (12, 112, 212) having a zoom objective lens with different focal lengths at different zoom positions, comprising: An object to be imaged (16) having a plurality of structures (18) disposed in a plane and separated from one another; A two-dimensional image sensor (32); a collecting optics (30) having a focal length f, and the image sensor (32) has a distance a from the collecting optics (30), where 0.9·f≦a≦1.1·f; a holder (13) for said optical system (12, 112, 212) arranged such that said optical system is located in a beam path between said object (16) and said collecting optics (30); Equipped with an apparatus, characterized in that the image sensor (32) and the collecting optics (30) are configured such that they are not affected by the zoom position of the optical system (212) and that all structures (18) can be imaged simultaneously onto the image sensor (32) by the optical system (12, 112, 212) and the collecting optics (30) without adjusting the collecting optics (30) and / or the image sensor (32) of the apparatus.

2. 2. The apparatus of claim 1, wherein the object is an illuminated reticle (16).

3. 3. Apparatus according to claim 1 or 2, characterized in that said structure (18) is a crosshair.

4. Apparatus according to any one of claims 1 to 3, characterized in that the holder (13) is arranged in the diverging beam path.

5. The device according to any one of claims 1 to 3, characterized in that a collimator (42) is arranged in the optical path between the object (16) and the holder (13).

6. 6. The apparatus of claim 5, wherein the collimator (42) is a conoscopic lens.

7. 7. The apparatus according to claim 5 or 6, characterized in that the object (16) is movable along an optical axis (34) of the apparatus (310) to vary the distance between the collimator (42) and the object (16).

8. Device according to any one of the preceding claims, characterized in that the image sensor (32) is movable along an optical axis (34) of the device.

9. A method for measuring optical characteristics of an optical system (12, 112, 212) having a zoom objective lens with different focal lengths at different zoom positions, comprising the steps of: providing a two-dimensional image sensor (32), a collecting optics (30) having a focal length f, the image sensor (32) having a distance a from the collecting optics (30) such that 0.9·f≦a≦1.1·f, and an object (16); inserting the optical system (12, 112, 212) to be measured in a beam path between the object (16) and the collecting optics (30); using the optical system (12, 112, 212) and the collecting optics (30) to simultaneously image the object (16) within a field of view (FOV) of the optical system (12, 112, 212) and the collecting optics (30) onto the image sensor (32) without being affected by the zoom position of the optical system (212) and without adjusting the collecting optics (30) and / or the image sensor (32); evaluating an image of the object (16) formed on the image sensor (32) to determine the optical characteristics; A method comprising:

10. The method of claim 9, wherein the optical characteristics of the optical system (212) are measured for at least two different focal lengths.

11. 10. The method of claim 9, wherein the optical system (312) is afocal and a collimator (42) is disposed in the optical path between the object (16) and the optical system (312).

12. The method according to any one of claims 9 to 11, characterized in that at least one optical property is selected from the group consisting of distortion, image field curvature, field of view and vignetting.

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