Equipment and methods for measuring the optical properties of optical systems.

Abstract

The optical properties measuring device of the optical system (12; 112; 212) has an object (16) to be photographed, with structures (18) arranged in a plane and separated from each other, a two-dimensional image sensor (32) and an optical collecting unit (30) with focal length f. The image sensor (32) is at a distance from the optical collecting unit (30) equal to 0.9·f ≤ a ≤ 1.1·f. The support (13) for the optical system (12; 112; 212) is arranged so that the optical system is placed in the beam path between the object (16) and the optical collecting unit (30). According to the invention, the image sensor (30) and the optical collecting unit (30) are designed in such a way that all structures (18) can be photographed by the optical system (12; 112; 212) and the optical collecting unit (30) onto the image sensor (32) simultaneously.

Description

[0001] Apparatus and method for measuring an optical property of an optical system

[0002] BACKGROUND OF THE INVENTION

[0003] 1. Field of the invention

[0004] The invention relates to a device and a method for measuring an optical property of an optical system. The optical property can be, for example, the imaging properties of a refractive or afocal optical system, as can be described by the modulation transfer function (MTF), the distortion, or the chief ray angle.

[0005] 2. State of the art

[0006] Devices for measuring MTF are known in the prior art. These devices generate a light pattern in a focal plane of the optical system to be measured. This pattern can consist, for example, of an arrangement of reticles, dots, or line patterns. A reticle illuminated by a light source and having a pattern of apertures can be used to generate the light pattern.

[0007] On the opposite side of the optical system, several independent cameras are arranged, each with a lens and an image sensor positioned in a focal plane of the lens. The cameras are distributed so that each camera captures the image of exactly one section of the light pattern (typically exactly one reticle) with its image sensor. This allows the modulation transfer function to be measured independently at multiple field positions.

[0008] Due to 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 each section of the light pattern requires a separate camera, which requires a certain amount of space, it becomes difficult, with larger numbers of field positions, to arrange a corresponding number of cameras densely enough to allow the measurement light to enter the corresponding cameras from the desired direction.

[0009] From WO 2016 / 180525 A1 (corresponds to US 2018 / 0136079 A) a device for measuring the MTF is known in which, by means of a special beam folding, more cameras can be arranged than is the case with the conventional arrangement.

[0010] However, even this approach does not allow the MTF to be measured for all field points. This is particularly problematic if the optical system being measured has a variable focal length and the MTF needs to be measured for different focal lengths. Since the individual images of the reticle lines are created at different locations when the focal length changes, the cameras must be realigned each time the focal length changes. This is time-consuming and therefore not cost-effective for the rapid measurement of optical systems produced in large quantities, such as machine-made optics for smartphone cameras.

[0011] Further devices for measuring an optical property are known from TW M579270 U, TW M583048 U, TW M579270 U and CN 206638403 U.

[0012] SUMMARY OF THE INVENTION

[0013] The object of the invention is to provide a device and a method for measuring an optical property of an optical system, in which the measurement can be carried out at as many different field points as possible with little equipment expenditure.

[0014] According to the invention, this object is achieved by a device for measuring an optical property of an optical system, which device comprises an object to be imaged, which has a plurality of structures arranged in a plane and separated from one another. The device further comprises a two-dimensional image sensor, a collecting optic with a focal length f, wherein the image sensor is at a distance a from the collecting optic with 0.9-f < a < 1.1-f, and a holder for the optical system. This holder is arranged such that the optical system is located in the beam path between the object and the collecting optic. The image sensor and the collecting optic are designed such that all structures can be imaged by the optical system and the collecting optics simultaneously onto the image sensor.

[0015] The invention is based on the idea that, at least in optical systems with a long focal length and a correspondingly small field of view (FOV), the entire object and thus several separate structures can be imaged simultaneously onto the image sensor via the optical system and the collecting optics. The large, high-resolution image sensors required for this – ideally with more than 10 megapixels – are now available at reasonable costs. Due to the generally higher optical conductance, the collecting optics are also more complex than in the camera lenses used previously.However, since only one image sensor and one collecting optics have to be provided and preferably arranged axially centered, the design and manufacturing effort for the device according to the invention is nevertheless significantly lower than for conventional devices in which a plurality of cameras are arranged dome-like above the optical system to be measured.

[0016] The object can be an illuminated reticle, as is known in the art. The structures are typically line patterns or single or double crosshairs.

[0017] It is particularly advantageous if the holder is arranged in the diverging beam path. The optical system to be measured can then have a variable focal length and, in particular, be designed as a zoom lens. While with conventional devices the images of the structures generally move completely or partially out of the image fields of the individual cameras when the focal length of the optical system to be measured is changed, with the device according to the invention the images can always be completely captured by the image sensor when the focal length is changed. Thus, no adjustments or adjustments whatsoever would have to be made when the focal length of the optical system is changed. The device can therefore 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.

[0018] In another embodiment, a collimator is arranged in the light path between the object and the holder. In this case, the optical system to be measured is not located in a diverging, but in a parallel beam path. Such an arrangement is advantageous when the optical system to be measured itself has no refractive power and is therefore afocal. Examples of such optical systems are lenses with telecentricity on both sides, prisms, or waveguides. The optical property to be measured can then be a photometric quantity such as an angle-dependent luminance or color distribution.

[0019] 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 test specimen surface. This feature of the conoscope has the advantage that no physical aperture needs to be inserted into the beam path near the test specimen.

[0020] The object can be arranged so that it can be moved along an optical axis of the device in order to change 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 the collimated beam path. In all other axial positions of the object, the beam path is only approximately collimated. In this way, one can determine, for example, under which optical conditions the optical system to be measured has the best optical properties. For this purpose, the object and the collimator are preferably arranged in a common housing.

[0021] If the collimator also has an adjustable focal length, the diameter of the area illuminated in the optical system can be changed by varying the collimator's focal length and the distance to the object. The measuring light emerging from the optical system to be measured does not need to be perfectly collimated (i.e., the image of the object lies at infinity). Even imaging at a great distance, e.g., more than two meters, allows for a highly precise measurement of the optical property without any significant compromise in measurement accuracy.

[0022] If deviations from the collimated beam path are no longer tolerable, this can be accommodated by moving the image sensor along the optical axis. The same effect can be achieved if the collecting optics have a variable focal length. The distance a between the collecting optics and the image sensor can therefore deviate by up to 10% from the focal length f of the collecting optics, which leads to the condition 0.9-f < a < 1.1-f. In some embodiments, the deviation is a maximum of 5%; in others, no deviation is permitted.

[0023] With regard to the method, the object posed at the outset is achieved by a method for measuring an optical property of an optical system, which comprises the following steps:

[0024] Providing a two-dimensional image sensor, a collecting optics with a focal length f, wherein the image sensor has a distance a from the collecting optics with 0.9-f < a < 1.1 -f, and an object having a plurality of structures arranged in a plane and separated from one another;

[0025] Inserting the optical system to be measured into the beam path between the object and the collecting optics,

[0026] Simultaneous imaging of the object within the field of view of the test object and the collection optics onto the image sensor using the optical system and the collection optics;

[0027] Evaluating an image of the object captured by the image sensor to determine its optical properties. The considerations and advantages explained above in connection with the device according to the invention apply accordingly to the method.

[0028] In one embodiment, the optical system has a variable focal length. The optical properties of the optical system are measured for at least two different focal lengths.

[0029] In another embodiment, the optical system is afocal. In this case, a collimator is arranged in the light path between the object and the optical system. The object can be moved along an optical axis of the device between successive measurements of the same optical system or between measurements of different optical systems.

[0030] The method according to the invention can be used to measure at least one optical property selected from the group consisting of: distortion, field curvature, field of view and marginal light falloff. Since the method according to the invention uses a large image sensor in order to be able to image an object in the entire field of view of the test object, a high measurement point density is achieved. This enables the aforementioned optical properties of the test object to be measured with high precision and in higher orders. For example, it is possible to determine a high-order distortion polynomial, which was only possible to a limited extent with the known measuring devices. In addition, the large image sensor makes it possible to image structures that extend across the entire image field, e.g. solid lines. This enables new approaches to evaluation, which is advantageous, among other things, when measuring the field of view.

[0031] BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In the following, exemplary embodiments of the invention are explained in more detail with reference to the drawings. In these drawings:

[0033] Figure 1 shows a schematic meridional section through a measuring device according to the prior art; Figure 2 shows a schematic meridional section through a measuring device according to a first embodiment;

[0034] Figure 3 is a plan view of the embodiment according to Figure

[0035] 2 reticles used as objects;

[0036] Figure 4 shows the device shown in Figure 3 with additionally drawn beams and a feed device for similar optical systems to be measured;

[0037] Figures 5a and 5b are meridional sections through the device shown in Figure 2 when measuring a zoom lens in different travel positions of the zoom lens;

[0038] Figure 6 shows a schematic meridional section through a measuring device according to a second embodiment for measuring an afocal optical system;

[0039] Figures 7a and 7b are meridional sections through a measuring device according to a third embodiment, in which the reticle is arranged to be axially displaceable, in different travel positions of the reticle;

[0040] Figures 8a and 8b meridional sections through a variant of the measuring device shown in Figures 7a and 7b, in which the collimator additionally has a variable refractive power, in different travel positions;

[0041] Figures 9a and 9b meridional sections through a measuring device according to a fourth embodiment, in which the image sensor is arranged to be axially displaceable, in different travel positions.

[0042] DESCRIPTION OF PREFERRED EMBODIMENTS

[0043] 1. State of the art To explain the functioning of the measuring device according to the invention, reference is first made to Figure 1, in which a measuring device according to the prior art is shown in a schematic meridional section and is designated overall by 10'.

[0044] The measuring device 10' is intended to measure the modulation transfer function (MTF) of an optical system, hereinafter referred to as the test object 12'. The test object 12' is indicated here only as a single lens; it will often be an optical system with several refractive and / or reflective optical elements. The test object 12' is held by a holder 13'. The holder 13' can include an adjustment device with which the test object 12' can be positioned axially centered and untilted in the beam path of the device 10'.

[0045] 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 through the ratio of the relative image contrast to the relative object contrast. When an object is imaged by an optical system, aberrations and diffraction inevitably lead to a reduction in image quality in the image plane. Manufacturing deviations, as well as assembly and alignment errors, also weaken the imaging performance of the test object 12'.

[0046] To measure the modulation transfer function, the test object 12' images an object; the modulation transfer function of the test object 12' can be deduced from the image of the object. 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', which is uniformly illuminated by a light source 20', represented as a light bulb, with the aid of a condenser 22'.

[0047] A reticle is a glass pane with a structured coating on one side. The structuring can be created, for example, using a photolithographically defined etching process. In Figure 1, several light-transmitting structures in the coating are designated 18'. The test specimen 12' is arranged in the measuring device 10' such that its optical axis is aligned with a reference axis 24' of the measuring device 10'. The reference axis 24' of the device 10' coincides with the optical axis of the condenser 22'. In addition, the test specimen 12' is axially positioned with the aid of the holder 13' such that the reticle 16' is arranged in the focal plane 26' of the test specimen 12'. As a result, the light pattern defined by the structures 18' is imaged from the test specimen 12' to infinity.

[0048] Two identically constructed cameras 28a', 28b' are arranged on a side of the test piece 12' opposite the light pattern generation device 14'. The cameras 28a', 28b' each contain a lens 30' and a spatially resolving image sensor 32' located in a focal plane of the lens 30'. This creates a section of the light pattern generated by the light pattern generation device 14' on the image sensor 32'. The section is 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 arranged at an angle to the reference axis 24'. As a result, the camera 28b' captures the image of one of the outer structures 18'.

[0049] Typically, additional cameras are arranged around the central camera 28', which are not shown in Figure 1 for clarity. These additional cameras capture images of the other structures 18'. By evaluating the images of the structures 18' generated by the image sensors 32' of the cameras 28a', 28b', the modulation transfer function of the test object 12' can be determined in a conventional manner.

[0050] The conventional setup shown in Figure 1 is particularly advantageous when the focal length of the test object is small and the field of view is correspondingly large. Cameras can then be arranged so that they can record light leaving the test object 12' at very large angles relative to the reference axis 24'. With the known measuring device 10' shown in Figure 1, no statement can be made 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. Figure 1 clearly shows that due to the limited installation space, the number of cameras cannot be increased indefinitely.

[0051] 2. First embodiment

[0052] Figure 2 shows, in a representation similar to Figure 1, a device according to the invention, designated 10, in a meridional section.

[0053] Components marked with unprimed reference numerals X correspond to components X' in Figure 1 and are only explained again if there are noteworthy differences.

[0054] In the device 10 according to the invention, the plurality of cameras 28a, 28b are replaced by a single camera 28, which also has a 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 principal plane H) is equal to the focal length f of the collecting optics 30. Measurement light that collimates and hits the collecting optics 30 is therefore focused on the image sensor 32.

[0055] The dimensions of the image sensor 32 and the collecting optics 30 are preferably selected such that the entire field of view of the test piece 12 is captured by the image sensor 32. As a result, all of the separate structures 18 on the reticle 16, which is shown in a plan view in Figure 3, can be imaged simultaneously by the test piece 12 and the collecting optics 30 onto the image sensor 32, provided the structures 18 are located in the field of view of the test piece. If, for example, additional field points are to be measured at positions located between the structures 18 during a different measurement, all that is required is to exchange the reticle 16 for another reticle that contains structures at the desired field positions. The use of structures that extend over the entire reticle is also possible in principle.

[0056] In Figure 4, the field of view (FOV) of the test object 12 is indicated by dashed lines. 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 typically rectangular image fields, the field of view is an infinite truncated pyramid, the apex of which lies in the entrance pupil of the test object 12. The aperture angles of the truncated pyramid are determined by the dimensions of the image field and the focal length of the test object 12.

[0057] Figure 4 assumes that the test piece 12 is being measured in the opposite direction of the light. During later use, the light passes through the test piece 12 as seen from above in Figure 4, which is why the field of view (FOV) is shown on the side of the image sensor 32. The field of view (FOV) corresponds to the image space on the image side, i.e., each point in the field of view corresponds to a point in the image space.

[0058] The device 10 is characterized in that the entire reticle 16 located in the field of view (FOV) or in the image space is simultaneously imaged onto the image sensor 32. The collecting optics 30 and the image sensor 32 are thus designed such that all field points that can be imaged by the test object 12 are actually imaged onto the image sensor 32. In this way, for example, a distortion of the test object 12 can be measured very easily and with high accuracy, since, unlike conventional devices of this type, no individual images are created, but rather the entire field of view / image field is captured. Typically, a reticle 16 whose structures 18 form a regular grid is used for distortion measurements.

[0059] The size of the field of view (FOV) can also be measured very easily, since the image sensor 32 is normally larger than the image field. Furthermore, any marginal light falloff can be easily detected using the device 10. Indicated by 38 in Figure 4 is a feed device with which a plurality of similar test pieces 12 can be fed to the device 10 in an automated quality assurance process and measured there with regard to their optical properties. The test pieces are conveyed step by step along the feed direction indicated by an arrow 40 such that the test pieces 12 are positioned one after the other in the beam path of the device 10.

[0060] Figures 5a and 5b show meridional sections through the device 10 shown in Figures 2 and 4 during the measurement of a zoom lens 112 in different zoom positions. The position of the object-side principal plane H1 of the zoom lens 112 is indicated by a dashed line. The advantages of the device 10 according to the invention are particularly evident when measuring the zoom lens 112.

[0061] When the zoom lens 112 is adjusted by axially displacing several lenses, the images of the structures 18 do not remain stationary, but rather move radially across the image plane. In Figures 5a and 5b, this can be seen from the position of the off-axis pixel on the image sensor 32. In conventional devices of this type, as shown in Figure 1, the images of the structures 18 would move out of the field of view of the individual cameras 28a, 28b and could no longer be evaluated. With the device 10 according to the invention, however, these images can be simultaneously captured and evaluated in all positions of the zoom lens 112, i.e., regardless of the image scale ß, without the device 10 having to be adjusted.

[0062] 3. Second embodiment

[0063] Figure 6 shows a meridional section of a device 210 according to the invention in accordance with a second exemplary embodiment. The device 210 is designed to measure optical properties of test objects 212 that are afocal. In the illustrated exemplary embodiment, the test object 212 is a bilaterally telecentric lens. However, prisms or waveguides, such as those used in AR or VR systems, are also afocal. In the device 210, a collimator 42 is arranged between the reticle 16 and the holder 13 for the test object 212, the reticle 16 being located in the focal plane 226 of the collimator. The collimator 42 images the reticle 16 to infinity, so that the test object 212 is arranged in the collimated beam path. Otherwise, the device 210 does not differ from the device 10 of the first exemplary embodiment.

[0064] The collimator 42 can be designed as a conoscopic lens. This allows a virtual aperture to be optically created in the test object plane without having to introduce a physical aperture into the beam path near the test object.

[0065] 4. Third embodiment

[0066] Figures 7a and 7b show meridional sections of a device 310 according to the invention in accordance with a third exemplary embodiment. The device 310 essentially corresponds to the device 210 shown in Figure 6. However, the collimator 42 is housed here together with the light pattern generating device 14 in a common housing 43. Furthermore, the reticle 16 of the light pattern generating device 14 can be moved along the optical axis 34 with the aid of an adjustment device 46. If the reticle 16 is located exactly in the focal plane of the collimator 42, as shown in Figure 7a, the collimator 42 images the structures 18 on the reticle 16 to infinity, so that the test specimen 12 is penetrated by collimated light. 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 diverges or converges.Figure 7b shows the case where the test specimen 12 is in the convergent beam path.

[0067] If the collimator also has a variable focal length, which is indicated in Figures 8a and 8b by an adjustment device 44, beam paths with different beam diameters can be set by simultaneously adjusting the collimator 42 and moving the reticle 16, as shown by a comparison of Figures 8a and 8b. The ability to adjust the size of the illuminated area in the test piece 12 without loss of light is sometimes advantageous for certain measurement tasks, such as the measurement of photometric quantities. 5. Fourth Embodiment

[0068] Figures 9a and 9b show meridional sections through a device 410 according to the invention according to a fourth exemplary embodiment. In this exemplary embodiment, the image sensor 32 can be moved along the optical axis 34 of the device 410 with the aid of an adjustment device 48. This is particularly advantageous when the measuring light emanating from the test piece 12 is not precisely collimated. The reason for this can be, for example, that the reticle 16 is not located exactly in the focal plane of the test piece 12, or that, in the exemplary embodiments shown in Figures 6 to 8, the test piece 12 is only approximately afocal.

[0069] Figure 9a shows the case where the light emanating from the test object 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. Figure 9b illustrates how a sharp image can be achieved by reducing the distance a, even though the light emanating from the test object 12 converges slightly. In general, the distance a deviates by a maximum of 10% from the focal length f. The device 410 can also be used to easily measure image field curvature. For this purpose, for example, several images of the reticle 16 can be recorded in different axial travel positions of the image sensor 32 and the image contrast can be measured as a function of the distance a.

Claims

PATENT CLAIMS Device for measuring an optical property of an optical system (12; 112; 212), comprising: an object (16) to be imaged, which has several structures (18) arranged in a plane and separated from each other, a two-dimensional image sensor (32), a collecting optic (30) with a focal length f, wherein the image sensor (32) has a distance a from the collecting optic (30) with 0.9-f < a < 1.1 -f, a holder (13) for the optical system (12; 112; 212), which is arranged such that the optical system is located in the beam path between the object (16) and the collecting optic (30), characterized in that the image sensor (30) and the collecting optic (30) are designed such that all structures (18) can be imaged simultaneously onto the image sensor (32) by the optical system (12; 112; 212) and the collecting optic (30). Device according to claim 1, characterized in that the object is a translucent reticle (16). Device according to claim 1 or 2, characterized in that the structures (18) are crosshairs.Device according to one of the preceding claims, characterized in that the holder (13) is arranged in the diverging beam path. Device according to one of claims 1 to 3, characterized in that a collimator (42) is arranged in the light path between the object (16) and the holder (13). Device according to claim 5, characterized in that the collimator (42) is a conoscopic lens. Device according to claim 5 or 6, characterized in that the object (16) is movable along an optical axis (34) of the device (310) to change the distance between the collimator (42) and the object (16). 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. Method for measuring an optical property of an optical system (12; 112; 212), comprising the following steps: Providing a two-dimensional image sensor (32), a collecting optic (30) with a focal length f, wherein the image sensor (32) has a distance a from the collecting optic (30) with 0.9-f < a < 1.1 -f, and an object (16); Introducing the optical system to be measured (12; 112; 212) into the beam path between the object (16) and the collecting optics (30), Simultaneous imaging of the object (16) within the field of view (FOV) of the test specimen (12; 112; 212) and the collecting optics (30) onto the image sensor (32) using the optical system (12; 112; 212) and the collecting optics (30); Evaluating an image of the object (16) generated on the image sensor (32) to determine its optical properties. The method according to claim 9, characterized in that the optical system (212) has a variable focal length, and that the optical properties of the optical system (212) are measured for at least two different focal lengths. - 17 - Method according to claim 9, characterized in that the optical system (312) is afocal, and that a collimator (42) is arranged in the light path between the object (16) and the optical system (312). Method according to any one of claims 9 to 11, characterized in that at least one optical property is measured, which is selected from the group consisting of: distortion, field curvature, field of view and marginal light falloff.

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