DEVICE FOR IMAGE BY AN OPTICAL SYSTEM UNDER TESTING AND SYSTEM AND METHOD FOR TESTING AN OPTICAL SYSTEM

Description

[0001] The invention relates to a device for imaging by an optical system to be tested and a testing system as well as a method for testing an optical system.

[0002] To measure the image quality of an optical system using a modulation transfer function (MFT), an object can be imaged onto a sensor via the optical system under test, and the MFT can then be calculated from the intensity distribution received by the sensor. However, a two-dimensional measurement of the MFT using a conventional test structure, often due to a non-rotationally symmetric focus shape, can typically result in increased sensor noise. Conventional test structures include, for example, a slit reticle, a cross reticle, a pinhole reticle, or an H-reticle.

[0003] DE 101 54 125 A1 relates to a measuring method and measuring system for measuring the image quality of an optical imaging system, wherein an interferometer is used, the pattern imaged by the test object lies in its object plane and is generated by a monitor.

[0004] US Patent 5,424,552 A describes a projection exposure system for processing a wafer, with a possibility to adjust the focus position of the optical system by setting a distance between the mask and a semi-transparent plate, including detection of the partially transmitted light, and further shows interchangeable aperture diaphragms.

[0005] WO 01 / 63233 A2 relates to a device for wavefront detection, with an interferometer in which a shadow mask, also referred to as a wavefront source, is imaged onto a grating via a system to be tested, with measurement being carried out interferometrically.

[0006] The JP S56 40739 A is a ring-shaped test pattern for MTF measurement, featuring circularly arranged stripe patterns that are detected by detectors when the test object is rotated. The stripe patterns have defined spatial frequencies. Furthermore, longitudinal cross-sections at predefined detector positions are evaluated.

[0007] US Patent 4,828,385 A relates to a device for measuring the refractive power of a lens, wherein an annular mask is placed in the beam path, which serves as a beam limiter and to which an annular cylindrical lens is rigidly connected.

[0008] Against this background, the approach presented here enables an improved testing system for testing an optical system according to claim 1 and an improved method for testing an optical system according to claim 6. Advantageous embodiments and further developments of the invention are set forth in the following dependent claims.

[0009] According to embodiments, a device for imaging by an optical system under test, or, in other words, a test structure for measuring the imaging quality of an optical system based on a modulation transfer function, contrast transfer function, or modulation transfer function (MTF), can be provided. The device or test structure has a geometrically ring-shaped design. The device can enable a two-dimensional, direction-dependent measurement of the MTF within the image plane while simultaneously providing high illumination of a sensor for image acquisition.

[0010] Thus, a two-dimensional MTF measurement can be advantageously achieved, in particular, by means of the device or test structure proposed herein, due to its geometrically ring-shaped design. Light transmission analogous to that of a cross-reticle or a cross-shaped test structure can also be achieved. In particular, the advantages of conventional test structures can be combined in a novel way, and previous disadvantages overcome. A further advantage of a ring-shaped test structure is that, in addition to a two-dimensional MTF measurement, a measurement of the effective focal length (EFL) is also possible.

[0011] A testing system for testing an optical system according to claim 1 is presented.

[0012] The optical system may include at least one lens or other optical element. The device may be shaped like a plate, disc, or the like. It may have a rectangular or curved circumferential contour. The device may also be described as an annular test structure, a ring reticle, or a reticle plate with an annular structure. The electromagnetic waves may be visible light or other electromagnetic radiation.

[0013] According to one embodiment, the second device section can be ring-shaped. In this case, the second device section can be arranged between a first sub-section and a second sub-section of the first device section. In other words, the second device section can be embedded between the first sub-section and the second sub-section of the first device section. For example, optionally, the first sub-section of the first device section can also be ring-shaped and surround the second device section. The second sub-section of the first device section can be circular and surrounded by the second device section.

[0014] According to another embodiment, the first device section can be arranged between a first sub-section and a second sub-section of the second device section. In other words, the first device section can be embedded between the first sub-section and the second sub-section of the second device section. For example, the first sub-section of the second device section can optionally be annular and surround the first device section. The second sub-section of the second device section can be circular and surrounded by the first device section.

[0015] According to the invention, one of the ring-shaped device sections is formed as a slot or annular gap. For example, the second device section can be formed as a slot or annular gap.

[0016] The evaluation unit is also designed to determine a two-dimensionally measured modulation transfer function of the optical system as the evaluation result, using the image of the device generated by the optical system under test. The evaluation unit can be configured to determine the evaluation result using a Fourier transform.

[0017] Furthermore, the evaluation device can be configured to determine, using the image produced by the optical system under test, an effective focal length and, additionally or alternatively, a direction-dependent magnification capability of the optical system as the evaluation result. Using the device, the evaluation device can be configured to perform a focal length measurement for different orientations and thus determine additional optical quantities such as distortion or anamorphic imaging, whereby the orientation of, for example, the distortion can also be determined.

[0018] Furthermore, a method for testing an optical system is presented, wherein the method can be carried out using an embodiment of a testing system mentioned herein, and wherein the method comprises the following steps: Generating an image of the device using the optical system under test; and evaluating the image of the device to determine an evaluation result for testing the optical system.

[0019] The testing procedure can be carried out using and / or in conjunction with an embodiment of a testing system mentioned herein. The evaluation step can be performed by an evaluation unit of the testing system.

[0020] The evaluation result can at least have or represent one parameter for the image quality of the optical system.

[0021] According to one embodiment, in the evaluation step, a two-dimensionally measured modulation transfer function of the optical system can be determined as the evaluation result using the image of the device. This allows for a precise and meaningful examination of the optical system.

[0022] In the evaluation step, the two-dimensionally measured modulation transfer function can be determined from a point spread function of the optical system, in particular by means of a Fourier transform. Here, the point spread function can be mathematically determined from a plurality of line spread functions of the optical system obtained in different cross-sectional planes.

[0023] Furthermore, during the evaluation step, using the image of the device, an effective focal length and, additionally or alternatively, a direction-dependent magnification capability of the optical system can be determined as the evaluation result. Additionally, the energy on a circular or square area can be determined as the evaluation result. The evaluation result can be determined based on the two-dimensionally measured modulation transfer function or the point spread function. Thus, a focal length measurement can be performed for different orientations, and additional optical quantities such as distortion or anamorphic imaging can be determined, including the orientation of, for example, the distortion.

[0024] An embodiment of the invention is shown purely schematically in the drawings and is described in more detail below. It shows Fig. 1 a schematic representation of an embodiment of a device for imaging by an optical system to be tested; Fig. 2 a schematic representation of an embodiment of a device for imaging by an optical system to be tested; Fig. 3 a schematic representation of an exemplary embodiment of a test system for testing an optical system; Fig. 4 a schematic representation of the planes of an optical system; Fig. 5 a schematic representation of an exemplary embodiment of a test system for testing an optical system; and Fig. 6 A flowchart of an exemplary embodiment of a method for testing an optical system.

[0025] In the following description of favorable embodiments of the present invention, the same or similar reference numerals are used for the elements shown in the various figures and having a similar effect, without repeating these elements.

[0026] Fig. 1 Figure 1 shows a schematic representation of an embodiment of a device 100 for imaging by an optical system to be tested. The device 100 is designed as an annular test structure, a ring reticle, or a reticle with an annular structure. The device 100, or test structure, comprises a first device section 110 with a first transmittance for electromagnetic waves and a second device section 120 with a second transmittance for electromagnetic waves. The second transmittance is higher than the first transmittance. Thus, the second device section 120 has a higher transmittance than the first device section 110. At least one of the device sections 110, 120 is annular in shape. In particular, one of the annularly shaped device sections 110, 120 is configured as a slot or annular gap.

[0027] According to the in Fig. 1 In the illustrated embodiment, the second device section 120 is ring-shaped. The second device section 120 is arranged or embedded between a first subsection 112 and a second subsection 114 of the first device section 110. In other words, the first device section 110 is subdivided into the first subsection 112 and the second subsection 114 by the second device section 120.

[0028] Fig. 2 Figure 1 shows a schematic representation of an exemplary embodiment of a device 100 for imaging by an optical system under test. The device in Fig. 2 corresponds to the device Fig. 1 except that, according to the in Fig. 2 In the illustrated embodiment, the first device section 110 is ring-shaped. The first device section 110 is arranged between a first sub-section 222 and a second sub-section 224 of the second device section 120.

[0029] Fig. 3 Figure 1 shows a schematic representation of an embodiment of a test system 300 for testing an optical system OS. The optical system OS is illustrated in the figure only by way of example by a lens. The test system 300 comprises a device 100 for imaging by the optical system OS under test and an evaluation unit 330. The device 100 corresponds to or is similar to the device shown in one of the figures described above. The evaluation unit 330 is configured to evaluate an image 340 of the device 100 generated by the optical system under test in order to determine an evaluation result 360 for testing the optical system OS. For this purpose, the evaluation unit 330 may have an optical sensor or an interface to an optical sensor for capturing the image 340 and at least one determination unit.

[0030] The evaluation unit 330 is configured according to an exemplary embodiment to determine a two-dimensionally measured modulation transfer function of the optical system OS as the evaluation result 360, using the figure 340 generated by the optical system OS under test. The evaluation result 360 is determined from the figure 340, for example, by means of a Fourier transform 350. Optionally, the evaluation unit 330 is also configured to determine an effective focal length and / or a direction-dependent magnification capability of the optical system OS as the evaluation result 360, using the figure 340 generated by the optical system OS under test.

[0031] Fig. 4 Figure 1 shows a schematic representation of the planes of an optical system. The optical system corresponds to or resembles the optical system mentioned with reference to one of the figures described above. Shown are an optical axis 401, an image plane 403 defined by two axes x and y with an axis point 405 and a field point 407, an exit pupil 409 of the optical system or lens, a sagittal plane 411 with a sagittal ray 413 extending along it, a tangent plane 415 or meridional plane, a principal ray 417, and an axial ray 419.

[0032] Fig. 5 Figure 3 shows a schematic representation of an embodiment of a test system 300 for testing an optical system OS. The test system 300 corresponds to or is similar to the test system from [reference missing]. Fig. 3 The test system 300 comprises the device 100, which is imaged by the optical system OS under test, and the evaluation unit 330. The evaluation unit 330 is designed to evaluate the image 340 of the device 100 generated by the optical system under test in order to determine an evaluation result 360 for testing the optical system OS.

[0033] Device 100 is shown in the illustration as merely an example, rather than the device from Fig. 1 as well as in the form of its object contrast. Figure 340 of the device 100 is illustrated in the representation both as an image of the device 100 and in the form of its image contrast, which is represented by two individual line spreading functions. The optical system OS is illustrated in the representation only by way of example by a lens between the device 100 and Figure 340, thus also between the object contrast and the image contrast. In the representation of Fig. 5 Furthermore, an exemplary cross-sectional plane A for radial sections through the device 100 is shown. The object contrast and the image contrast are determined according to a radial section for such a cross-sectional plane A.

[0034] The evaluation device 330 is configured according to the embodiment shown here to determine a two-dimensionally measured modulation transfer function of the optical system as the evaluation result 360, using the data generated by the optical system OS under test (Figure 340 of the device 100). The modulation transfer function indicates which contrast (or modulation) can be transmitted by the optical system (OS) as a function of the spatial frequency R, represented in line pairs per millimeter (Ip / mm). The evaluation device 330 is configured to determine the two-dimensionally measured modulation transfer function, i.e., the evaluation result 360, from a point spread function 555 of the optical system OS, in particular by means of a Fourier transform 350.The evaluation unit 330 is designed to mathematically determine the point spread function 555 from a plurality of line spread functions 545 of the optical system OS obtained in different cross-sectional planes A. For this purpose, the evaluation unit 330 is designed to superimpose the individual line spread functions of a radial section in a first superposition 532 or averaging to form a superimposed line spread function 545, and to repeat this with several radial sections for at least one further cross-sectional plane A in order to generate the point spread function 555 by a second superposition 534 or averaging.

[0035] Fig. 6 Figure 6 shows a flowchart of an embodiment of method 600 for testing an optical system. Method 600 can be carried out in conjunction with and / or using the test system from one of the figures described above or a similar test system. Method 600 comprises a generation step 602 and an evaluation step 604. In generation step 602, an image of the device is generated using the optical system under test. In evaluation step 604, the image of the device is evaluated to determine an evaluation result for testing the optical system.

[0036] The following section summarizes and explains, in other words, and / or briefly presents examples of implementation as well as the fundamentals and advantages of such implementations.

[0037] In particular, a device 100, designed as a ring-shaped test structure for direction-dependent MTF measurement, is presented for imaging by an optical system OS under test. Specifically, the device 100 is designed as a test structure for imaging by an optical system OS under test for the purpose of measuring the modulation transfer function (MTF) and / or the effective focal length and / or the direction-dependent magnification capability of the optical system OS. The test structure or device 100 comprises a high-transmission region, the second device section 120, and a low-transmission region, the first device section 110, one of which has a geometrically ring-shaped structure. Furthermore, a test system 300 for testing an optical system OS is presented.Using the test system 300 and in particular the evaluation device 330, a method for determining the effective focal length and / or the direction-dependent magnification or the direction-dependent MTF of the optical system OS using the test structure or device 100 is also provided, wherein an image 340 of the test structure or device 100 generated by the optical system OS to be tested is evaluated.

[0038] The fundamentals of measuring or determining the image quality of an optical system (OS) using the MTF are explained below. The basic measurement procedure can also be described using... Fig. 3 This is shown schematically. An object, here the test structure or device 100, is imaged onto a sensor via the optical system OS under test, and the MTF is calculated from the intensity distribution received by the sensor (Figure 340). To determine the MTF over a wide range of spatial frequencies, it is recommended to use a narrow slit as the object, such as the annular slit of device 100. Imagering a straight line, for example, yields an intensity distribution, which is naturally referred to as the line spread function (LSF). The MTF of the test object is then obtained from the LSF using the Fourier transform. Because the focus shape of an optical system OS is often not rotationally symmetric, it is advantageous to measure the imaging performance in multiple orientations.This can be achieved simply and in any number of orientations, as shown in the exemplary embodiments. However, a conventional approach is to orient two directions according to the sagittal plane 411 and the tangent plane 415, as shown in . Fig. 4 The simplest option would be to use a cross as the test object. However, unlike device 100, when using a slit or cross as the test object, the MTF measurement can only be performed one-dimensionally, perpendicular to the respective slit. Imaging performance in other orientations cannot be determined. For a two-dimensional MTF measurement, which can include any direction, a point-like test structure (pinhole reticle) is conventionally used. The intensity distribution in the image of such a point-like test structure is called the point response, point image function, point distribution function, or point spread function (PSF). With a point-like test structure, unlike device 100, the amount of light incident on the sensor can be small, which can amplify sensor noise as a potential source of error.For example, enlarging a point-shaped test structure would lead to a reduction in the spatial frequencies that can be used in the measurement.

[0039] One advantage of using the device 100, or the ring-shaped test structure as described in the exemplary embodiments, is that, in addition to a two-dimensional MTF measurement, a measurement of the effective focal length (EFL) is also possible. Traditionally, a modified cross-reticle would be used for this purpose. An example would be the use of an H-reticle. The focal length can be deduced from the line spacing in the image by increasing the magnification. However, with the test structure or device 100, a focal length measurement can also be performed for different orientations, and thus additional optical quantities such as distortion or anamorphic imaging can be determined. The advantage here is that the orientation of, for example, the distortion can also be determined.

[0040] The testing procedure 600 can also be considered a method for determining a parameter for the image quality of an optical system OS using Figure 340 or the device of the test structure 100. One variant of such an evaluation procedure is the calculation of a point spread function 555 or point image function (PSF) from Figure 340 of the test structure or device 100. In other words, according to the exemplary embodiments, the use of the ring reticle or an operating mode of the device 100 is provided for determining a point spread function 555 or point image function (PSF), with the help of which the image parameters of an optical system OS can be determined.

[0041] Fig. 5 Figure 340 schematically shows how a point spread function 555 (PSF) is determined using the ring reticle or the device 100, from which, for example, a two-dimensional MTF is derived as an evaluation result 360. As shown in the Fig. 5As can be seen, the optical system 100 under test, which is simplified to a single lens, produces a blurred image of the ring structure as shown in Figure 340. In any cross-sectional plane A of the ring structure, both the object contrast and the image contrast produced by the imaging are discernible. The image contrast exhibits a blurring effect caused by the lens, which is represented by the rounded edges in the intensity distribution. Due to the imaging of a ring structure, i.e., the device 100, the intensity distribution has two separate line image functions (LIFs), which are referred to as the left and right LIFs for the sake of simplicity. A resulting LIF 545 is then calculated mathematically from the two individual line image functions, e.g., by taking an average value.By repeating this process for a multitude of cross-sectional planes A through the ring structure, several superimposed LSF 545 values ​​are determined, which are then combined to form a PSF 555 that describes the two-dimensional intensity distribution of a point in the image plane. Since this is strictly speaking not a measured but a calculated PSF 555, it can also be referred to as a "pseudo"-PSF. From this pseudo-PSF, or point spreading function 555, the two-dimensional MTF of the optical system OS to be measured or tested is determined using known methods, for example, a Fourier transform 350. Further imaging parameters of the optical system OS can also be determined using the PSF 555, such as the energy on a circular or square surface, also known as encircled or square energy.

[0042] The use of the ring structure or device 100 offers a particular advantage over the use of a pinhole in that a large number of pixels in the sensor plane are illuminated, thus compensating for disruptive moiré effects. Furthermore, the ring reticle or device 100 provides a higher amount of light than a conventional pinhole. Another advantage of the ring structure or device 100 is that the magnification factor of the optics or optical system OS being measured can be easily determined.

Claims

1. Test system (300) for testing an optical system (OS), wherein the test system (300) has the following features: a device (100) for imaging through the optical system (OS) to be tested, wherein the device (100) comprises a first device portion (110) having a first transmittance for electromagnetic waves and a second device portion (120) having a second transmittance for the electromagnetic waves, wherein the second transmittance is higher than the first transmittance, wherein at least one of the device portions (110, 120) is ring-shaped; and an evaluation device (330), characterized in that one of the ring-shaped device portions (110, 120) is shaped as a slit or ring gap, wherein the evaluation device (330) is designed to evaluate an image representation (340) of the entire device (100) generated by the optical system (OS) to be tested, in order to determine an evaluation result (360) for the testing of the optical system (OS).

2. Test system (300) according to Claim 1, characterized in that the second device portion (120) is ring-shaped, wherein the second device portion (120) is arranged between a first partial region (112) and a second partial region (114) of the first device portion (110).

3. Test system (300) according to Claim 1, characterized in that the first device portion (110) is ring-shaped, wherein the first device portion (110) is arranged between a first partial region (222) and a second partial region (224) of the second device portion (120).

4. Test system (300) according to any of Claims 1 to 3, characterized in that the evaluation device (330) is designed to determine a two-dimensionally measured modulation transfer function of the optical system (OS) as the evaluation result (360) using the image representation (340) of the device (100) generated by the optical system (OS) to be tested.

5. Test system (300) according to any of Claims 1 to 4, characterized in that the evaluation device (330) is designed to determine an effective focal length and / or a direction-dependent magnification capability of the optical system (OS) as the evaluation result (360) using the image representation (340) of the device (100) generated by the optical system (OS) to be tested.

6. Method (600) for testing an optical system (OS), wherein the method (600) can be carried out using a test system according to any of Claims 1 to 5, wherein the method (600) comprises the following steps: generating (602) an image representation (340) of the entire device (100) by means of the optical system (OS) to be tested; and evaluating (604) the image representation (340) of the device (100) in order to determine an evaluation result (360) for the testing of the optical system (OS).

7. Method (600) according to Claim 6, characterized in that in step (604) of evaluating, using the image representation (340) of the device (100), a two-dimensionally measured modulation transfer function of the optical system (OS) is determined as the evaluation result (360).

8. Method (600) according to Claim 7, characterized in that in step (604) of evaluating, the two-dimensionally measured modulation transfer function is ascertained from a point spread function (555) of the optical system (OS), in particular by means of a Fourier transformation (350), wherein the point spread function (555) is mathematically determined from a plurality of line spread functions (545) of the optical system (OS) obtained in different cross-sectional planes (A).

9. Method (600) according to any of Claims 6 to 8, characterized in that in step (604) of evaluating, using the image representation (340) of the device (100), an effective focal length and / or a direction-dependent magnification capability of the optical system (OS) are / is determined as the evaluation result (360).