Optical testing device and method of operating an optical testing device

Abstract

The present invention discloses an optical testing apparatus (100) comprising: a light source (105) for emitting a light beam (110); and a beam shaping unit (115) having a diffraction optical element (DOE, 120) for shaping the light beam (110) and guiding it to a coupling input region (125) of an optical workpiece (130) to couple the shaped light into the optical workpiece (130) to be inspected. Furthermore, the optical testing apparatus (100) comprises: a workpiece holding element (135) for holding the optical workpiece (130); and a detector (145) for evaluating the light (110) emitted from the coupling output region (140) of the optical workpiece (130) to inspect the optical properties of the workpiece (130).

Description

Technical Field

[0001] The present invention provides an optical testing apparatus according to an independent claim and a method for operating the optical testing apparatus. Prior Technology

[0002] To test an optical workpiece (i.e., a workpiece that is at least partially transparent to light), the workpiece can be illuminated with a beam of light having a certain intensity distribution and / or wavelength, and then tested to see how this intensity distribution changes or how light of a certain wavelength is attenuated, so as to be able to detect one or more corresponding parameters of the workpiece.

[0003] In this context, the present invention aims to improve the inspection of optical components.

[0004] This objective is achieved through the object of a separate request. Summary of the Invention

[0005] The method described herein produces an optical testing apparatus having the following characteristics: - A light source, used to emit a beam of light; - A beam shaping unit with diffraction optical elements for shaping the light beam and guiding it to the coupling input region of the optical workpiece, so as to couple the shaped light into the optical workpiece to be inspected; - A workpiece holding element used to hold optical workpieces; - A detector used to evaluate the light emitted from the coupled output region of an optical workpiece to examine the optical properties of the workpiece with respect to the characteristics to be examined.

[0006] A light source can be understood as, for example, a laser light source that emits a laser beam as a light source. For example, a beam shaping unit can be understood as a unit that uses diffraction optics to shape a light beam, particularly a laser beam, and outputs it to the coupling input region of an optical workpiece. Specifically, the diffraction effect is used to deflect light at a predetermined wavelength illuminating the optical workpiece at a certain angle (e.g., normal incidence), and thereby illuminating the coupling input region at a predetermined angle and / or a predetermined angle range, such that the shaped light propagates in the optical workpiece along a desired optical path and is coupled out again in the coupling output region of the optical workpiece and can enter the detector. The optical workpiece can be understood here as a waveguide, optical guide, or optical fiber designed to transmit light with desired physical properties. For example, such an optical workpiece can be used as part of a data goggle, an augmented reality (AR) or virtual reality (VR) display system, a near-eye device (NED), or another optical element in which optical functions with high optical quality are to be achieved. The received light is then tested in the detector itself for the characteristics to be detected. The characteristic to be detected can be, for example, MTF, PSF, LSF and / or ESF parameters or other parameters such as principal angle (CRA) or chromatic aberration, which can be used to evaluate the quality of optical components.

[0007] The detector may include one camera, or at least two cameras, or multiple cameras, or a portion of the detector unit. The one or more cameras may further include one or more telescopes and may have a fixed or variable focal length. The one or more cameras may be implemented as electronic cameras, such as CCD or CMOS cameras. Alternatively, the camera may be implemented as a color camera. In other embodiments, the detector may be implemented as a conodolite, spectrometer, or spectrophotometer.

[0008] The method presented herein is based on the understanding that by using diffractive optics in a beam shaping unit, a specific desired deflection or shaping of the beam, particularly with respect to the angle illuminating the sample under test, can be performed in a technically very simple and cost-effective manner, where parameters highly relevant to the function of the optical workpiece can be tested simply and reliably. In the prior art, one or more illumination units are moved or rotated to illuminate the sample under test at multiple incident or field angles. U.S. Patent Publication No. 20220163423 outlines a measurement system for waveguides used in AR / VR devices. The system includes an optical engine comprising a light source 320 and a lens 310. In one embodiment (see Figure 3E), the light source and lens can be rotated or tilted to adjust the incident angle onto the device under test 100.

[0009] A particularly advantageous embodiment of the method proposed herein is that the beam shaping unit is designed to actively alter the optical properties of diffractive optical elements and / or use another diffractive optical element to shape the beam. For example, the shape or structure of the diffractive optical element can be altered by applying or changing a voltage, such that the pattern or deflection of the light beam when illuminated by light from the light source differs from that when no voltage is applied or changed. It is also conceivable to arrange several corresponding diffractive optical elements in the beam shaping unit, which are brought into the beam path of the light source as needed, so that the desired shape or deflection of the beam emitted into the coupling input region can be set by replacing the corresponding diffractive optical elements. This allows for quick and easy modification of the light according to the desired application.

[0010] A particularly advantageous embodiment of the method proposed herein is that the beam shaping unit is designed to project light patterns, particularly dot patterns, cross patterns, or ring patterns, onto the coupling input region by means of diffraction optics. This embodiment of the method proposed herein offers the advantage that, by defining a specific light pattern and illuminating the sample under test with the specific light pattern at a predetermined set of field angles, it is possible to test the physical effects related to the properties under investigation particularly well.

[0011] According to another embodiment of the method proposed herein, the light source and / or beam shaping unit is designed to be adjusted along at least one axis, particularly shifted and / or rotated about at least one axis. For example, the light source and / or beam shaping unit can be moved vertically or laterally, or it can be tilted or rotated about one or more axes. Such an embodiment provides the advantage that, due to the flexible movement of the light source and / or beam shaping unit, the beam formed by the beam shaping unit can be flexibly coupled to the coupling region of the workpiece.

[0012] Another advantageous embodiment of the method proposed herein is that the workpiece holding element is designed to be adjusted along at least one axis, particularly to be displaced and / or rotated about at least one axis. Such an embodiment also allows for flexible workpiece positioning, enabling the detection of a certain characteristic and / or parameter of the workpiece using the testing apparatus described herein.

[0013] Another advantage is the implementation of the method proposed herein, in which the workpiece holding element is designed to hold a flat optical transmission and / or reflection workpiece, such as a waveguide. The optical workpiece to be tested includes a coupling input region and a coupling output region. This implementation allows for the installation and therefore testing of frequently used optical components, enabling flexible use of the test apparatus proposed herein.

[0014] In another embodiment of the method proposed herein, the detector is designed to be adjusted along at least one axis, particularly shifted and / or rotated about at least one axis. This embodiment also allows the detector to flexibly adapt to different beam angles of the beam emitted from the coupling output region of the optical workpiece. This makes it particularly easy to detect certain characteristics or parameters of the workpiece when coupled with a beam or light pattern of a predetermined shape.

[0015] In one embodiment of the method proposed herein, the detector may include a camera comprising an image sensor and an imaging optics system, such as a telescope. The camera may have a fixed or variable focal length. Depending on the requirements, the camera may be implemented as a color camera with different spectral channels. In this way, the wavelength-dependent characteristics of the optical workpiece under test can be studied.

[0016] Another advantageous embodiment of the method proposed herein is that the detector comprises at least two partial detector units or cameras, each designed to receive light emitted from different directions from the coupled output region, and particularly, wherein the at least two partial detector units or cameras are arranged adjacent to each other. Such an embodiment enables the parallel measurement or recording of workpiece values ​​or characteristics at different angles relative to the optical axis, resulting in a generally shorter minimum measurement duration and / or enabling significantly broader measurements, including, for example, measuring or recording different parameters or characteristics.

[0017] According to another embodiment of the method proposed herein, the detector is adapted to move at least partially within a range of motion. For example, this movement can occur along the optical axis of the detector and in at least two directions perpendicular to the optical axis of the detector, such that the detector is aligned with the decoupled region at different positions and in different directions. This embodiment of the method proposed herein offers the advantage of being able to capture and evaluate beams coupled out in different directions to detect desired characteristics or corresponding parameters.

[0018] The advantages mentioned above can also be implemented in the form of a process method for operating the optical testing apparatus according to the version presented herein, which includes the following steps: - The light beam is output from the light source to the diffraction optical element of the beam shaping unit; and - Evaluate the beam emitted from the coupled output region of the optical workpiece to be tested to examine the characteristics of the workpiece to be inspected.

[0019] After passing through the diffraction optics, the light beam can be emitted into the coupling input region and coupled into the workpiece. In the coupling output region, the light beam propagating through the workpiece can be recorded by a detector, allowing the recorded light to be evaluated accordingly to detect desired parameters or characteristics.

[0020] The method proposed herein also includes a control device or control unit designed to perform, control, or implement the steps of variations of the method proposed herein in a corresponding unit or device. This embodiment of the invention in the form of a control device can also solve the objectives of the invention quickly and efficiently.

[0021] For this purpose, the control device may include: at least one computing unit for processing signals or data, at least one memory unit for storing signals or data, at least one interface for reading sensor signals from a sensor or for outputting control signals to an actuator, and / or at least one communication interface embedded in a communication protocol for reading or outputting data. The computing unit may be, for example, a signal processor, a microcontroller, etc., wherein the memory unit may be flash memory or magnetic memory. The communication interface may be designed to read or output data wirelessly and / or wiredly, wherein a communication interface capable of wired data reading or output can, for example, read data electrically or optically from a corresponding data transmission line, or register data to a corresponding data transmission line.

[0022] In the current context, a control device can be understood as an electrical device that processes sensor signals and outputs control signals and / or data signals based on the sensor signals. The control device may have an interface that is either hardware-based or software-based. In the case of a hardware-based design, the interface may, for example, be part of a so-called ASIC system that contains various functions of the control device. However, the interface may also be a separate integrated circuit or at least partially composed of discrete components. In a software-based design, for example, the interface may be a software module that exists on a microcontroller along with other software modules.

[0023] Another advantage is a computer program product or computer program having code that can be stored on a machine-readable medium or storage medium (such as semiconductor memory, hard disk memory, or optical memory), and particularly when the program product or program is executed on a computer or device, the code is used to perform, implement, and / or control the steps of the method described in one of the embodiments described above. Simple Explanation of the Diagram

[0024] At least one embodiment of the invention presented herein is explained in more detail below with reference to the accompanying drawings. In the drawings: [Figure 1a] illustrates an embodiment of the optical testing apparatus; [Figure 1b] shows an illustration of the illumination section of the optical testing device; [Figure 2] shows a schematic diagram of an implementation of the optical testing device; [Figure 3] shows a schematic diagram of a light source and a beam shaping unit as an illumination unit for an embodiment of the method described herein; [Figure 4] shows a schematic diagram of a workpiece holding element used in an embodiment of the method described herein; [Figure 5] shows a schematic diagram of a detector used in an implementation of the method described herein; [Figure 6] shows a flowchart illustrating an example implementation of the method for operating the optical testing apparatus; and [Figure 7] shows a block illustrating an example implementation of a control unit for operating an optical testing apparatus.

[0025] The same or similar elements are identified in the following description by the same or similar reference numerals, wherein repeated descriptions of the elements are omitted. Implementation

[0026] Figure 1a illustrates an embodiment of the optical testing apparatus 100, in which a light source 105, such as a laser light source in the form of a laser diode, is used. The light source 105 can be relatively flat and small, and therefore does not require much installation space. A beam 110 is emitted from the light source 105, and then the beam is modified by a beam shaping unit 115. The beam shaping unit 115 includes a diffraction optics element 120, which is designed to deflect the light beam 110 by diffraction and / or form a light pattern therefrom. This light is then coupled at a coupling input region 125 into an optical workpiece 130, which is, for example, placed or fixed on a workpiece holding element 135. The light beam 110 formed at this point can then propagate in the workpiece 130, for example, via total internal reflection, and is coupled out of the workpiece 130 in an output coupling region 140 and received or evaluated by a detector 145. The evaluation can be performed, for example, by having light received by one and / or more partial detectors or cameras 150 and evaluated regarding its wavelength, intensity, and / or light pattern, so that by means of such evaluation and the original parameter information of the beam 110 before or after the beam shaping unit 115, such changes in the light of the beam in the workpiece 130 can be inferred, and thus the corresponding characteristics of the workpiece, in particular the corresponding parameters, can be subsequently detected or determined.

[0027] Individual sensors of some detectors 150 or 145 may be movably arranged, for example, to receive light beams 110 emitted from different directions of the coupled output region 140, for example, to infer the corresponding propagation behavior of light in the workpiece 130, particularly optical properties (e.g., MTF) observed from different field positions or field angles. It is also conceivable that sensors of detector 145 or some detectors 150 are designed to detect light of different wavelengths, such that, for example, color-related optical parameters of the workpiece 130, such as color MTF, can also be evaluated.

[0028] It is also conceivable that the beam shaping unit 115 is designed to actively alter or modify the diffraction optical element 120. This can be achieved, for example, by applying a voltage to the diffraction optical element 120 via the control unit 155, thereby changing the diffraction characteristics of the diffraction optical element and setting a different deflection characteristic for the beam 110 compared to the original deflection characteristics. It is also conceivable that the control unit can introduce another diffraction optical element 160 into the beam path of the beam 110, which then has a different deflection characteristic than the diffraction optical element 120. This can also cause changes in the beam shaping of the beam shaping unit 115, thereby enabling different illumination scenarios for the optical workpiece under test.

[0029] The system used for testing optical components measures parameters such as chromatic aberration, MTF / PSF / LSF / ESF, or other efficiency parameters. The illumination pattern and angle can be easily adjusted according to the sample requirements.

[0030] The DOE-collimators disclosed in the prior art are used to calibrate assembled camera systems. In these collimator-DOE combinations, only the use of dot patterns is disclosed. Measurement systems used to test AR / VR waveguides either utilize mechanical displacement to change the angle of incidence of light onto the incident pupil of the DUT or use multiple collimators. Mechanically pivoting the light source results in prolonged measurement time, and using multiple collimators limits the available angle of incidence. Most measurement systems for waveguides do not use laser illumination sources; similarly, the source does not use DOEs to create patterns for measuring different field locations.

[0031] Figure 1b provides a detailed illustration of the illumination of the sample under test. In the embodiment shown in Figure 1b, light source 105 emits collimated laser 110. Light source 105 may include one or more laser emitters. Collimated light 110 illuminates diffracting optics 120 (DOE) and is deflected or "fanned out" at a predetermined deflection angle. Thus, the sample under test is illuminated by collimated light at multiple field angles. In the embodiment described herein, the illumination pattern 116 is a dot pattern. Other patterns, such as cross patterns, ring patterns, or bow-shaped patterns, can also be implemented.

[0032] Figure 2 shows a schematic diagram of an embodiment of the optical testing apparatus 100, as briefly explained, for example, in the schematic diagram of Figure 1a. Compared to the diagram in Figure 1a, it is now shown in Figure 2 that, for example, the light source 105 and the beam shaping unit 115 are arranged together on the first holder 200, forming a corresponding holder mechanism 205 to allow the holder 200 to be vertically or horizontally displaced or rotated about different axes. Thus, for example, the light source and the beam shaping unit 115 can be moved or rotated such that the beam 110 is coupled into the workpiece 130 at a desired position and / or at a certain angle in the coupling input region 125, and then exits the workpiece 130 again in the coupling output region 140 to be captured by the detector 145. Therefore, the detector 145 is also arranged on or attached to the detector mechanism 210, which is designed, for example, to resemble the holder mechanism 205, and thus the detector 145 can also be vertically or horizontally moved or rotated about different axes. In this way, the beam 110 emitted from the decoupling region 140 can also be recorded by the detector 145 and then evaluated accordingly.

[0033] Figure 3 shows a schematic diagram of a light source 105 and a beam shaping unit 115 as an illumination unit 300 for an embodiment of the method described herein. It can be seen that, in addition to the light source 105, a grading unit 305 is provided to enable the measurement process using different illumination patterns. Following the grading unit 305 is a DOE 120. Downstream of the diffractive optical element 120, an optical system 310 is positioned in the beam path, which includes, for example, one or more lenses and relays the beam formed by the diffractive optical element 120 toward the sample under test via a variable or interchangeable aperture 315. The light source 105 is attached to a holder mechanism 205, which is designed to allow the light source 105 to move along several directions or along multiple axes, which are schematically shown herein as the x-direction, x-direction, and z-direction. Accordingly, an axis 320 around which the light source 105 can rotate is also shown.

[0034] Figure 4 shows a schematic diagram of a workpiece holding element 135 used in an embodiment of the method described herein, the workpiece holding element including a holding element 400 for holding a workpiece 130. The holding element 400 can rotate about an axis 405 and can also move, for example, in the x-direction and / or y-direction, as schematically shown in Figure 4.

[0035] Figure 5 shows a schematic diagram of a detector 145 used in an embodiment of the method described herein, attached to a detector mechanism 210 that also allows the detector 145 to move in the x, y, and / or z directions, wherein movement in the z direction is not explicitly shown in Figure 5. The detector mechanism 210 also allows the detector 145 to rotate on one or more axes 500, such that the optical system 510 of the detector 145, along with the aperture 515, can move flexibly. This movement of the detector allows, for example, scanning the eyepiece of the sample under test. The aperture 515 can also be variable or interchangeable. One or more partial detectors 150 can then be arranged within the detector 145 itself to record or analyze and thus evaluate the corresponding parameters of the received light.

[0036] Figure 6 shows a flowchart of an example implementation of a method 600 of an optical testing apparatus according to a previously described version, the method 600 including a step 610 of outputting a light beam from a light source to a diffraction optical element (DOE) of a beam shaping unit, and a step 620 of evaluating the light emitted from the coupled output region to inspect the characteristics to be inspected on the workpiece.

[0037] Figure 7 shows a block diagram of an example embodiment of a control unit 700 for operating an optical testing apparatus according to a previously described version, wherein the control unit 700 includes a unit 710 for outputting a light beam from a light source to a diffraction optical element of a beam shaping unit, and a unit 720 for evaluating light emitted from a coupled output region to examine desired optical properties of the workpiece under test.

[0038] In summary, it can be noted that, in this description, an illumination setup with light (especially laser) and diffractive optical elements (DOE) can be included in the test apparatus to perform high-quality measurements of specific parameters of optical workpieces (especially waveguides or light guides for AR / VR display systems or NEDs).

[0039] A key aspect of the proposed method lies in the use of a DOE (Displacement Optical Array) in conjunction with a light source (i.e., a laser) to illuminate an optical workpiece (such as a waveguide sample) with TIR (Transient Infrared) to measure parameters such as MTF / PSF. Illuminating the sample under test with a light source (i.e., a laser) can be performed at a wide angle to cover the entire field of view.

[0040] In contrast, previous technology systems for testing AV / VR optics used different types of light engines to generate images for testing the component. Some of these used LBS projectors, which did not allow for the accurate or complete measurement of different optical parameters using the light sources currently used in some systems on the market.

[0041] This concept proposes replacing LEDs with a light source (especially a laser system) in conjunction with diffractive optical elements (DOEs). The idea of ​​using a combination of a light source (especially a laser) and a DOE, different from those used in cameras, to illuminate the optical elements is a novel approach. Image formation, for example, of the parameters discussed, used in MTF measurements, is accomplished by a marker placed in front of the laser source. Alternatively, the aperture of the laser or fiber optic cable can be used as the marker. A lens system and the DOE element are used to split a single beam into different beams that cover the entire field of view of the sample under test (i.e., the workpiece). For example, the lens system, as part of the optical system, is used to propagate the first-order image of the marker and the second, third, and higher-order diffracted images of the marker.

[0042] As mentioned earlier, the system has three main parts: 1. Irradiated area 300, This is the main part of the proposed method. A laser is used as the light source 105. Markers with patterns, crosses, rings (ring shapes), bows, etc., are used in the measurement 305, along with a diffraction optics element 120 and an optical system 310. This optical system is used to propagate the light coupled from the laser output (i.e., the light source 105) toward the DOE 120 and then toward the entrance pupil of the sample (i.e., the workpiece 130). The final part of the system is an interchangeable aperture 315 located in front of the entrance pupil of the sample. The system has a degree of freedom of movement that allows not only movement along the x / y / z axes but also rotation via at least two rotation axes 320 to cover different fields of view.

[0043] 2. Sample arm, (i.e., workpiece holder 135), which includes a sample holder 400, which may vary depending on the customer. The sample holder 400 allows for the measurement of different types of samples or workpieces 130 within the system. The sample holder 400 also has degrees of freedom of movement along the x / y / z axes and a rotation axis 405.

[0044] 3. Detector 145 This can be any type of detector used in similar measurement systems, such as a monochrome camera to a spectrometer or photodiode. The same applies to the optical system 510 attached to detector 145, ranging from a single-focal camera with a FoV of 2 degrees to a conic polarimeter (with a FoV up to 120 degrees), which will cover the entire field of view in a single measurement. Optical system 510 also includes interchangeable apertures 515. Detector 145 and optical system 510 can be fixed, have degrees of freedom of movement along the x / y / z axes via detector mechanism 210, and include different rotary stages 500.

[0045] Using the optical testing apparatus 100 presented herein, several parameters can be measured by the system, including, for example, MTF, CRA, focus scan, overfocus measurement, relative efficiency, external environment measurement, and color coordinates, absolute brightness, etc., depending on the camera;

[0046] 100: Optical testing equipment 105: Light source 110: Beam 115: Beam Shaping Unit 116: Illumination Pattern 120: Diffraction optical element 125: Coupled Input Region 130: Workpiece 135: Holder 140: Decoupling region 145: Detector 150: Partial detectors 155: Control Unit 160: Another diffractive optical element 200: Holder 205: Holder mechanism 210: Detector mechanism 300: Irradiation unit 305: Measurement 310: Optical System 315: Interchangeable aperture 320: Axis 400: Sample holder 405: Axis 500: Rotary Table 510: Optical System 515: Aperture 600: Method 610: Steps 620: Steps 700: Control Unit 710: Unit 720: Unit

[0047] none

Claims

1. An optical testing apparatus (100) for inspecting the optical properties of an optical workpiece (130), the apparatus comprising: - A light source (105) for emitting a light beam (110); - A beam shaping unit (115) having a diffraction optical element (120) for shaping the light beam (110) and guiding it to a coupling input region (125) of an optical workpiece (130) to couple the shaped light into the optical workpiece (130), wherein the optical workpiece is a waveguide for an augmented reality or virtual reality display system; - A workpiece holding element (135) for holding the optical workpiece (130); - A detector (145) for evaluating the light (110) emitted from the coupling output region (140) of the optical workpiece (130). The device is characterized by further including a marking unit (305) and an optical system (310), wherein the marking unit, the optical system and the beam shaping unit (115) are arranged to project the image of the marking unit onto the coupling input region (125) via the entrance pupil of the optical workpiece (130); and wherein the detector (145) is configured to check the optical characteristics by evaluating the quality of the optical workpiece (130) after it has been emitted from the coupling output region (140), wherein the optical characteristics are one or more of the modulation transfer function (MTF), point dispersion function (PSF), line dispersion function (LSF), edge dispersion function (ESF) and principal ray angle (CRA).

2. The optical testing apparatus (100) as described in claim 1, wherein, The marking unit (305) has a pattern selected from cross, ring or bow.

3. The optical testing apparatus (100) as described in any one of claims 1 to 2, wherein, The optical system (310) includes one or more lenses and a variable or interchangeable aperture (315).

4. The optical testing apparatus (100) as described in claim 1 or 2, wherein, The beam shaping unit (115) is designed to modify the diffraction optics (120) and / or use another diffraction optics (160) to shape the beam (110).

5. The optical testing apparatus (100) as described in any one of claims 1 to 2, wherein, The light source (105) and the beam shaping unit (115) are designed to be adjusted on at least one axis (x, y, z, 320).

6. The optical testing apparatus (100) as described in any one of claims 1 to 2, wherein, The workpiece holding element (135) is designed to be adjusted on at least one axis (x, y, z, 405).

7. The optical testing apparatus (100) as described in any one of claims 1 to 2, wherein, The detector (145) is designed to be adjusted on at least one axis (x, y, z, 500).

8. The optical testing apparatus (100) as described in any one of claims 1 to 2, wherein, The detector (145) is designed to receive light emitted from different directions and / or with different wavelengths from the coupled output region (140) to examine chromatic aberration or color-related optical parameters.

9. The optical testing apparatus (100) as described in any one of claims 1 to 2, wherein, The detector (145) includes at least two partial detector units (150), each of which is designed to receive light emitted from different directions from the coupled output region (140).

10. The optical testing apparatus (100) as described in any one of claims 1 to 2, wherein, The detector (145) or at least one component (150, 510) of the detector (145) is adapted to move at least partially within the range of motion.

11. A method (600) of operating an optical testing apparatus (100) as claimed in any one of claims 1 to 10, the method (600) comprising the steps of: - projecting (610) the image of the calibrating unit (305) onto the coupling input region (125) of the optical workpiece (130) via the beam shaping unit (115); and - evaluating (620) the beam (110) after exiting the coupling output region (140) by means of the detector (145) to examine the optical characteristics of the optical workpiece (130), wherein the optical characteristics are one or more of modulation transfer function (MTF), point dispersion function (PSF), line dispersion function (LSF), edge dispersion function (ESF) or principal ray angle (CRA).

12. The method (600) as described in claim 11, wherein, The image projection (610) of the marking unit (305) includes a projection of a pattern selected from cross, ring or bow.

13. A control unit (155, 700) adapted to control and / or perform the steps of the method (600) as described in any one of claims 11 to 12.

14. A computer program including code adapted to control and / or perform the steps of the method (600) as described in any one of claims 11 to 12 when the computer program is executed on a control unit (155, 700).

15. A machine-readable storage medium having a computer program as described in claim 14 stored thereon.

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