Polarization selective imaging

Abstract

The invention provides polarization selective imaging. The invention relates to a measuring device for imaging an object. A measurement device includes a light emitting unit configured to emit polarized measurement light, a light receiving unit configured to receive and detect reflected measurement light, and a support structure including a retroreflective layer and a polarization state manipulation layer. The support structure provides retroreflection of the measurement light and changes the polarization state of the measurement light such that the polarization state of the incident measurement light differs relative to the polarization state of the retroreflection measurement light. The measurement device includes a polarization selection unit configured to provide measurement light selecting a reflection having a first polarization state or a second polarization state. Providing selection by changing an emission selection state of a polarization selection unit to selectively provide emission of measurement light having a first polarization state or a second polarization state and / or by changing a transmission selection state of a polarization selection unit to selectively provide transmission of reflected measurement light only of the first polarization state or the second polarization state and selecting.

Description

Technical Field

[0001] The invention relates to a measuring machine comprising an optical probe with a polariser, a reflecting surface and a control unit for controlling the optical probe and generating an image. Background Art

[0002] A measuring machine or coordinate measuring machine (CMM) according to the invention is a machine configured to image an object and / or to measure the 3D coordinates of certain points of a workpiece, in particular the entire surface topography. In the sense of the invention, a surface can denote the 2D exterior of a workpiece, but can equally denote a surface area, i.e. an area of ​​finite thickness. For the sake of simplicity and transparency, from here on, a surface will be used as a 2D surface, and specific features for measuring a surface area can be applied accordingly.

[0003] CMMs are important in various industries, for example in production measurement, quality control or reverse engineering. They can be used, for example, to determine the deviation of the geometry of a manufactured product from a design model, for example to determine whether the deviation is within the manufacturing tolerance. Such measurements are usually performed automatically or semi-automatically based on a computer-generated or operator-selected measurement path, wherein such a measurement path is provided relative to the design model.

[0004] Another increasingly important application of CMM is reverse engineering of objects. For such applications, there is no design model, but the operator can command the 3D movement of the probe head by manual steering commands using, for example, a jogbox / joystick. Alternatively, the operator can directly manipulate a handheld sensor.

[0005] Typically, a CMM has a main structure, a detection system, and a data collection and data processing system.

[0006] The main structure usually includes a group of actuators responsible for positioning the detection system. A widespread example of CMM is a 3-axis system. DE4325347 discloses such a CMM system. Here, the main structure includes a base (basis) having a measuring table and a movable frame. The workpiece can be positioned or mounted on the measuring table. The movable frame is mounted on the base so that it can move along a first axis, and the frame includes an arm, which is mounted so that it can move along a second axis perpendicular to the first axis. The detection system includes a probe head mounted on the arm so that it can move along a third axis perpendicular to the first axis and the second axis. This construction enables the probe head to be manipulated in all three dimensions, thereby allowing the relevant 3D coordinates of the object to be measured. Contemporary three-axis systems also typically include components, such as stacked rotation tables, to provide 5 degrees of freedom regarding the relative posture of the probe head and the workpiece.

[0007] Another typical embodiment of CMM is the so-called articulated arm coordinate measuring machine (AACMM). AACMM includes a base to be fixed and an arm including a plurality of arm segments connected by joints. The joint provides mobility to the movable end of the arm relative to the base, and a probe head can be attached thereto. The probe head is usually manually guided by an operator. Due to its design principle, such a system is not as accurate as the above-mentioned 3-axis system or 5-axis system, on the other hand, it provides higher flexibility. For example, EP2916099B1 discloses this AACMM instrument.

[0008] The detection system of a CMM can be based on contact technology or non-contact technology. In the first case, a mechanical probe, usually implemented as a stylus, achieves direct mechanical contact with the workpiece and the probe is guided through a given measurement path, while the endpoint coordinates of the stylus are derived from sensor readings about the state of the CMM (e.g. encoder readings). Non-contact technology can be based on projecting a main measurement beam on the work object and aligning secondary beams emanating from the surface area of ​​the object. One advantage of non-contact technology is that the work object is less likely to be damaged due to the lack of mechanical contact. In addition, unlike stylus-based methods that only align the coordinates of a single point, non-contact methods allow parallel acquisition of extended areas.

[0009] Another typical embodiment of a measuring machine for imaging an object is a so-called vision machine. Such vision machines are known for selectively detecting transmitted or reflected light irradiation. The vision machine can be embodied as a telecentric imaging system for imaging and measuring an object on a measuring table. Another type of vision machine provides for movement of the optical system relative to the base, and this enables imaging of a larger area of ​​the object to be measured. The movement of the optical system can be provided by a drive device and an encoding device (as in a typical CMM).

[0010] Conventional imaging of objects by providing the option of detecting backlighting (transmitted light) or detecting incident light for specific imaging of the object is usually accompanied by a great technical effort, especially for backlighting. This is because additional illumination optics (light setup) below the object are required.

[0011] EP0531322B1 proposes a specific arrangement of a reflector behind an object and the use of polarized light and a specifically arranged polarization element for selecting the detection of light reflected at the object or light reflected by the reflector. However, this approach still results in a reduction in the contrast of both image types, i.e. imaging the transmitted light or detecting the incident light. Summary of the invention

[0012] It is therefore an object of the present invention to provide a measuring machine which reduces the above-mentioned disadvantages.

[0013] Another object of the present invention is to provide a measuring machine that can more accurately image a measured object.

[0014] Top-light illumination is often suitable for imaging the surface of an object. Here, a vertical surface orientation may be preferred, so that the surface normal vector is parallel to the optical axis of the lens. In order to match illumination angles and imaging angles over the entire field of view, the lens and illumination are often (almost) telecentric, i.e., all chief rays are parallel to the optical axis.

[0015] Close to the pupil plane of the telecentric lens, the top light can be in-coupled coaxially to the optical axis via a beam splitter (cube or plate) in the optical path. The emitted top light can have an angular illumination spectrum identical to the NA of the lens. After reflection at the (vertically shiny) surface, the received light can thus perfectly match and completely fill the entrance pupil of the lens, resulting in a sharp and high-contrast image of the object structure that allows high-precision measurements.

[0016] Top lighting can also be used to illuminate diffuse surfaces, and can be used to reduce shadows when shining into deep holes or similar structures.

[0017] In contrast, thin objects such as sheet metal parts or PCBs are typical candidates for bottom light illumination to determine, for example, drill hole location and diameter. For optimal illumination conditions, the angular spectrum (NA) of the light can be matched to the NA of the lens so that the entrance pupil of the telecentric lens is filled.

[0018] Prior art vision machines (vision CMMs) may have an illumination table consisting of a glass plate with a hollow space underneath to mechanically move the light setup, for example by motorizing the illumination modules individually or by suspending the illumination head on a bridge of the CMM. More than 1m 2 Larger size glass sheets will become quite heavy (hundreds of kilograms at 10mm thickness) and will be affected by bending due to their weight. Therefore, this approach limits the size of vision-based CMMs and is not scalable for use in larger machines for tactile and non-contact 3D metrology.

[0019] The invention is based on the idea of ​​providing a retroreflective element behind the object to be imaged or measured. Thus, by illuminating the object from above, the light is reflected at the upper surface of the object, but also by retroreflection from behind the object. Thereby, top light from above can be converted into bottom light from below the object in areas where the object shows opening edges, such as drill holes, etc. A combination of top-light imaging and bottom-light imaging becomes available by using only one light source and also avoiding the provision of a complex bottom-light device.

[0020] Furthermore, polarized light and additional polarization elements are used to provide the option of detecting measurement light reflected at or behind the object.

[0021] The present invention relates to a measuring device for imaging an object. The measuring device comprises: a light emitting unit configured to emit polarized measuring light, in particular linearly polarized measuring light; a light receiving unit configured to receive and detect reflected measuring light; and a supporting structure comprising a retroreflective layer and a polarization state manipulation layer.

[0022] The support structure provides retroreflection of the measurement light and changes the polarization state of the measurement light such that the polarization state of the incident measurement light is different relative to the polarization state of the retroreflected measurement light.The support structure may in particular also provide support for the object.

[0023] The measuring device comprises a polarization selection unit configured to provide selection of reflected measurement light having a first polarization state and / or reflected measurement light having a second polarization state for receiving the reflected measurement light by the light receiving unit.

[0024] The reflected measurement light can be provided by reflection of the measurement light at the object and / or by retroreflection of the measurement light by the support structure. In other words, the reflected measurement light can be understood as the measurement light reflected by the object or by the retroreflective layer or a combination of both (compared to the emitted measurement light directed toward the object).

[0025] In one embodiment, the polarization selection unit may be a part of the light receiving unit, in particular a part of a camera.

[0026] In one embodiment, the light emitting unit may be configured to emit measurement light in a first polarization state and / or measurement light in a second polarization state, wherein the measurement light in the first polarization state and the measurement light in the second polarization state may be provided simultaneously or sequentially.

[0027] In one embodiment, the light emitting unit may include at least two light sources, such as laser diodes, one of which is configured to emit a measurement light of a first polarization state and the other is configured to emit a measurement light of a second polarization state. The polarization selection unit may be represented by a switchable light emitting unit. In one embodiment, the polarization selection unit may be configured to control the light emitting unit so as to emit a measurement light of a first polarization state or a measurement light of a second polarization state. In particular, the polarization selection unit is represented by a controller, i.e., the polarization selection unit can control the emission of light.

[0028] In one embodiment, the light emitting unit, the support structure and the light receiving unit define a beam path for the measuring light.

[0029] According to one embodiment, the polarization selection unit may be arranged along the beam path between the light emitting unit and the support structure.

[0030] According to one embodiment, the polarization selection unit may be arranged along the beam path between the support structure and the light receiving unit.

[0031] In one embodiment, the measuring device may include a first polarizer arranged along the beam path between the light emitting unit and the support structure and / or a second polarizer arranged along the beam path between the support structure and the light receiving unit.

[0032] The polarizer may provide for filtering light of a specific polarization state and transmitting measurement light of only one specific polarization state.

[0033] In one embodiment, the polarization selection unit may be configured as a polarization beam splitter that provides separation of reflected measurement light having a first polarization state and reflected measurement light having a second polarization state, and the light receiving unit may include two sensor units, one of which is configured and arranged to detect the measurement light of the first polarization state, and one of which is configured and arranged to detect the measurement light of the second polarization state.

[0034] In one embodiment, the polarization selection unit may be configured to provide selection of reflected measurement light having the first polarization state or having the second polarization state. Selecting reflected measurement light may be understood as selecting a portion of the measurement light allowed to be transmitted to the light receiving unit.

[0035] The selection of the reflected measurement light may be provided by selectively providing the emission of the measurement light having the first polarization state or providing the emission of the measurement light having the second polarization state by changing the emission selection state of the polarization selection unit. Alternatively or in addition, the selection of the reflected measurement light may be provided by selectively providing the transmission of the reflected measurement light of only the first polarization state or the second polarization state by changing the transmission selection state of the polarization selection unit.

[0036] Hence, the polarization selection unit may provide for selecting a specific kind or part of the measuring light to be emitted by the light emitting unit.Alternatively or additionally, the polarization selection unit may provide for selecting a propagation of a specific part of the reflected measuring light to the sensor of the light receiving unit.

[0037] In one embodiment, the polarization selection unit can be arranged as a controller for controlling the light emitting unit. For example, here, the polarization selection unit can be configured to control the light emitting unit, in particular a switchable polarization filter or two light sources of the light emitting unit. The switchable polarization filter can be provided by a mechanically rotatable linear polarizer.

[0038] In particular, the polarization selecting unit may comprise a fixed polarizer followed by a switchable polarization rotation device such as an LCD acting as a rotator (twisted nematic) or a half-wave plate.

[0039] In one embodiment, the polarization selection unit can be provided by a light emitting unit. The invention also relates to such an embodiment, wherein the selection of the emitted measurement light can be provided by the light emitting unit. Such a function can be provided by an arrangement of at least two different light sources (e.g. laser diodes) and providing switching of the light emission between the light sources.

[0040] The light emitting unit may also be configured or controlled to provide emission of measurement light in at least two different polarization states simultaneously.

[0041] In one embodiment, the polarization selection unit may comprise a movable polarization filter element (e.g., a polarizer) for filtering the emitted measurement light or the reflected measurement light, in particular arranged to be rotatable around an optical axis (corresponding to the propagation direction of the measurement light). For example, in order to select a specific part of the reflected measurement light, the polarization filter element may be arranged together with an actuator (motor) to provide rotation of the filter element.

[0042] In one embodiment, the polarization selection unit can be arranged as a switchable optical element, in particular a liquid crystal device (LCD). An LCD (in particular a twisted nematic LCD) can provide for changing the polarization state of the transmitted light by 90°. If combined with a conventional polarizer (polarizer behind the LCD), the LCD can be used to select a specific portion of the measurement light (for example, in the case where the measurement light propagating to the LCD comprises two differently polarized portions) to correspond to the polarization condition of the polarizer and to be continuously transmitted through the polarizer.

[0043] In one embodiment, the polarization selection unit can be configured as a polarizer (e.g., a polarization matrix) which is combined with the sensor unit of the light receiving unit and provides pixel-selective detection of reflected measurement light of a first polarization state and a second polarization state. In particular, the polarization selection unit can be provided by a polarization-sensitive sensor. Thus, each pixel or pixel group of the sensor can be irradiated with measurement light of a specific polarization state. By reading out and / or processing only pixels irradiated with measurement light of a desired or favorable polarization, a corresponding image can be generated, for example covering only light reflected by an object or a supporting structure.

[0044] In particular, by using a polarizer combined with a sensor unit, the polarization selection unit can be implemented at the algorithm level. When using at least three (typically four) polarization filters with different polarization directions, the polarization direction can be calculated and two or even more polarization states can be distinguished.

[0045] The polarization selection unit may provide for emission or transmission of light of a specific polarization state.

[0046] In one embodiment, the measurement light of the first polarization state may include a linear polarization of a first orientation, and the measurement light of the second polarization state may include a linear polarization of a second orientation orthogonal to the first orientation. In particular, the light emitting unit may include a polarization filter of a first orientation, and the light receiving unit may include a polarization filter of a second orientation orthogonal to the first orientation.

[0047] In one embodiment, the polarization state manipulation layer may provide circular polarization of the measurement light by passing through the polarization state manipulation layer. In particular, the polarization state manipulation layer may include a λ / 4 wave plate or a λ / 4 foil.

[0048] In one embodiment, the retroreflective layer may include retroreflective tape.

[0049] In one embodiment, the measuring device may include a control and processing unit configured to control at least the polarization selection unit and the light receiving unit, and the control and processing unit may include an image capture function. The image capture function may be configured to emit the measuring light by controlling the light emitting unit, detect the reflected measuring light by controlling the polarization selection unit to select the measuring light having a first polarization state and correspondingly capture a first image by the light receiving unit, and detect the reflected measuring light by controlling the polarization selection unit to select the measuring light having a second polarization state and correspondingly capture a second image by the light receiving unit.

[0050] In particular, for example when using a polarization-sensitive sensor, the first image and the second image may be captured within a single exposure.

[0051] In particular, the first image and the second image may each cover at least a portion of the object and the support structure, wherein the respective images cover a common area of ​​the object.Thereby, processing of the two images provides a measurement of the object, ie a measurement of the common area of ​​the object.

[0052] In one embodiment, the detected reflected measurement light may be partially reflected at the object and partially reflected by the support structure, wherein the measurement light reflected at the object comprises a polarization state different from a polarization state of the measurement light reflected by the support structure.

[0053] Thus, the control and processing unit, in particular the image capture functionality, may provide for capturing two different images. Each image comprises image information relating to a specific portion of the measurement light, for example image information relating to a specific polarization of the measurement light. Thus, one image may comprise image information relating to the measurement light reflected at the object and the other image may comprise image information relating to the measurement light reflected by the support structure. In particular, a top light image and a bottom light image may be captured.

[0054] In one embodiment, the control and processing unit may comprise an image processing function configured to subtract image information of the first image from image information of the second image.

[0055] As explained in more detail below, a subtraction of the image information may be provided to improve the contrast of the resulting image. The contrast may be reduced because the polarization manipulation layer may not function properly. As a result, the received measurement light (bottom light image) reflected at the support structure will be less bright. Furthermore, the retroreflected light (reflected by the support structure) may not be completely eliminated because the polarization state may be inaccurate due to a possible depolarization effect of the reflective foil. As a result, the contrast of the object surface to the background light results in being too low for many applications.

[0056] Therefore, by subtracting the two images, the contrast can be further improved and thereby more accurate measurements of, for example, edges and sizes can be obtained.

[0057] The subtraction of the image information can preferably be performed by applying a weighting factor for the subtraction. In order to provide maximization of contrast (making the light from the unwanted measurement mode as dark as possible in the image), for example, an image comprising 20% ​​of the unwanted intensity can be taken into account by a weighting factor of 0.2 to eliminate this component compared to another image.

[0058] In one embodiment, the image information may include pixel-related values ​​regarding the color and / or brightness of the pixels.

[0059] In one embodiment, the image information may include information related to the image region, in particular contrast.

[0060] In one embodiment, the measuring device can be embodied as a coordinate measuring machine, in particular a vision machine. A vision machine should be understood as a type of CMM, i.e. a specific CMM. Alternatively, the measuring device can be provided by a telecentric imaging system, a microscope, etc.

[0061] Therefore, the present invention also relates to a CMM as a measuring device for measuring a plurality of measuring points on a surface of an object. A coordinate measuring machine may comprise a base, a probe head with an optical probe and a machine structure, in particular a portal (bridge) or SCARA type machine structure, with structural components for connecting the probe head to the base. The CMM may also be embodied as a horizontal arm CMM or a mobile stage bridge CMM (e.g., Optiv and PMM-C and gantry CMM). A Scara type machine is an industrial robot. The acronym SCARA stands for Selective Compliance Assembly Robot Arm or Selective Compliance Articulated Robot Arm.

[0062] The base may be provided as a measuring table, for example made of granite, or as a physical reference point, for example a pedestal or a setting position of a CMM.A support structure may preferably be arranged at the base.

[0063] The probe head may be configured as an element for carrying a measurement probe (e.g. a tactile or optical probe) or a tool. The probe head may also be embodied to provide rotation of the measurement probe around one, two or three axes. In the case where the CMM is constructed as a gantry (3-axis) CMM, the probe head is preferably mounted to a z-ram that provides movability of the probe head in the z direction.

[0064] The optical probe may preferably be arranged at the probe head so as to provide a measurement of the distance to an object or structure (1D data), the lateral dimensions of the object or structure (2D data), or both (3D data) using the optical probe. Thus, the optical probe may be movable relative to the base. The optical probe may be embodied as a camera system.

[0065] The optical probe may include an optical system including a light emitting unit and a light receiving unit and, for example, a telecentric lens.

[0066] Illumination of the object can be a coaxial top beam coupled into a telecentric lens via a beam splitter or combiner. Typically, optical probes have a lateral object resolution of a few microns.

[0067] Therefore, as described above, optical elements such as a light emitting unit and a light receiving unit may be preferably arranged together with the optical measuring probe.

[0068] As mentioned above, the machine structure may include specific elements that are movable relative to each other to provide positioning of the probe head within the measurement volume of the CMM. Relative movability should be understood as relative linear movement or relative rotational movement, but should not be limited to a specific design of the CMM, but should also cover CMMs and robots known in the prior art.

[0069] The coordinate measuring machine further comprises at least one drive mechanism for providing movability of the machine structure and the probe head relative to the base. The drive mechanism may be configured as a motor, a stepper motor, an actuator, etc. In particular, the coordinate measuring machine may comprise at least three drive devices, which are arranged such that the probe head can be moved in three directions, for example in the directions of the x-axis, the y-axis and the z-axis.

[0070] The coordinate measuring machine further comprises at least one encoding unit configured to provide position data about the position of the machine structure and / or the probe head relative to the base. In particular, the coordinate measuring machine comprises a set of encoders providing the relative position of the machine part on a specific coordinate axis. The at least one encoding unit may be provided by a linear encoder, a rotary encoder or a sensor unit capable of measuring the distance between two points.

[0071] The present invention also relates to a method for imaging an object by means of a measuring device as described above. The method comprises the following steps: providing the object on a support structure; illuminating the object by directing measuring light to the object by means of a light emitting unit; receiving and detecting the measuring light reflected at the support structure and reflected by the object by means of a light receiving unit; and capturing a first image and a second image of the object by means of the light receiving unit.

[0072] In particular, the first image and the second image may be captured within a single exposure, for example by using a polarization sensitive sensor.

[0073] According to the method, the reception and detection of the measurement light is provided by selecting the measurement light having a first polarization state and capturing a first image by capturing a reflection measurement during the selection of the measurement light having the first polarization state. In addition, the measurement light having a second polarization state is selected and the second image is captured by capturing a reflection measurement during the selection of the measurement light having the second polarization state.

[0074] The invention also relates to a computer program product which, when executed by a control and processing unit and / or by a controller, enables automatic execution and control of the steps of the above method. BRIEF DESCRIPTION OF THE DRAWINGS

[0075] By way of example only, specific embodiments of the present invention will be described more fully hereinafter with reference to the accompanying drawings, in which:

[0076] Figure 1 A schematic diagram of an embodiment of the measuring machine according to the invention is shown as a three-axis machine.

[0077] Figure 2 An embodiment of an optical system and a support structure of a measuring machine according to the present invention is shown.

[0078] Figure 3 An embodiment of an optical system and a support structure of a measuring machine according to the present invention is shown.

[0079] Figure 4a and Figure 4b The advantageous effects of providing a retroreflective layer compared to normal (specular) reflection are depicted.

[0080] Figure 5a and Figure 5bAnother embodiment of the optical system and the support structure of the measuring machine according to the present invention is shown.

[0081] Figure 6 Another embodiment of the optical system and the support structure of the measuring machine according to the present invention is shown. DETAILED DESCRIPTION

[0082] Figure 1 A schematic diagram of an embodiment of a measuring machine 1 (in particular a vision machine or CMM) according to the invention as a three-axis machine is shown. The measuring machine 1 comprises a measuring table 2 for supporting an object 7 to be measured. The measuring table 2 provides a base for the machine. Although not shown in the depicted embodiment, the measuring table 2 may include elements that assist in the positioning of the workpiece 7. The measuring table 2 may also include further elements to clamp or otherwise fix the workpiece 7 (in particular by a vacuum chuck). Such further elements may be temporarily mounted to the measuring table 2.

[0083] The measuring machine 1 comprises a machine structure 3 which is embodied as a movable frame mounted on a measuring table 2 such that it can be moved along a first axis (x-axis) by a first set of motors. The machine structure 3 comprises an element spanning the measuring table 2, the so-called bridge 4. Furthermore, the CMM comprises a carrier which is movably mounted along the bridge 4. The mounting of the carrier is provided such that it can be moved along a second axis (y-axis) perpendicular to the first axis.

[0084] The support carries another element 5 which is arranged and configured to move along a third axis (z axis). Such an element 5 may also be referred to as a (z) ram or rod.

[0085] Furthermore, the measuring machine 1 comprises a drive mechanism for providing movability of the machine structure 3 and the probe head 6 relative to the base 2. The drive mechanism may comprise specific drive units providing relative movement of specific parts of the machine structure 3, such as a motor providing movement of the entire machine structure 3 (bridge 4) relative to the base 2, a motor providing movement of the support relative to the bridge 4 and a motor providing movement of the z slide 5 relative to the support.

[0086] The measuring machine 1 further comprises at least one encoding unit configured to provide position data about the position of the machine structure 3 and / or the probe head 6 relative to the base 2. The encoding unit may comprise several encoders providing information about the position of the bridge 4 relative to the base 2, the position of the support relative to the bridge 4 and the position of the z-slide 5 relative to the support.

[0087] The optical probe 6 is mounted on the end of the z ram 5. The optical probe 6 may be part of a replaceable probe head. The optical probe 6 is configured to acquire data related to the surface of the workpiece 7. Therefore, the optical probe 6 includes a light receiving unit (sensor) to receive and detect an optical signal, such as light reflected at the surface of the workpiece 7 or a measurement light.

[0088] In one embodiment, the measuring machine 1 may include additional joints and / or actuators to provide additional degrees of freedom for moving the optical probe head 6. For example, a swivel joint may be arranged to provide rotation of the probe head and / or the optical probe 6.

[0089] The optical probe 6 comprises a light emitting unit for irradiating the workpiece 7 with polarized light and / or for projecting polarized measurement light toward the workpiece 7. The light receiving unit is configured to detect reflection of the light emitted by the light emitting unit.

[0090] The measuring machine 1 further includes a retroreflective layer 11 and a polarization state manipulation layer 12. The combination of these layers provides retroreflection of the measuring light emitted by the light emitting unit and switching of the polarization state of the measuring light when the measuring light passes through the polarization state manipulation layer 12.

[0091] The polarization state manipulation layer 12 is preferably designed to change the linear polarization direction of the light passing through the polarization state manipulation layer 12 twice, a total of 90°, that is, to change the orientation of the electric field vector relative to the axis of the light propagating around the axis by 90°. In other words, measuring light with parallel polarization results in reflected measurement light with perpendicular (orthogonal) polarization.

[0092] The polarization state manipulation layer 12 may include or consist of a quarter wave plate foil (also called a retarder foil). The light passing through the wave plate foil becomes circularly polarized after the first pass, and after reflection and a second pass through the wave plate foil, it becomes vertically polarized compared to the incident polarization state of the top light.

[0093] The retroreflective layer 11 may include or consist of a retroreflective tape.

[0094] By means of such a support structure comprising a retroreflective layer and a polarization state manipulation layer, an improved selective detection of light reflected on top of the object 7 and light reflected by the support structure is achieved. A corresponding configuration of the optical probe provides such selection of reflected light. The configuration of the system is described herein, for example, with respect to the following figures.

[0095] In particular, the retroreflective layer 11 may also influence the polarization state of the measurement light.Here, the polarization state manipulation layer 12 may be adjusted such that the combined effect of the two layers 11, 12 influences the polarization such that it is orthogonal to the polarization of the incident light.

[0096] The measuring machine 1 further comprises a control and processing unit 9. The control and processing unit is configured to control a drive of the measuring machine 1 to provide a precise positioning of the optical probe 6 relative to the object 7. The control and processing unit is further configured to control data acquisition by the optical probe 6. In particular, the control and processing unit 9 may be configured to control the polarization selection unit to provide data acquisition of both light reflected at the object and light reflected by the support structure.

[0097] The method of controlling the polarization selection unit according to the present invention is described in more detail below.

[0098] Figure 2 An optical system 20 and a support structure 10 of a measuring machine 1 according to the invention are shown. The optical elements of the optical system 20 for providing measuring light and detecting reflected measuring light may be provided by an optical probe 6 of the measuring machine.

[0099] The support structure 10 includes a retroreflective layer 11 and a polarization state manipulation layer 12. The combination of these components provides retroreflection of incident light and perpendicular polarization of reflected light (eg, second polarization state) compared to the incident polarization state of the incident light (eg, first polarization state).

[0100] The support structure 10 also comprises a laminated glass sheet 13 for protecting the underlying layers. However, such a glass sheet 13 is an optional feature.

[0101] As shown in the figure, the retroreflective properties of the retroreflective layer 11 (e.g., tape) provide for converting top light from above into bottom light from below the object 7 in areas where the object shows opening edges (such as drilled holes, etc.). The bottom light arrangement may include a quarter wave plate foil (also called a retarder foil) followed by a retroreflective tape. The light passing through the wave plate foil becomes circularly polarized after the first pass, and after reflection and a second pass through the wave plate foil, it becomes vertically polarized compared to the incident polarization state of the top light.

[0102] The main advantage compared to e.g. a plane mirror placed below the object is the inversion of the light direction, allowing sharper imaging of edges, which is one of the main applications of bottom lighting. Figure 3 The advantageous effect of retroreflection compared to co-reflection by a mirror is described.

[0103] The measuring machine 1 (particularly the optical system 20) further comprises a light emitting unit 21. In the present embodiment, the light emitting unit 21 comprises a light source 22 and a first polarizer 23. Therefore, the light emitting unit 21 is configured to emit polarized light of a specific first polarization state.

[0104] The introduction of this polarization technology allows the distinction between light reflected from the top surface and the back-reflected bottom light. The light emitting unit 21 can be set as a laser diode (which inherently produces polarized emission) or an alternative light source combined with a polarization filter (as shown), such as polarization filtered LED light. This polarized light can be coupled into a telecentric lens like a conventional coaxial top light.

[0105] The beam splitter 24 is arranged to couple in the top light emitted by the light emitting unit 21 .

[0106] The second polarizer 25 is disposed between the beam splitter 24 and the light receiving unit 30. The light receiving unit 30 may include an image sensor. The polarizer 25 is configured to select a desired polarization state, ie, to provide transmission of light of a specific polarization state.

[0107] In case the transmissive state of the polarizer 25 is aligned with the bottom light, this appears bright compared to the dark areas of the surface of the object 7. By a 90° rotation of the second polarizer 25 the conditions are reversed: the surface of the object 7 appears bright and the bottom light is blocked.

[0108] The second polarizer 25 provides a polarization selection unit, wherein it comprises a movable polarization filter element for filtering the emitted measurement light or the reflected measurement light. The polarization filter element is arranged rotatable around an optical axis, eg a propagation axis of the reflected measurement light.

[0109] The second polarizer 25 provides selective transmission of the reflected measurement light having the first polarization state or having the second polarization state by corresponding rotation. Thus, the second polarizer 25 selectively provides transmission of only the reflected measurement light of the first polarization state or the second polarization state by changing the polarization-selective transmission selection state (here by rotation of the polarization filter).

[0110] The polarization state of the emitted measurement light is typically not changed due to reflection at the object 7. Thus, by setting the polarizer 25 parallel to the illumination polarization state, the surface of the object 7 is visible and the light propagating through the polarization state manipulation layer 12 is attenuated.

[0111] In contrast, the polarization state of the emitted measurement light changes by 90° under the condition of double interaction with the polarization state manipulation layer 12, and the respectively reflected measurement light includes a different polarization state. Therefore, the 90° rotation of the polarizer 25 suppresses the light from the surface and makes the light from the bottom light retroreflection band visible after transmitting through the polarization state manipulation layer 12 (e.g., Figure 2 ). Thereby, a typical bottom-light appearance of the object 7 can be provided, ie with a very high contrast between the workpiece and the background.

[0112] Thus, an operator of the measuring machine can change between a top-light or bottom-light appearance of the object 7 by rotating the second polarizer 25 or by rotating the first polarizer 23. A polarization selection unit can be provided by the first polarizer 23 when the first polarizer 23 is rotated.

[0113] The measuring machine further comprises a control and processing unit 9. The control and processing unit 9 is configured to control the polarization selection unit, ie here the second polarizer 25. The control and processing unit may further be configured to control the light emitting unit 21 and / or the light receiving unit 30.

[0114] The control and processing unit 9 includes an image capture function, which is configured to emit measurement light by controlling the light emitting unit 21, detect the reflected measurement light by selecting the measurement light having a first polarization state by controlling the polarization selection unit 25 and correspondingly capturing a first image by the light receiving unit 30, and detect the reflected measurement light by selecting the measurement light having a second polarization state by controlling the polarization selection unit 25 and correspondingly capturing a second image by the light receiving unit 30.

[0115] Due to typical depolarization effects on the sides of the polarization state manipulation layer 12 and / or the retroreflective layer 11, the contrast of the image may be reduced. This is, for example, because the wave plate foil does not work properly. In particular, the contrast may also be reduced due to depolarization, for example due to volume scattering in the material.

[0116] Possible depolarization of the polarization state manipulation layer 12 (e.g., wave plate foil or film) and / or the retroreflective layer 11 may be caused, for example, by manufacturing-related deviations in the microgeometry of the retroreflective structures (e.g., due to stamping conditions). This is the so-called dihedral error, i.e., the reflective surfaces are not completely orthogonal to each other, which may lead to a slight divergence and also to a slight rotation of the polarization of the retroreflected light.

[0117] Furthermore, diffraction effects at the edges of the microstructures may also lead to smearing and incorrectly aligned polarization of the back-propagating light.

[0118] For example, in bottom light applications, the background light is less bright. However, the light from the object surface can be almost completely terminated. This is especially true for technical (metal) materials. In contrast, the backlight is bright enough and has a large amplitude. A contrast ratio of about 90% can still be achieved.

[0119] However, the situation is different for top light measurement. Since the polarization state is usually inaccurate due to the depolarization of the polarization state manipulation layer 12 (e.g., the polarization state manipulation layer 12 of the reflective foil), the background light cannot be eliminated at the polarizer 25. Therefore, a relatively large amplitude remains, in particular, an amplitude greater than that of the surface of the object 7. Therefore, the contrast between the reflected light from the surface of the object 7 and the background light can reach about 30%, which is too low for many measurement or imaging applications.

[0120] The above problems can be overcome by capturing at least two images of the object as described above. Furthermore, the control and processing unit comprises an image processing function configured to subtract the image information of the first image from the image information of the second image. In particular, the image information of the bottom light image is subtracted from the top light image.

[0121] By taking this difference image, the background light is eliminated and a contrast ratio of about 100% can be provided.

[0122] In particular, the subtraction may not be a simple difference operation, but a weighted average or linear combination of the two original images, thereby eliminating the backlight component in the top light image.

[0123] In particular, automatic field calibration of the workpiece to be measured can be performed by automatically classifying pixels as retroreflective or non-retroreflective by comparison with the known polarization state of the retroreflected light. Based on this, the weights can be optimized to maximize the contrast.

[0124] For a corresponding integration in the measuring machine, the image subtraction must be performed relatively quickly and automatically, so that the movement of the optical probe for continuous scanning of, for example, the object surface does not have to be slowed down or disturbed. For this purpose, a fast polarization switching (e.g., by using an electrically switchable LCD and taking two images sequentially and then performing the image subtraction in software) can be provided instead. Figure 4a and Figure 4b shown).

[0125] Alternatively, the image sensor may be configured to allow two exposures of different polarization states directly after each other without image readout in between (quasi-simultaneous).

[0126] Alternatively, the image sensor may be configured to allow for capturing a polarization-sensitive image and subtracting two orthogonal images taken simultaneously in one exposure.

[0127] Figure 3 An optical system 20 and a support structure 10 of a measuring machine 1 according to the invention are shown. The optical elements of the optical system 20 for providing measuring light and detecting reflected measuring light may be provided by an optical probe 6 of the measuring machine.

[0128] This implementation method and Figure 2 The embodiment of is distinguished by having a polarization beam splitter 29 arranged between the support structure 10 and the light receiving unit. The light receiving unit comprises two sensor units 31 and 32. The beam splitter 29 and the sensor units 31, 32 are arranged relative to each other so that each sensor provides detection of measurement light of a specific polarization state. Therefore, the sensors 31, 32 are arranged relative to the beam splitter 29 in such a way that each image sensor 31, 32 detects only one orthogonal polarization state of light.

[0129] The advantage of this setup is that different images with different polarization states can be captured simultaneously, where optical losses are avoided.

[0130] Figure 4a and Figure 4b The advantageous effect of providing a retroreflective layer 11 compared to conventional reflection by eg a mirror 8 or the like is shown.

[0131] The advantage of the retroreflective layer 11 compared to a flat mirror 8 placed under the object 7 is the reversal of the light direction, allowing a sharper imaging of the edges. For a favorable (sharp) imaging of the edges, it is important that as many edge rays as possible propagate back to the entrance pupil 40 (e.g., defined by the numerical aperture (NA) of the telecentric lens) and in this way contribute to the image formation process. Furthermore, the orientation of the retroreflective layer is less sensitive to misalignment, whereas a mirror would have to be aligned perfectly perpendicular to the direction of the emitted light.

[0132] Figure 4a Involving a conventional mirror 8 placed below an object of interest 7 having the edge of a borehole to be measured. The edge ray 42 is reflected by the mirror, propagates out and does not enter the entrance pupil 40 in the reverse propagation. In this illustration, only the chief ray 41 propagates back into the lens (assuming that the mirror is well aligned with the optical axis of the telecentric lens). Of course, some parts of the light on one side between the chief ray 47 and the outer edge ray 42 are reflected so that they can be captured, but there will be a significant intensity drop near the edge and the lens entrance hole will not be filled symmetrically. For larger distances between the object 7 and the mirror 8, this reduction in the angular illumination spectrum becomes even stronger. As a result, the image of the edge will become (more) blurred and the position of the edge cannot be well determined, resulting in reduced measurement accuracy.

[0133] In case the mirror 8 is replaced by, for example, a retroreflective layer 11 having a fine microstructure, the direction of the edge rays 42 is preserved and they can propagate back into the entrance pupil 40 after reflection at the layer 11, as can be seen from Figure 4b Thus, in this case, the image of the edge will become sharp, with higher contrast, and the entrance pupil 40 is filled, allowing the edge position to be determined accurately. This can become very important in metrology applications to determine, for example, the diameter of a drill hole.

[0134] Preferably, the retroreflective layer 11 is arranged at a defined distance from the object 7 so that the microstructure of the retroreflective layer 11 is out of focus for the imaging optics and the reflected light becomes uniform. Thus, the fine microstructure of the retroreflective layer 11 itself does not interfere with the imaging of the edge of the object.

[0135] Figure 5a and Figure 5b An optical system 20 and a support structure 10 of a measuring machine 1 according to the invention are shown. The optical elements of the optical system 20 for providing measuring light and detecting reflected measuring light may be provided by an optical probe of the measuring machine.

[0136] The support structure 10 comprises a retroreflective layer 11 and a polarization state manipulating layer 12. The support structure 10 also comprises a laminated glass sheet 13 for protecting the underlying layers. However, such a glass sheet 13 is an optional feature.

[0137] As above Figure 2 As described in the context of , the retroreflective properties of the retroreflective layer 11 provide for converting top light from above into bottom light from below the object 7 in the region where the edge of the opening is shown in the object 7. The support structure 10 may comprise a retarder foil 12 followed by the retroreflective tape or foil 11. The light passing through the waveplate foil becomes circularly polarized after a first pass and after reflection and a second pass through the waveplate foil it becomes vertically polarized compared to the polarization state of the incident measurement light.

[0138] The measuring machine 1 (in particular, the optical system 20) further includes a light emitting unit 21. In the present embodiment, the light emitting unit 21 includes two light sources 22 and 26 and a polarization beam splitter 27 (PBS). The light sources 22 and 26 are configured to emit light so that the light of the light source 22 provides polarized light with an orthogonal polarization compared to the light emitted by the light source 26. Therefore, by means of the polarization beam splitter 27, two orthogonally polarized light sources can be combined.

[0139] The introduction of this polarization technique allows for the distinction between light reflected from the top surface of the object 7 and light reflected back from the support structure 10. The light sources 22 and 26 can be provided as laser diodes (which inherently can produce polarized emission) or as alternative light sources combined with polarization filters, such as polarization filtered LED light. This polarized light can be coupled into a telecentric lens like a conventional coaxial top light.

[0140] A further beam splitter 24 is arranged for coupling the top light emitted by the light emitting unit 21 in the imaging optics.

[0141] The polarizer 25 is disposed between the beam splitter 24 and the light receiving unit 30. The light receiving unit 30 may include an image sensor. The polarizer 25 is configured to select a desired polarization state, ie, to provide transmission of light of a specific polarization state.

[0142] In this embodiment, the transmission state of polarizer 25 is selected to correspond to one of the polarization states provided by one of light sources 22 or 26 .

[0143] As shown in the figure, the transmission state of the polarizer 25 here corresponds to the polarization direction of the measurement light emitted by the light source 26 .

[0144] Figure 5a The illumination of the light source 22 is shown. The measurement light reflected at the surface of the object 7 and arriving as reflected measurement light provides the same polarization state as the emission. Since the polarizer 25 only allows the transmission of the measurement light of the light source 26, i.e. the transmission of the measurement light of orthogonal polarization compared to the light source 22, the measurement light reflected at the surface of the object 7 is blocked.

[0145] However, a portion of the measuring light that passes through the object 7 (e.g., via the drilled hole) and the polarization state manipulation layer 12 (twice) and is retroreflected by the layer 11 reaches the polarizer 25 in an orthogonally changed polarization state and passes through the polarizer 25. Therefore, the retroreflected portion of the measuring light can be received and detected by the light receiving unit 30.

[0146] As a result, the bottom light becomes bright compared to the dark area of ​​the surface of the object 7. Thus, a bottom light image can be captured.

[0147] Figure 5b The illumination of the light source 26, i.e. the measurement light whose polarization state is rotated by 90°, is shown. Here, a part of the measurement light reflected at the surface of the object 7 is transmitted through the polarizer 25 and can be captured by the light receiving unit. Another part of the measurement light reflected by the retroreflective layer 11 (and therefore including the polarization state changed due to the interaction with the polarization state manipulation layer 12) is blocked by the polarizer 25.

[0148] As a result, the top light becomes bright compared to the dark area of ​​the surface of the object 7. Thus, a top light image can be captured.

[0149] This embodiment provides for electronic switching of the light sources 22 and 26, thus allowing selection of a top-light or bottom-light appearance.

[0150] The system further comprises a control and processing unit 9 which is configured to control the switching of the light sources 22 and 26. Thus, the control and processing unit 9 may provide for selectively generating a bottom image or a top image.

[0151] Here, the light emitting unit 21 may provide a polarization selection unit configured to provide a measurement light having a first polarization state or having a second polarization state. The emission of the measurement light having a first polarization state or having a second polarization state may be selectively provided by changing the emission selection state of the polarization selection unit (e.g., activating the light source 22 or the light source 26).

[0152] Alternatively, in order to provide different polarization states, a rotatable polarizer or a switchable LCD may be combined with the light emitting unit to provide a change in the polarization state of the emitted measurement light (not shown). Alternatively, the polarization selection unit may be provided by a control and processing unit 9 configured to control the light emitting unit 21, for example by implementing an image capture function.

[0153] In one embodiment, the control and processing unit may further be configured to provide image subtraction in order to improve the contrast of the top light image, in particular as described above. This may provide compensation for possible depolarization effects of the support structure.

[0154] Figure 6 Another embodiment of the system according to the present invention is shown. This embodiment mainly corresponds to Figure 2 In an embodiment of the invention, however, the polarizer 25 is arranged fixed and a further switchable optical element 28 is arranged along the beam path between the object 7 and the light receiving unit 30. The control and processing unit is further configured to control the switchable optical element 28.

[0155] The switchable optical element 28 is provided by a liquid crystal device 28 (LCD) in front of the second polariser 25, i.e. between the beam splitter 24 and the polariser 25. The LCD 28 provides electronically controlled switching between top and bottom light images without any structural changes, such as mechanical drive for rotating the polarisers.

[0156] LCD 28 can function like a single pixel device that can be electronically controlled to rotate the polarization state of incident light like a half wave plate. For example, a twisted nematic cell can be used to rotate the polarization or not rotate it in a relaxed state by applying a voltage. However, it should be understood that other types of LCD modes are also possible.

[0157] Thus, the transmission of the reflected measurement light reflected by the surface of the object 7 or the transmission of the reflected measurement light retroreflected by the support structure 10 can be selected by switching the activation state of the LCD. Relatively fast switching between top light detection and bottom light detection can be provided. Therefore, by using such an LCD 28, fast measurement or scanning of the object 7 can be provided.

[0158] Hence, the switchable optical element 28 (eg an LCD) provides a polarization selection unit according to the invention.

[0159] As an alternative (instead of a switchable LCD), an image sensor with a polarization matrix in front of each pixel can be provided. The polarization matrix can be configured similarly to the RGB filters in front of the pixels of a color image sensor ("Bayer pattern"). Here, a specific image sensor is provided with a polarization selection unit according to the invention.

[0160] By providing the correct orientation to the polarization of the emitted light and the polarization matrix at the sensor, such a sensor allows simultaneous detection of two polarization states in a single exposure. Selecting the correct pixels allows separation of the two polarization states and thereby can provide simultaneous measurement of top and bottom light images.

[0161] In particular, such a polarization-sensitive sensor can include four polarization filters with different polarization directions at angles 0°, 90°, 180° and 270°. By using the four-phase shift method, the polarization contrast and direction can be calculated. Thus, the detection of two polarization states can be provided by numerical evaluation rather than by selecting pixels. Thus, the polarization direction of the emitted light does not have to be fixed.

[0162] In one embodiment, the light emitting unit may include or be provided by, for example, a laser, a laser diode, an LED or an OLED. In one embodiment, the light receiving unit may include or be provided by, for example, a CMOS or CCD image sensor.

[0163] Although the present invention has been described in the foregoing with reference to some specific embodiments, it must be understood that many modifications and combinations of different features of the embodiments are possible. All these modifications are within the scope of the appended claims.

Claims

1. A measuring device for imaging an object (7), the measuring device comprising: A light emitting unit (21), the light emitting unit (21) being configured to emit polarization measurement light; a light receiving unit (30) configured to receive and detect the reflected measurement light; as well as A support structure (10), comprising a retroreflective layer (11) and a polarization state manipulation layer (12), wherein the support structure (10) provides: □ the retroreflection of the measuring light, and □ changing the polarization state of the measuring light so that the polarization state of the incident measuring light is different from the polarization state of the reflected measuring light, It is characterized in that The measuring device comprises a polarization selection unit configured to provide reflected measurement light having a first polarization state and / or reflected measurement light having a second polarization state for reception by the light receiving unit (30).

2. The measuring device according to claim 1, wherein: The reflected measuring light is provided by reflection of the measuring light at the object (7) and / or by retroreflection of the measuring light by the support structure (10).

3. The measuring device according to claim 1 or 2, wherein: The light emitting unit (21) is configured to emit measurement light in the first polarization state and / or measurement light in the second polarization state, wherein the measurement light in the first polarization state and the measurement light in the second polarization state can be provided simultaneously or successively, in particular, The light emitting unit (21) comprises at least two light sources (22, 26), one of the at least two light sources (22, 26) is configured to emit measurement light in the first polarization state, and the other of the at least two light sources (22, 26) is configured to emit measurement light in the second polarization state, and / or The polarization selection unit is configured to control the light emitting unit so as to emit the measurement light in the first polarization state or the measurement light in the second polarization state.

4. The measuring device according to any one of the preceding claims, wherein The light emitting unit (21), the supporting structure (10) and the light receiving unit (30) define a beam path for the measuring light, wherein the polarization selection unit is arranged along the beam path. between the light emitting unit (21) and the supporting structure (10), or Between the support structure (10) and the light receiving unit (30), In particular, the measuring device comprises a first polarizer (23) arranged along a beam path between the light emitting unit (21) and the supporting structure (10) and / or a second polarizer (25) arranged along the beam path between the supporting structure (10) and the light receiving unit (30).

5. The measuring device according to any one of claims 1 to 4, wherein: the polarization selection unit is a polarization beam splitter arranged to provide separation of the reflected measurement light having a first polarization state and the reflected measurement light having a second polarization state, and The light receiving unit (30) includes two sensor units, one of the two sensor units is configured and arranged to detect the measuring light of the first polarization state, and one of the two sensor units is configured and arranged to detect the measuring light of the second polarization state.

6. The measuring device according to any one of claims 1 to 4, wherein: The polarization selection unit is configured to provide selection of the reflected measurement light having the first polarization state or having the second polarization state by: selectively providing emission of the measurement light with a first polarization state or with a second polarization state by changing an emission selection state of the polarization selection unit, and / or · selectively providing transmission of reflected measurement light of only the first polarization state or the second polarization state by changing the transmission selection state of the polarization selection unit.

7. The measuring device according to claim 6, wherein: The polarization selection unit a controller configured to control the light emitting unit (21), or provided by the light emitting unit (21), or comprising a movable polarization filter element (23, 25) for filtering the emitted measurement light or the reflected measurement light, in particular being arranged so as to be rotatable about the optical axis or so as to be linearly movable, or is arranged as a switchable optical element (28), in particular as a liquid crystal device, or is arranged as a polarizer which is combined with the sensor unit of the light receiving unit (30) and provides pixel-selective detection of the reflected measurement light in the first polarization state and the second polarization state.

8. The measuring device according to any one of the preceding claims, wherein The measurement light in the first polarization state includes a linear polarization of a first orientation, and the measurement light in the second polarization state includes a linear polarization of a second orientation orthogonal to the first orientation, in particular, wherein the light emitting unit (21) includes a polarization filter of the first orientation, and the light receiving unit (30) includes a polarization filter of the second orientation orthogonal to the first orientation.

9. The measuring device according to any one of the preceding claims, wherein The polarization state manipulation layer (12) provides circular polarization of the measurement light by passing through the polarization state manipulation layer (12), in particular wherein the polarization state manipulation layer (12) comprises a λ / 4 wave plate or a λ / 4 foil, and / or The retroreflective layer (11) comprises a retroreflective foil, in particular a microsphere retroreflective foil.

10. The measuring device according to any one of the preceding claims, wherein The measuring device comprises a control and processing unit (9) configured to control at least the polarization selection unit and the light receiving unit, the control and processing unit (9) comprising an image capture function, the image capture function being configured to: · emitting the measuring light by controlling the light emitting unit (21), · detecting the reflected measuring light by controlling the polarization selection unit to select the measuring light having the first polarization state and correspondingly capturing a first image by the light receiving unit (30), and · detecting the reflected measuring light by controlling the polarization selection unit to select the measuring light having the second polarization state and correspondingly capturing a second image by the light receiving unit (30).

11. The measuring device according to claim 10, wherein: the first image and the second image each cover at least a portion of the object (7) and the support structure (10), wherein the respective images cover a common area of ​​the object (7), and / or The detected reflected measurement light is partly reflected at the object (7) and partly reflected by the support structure (10), wherein the measurement light reflected at the object (7) comprises a polarization state different from the polarization state of the measurement light reflected by the support structure (10).

12. The measuring device according to claim 10 or 11, wherein: The control and processing unit (9) comprises an image processing function, which is configured to subtract image information of the first image from image information of the second image, in particular wherein the image information comprises: Pixel-related values ​​regarding the color and / or brightness of the pixel, and / or Information about the image area, especially contrast.

13. The measuring device according to any one of the preceding claims, wherein The measuring device is embodied as a coordinate measuring machine, in particular as a vision machine.

14. Method for imaging an object (7) by means of a measuring device according to any of the preceding claims, wherein: The method comprises the following steps: providing said object (7) on said support structure (10), illuminating the object (7) by directing measuring light to the object by means of the light emitting unit (21), receiving and detecting, by the light receiving unit (30), the measurement light reflected at the support structure (10) and reflected by the object (7), and capturing a first image and a second image of the object by means of the light receiving unit (30), It is characterized in that Receiving and detecting the measuring light is provided by: selecting a measurement light having a first polarization state, capturing said first image by capturing a reflection measurement during selection of measurement light having said first polarization state, selecting the measurement light to have a second polarization state, and - Capturing the second image by capturing a reflection measurement during selection of measurement light having the second polarization state.

15. A computer program product which, when executed by a control and processing unit and / or a controller, causes the steps of the method according to claim 14 to be automatically performed and controlled.

Images