Apparatus for chromatic confocal measurement of distances
Patent Information
- Application Number
- EP2023735765
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2022-07-13
- Filing Date
- 2023-06-28
- Publication Date
- 2025-05-21
Smart Images
Figure 1.1
Abstract
Description
[0001] Device for chromatic confocal measurement of distances
[0002] BACKGROUND OF THE INVENTION
[0003] 1. Field of the invention
[0004] The invention relates to a device for the chromatic confocal measurement of distances at several points, preferably arranged along a line, on surfaces of workpieces and other objects.
[0005] 2. Description of the state of the art
[0006] Chromatic confocal distance measurement devices have been used in industrial metrology for many years when distances to workpieces or other objects need to be measured non-contact and with high accuracy. For transparent objects, such devices can also be used to measure wall thicknesses or other thicknesses, since a distance can usually be determined for each optical interface, and the thickness is calculated as the difference between the measured distances.
[0007] Conventional devices for chromatic confocal distance measurement contain a light source that generates polychromatic measuring light and directs it onto a pinhole with a very small aperture. The aperture is imaged onto the surface to be measured by imaging optics. At least part of the imaging optics exhibits significant longitudinal chromatic aberration. Differently colored images of the aperture are therefore arranged one behind the other on the optical axis of the imaging optics. Only the spectral portion of the measuring light for which the image of the aperture lies exactly on the surface to be measured is reflected back by the surface in such a way that it can enter the same aperture or an aperture arranged optically conjugate to it. The wavelength of this spectral portion is recorded using a spectrometer. Each wavelength is assigned a specific distance from the imaging optics.Although the other spectral components are also partially reflected by the object surface, they cannot be focused by the imaging optics into the same or an optically conjugated aperture and therefore do not contribute significantly to the part of the measuring light that is spectrally analyzed by the spectrometer.
[0008] US 2018 / 0038680 A1 discloses a chromatic confocal measuring device that allows distances or thicknesses to be measured simultaneously at multiple points. The otherwise common pinhole aperture is replaced by a linear or planar arrangement of optical fiber ends. Measuring distances or thicknesses simultaneously at multiple locations significantly reduces measurement time. Even for larger surfaces, a complete surface profile can then be created in a short time.
[0009] The lateral resolution of such a multi-point measuring device, i.e. the density of the measuring points on the surface of the object, is limited by the fact that measuring light reflected from the object which strikes the end of an optically conjugated optical fiber must not simultaneously strike a neighboring optical fiber. If such optical crosstalk occurs, clear distance values can no longer be assigned to the individual measuring points. One of the factors that contributes to optical crosstalk is that, for physical reasons, the image of a fiber end can never be exactly sharp, even with optimally corrected imaging optics, but due to diffraction phenomena takes the form of a flat diffraction disk which has several clearly recognizable secondary maxima.
[0010] In the known devices for chromatic-confocal multi-point measurement, the fiber ends are therefore relatively far apart, which is why a high density of measuring points is not possible.
[0011] SUMMARY OF THE INVENTION
[0012] The object of the invention is to provide a device for the chromatic confocal measurement of distances to multiple points on a surface of an object, which device has a high lateral resolution. According to the invention, this object is achieved by a device for the chromatic confocal measurement of distances at multiple points on a surface of an object, which device has a light source configured to generate polychromatic measuring light. A light pattern generation device is configured to generate a light pattern from the measuring light, which extends in a first plane along one dimension or along two dimensions. A chromatically uncorrected objective is configured to generate images of the light pattern in image planes whose axial position is wavelength-dependent due to chromatic longitudinal aberration.A static spatial filter is arranged in a second plane and has a filter function corresponding to the light pattern (but not necessarily identical), with the second plane being optically conjugate to the first plane. A preferably non-polarizing beam splitter cube is arranged in the light path between the light pattern generation device and the objective and guides measurement light, which has been reflected from the surface and passed through the objective, to the spatial filter. A spectrometer has several input channels configured to spectrally analyze reflected measurement light that has passed through a point of the static spatial filter assigned to the respective input channel. An evaluation device is configured to calculate distances from points on the surface to the measuring device from wavelengths measured by the spectrometer.According to the invention, the device further comprises a pixel-by-pixel switchable spatial filter which has a plurality of pixels and either forms the light pattern generating device or is arranged in a third plane which is optically conjugate to the first plane and the second plane, wherein each pixel can be converted into a first switching state in which the pixel blocks measuring light and into a second switching state in which the pixel does not block measuring light.
[0013] The pixel-by-pixel switchable spatial filter makes it possible to dim or deflect a portion of the measurement light that would otherwise cause optical crosstalk at one point during the measurement so that it cannot reach the spectrometer. At another point in time, another portion of the measurement light is suppressed. In this way, very high lateral resolution can be achieved by combining multiple successive measurements. In the simplest case, the pixel-by-pixel digitally switchable spatial filter is controlled in such a way that it switches between a first switching configuration and a second switching configuration during a measurement. In the first switching configuration, no two adjacent pixels are in the same switching state, and the second switching configuration is complementary to the first switching configuration. This means that only two individual measurements are required.
[0014] To achieve even higher lateral resolutions, it may be necessary to increase the distances between the active pixels, so that for each switching configuration, at least n = 2, 3, ... pixels are in an inactive switching position between two active pixels. Accordingly, n + 1 different switching configurations must follow one another until all pixels have been in an active switching position once.
[0015] The control with different switching configurations can be determined not only by the required lateral resolution, but also by the properties of the object surface to be measured. For surfaces with a favorable signal-to-noise ratio, more crosstalk can be tolerated, so that at a given time, half or even all of the spatial filter's pixels can be in the active second switching state. The more disruptive the crosstalk, the greater the distance between pixels in the active second switching state should be at a given time.
[0016] The optical conjugation between the third plane, in which the pixel-by-pixel digitally switchable spatial filter is located, and the first plane, in which the light pattern generation unit is located, ensures that pixels of the pixel-by-pixel digitally switchable spatial filter can be clearly assigned to the individual measurement points. Planes that can be converted into one another by optical imaging are referred to as optically conjugated. As a rule, therefore, an objective comprising one or more lenses or mirrors is located between two optically conjugated planes, which creates the optical imaging. In special cases, however, optical conjugation can also be achieved by other optical systems instead of objectives. One example of this is the opposite ends of optical fibers, which can both be considered optically conjugated because the point-like light distribution is identical at both ends.Optical images can therefore also be created using bundles of optical fibers, as is known from endoscopes, for example. Two planes lying directly behind one another can also be considered optically conjugate in a broader sense.
[0017] If the pixel-by-pixel digitally switchable spatial filter is arranged independently of the light pattern generation device and in the third plane, it can be located in the light path between the light pattern generation device and the beam splitter cube. Such an arrangement is advantageous because the image of the light pattern is sharper there than after re-imaging through the lens.
[0018] The device then preferably has an imaging optics that images the first plane onto the third plane. Such an imaging optics is particularly advantageous when the light pattern generation device comprises an arrangement of optical fiber ends or a diaphragm with at least one opening. The measuring light usually emerges from the fibers or the opening in a highly diverging manner. If the pixel-by-pixel digitally switchable spatial filter is arranged directly at the fiber ends or in front of the opening of a diaphragm, optical conjugation in the broader sense is achieved. However, many particularly suitable pixel-by-pixel digitally switchable spatial filters, such as micromirror arrays, cannot be arranged so close to the light pattern generation device that optical conjugation can still be spoken of.The imaging optics allow, on the one hand, the use of a large numerical aperture and, on the other hand, the use of micromirror arrays and other switchable spatial filters, which can only be arranged at a greater distance from the light pattern generation device due to beam folding or the required installation space.
[0019] The light pattern can comprise several two-dimensionally distributed light points or at least one continuous light line or one composed of light points. The pixels of the switchable spatial filter divide a continuous light line into individual measurement points.
[0020] In one embodiment, the light pattern generation device comprises a planar array of micro-converging lenses. The converging lenses focus the measurement light onto the respective associated pixels of the switchable spatial filter, thereby effecting optical conjugation in the broader sense.
[0021] In another embodiment, the pixel-by-pixel digitally switchable spatial filter forms the light pattern generation device. Such a design is particularly simple and allows the use of spatial filters in the form of micromirror arrays without the need for additional imaging optics.
[0022] The spatial filter should then be illuminated with light with an angular distribution that matches the numerical aperture of the chromatically uncorrected objective. This works particularly well if the measuring light propagates as a free beam between the light source and the pixel-by-pixel digitally switchable spatial filter.
[0023] To avoid light loss, the spatial filter, which acts as a light pattern generator, should be illuminated by the light source in such a way that as much light as possible hits the spatial filter. Depending on the type of light source, a simple collecting lens is sufficient to focus the light emanating from the light source's exit surface so that it hits the spatial filter with minimal loss. If the light pattern generated by the spatial filter comprises a line of light composed of several light points, the collecting lens can be a cylindrical lens or otherwise anamorphic.
[0024] In one embodiment, a collimator is arranged in the light path between the light pattern generation device and the beam splitter cube, which collimates the measurement light entering the beam splitter cube. Due to the confocal imaging, the measurement light reflected from the object also passes through the beam splitter cube in a collimated manner. A collimated passage is advantageous because it prevents the beam splitter cube from generating spherical aberration.
[0025] The pixel-by-pixel digitally switchable spatial filter is preferably a micromirror array. Alternatively, it can also be a transmissive MEMS component, such as that described in US Pat. No. 8,054,527 B2. An LCD panel can also be considered as a pixel-by-pixel digitally switchable spatial filter, but has disadvantages due to its polarizing properties. For the same reason, it is advantageous if the beam splitter cube is non-polarizing. The splitting ratio of the beam splitter cube then does not depend on the polarization state and thus on the properties of the object surface.
[0026] In one embodiment, the polychromatic light source contains a luminophore, as known from EP 3 764 056 A1 (corresponding to US 10,731,965 B1). Such a light source preferably has an elongated light exit surface. If optical fibers are used for light guidance, their ends can form a rectangular or circular arrangement and receive light emitted from an adjacent square or circular light exit surface of the luminophore.
[0027] In a luminophore-based light source, a pump source, usually a laser or LED, excites a phosphor called a luminophore, which emits light through a physical process, particularly phosphorescence, fluorescence, or scintillation. Such a light source has the advantage of being able to illuminate the entire confocal aperture while maintaining high efficiency and high light yield.
[0028] To reduce optical crosstalk caused by stray light or similar, additional spatial filters can be arranged at positions that are optically conjugated to the first, second and third planes.
[0029] The spectrometer can be synchronized with the pixel-by-pixel digitally switchable spatial filter in such a way that input channels assigned to pixels in the second switching state are logically or physically deactivated. This prevents measurement light that enters neighboring input channels due to crosstalk from being taken into account in the evaluation. The simplest way to deactivate the relevant input channels is to prevent the intensities recorded there from being read out.
[0030] If the pixel-by-pixel digitally switchable spatial filter is arranged in the light path between the light source and the beam splitter cube, another pixel-by-pixel digitally switchable spatial filter can be arranged in the light path between the beam splitter cube and the spectrometer in a fourth plane that is optically conjugate to the first plane and the second plane. Each pixel of the additional pixel-by-pixel digitally switchable spatial filter can be switched to a first switching state, in which the pixel blocks measurement light, and to a second switching state, in which the pixel does not block measurement light.
[0031] The reason for this measure is that the image of the light pattern loses sharpness with each optical imaging. If, for example, two neighboring diffraction disks overlap only negligibly on the object surface, the overlap can already be significant after re-imaging through the lens at the level of the spectrometer. The additional spatial filter, which can be digitally switched pixel by pixel, can remedy this by cropping the relevant light beams so that no crosstalk occurs at the spectrometer even after re-imaging.
[0032] BRIEF DESCRIPTION OF THE DRAWINGS
[0033] In the following, exemplary embodiments of the invention are explained in more detail with reference to the drawings. In these drawings:
[0034] Figure 1 shows a device according to the invention for measuring distance according to a first embodiment in a schematic meridional section;
[0035] Figure 2 shows a pinhole diaphragm used as a light pattern generating device in the device according to Figure 1, in a plan view;
[0036] Figures 3a and 3b show a transmissive MEMS spatial filter contained in the device according to Figure 1 in an enlarged schematic section in different switching configurations;
[0037] Figure 4 shows another embodiment of a pinhole diaphragm which can be used as a light pattern generating device in the device according to Figure 1, in a plan view;
[0038] Figure 5 shows a further exemplary embodiment of a pinhole diaphragm usable as a light pattern generating device in the device according to Figure 1, in a plan view; Figure 6 shows a device according to the invention for distance measurement according to a second exemplary embodiment, in a schematic meridional section;
[0039] Figures 7a and 7b show a reflective MEMS spatial filter contained in the device according to Figure 6 in an enlarged schematic section in different switching configurations;
[0040] Figure 8 is a schematic representation of an embodiment in which the light pattern generating device is formed by a linear arrangement of microlenses;
[0041] Figures 9a and 9b show further possibilities for possible switching configurations in schematic representations based on Figures 3a and 3b;
[0042] Figure 10 shows a section of another variant with two transmissive MEMS spatial filters; and
[0043] Figure 11 shows a device according to the invention for distance measurement according to a third embodiment in a schematic meridional section, in which the pixel-by-pixel digitally switchable spatial filter forms the light pattern generating device.
[0044] DESCRIPTION OF PREFERRED EMBODIMENTS
[0045] 1. Structure of the measuring device
[0046] Figure 1 shows a device for distance measurement according to the invention and designated overall by 10 according to a first embodiment in a schematic meridional section.
[0047] The device 10 has a light source 12 that generates polychromatic measuring light ML.
[0048] The measurement light ML preferably has a continuous spectrum. However, the use of comb spectra or other discrete spectra is also generally possible, provided the wavelength peaks are sufficiently close together. In the illustrated embodiment, the light source 12 comprises a superluminescent diode 14 and a converging lens 16 arranged behind it in the direction of light propagation.
[0049] The measuring light ML generated by the light source 12 strikes a light pattern generating device 18, which generates a light pattern from the incident measuring light ML. In the illustrated embodiment, the light pattern generating device 18 comprises a pinhole 20, shown in plan view in Figure 2. The pinhole 20 has several apertures 22 arranged along a line. When illuminated with the measuring light ML, the apertures 22 represent point light sources from which the measuring light ML emerges divergently.
[0050] The aperture openings 22 are arranged in a first plane E1, which coincides with an object plane of an imaging optics 24. In the illustrated embodiment, the imaging optics 24 is designed as a 4f optic. Such a 4f optic is characterized in that two objectives or lenses L1, L2 are arranged such that the rear focal plane of lens L1 coincides with the front focal plane of lens L2. This leads to a bilaterally telecentric beam path, with the result that the angular distribution at the aperture openings 22 is transferred into the image plane E2 of the imaging optics 24 without tilting. If, as in the illustrated embodiment, the focal length of the lenses L1, L2 is the same, the imaging scale ß = -1.
[0051] A transmissive MEMS spatial filter 26 is arranged in the image plane E2 of the imaging optics 24, where MEMS is an acronym for micro-electro-mechanical system. Such filters 26 comprise a plurality of regularly arranged elements or pixels 28, which can be individually mechanically moved to selectively clear or block the light path. Suitable examples of such MEMS filters are known per se and are described, for example, in US 2006 / 0012781 A1. Alternatively, an LCD panel can be used, although this has a polarizing effect, which can be disadvantageous for some measurement tasks. Figures 3a and 3b show the transmissive MEMS spatial filter 26 in an enlarged schematic section. In the illustrated embodiment, the pixels 28 of the transmissive MEMS spatial filter 26 are arranged along a line such that each pixel 28 is assigned exactly one aperture 22 of the light pattern generation device 18.This assignment results from the imaging of the apertures 22 onto the pixels 28 by the imaging optics 24.
[0052] The individual switchability of the individual pixels 28 is indicated in Figure 3a by light and dark backgrounds. A pixel 28 with a dark background is in a first switching state, in which the pixel 28 blocks measurement light ML. A pixel 28 with a light background is in a second switching state, in which the pixel 28 is transparent to measurement light. The switching states of the pixels 28 are specified by a control and evaluation device 30. The change between different switching states, as can be seen from a comparison of Figures 3a and 3b, typically occurs within a few microseconds.
[0053] For pixel 28 shown on the far left in Figure 3a, which is in the second switching state, additional rays indicate that the angular distribution of the measurement light ML is maintained as it passes through the transmissive MEMS spatial filter 26. The transmissive MEMS spatial filter 26 thus filters only in spatial space, but not in angular space. Consequently, the measurement light ML exits the output of pixel 28 with the numerical aperture NA of the imaging system 24.
[0054] The pixels 28 of the transmissive MEMS spatial filter 26 thus also represent point light sources, which, however, in contrast to the apertures 22 of the light pattern generating device 18, can be individually switched on and off.
[0055] Figures 3a and 3b illustrate two different switching configurations that can be switched between in this exemplary embodiment. In the first switching configuration, shown in Figure 3a, no two adjacent pixels 28 are in the same switching state. The switching state thus alternates from pixel to pixel. The second switching configuration according to Figure 3b is complementary to the first switching configuration in Figure 3a. There, too, therefore, no two adjacent pixels 28 are in the same switching state, which alternates from pixel to pixel. However, the switching pattern is offset by one pixel, so that one can switch between the two switching configurations according to Figures 3a and 3b by switching to the other switching state at each pixel 28. The advantages associated with the transmissive MEMS spatial filter 26 and the switching of the switching configurations are explained further below in Section 2.
[0056] A collimator lens 32 is arranged in the light path behind the transmissive MEMS spatial filter 26, the front focal plane of which coincides with the plane E2. The beams emerging from the pixels 28 are therefore collimated upon passing through the collimator lens 32.
[0057] The collimated beams strike a non-polarizing beam splitter cube 33, which has a beam splitter surface 34 inclined by 45° to the optical axis. A predetermined proportion of the incident light, e.g. 50%, is reflected by the beam splitter surface and is lost for measurement. The remaining measuring light ML passes through the beam splitter cube without deflection due to the collimated beam path and strikes a chromatically uncorrected objective lens 36, which has an object plane lying at infinity. Together with the imaging optics 24 and the collimator lens 32, the objective lens 36, due to its chromatic longitudinal aberration, images the diaphragm openings 22 arranged in the plane E1 into image planes B1 to B3, the axial positions of which are wavelength-dependent. In Figure 2, this is indicated by different dashed lines. Each wavelength is assigned exactly one image plane.If - as in the present embodiment - the spectrum generated by the light source 12 is continuous, a continuous sequence of image planes is created.
[0058] If the image of an aperture 22 lies in an image plane at the level of which an at least partially reflective surface 38 of an object 40 is located, the light beam incident there is reflected back into itself and travels the same path via the lens 36 back to the beam splitter surface 34 of the beam splitter cube 33. For the constellation shown here, for example, this requirement is met for the aperture 22 located on the optical axis OA of the device 10 at a wavelength for which the image of the aperture 22 lies in the image plane B2. For the light beam 43 that has passed through a different aperture 22, this requirement is met for the object 40 assumed here at a different wavelength for which the image of the aperture 22 in question lies in the image plane B1. Of course, not only light of a single wavelength is reflected at the surface 38.However, there is only one wavelength at which the measuring light beam is reflected, because the focus lies exactly on the surface 38 only for this wavelength.
[0059] At the beam splitter surface 34, a predetermined portion of the reflected measuring light ML is reflected by 90° and focused by a converging lens 42 onto a static spatial filter 44, which is arranged in a third plane E3 and has a filter function corresponding to the light pattern. The static spatial filter 44 is transparent to the measuring light ML at certain locations (or reflective in the case of a reflective spatial filter), while it blocks the measuring light ML at the remaining locations. In the simplest case, the static spatial filter 44 is the same pinhole diaphragm that was used for the light pattern generation device 18, possibly reduced or enlarged by the image scale of the intermediate optics. The entire optics in the light path between the light pattern generation device 18 and the static spatial filter 44, ieThe imaging optics 24, the collimator lens 32, the objective lens 36, and the focusing lens 42 cause the light pattern generation device 18 to be imaged onto the static spatial filter 44. The planes E1, E2, and E3 are therefore optically conjugated. Intermediate images are created at the level of the transmissive MEMS spatial filter 26 and in the image planes B1 to B3 on the surface 38 of the object 40.
[0060] The static spatial filter 44 ensures that only measurement light beams that have been reflected at a specific wavelength from the surface 38 can be further evaluated. Measurement light ML that does not meet this requirement is blocked by the static spatial filter 44 according to the chromatic-confocal measurement principle.
[0061] Arranged in the light path behind the static spatial filter 44 is a spectrometer 46 with multiple input channels that spectrally analyze reflected measurement light ML that has passed through a point of the static spatial filter 44 associated with the respective input channel. The spectrometer 46 typically contains a dispersive optical element, e.g., an optical grating or a prism 47, and a line-like arrangement 48 of light-sensitive cells for each input channel, as schematically indicated in Figure 1. Since spectrometers 46 of this type are known per se, further explanations are unnecessary here.
[0062] 2. Function
[0063] During a measurement, the control and evaluation device 30 calculates distances from points on the surface 38 to the measuring device 10 from wavelengths measured by the spectrometer 46, as is known per se for chromatic confocal measuring devices. These points are images of the point light sources, i.e., the apertures 22. Each point light source is assigned its own input channel, so that the distance measurement can, in principle, be performed simultaneously for all point light sources.
[0064] However, when measuring the distance for all point light sources simultaneously, optical crosstalk can occur if the point light sources are very close to one another. The reason for this is that the images of the point light sources are blurred due to diffraction, even with optimal optical imaging, and can therefore overlap. This is shown schematically in the enlarged section C of the static spatial filter 44. In section C, it can be seen that the gray-shaded light beam LB in the plane E3 is not focused on a single aperture 22', but partially extends across the two neighboring apertures 22", 22'". Measuring light ML entering the wrong apertures 22", 22'" would be detected by the corresponding input channels of the spectrometer 46 and falsify the distance measurement.For example, not only a light beam LB' indicated by dashed lines would fall into the aperture 22'", but also part of the light beam LB.
[0065] Optical crosstalk could be prevented by increasing the distances between the point light sources (i.e., here, the apertures 22). However, this measure would be at the expense of the lateral resolution of the device 10. According to the invention, the control and evaluation device 30 therefore controls the transmissive MEMS spatial filter 26 such that the measurement is divided into two partial measurements. During the first partial measurement, the pixels 28 of the transmissive MEMS spatial filter 26 are in one of the two switching configurations shown in Figures 3a and 3b. During the second partial measurement, the pixels 28 are in the other switching configuration.For the adjacent light beam LB', which is indicated by dashed lines in section C of Figure 1, this means that it is prevented from propagating by the transmissive MEMS spatial filter 26 and cannot enter input channels of the spectrometer 46 that are not assigned to this light beam LB'; this applies accordingly to the other light beam, which is not indicated in section C, but which is adjacent to the light beam LB on the other side.
[0066] At the same time, the spectrometer 46 is synchronized with the pixel-by-pixel digitally switchable spatial filter 26 such that the two input channels assigned to the adjacent apertures 22" are deactivated. The portion of the light beam LB incident on the light-sensitive cells of these two input channels is therefore not evaluated.
[0067] By splitting the measurement into two individual measurements, alternating between the switching configurations shown in Figures 3a and 3b, optical crosstalk is effectively prevented. This allows high lateral resolution to be achieved while approximately doubling the measurement time. The control and evaluation device 30 can be programmed so that this mode is only used for measurements where high lateral resolution is required and the object has a surface that is particularly conducive to optical crosstalk. If the lateral resolution requirements are lower, only one measurement is performed using one of the switching configurations shown in Figures 3a and 3b.
[0068] 3. Further examples
[0069] Figure 4 shows, in a representation similar to Figure 2, a slit diaphragm 20' with a slit-shaped aperture 22', which can be used as an alternative to the pinhole 20 shown in Figure 2 as a light pattern generating device 18. Measurement light ML, which would lead to optical crosstalk, is also prevented from propagating here by means of the transmissive MEMS spatial filter 26.
[0070] Figure 5 shows, in a representation similar to Figures 2 and 3, a 2D pinhole 20", which can also be used as an alternative to the pinhole 20 shown in Figure 2 as a light pattern generating device 18. The apertures 22" in the 2D pinhole 20" are distributed in a uniform two-dimensional grid.
[0071] The transmissive MEMS spatial filter 26 must then have a corresponding two-dimensional arrangement of pixels 28 so that each aperture 22" is assigned a switchable pixel. The static spatial filter 44 and the spectrometer 46 must also be expanded two-dimensionally accordingly, as is known in the prior art.
[0072] Figure 6 shows a second embodiment of a measuring device, designated overall by 10'. This embodiment differs from the device 10 shown in Figure 1 primarily in the following points:
[0073] Firstly, the light source 12 is not a superluminescent diode, but rather a luminophore 52 which, in the illustrated embodiment, has an elongated light exit surface and is illuminated by a pump source 54, which may be a laser, for example. In the illustrated embodiment, the polychromatic measuring light emerging from the light exit surface of the luminophore 52 is first collimated and then coupled into the ends of a linear arrangement of parallel optical fibres 56 with the aid of a cylindrical lens 55. A particularly high light intensity can be achieved with the luminophore 52. In one variant, the luminophore 52 has a round or square light exit surface. The ends of the optical fibres 56 then form a round or square arrangement rather than a linear arrangement; the cylindrical lens 55 is omitted in this variant.The measurement light ML guided in the optical fibers 56 exits at the opposite ends of the fibers 56. These ends are arranged along a line perpendicular to the paper plane and simultaneously represent the light pattern generation device 18. The imaging optics 24 images the fiber ends via a planar deflection mirror 58 onto a switchable spatial filter 26', which in the illustrated embodiment is designed as a one-dimensional digital micromirror array (DMD). The micromirror array extends perpendicular to the paper plane of Figure 6.
[0074] In this embodiment, the static spatial filter 44 is also formed by a linear arrangement of optical fiber ends 60, with the line extending perpendicular to the plane of the paper. The optical fibers 60 guide the measurement light to the spectrometer 46.
[0075] Optical fiber connectors 62, 62', 64, 64' can be integrated into the optical fibers 56, 60. In this way, the parts of the device 10' outlined by dashed lines can be housed in separate housings, forming a mobile measuring head 66 and a stationary control unit 68.
[0076] Figures 7a and 7b illustrate, in representations similar to Figures 3a and 3b, the two switching configurations of the reflective switchable spatial filter 26'. Each pixel 28 is formed by a micromirror that can assume two stable switching states. Pixels 28 with a gray background are in the second switching state and reflect the incident measurement light ML so that it falls onto an absorber. Pixels 28 with a white background reflect the incident measurement light ML so that it can enter the lens 36 via the beam splitter cube 33.
[0077] Figure 8 schematically illustrates an embodiment in which the light pattern generation device 18 is formed by a linear arrangement of microlenses 70. The light source comprises a collimator lens 72, which collimates the measurement light onto the microlenses 70. These focus the measurement light ML onto an associated pixel 28 of the transmissive, switchable spatial filter 26. Therefore, no imaging optics 24 are required in this embodiment. Since the microlenses 70 are arranged directly in front of the transmissive, switchable spatial filter 26, the plane E1 of the microlenses 70 virtually coincides with the plane E2, which corresponds to optical conjugation. The numerical aperture NA of the microlenses 70 is adapted to the numerical aperture NA of the objective 36.
[0078] Figures 9a and 9b illustrate further possibilities for possible switching configurations in schematic representations based on Figures 3a and 3b. If the images of the point light sources at the level of the static spatial filter 44 are particularly large, these images may extend not only across the immediately adjacent apertures 22', but across two or even three adjacent apertures 22'. In this case, the measurement should be divided into not two, but three or more individual measurements to prevent optical crosstalk.
[0079] Figures 9a and 9b assume that a total of four individual measurements are planned. Therefore, in each switching configuration, only one of four pixels 28 adjacent along a direction is in the second switching state, while the remaining three pixels are in the first switching state and block the incident measurement light. Two of the four switching configurations, which can be set consecutively, are shown in Figures 9a and 9b.
[0080] Figure 10 shows a section of another variant; the parts of the device 10 not shown in Figure 10 correspond to those in Figure 1.
[0081] In this variant, a further transmissive MEMS spatial filter 74 is arranged in a fourth plane E4 in the light path between the beam splitter cube 33 and the spectrograph 46, which is optically conjugate to the first plane E1 and the second plane E2. A further imaging optics 75 images the further transmissive MEMS spatial filter 74 in the plane E4 onto the static spatial filter 44 in the plane E3. The pixels of the further transmissive MEMS spatial filter 74 also have the property of being convertible into a first switching state, in which the pixel blocks measurement light ML, and into a second switching state, in which the pixel does not block measurement light ML. In one variant, the static spatial filter 44 is omitted; the entrance slit of the spectrograph 46 is then located in the plane E3.Such an additional transmissive MEMS spatial filter 74 is advantageous when the input channels in the spectrograph cannot be individually deactivated and the images of the point light sources would partially overlap on the static spatial filter 44 despite the MEMS spatial filter 26 arranged at the front of the beam path behind the lens 34. The additional transmissive MEMS spatial filter 74 "cleans" the light distribution and thus ensures that the images of the point light sources do not overlap on the static spatial filter 44. The switching configurations of the additional transmissive MEMS spatial filter 74 are always adapted to the switching configurations of the front transmissive MEMS spatial filter 26.
[0082] However, if the spectrograph has individually deactivatable input channels, the additional MEMS spatial filter 74 can be omitted.
[0083] Figure 11 shows, in a representation based on Figure 1, a second exemplary embodiment of a measuring device, which is designated overall by 10". While in the previously described exemplary embodiments the pixel-by-pixel digitally switchable spatial filter 26, 26' is an assembly independent of the light pattern generating device 18, in the device 10" the pixel-by-pixel digitally switchable spatial filter 26" forms the light pattern generating device 18. As a result, no separate light pattern generating device is required, with the result that no imaging optics are required which images a light pattern generating device onto the pixel-by-pixel digitally switchable spatial filter 26".
[0084] In the device 10" in the illustrated embodiment, the measuring light ML generated by the light source 14 is first collimated by a converging lens 16. A further converging lens 16' focuses the measuring light ML such that it just completely illuminates the pixel-by-pixel digitally switchable spatial filter 26". In Figure 11, it is assumed that the pixel-by-pixel digitally switchable spatial filter 26" is square and that a circularly delimited area is illuminated by the measuring light ML. In a linear spatial filter, i.e., a pixel-by-pixel digitally switchable spatial filter extending only in one direction, the converging lens 16' can be designed as a cylindrical lens or as another anamophotic optical element having a different refractive power along orthogonal directions. In this way, the linear spatial filter can be illuminated in a strip shape in order to minimize light losses.Alternatively, a light source with an elongated exit surface can be used, which is imaged onto the linear, pixel-by-pixel digitally switchable spatial filter and illuminates it accordingly.
Claims
PATENT CLAIMS Device for the chromatic confocal measurement of distances to a plurality of points on a surface (38) of an object (40) which at least partially reflects incident measuring light (ML), comprising: a) a light source (12) configured to generate polychromatic measuring light (ML), b) a light pattern generating device (18) configured to generate a light pattern from the measuring light which extends in a first plane (E1) along one dimension or along two dimensions, c) a chromatically uncorrected objective (36) configured to generate images of the light pattern in image planes (B1, B2, B3) whose axial position is wavelength-dependent due to chromatic longitudinal aberration, d) a static spatial filter (44) arranged in a second plane (E3) and having a filter function corresponding to the light pattern, wherein the second plane (E2) is optically conjugate to the first plane (E1),e) a beam splitter cube (33) arranged in the light path between the light pattern generating device (18) and the objective lens (36) and guiding measuring light (ML) that has been reflected by the surface (38) and has passed through the objective lens (36) to the static spatial filter (44), f) a spectrometer (46) having a plurality of input channels, each of which is configured to spectrally analyze reflected measuring light that has passed through a point of the static spatial filter (44) associated with the respective input channel, and g) an evaluation device (30) configured to calculate distances from points on the surface (38) to the device (10; 10') from wavelengths measured by the spectrometer (46), characterized by a pixel-by-pixel digitally switchable spatial filter (26; 26'; 26"), which has a plurality of pixels (28) and either forms the light pattern generating device (18) or is arranged in a third plane (E2) that is optically conjugate to the first plane (E1) and the second plane (E3), wherein each pixel (28) can be converted into a first switching state, in which the pixel (28) blocks measurement light (ML), and into a second switching state, in which the pixel (28) does not block measurement light (ML). Device according to claim 1, characterized in that the pixel-by-pixel digitally switchable spatial filter (26; 26'; 26") changes between a first switching configuration and a second switching configuration during a measurement, wherein in the first switching configuration no two adjacent pixels (28) are in the same switching state, and wherein the second switching configuration is complementary to the first switching configuration.Device according to one of claims 1 or 2, characterized in that the pixel-by-pixel digitally switchable spatial filter (26") forms the light pattern generation device (18), and in that the measuring light (ML) propagates as a free beam between the light source (14) and the pixel-by-pixel digitally switchable spatial filter (26"). Device according to claim 1 or 2, characterized in that the pixel-by-pixel digitally switchable spatial filter (26; 26') is arranged independently of the light pattern generation device (18) and in the third plane (E2). Device according to claim 4, characterized in that the pixel-by-pixel digitally switchable spatial filter (26; 26') is located in the light path between the light pattern generation device (18) and the beam splitter cube (33). Device according to one of claims 4 or 5, characterized in that the device (10; 10') has an imaging optics (24) which images the first plane (E1) onto the third plane (E2).
7. Device according to one of claims 4 to 6, characterized in that the light pattern comprises a plurality of two-dimensionally distributed light points (22; 22") or at least one continuous light line or one composed of light points (20; 22').
8. Device according to one of claims 4 to 7, characterized in that the light pattern generating device (18) comprises an arrangement of ends of optical fibers (56).
9. Device according to one of claims 4 to 7, characterized in that the light pattern generating device (18) comprises a diaphragm (20; 20'; 20") with at least one opening (22; 22', 22").
10. Device according to one of claims 4 to 7, characterized in that the light pattern generating device (18) comprises a planar arrangement of micro-collecting lenses (70).
11. Device according to one of the preceding claims, characterized in that a collimator (32) is arranged in the light path between the light pattern generating device (18) and the beam splitter cube (33), which collimator collimates measuring light (ML) entering the beam splitter cube (33).
12. Device according to one of the preceding claims, characterized in that the pixel-by-pixel digitally switchable spatial filter (26') is a micromirror array.
13. Device according to one of the preceding claims, characterized in that the polychromatic light source (12) contains a luminophore (52).
14. Device according to one of the preceding claims, characterized in that the spectrometer (46) is synchronized with the pixel-by-pixel digitally switchable spatial filter (26; 26'; 26") such that input channels assigned to pixels (28) in the second switching state are deactivated.
15. Device according to one of the preceding claims, characterized in that the pixel-by-pixel digitally switchable spatial filter (26; 26'; 26") is arranged in the light path between the light source (14) and the beam splitter cube (33), a further pixel-by-pixel digitally switchable spatial filter (74) is arranged in the light path between the beam splitter cube (33) and the spectrometer (46) in a fourth plane (E4) which is optically conjugate to the first plane (E1) and the second plane (E2), and that each pixel of the further pixel-by-pixel digitally switchable spatial filter (74) can be converted into a first switching state in which the pixel blocks measuring light (ML), and into a second switching state in which the pixel does not block measuring light (ML).