Method and device for determining an object-dependent focus position, method for measuring an object and coordinate measuring machine

Abstract

The invention relates to a method and a device for determining an object-dependent focus position of an optical detection system (2) for imaging an object (3), which comprises at least one optical detection device (5) and at least one optical element (4) for imaging, comprising the steps: - Generating a first image of the object (3) by detecting a first radiation reflected from the object (3), - Generating at least one further image of the object (3) by detecting further radiation reflected from the object (3), wherein the first radiation and the further radiation have different wavelengths or comprise wavelengths from different wavelength ranges, - Determining the pixel-specific intensity values ​​in the first and in at least one further image for at least one pixel in the images, Determining the image point-specific focus offset at least as a function of the intensity values ​​and a predetermined chromatic longitudinal deviation (CHL) of the at least one optical element (4) as well as a method for measuring an object and a coordinate measuring machine.

Description

[0001] The invention relates to a method and a device for determining an object-dependent focus offset as well as a method for measuring an object and a coordinate measuring machine.

[0002] In coordinate metrology, optical methods are used, among other things, to measure an object. Typically, the object is illuminated by one or more light sources, and the radiation reflected or scattered by the object is detected by an optical detection device with an image sensor, generating a generally two-dimensional image. In most cases, optical elements, e.g., lenses, are arranged in an observation beam path for this purpose.

[0003] A known method for generating measured values ​​is the determination of a focus value, i.e. the distance of a set focus point, from an image sensor or another reference point of an optical detection system, whereby a value of a coordinate of a focused point in a reference coordinate system can then be determined from this focus value and also determinable position and / or orientation information of the optical detection system.

[0004] Also known is the so-called longitudinal chromatic aberration (CHL), which describes an image error in optical systems that has its origin in the wavelength-dependent refraction of light. CHL describes - put simply - the dependence of the focus position on the wavelength arriving at the lens or the spectrum of the radiation arriving at the lens. In this way, different focus positions can be set for different wavelengths, e.g. using an autofocus function, which also produces different measured values ​​in the method for generating measured values ​​explained above. It is known that contributions to the longitudinal chromatic aberration can be determined by elements of the optical detection system, such as lenses, e.g. through appropriate measurement or calibration.This makes it possible to take into account the contribution to longitudinal chromatic aberration generated by the specific optical element when determining measured values.

[0005] The problem, however, is that different objects can contribute to longitudinal chromatic aberration in different ways, as different objects reflect radiation differently, especially with different spectra. This, in turn, means that the focus offset is object- or sample-dependent.

[0006] The focus offset refers to the difference between a true focus position that would have been determined without distortions such as the explained longitudinal chromatic aberration, and the focus position that is determined or is established under the influence of these distortions.

[0007] DE 10 2019 214 865 A1 is known from the prior art and discloses a lighting system, an optical measuring device and a method for the meteorological measurement of workpieces.

[0008] Also known is DE 37 43 584 A1, which relates to an optical spectrometer with a light source exciting the sample, an optical filter located along at least one beam path of the beam emanating from the sample, and an optoelectric converter with a downstream signal processing and evaluation circuit.

[0009] Also known is US 2021 / 0302632 A1, which discloses an optical filter arrangement for use in multi-wavelength systems, and in particular relates to a focus-corrected filter device for such multi-wavelength systems.

[0010] Also known is US 5,940,183 A, which discloses a filter arrangement and a detection device, wherein a plurality of filters, each with a unique beam wavelength, are used.

[0011] Also known is US 10,401,738 B2, which relates generally to overlay metrology and in particular to overlay metrology with multiple parameter configurations.

[0012] Also known is DE 10 2010 064 387, which discloses an imaging system and an imaging method.

[0013] DE 10 2019 208 114 B4 discloses a device for 3D measurement of object coordinates, a method for 3D measurement of object coordinates of a measurement object or parts of the measurement object, a computer program and a computer program product.

[0014] WO 2008 / 125605 A2 discloses a method and an arrangement for generating optical sectional images.

[0015] WO 2014 / 023364 A1 discloses a measuring device for measuring a measurement object, comprising an illumination device for illuminating the measurement object with an illumination pattern, a pattern generation device with at least one pattern generation element for producing a spatially varying intensity distribution of the illumination pattern, and an optical sensor arrangement for detecting the illumination pattern reflected and / or scattered by the measurement object.

[0016] US 2009 / 0251567 A1 discloses a method for processing a relative illumination phenomenon on a digital image.

[0017] DE 10 2009 025 815 A1 discloses a measuring arrangement for the three-dimensional measurement of at least a part of an object, in particular a semi-transparent object such as a tooth or a section thereof, comprising a light source with a continuous, in particular broadband spectrum, a device for generating a multifocal illumination pattern, an objective with large chromatic aberration for imaging foci of the illumination pattern onto the object, a detection unit such as a camera chip for determining the wavelength spectra of the foci imaged onto the object via the objective, and a spectrally dispersive device onto which light remitted by the object can be imaged via the objective.

[0018] The technical problem is to create a method and a device for determining an object-dependent focus offset of an optical detection system for imaging an object, which enable reliable and accurate determination of the focus offset. Furthermore, the technical problem is to create a method for measuring an object with an optical coordinate measuring machine and a coordinate measuring machine that generates a precise measured value.

[0019] The solution to the technical problem is achieved by the subject matter having the features of the independent claims. Further advantageous embodiments of the invention are set forth in the subclaims.

[0020] A method is proposed for determining an object-dependent focus offset of an optical detection system for imaging an object. The object can also be referred to as a test object. The optical detection system comprises at least one optical detection device, in particular an image detection device, which can comprise, for example, an image sensor such as a CMOS or CCD sensor. The image sensor can have a sensor surface onto which radiation impinges and is then converted into an electrical signal. The optical detection device can, in particular, generate a two-dimensional image of the object. However, a one-dimensional linear image of the object can also be generated, e.g., using a detection device designed as a line sensor.The optical detection system further comprises at least one optical element for imaging, wherein the rays detected to generate the image pass through the optical element. Of course, the optical detection system may comprise a plurality of such optical elements. In particular, the optical detection system may comprise optical elements configured to adjust or change a focal point of the optical system. For this purpose, these optical elements may, for example, be movably mounted. Their position and / or orientation may be adjustable to change the focal point.

[0021] The procedure includes the following steps: In a first step, a first image of an object is generated by detecting first radiation reflected from the object. For this purpose, the object can be illuminated, in particular with radiation from a predetermined spectrum, e.g. a white light spectrum or in a spectrum of visible light, but also with a different spectrum. The spectrum can comprise wavelengths from a minimum wavelength to a maximum wavelength. The minimum wavelength can be in a range from 350 nm to 400 nm and the maximum wavelength can be in a range from 700 nm to 1000 nm. However, partial spectra from this spectrum can also be used for illumination. For this purpose, the optical detection system can comprise at least one illumination device for illuminating the object.

[0022] In a second step, at least one further image of the object is generated by detecting further radiation reflected from the object. The first radiation and the further radiation have different wavelengths. Alternatively, the first radiation and the further radiation comprise wavelengths from different wavelength ranges. The different wavelength ranges can in particular be disjoint wavelength ranges. However, it is also conceivable for the different wavelength ranges to overlap in sections. However, the wavelength range of the first radiation comprises at least one wavelength that is not part of the wavelength range of the further radiation and / or vice versa.

[0023] To generate the image with the first and the further radiation, in particular to generate radiation with different wavelengths or from different wavelength ranges, the radiation reflected or scattered by the object can be filtered, whereby the object can advantageously be illuminated with the same illumination properties to generate the first and the further image in this scenario. However, it is also conceivable that to generate the first and the further image, the object is illuminated with different wavelengths or with radiation from different, in particular disjoint, wavelength ranges, whereby these partial spectra can be between the minimum wavelength and the maximum wavelength within the previously explained spectrum.

[0024] The various images can be generated sequentially, wherein the wavelengths or a central wavelength of a wavelength range of the radiation are changed, i.e. increased or decreased, by a predetermined amount after the generation of an image in order to generate the next image. This amount can be, for example, less than or equal to 5 nm, 10 nm or 50 nm. If the images are generated with radiation from different wavelength ranges, the width of the wavelength range for generating the various images can preferably be the same. However, it is also conceivable for these to change, e.g. by a predetermined amount. The wavelengths of the filtered radiation, in particular the central wavelength, can be changed such that the radiation filtered to generate the images covers the entire illumination spectrum or a selected sub-range thereof.

[0025] In a third step, the pixel-specific intensity values ​​in the first and at least one further image are determined for at least one pixel in the images. In other words, an intensity value curve can be determined for exactly one pixel, for selected but not all pixels, or for all pixels across different wavelengths or wavelength ranges.

[0026] In a fourth step, the pixel-specific focus offset is determined at least as a function of the intensity values ​​of the pixel in the generated images and a predetermined longitudinal chromatic aberration of the at least one optical element. The result of this fourth step is a radiation- and pixel-specific focus offset. The object-dependent focus offset can be determined as this radiation- and pixel-specific focus offset, in particular for each pixel.

[0027] Depending on this focus offset, a measured value determined by an optical coordinate measuring machine with the optical detection system can then be corrected. The measured value can be determined by determining a focus value, which is set manually or using an autofocus function. This was explained previously. Such a measured value can then be corrected depending on the focus offset determined as described. For example, the focus value can be corrected depending on the focus offset. This can be done, for example, by adding the focus offset to or subtracting it from the focus value. The corrected focus value can then be used in the known way to determine the corrected measured value. A corrected measured value can be determined for each pixel.As explained in more detail below, a resulting focus offset can also be determined for multiple pixels, and then a corrected measured value can be determined based on this resulting focus offset. The method thus advantageously enables the rapid and reliable determination of an object-dependent focus offset of an optical detection system, which in turn enables reliable and accurate correction when generating measured values.

[0028] In a preferred embodiment, the radiation used to generate the images is provided by radiation filtering. For this purpose, at least one optical filter element can be arranged in a beam path of the optical detection system, in particular in an illumination beam path extending from an illumination device to the object, and / or in an observation beam path extending from the object to the image detection device. Of course, several such optical filter elements can also be arranged in the beam path. An optical filter element can have high-pass, low-pass, band-pass, or band-stop properties. It is conceivable that different optical filter elements are arranged in the beam path to generate the various images, e.g. automatically.Radiation filtering advantageously results in reliable generation of images using rays of different wavelengths.

[0029] As explained above, it is alternatively or cumulatively also possible that the radiation properties, in particular the spectrum, of the radiation used to generate the images and which is provided by an illumination device is changed to generate the different images.

[0030] In a further preferred embodiment, the radiation filtering takes place in an illumination beam path and / or in an observation beam path of the optical detection system. Arranging at least one optical filter in the illumination beam path advantageously results in lower requirements for the properties of optical filters compared to an arrangement outside the illumination beam path, e.g., in the observation beam path; e.g., tolerances for desired properties can be greater. Arranging at least one optical filter element in the observation beam path advantageously reduces thermal stress on the optical filter caused by the light source.

[0031] In a further preferred embodiment, the radiation used to generate the images is provided or generated by filtering with at least one gradient filter or with at least one variable filter.

[0032] A gradient filter can change spectral filter properties along a dimension. For example, a spectral passband can be spectrally shifted for each physical point of an incident surface along a dimension. In other words, different filter properties are obtained at least along one dimension of a gradient filter, with which the radiation radiated onto the gradient filter is filtered. In other words, different filter properties can be provided for different incident points of the radiation on the incident surface of the gradient filter. In particular, different central wavelengths can be provided for different incident points, where a central wavelength can be a property of a high-pass, low-pass, band-pass, or band-stop behavior of the gradient filter.It is also possible to use two gradient filters arranged one behind the other in the beam direction to provide the resulting bandpass properties of the filter arrangement. In this case, a first gradient filter can have high-pass properties and the second gradient filter can have low-pass properties.

[0033] Preferably, the at least one gradient filter or the plurality of gradient filters are each movably mounted. In particular, a position of the at least one gradient filter can be changed, in particular in order to change an incident point or incident section of the radiation reflected / scattered by the object onto the incident surface of the at least one gradient filter. Thus, by changing the position, the radiation properties used for imaging can also be changed, without the need for complex replacement of the filter elements for generating the two images in the beam path. This advantageously results in a reliable and simple provision of radiation with a desired wavelength or wavelengths from a desired wavelength range, wherein radiation with different radiation properties can be provided by changing the position of the gradient filter.

[0034] Alternatively, the radiation is provided by filtering with at least one variable filter. Such variable filters, which can also be referred to as continuously variable filters, also change the spectral properties along an incident area of ​​the radiation to be filtered. Linearly variable filters are particularly well known; they change the spectral properties of the filter in the wavelength range for each point along an axis, particularly in a linear manner. Circular filters are also known; in these filters, the spectral properties of the filter are shifted in the wavelength range for each physical point along a circular line. Variable filters can have low-pass, high-pass, band-pass, or band-stop filter properties. Thus, the statements regarding gradient filters also apply accordingly to variable filters.

[0035] The use of movable filters in particular advantageously results in an easy-to-implement generation of images, in particular without the need for pulsed illumination with different wavelengths, which can have unfavorable biocompatibility.

[0036] In a preferred embodiment, a variable filter is a linear or circular filter. This was explained previously.

[0037] In a preferred embodiment, a weighted curve is determined by weighting a (previously known) wavelength-dependent curve of the predetermined longitudinal chromatic aberration of the at least one optical element with a wavelength-dependent curve of the determined intensity values, wherein the focus offset is determined as the mean value of the weighted curve. Intensity values ​​of the wavelength-dependent curve can be standardized, e.g., to a maximum achievable intensity value.

[0038] The wavelength-dependent profile can extend over a predetermined wavelength range, for example, over the wavelength range that includes all wavelengths of the previously explained illumination spectrum, in particular a wavelength range from 400 nm to 700 nm. This weighted profile can be determined pixel-specifically, whereby the pixel-specific focus offset is determined as the mean value of the weighted profile. The wavelength-dependent profile of the intensity values ​​for a pixel can be determined by recording the intensity at different times, with radiation with different wavelengths being imaged through the pixel at these different times. This results in a simple and reliable determination of the focus offset.

[0039] In a further embodiment, the focus offset is additionally determined as a function of a reference receiver spectrum. A receiver spectrum refers to the spectrum of the radiation received by the optical detection device.

[0040] This makes it possible to divide the spectrum reflected from the object, i.e., the object receiver spectrum, by this reference receiver spectrum, whereby the two spectra are different from each other. This can be done by dividing the intensity values ​​of the wavelength-dependent curve by wavelength-specific intensities of the reference receiver spectrum or a quantity dependent on them. The wavelength-dependent curve of the predetermined longitudinal chromatic aberration can then be weighted with the wavelength-dependent curve of the determined intensity values.

[0041] The reference receiver spectrum can be a previously known spectrum, for example, determined during measurement or calibration, and stored for retrieval. Alternatively, the reference receiver spectrum can be determined before each measurement by illuminating a white standard, e.g., made of barium sulfate (BaSo4).

[0042] When determining the reference receiver spectrum, the spectral transmission properties of all optical elements in the beam path as well as the properties of the image capture device are also taken into account or recorded. The optical element in the beam path can, in particular, be an element of the illumination optics.

[0043] It can be assumed that the focus offset results from a product of the illumination spectrum with the transmission transfer function of optical elements in the beam path and / or the reflection transfer function of the object to be imaged and / or the transfer function representing the longitudinal chromatic aberration of the optical element.

[0044] For example, when determining the wavelength-dependent intensity values, a previously known property of the illumination spectrum, e.g. the intensity at a selected wavelength, as well as a transmission loss and a sensitivity of the sensor used, which can also be wavelength-dependent, can be taken into account.

[0045] By additionally taking the reference receiver spectrum into account, a more precise determination of the focus offset is advantageously achieved.

[0046] In a further embodiment, a frame rate of the image capture device corresponds to a generation rate of the images with different wavelengths or different wavelength ranges, or an integer multiple of the generation rate. If the frame rate is, for example, 60 Hz, the generation rate can also be 60 Hz, whereby 60 images, each with different wavelengths or with wavelengths from different wavelength ranges, are generated in one second. However, the generation rate can also be 30 Hz, whereby 30 images, each with different wavelengths or with wavelengths from different wavelength ranges, are generated in one second. In this case, for example, two images generated consecutively by the image capture device with the same wavelength or with the same wavelength ranges can always be generated and fused to determine a resulting image, e.g.by determining an average of the intensity values ​​in the two images. The two subsequent images can then be generated with a different wavelength or wavelength range and also fused to determine a resulting image, e.g., by determining an average of the intensity values ​​in the two images.

[0047] This advantageously results in synchronization of the frame rate and the generation rate, thus determining the object-dependent focus offset. For example, it is possible for each image generated by the image capture device to be generated with different radiation properties within a predetermined time interval. However, it is also conceivable for multiple images, in particular a predetermined number of images, to be generated with identical radiation properties of the incident radiation within the time interval of a predetermined duration, and for a resulting image to be determined from these images, with the remaining subsets of the generated images then being acquired again with different radiation properties.

[0048] Further proposed is a method for measuring an object using an optical coordinate measuring machine. The optical coordinate measuring machine comprises an optical detection system designed as previously explained for imaging the object. Furthermore, the optical coordinate measuring machine can comprise a positioning device for setting various positions and / or orientations of the optical detection system, in particular of the optical detection device, in space. In the method, a measured value is determined as a function of a focus position of the optical detection system of the coordinate measuring machine. The measured value can, for example, represent a coordinate of a point or section of the object in a reference coordinate system. The measured value can also be determined as a function of a position and / or orientation of the optical detection device or the optical detection system.Position and / or orientation information can be acquired, for example, by a position detection device. Alternatively or cumulatively, at least one manipulated variable of the positioning device can be evaluated to determine the position and / or orientation information.

[0049] Furthermore, a focus offset is determined according to a method for determining an object-dependent focus offset according to one of the embodiments described in this disclosure, wherein the corrected measured value is determined as a function of the focus offset. In particular, this can be determined using a corrected focus position. Thus, the focus position of the optical detection system taken into account for determining the measured value can be corrected, in particular by adding the focus offset to the focus position or subtracting it from this focus position, and the corrected measured value is then determined as a function of the thus corrected focus position. The focus position can in particular be determined as a focus value.

[0050] This advantageously results in a reliably accurate determination of measured values, especially in coordinate measurement.

[0051] In a further embodiment, the focus offset is determined before the measured value is generated. In particular, the determination of the focus offset can be integrated into the processes required for the measurement. For example, the focus offset can be determined during a so-called focus move, wherein the images required to determine the focus offset are generated during the focus move. The focus move here refers to a change in the position and / or orientation of the optical detection system for the automatic setting of a focus value, i.e. for autofocusing. In particular, the focus move can be used to set a focus value at which the object is in focus. The focus offset can then be saved, e.g. in the form of a database, wherein in particular each pixel can be assigned a focus offset. The database can also be referred to as a focus correction map.

[0052] It is possible for the optical capture device to be a multi-channel capture device, e.g., an RGB camera. In this case, the pixel-specific focus offset can be determined for each image channel. This allows a color-channel-dependent focus offset to be determined. However, it is also possible for the optical capture device to be a monochromatic capture device.

[0053] It is possible that the optical detection device is a multi-channel detection device, e.g. an RGB camera.

[0054] The described method enables a rapid generation of accurate measured values, in particular since the determination of the focus offset for correction can be carried out in a preparatory step for generating the measured values.

[0055] In a further embodiment, a resulting focus offset is determined as a function of several pixel-specific focus offsets, wherein the corrected measured value is determined as a function of the resulting focus offset. For example, the resulting focus offset can be determined as the mean value of several pixel-specific focus offsets, wherein the pixels whose focus offsets are taken into account can be selected by a user. For this purpose, a user can, for example, use appropriate input devices, e.g. a mouse, a keyboard or a pen, to select the pixels desired for focus determination, wherein then, as explained above, the resulting focus offset can be determined for correcting the measured value. Of course, it is also possible for the pixels to be selected automatically.

[0056] Further proposed is a device for determining an object-dependent focus offset of an optical detection system for imaging an object, which comprises at least one optical detection device and at least one optical element for imaging. This has already been explained above. The device comprises an evaluation device and means for generating images of the object by detecting radiation reflected from the object with different wavelengths or wavelengths from different wavelength ranges. The device is configured to carry out a method according to one of the embodiments described in this disclosure. In particular, the previously explained determination of pixel-specific intensity values ​​and the determination of the pixel-specific focus offset can be carried out by means of the evaluation device.By means of the means for generating images of the object, the previously explained steps of generating a first image and at least one further image can be performed. The device advantageously enables the reliable and precise determination of the focus offset.

[0057] In a further embodiment, the device comprises at least one radiation filter element for filtering radiation, wherein the object is exposed to the radiation filtered by the filter element or radiation reflected from the object is filtered by the filter element. This and corresponding advantages have already been explained above.

[0058] Further proposed is a coordinate measuring machine for optically measuring an object, comprising an optical detection system, a device for determining an object-dependent focus offset according to one of the embodiments described in this disclosure, and an evaluation device. The coordinate measuring machine is configured to perform a method for measuring an object according to one of the embodiments described in this disclosure. This enables, as also explained above, an accurate and reliable determination of measured values.

[0059] The invention is explained in more detail using exemplary embodiments. The figures show: Fig. 1 a schematic representation of a device according to the invention, Fig. 2 a schematic representation of a device according to the invention in a further embodiment, Fig. 3 an exemplary curve of a chromatic longitudinal deviation of an optical element as well as of intensity values ​​of a pixel over the wavelength, Fig. 4 a schematic flow diagram of a method according to the invention for determining an object-dependent focus offset, Fig. 5 a schematic flow diagram of a method according to the invention for measuring an object, Fig. 6a shows an exemplary spectrum of the radiation emitted by a lighting device, Fig. 6b an exemplary spectrum of the radiation emitted by an optical element in the illumination beam path, Fig. 6c shows an exemplary spectrum of radiation reflected from a white standard under illumination of the radiation radiated by the illumination device through the optical element in the illumination beam path, Fig. 6d shows an exemplary spectrum of radiation reflected and filtered by an object under illumination of the radiation radiated by the illumination device through the optical element in the illumination beam path, Fig. 6e an exemplary representation of a chromatic longitudinal deviation of an optical element in the imaging beam path, Fig. 6f shows an example of the superposition of the longitudinal chromatic aberration with the reflected and filtered radiation.

[0060] In the following, the same reference symbols designate elements with the same or similar technical features.

[0061] Fig. 1 shows a schematic representation of a device 1 according to the invention for determining an object-dependent focus offset FA of an optical detection system 2 for imaging an object 3. The optical detection system 2 comprises an optical element 4, designed, for example, as a lens, with a previously known chromatic longitudinal deviation. The optical detection system 2 further comprises an optical detection device 5, for example an image detection device, which can, for example, comprise an image sensor designed as a CCD sensor with a sensor surface. The device 1 further comprises an evaluation device 6, which can be designed as a computing device, in particular as a microcontroller, or integrated circuit or can comprise such. The optical detection device 5 and the evaluation device 6 are connected via a data and / or signal line.

[0062] Also shown is an illumination device 7, which can generate radiation for illuminating the object 3. The illumination device 7 can likewise be an element of the optical detection system 2. Also shown is a further optical element 8, through which radiation generated by the illumination device 7 is guided onto the object 3. This further optical element 8, which can be designed as a lens, for example, can also be an element of the optical detection system 2. Arrows indicate the path of the radiation generated by the illumination device 7. After being generated by the illumination device 7, this radiation radiates through the further optical element 8 and then radiates onto the object 3.Radiation reflected or scattered by the object 3 passes through the optical element 4 onto the optical detection device 5, which can then generate a particularly two-dimensional image with a predetermined number of pixels, wherein intensity values ​​of the pixels represent an intensity of the radiation radiated onto the sensor surface of the optical detection device 5. The optical detection device 5 can be a multi-channel or a monochromatic detection device. Thus, the optical detection device can provide signals in multiple channels, which present the intensity in different spectral ranges, e.g., a red component, a green component, and a blue component of the radiation radiated onto the sensor surface.

[0063] The spectrum of the radiation entering the optical element 4 depends on an illumination spectrum, transmission properties of the illumination optics, i.e. of the further optical element 8, as well as the reflection / transmission properties of the object 3. Since at least the latter are unknown at the time of imaging, a test object-dependent focus offset or measurement deviation results, which is to be corrected.

[0064] The Fig. The optical detection system 2 shown in Figure 1 and the evaluation device 6 enable the execution of the Fig. 4 for determining an object-dependent focus offset of the optical detection system 2. In particular, different images of the object 3 can be generated sequentially by detecting the radiation reflected by the object 3, wherein the radiation used to generate these images each has different wavelengths or comprises wavelengths from different wavelength ranges. For this purpose, for example, the illumination device 7 can be controlled such that radiation with different radiation properties is generated to generate different images.

[0065] Fig. Figure 2 shows a schematic representation of a device 1 according to the invention in a further embodiment. In contrast to the device 1 shown in Fig. 1, the optical detection system 1 comprises a first optical filter element 9a and a further optical filter element 9b. It is shown that these filter elements 9a, 9b are arranged in an observation beam path, wherein this observation beam path guides the radiation reflected or scattered by the object 3 to the sensor of the optical detection device 5. It is shown that the filter elements 9a, 9b are arranged in front of the first optical element 4 in the beam direction. It is of course possible to arrange both filter elements 9a, 9b behind the first optical element 4 or the first optical element 9a in front of and the further optical element 9b behind the first optical element 4. Further alternatively, it is possible to arrange the optical filter elements 9a, 9b at least partially in the illumination beam path which guides the radiation from the illumination device 7 to the object 3.

[0066] In the illustrated embodiment, both the first filter element 9a and the further filter element 9b are designed as variable filter elements. Furthermore, the two filter elements 9a, 9b are movably mounted in the (observation) beam path. In particular, an incident surface of the filter elements 9a, 9b can be moved perpendicular to the main beam direction of the radiation reflected or scattered by the object 3, whereby the filter properties of the variable filter elements 9a, 9b change depending on the incident location or section, i.e., radiation incident at different incident locations is filtered in different ways. The first filter element 9a has high-pass properties, while the further filter element 9b has low-pass properties. Of course, a reverse order of these properties is also conceivable.Collectively, the filter elements 9a, 9b constitute a bandpass filter whose center frequency can be changed by moving the first and / or the further filter element 9a, 9b. The same applies to the size of the passband of the provided bandpass filter.

[0067] To generate the different images of the object, at least one of the two filter elements 9a, 9b can be moved into different positions, thereby changing the properties of the bandpass filter (center frequency and passband) and thus also the spectral properties of the radiation incident on the sensor surface of the optical detection device 5. For the sake of completeness, it should be added that while the use of two linear filter elements 9a, 9b represents a preferred embodiment, the generation of different images is also possible in other ways. For example, it is conceivable that only a single bandpass filter element, in particular with variable filter properties, is arranged in the beam path to provide the radiation generated to generate the images.

[0068] Alternatively, instead of linear filter elements 9a, 9b, a gradient filter element can also be used. The variable filter elements 9a, 9b are linearly movable in the illustrated embodiment. It is also possible to use a circularly variable filter element, which performs a rotational movement to change the filter properties.

[0069] For the movement of the filter elements 9a, 9b, the device 1, in particular the optical detection system 2, can comprise suitable actuators (not shown), which can be controlled, for example, by the evaluation device 6 in order to generate the various images explained above.

[0070] Fig. Figure 3 shows a schematic curve of the longitudinal chromatic aberration CHL over the wavelength λ in the range from 400 to 700 nm. This curve can be dependent on a zoom setting of the optical detection system 1. The solid line in Fig. 3 represents the longitudinal chromatic aberration CHL induced by the first optical element 4 for different wavelengths (curve 10). A dashed line schematically represents a normalized curve 11 of the intensity values ​​for an image point across different wavelengths. Curve 11 is created by generating images with different wavelengths, as previously explained. For example, it is conceivable to increase the wavelengths for generating different images or the central wavelengths of wavelength ranges for generating these different images by a predetermined amount, for example 5 nm, in a predetermined wavelength range, here for example from 400 to 700 nm, and thus generate a plurality of images. The intensity value of the corresponding image point can then be determined in these images.This curve therefore contains information about the object-induced effects on the focus offset, which is caused by the object reflecting or scattering three different wavelengths in different ways.

[0071] A resulting focus offset can then be determined by weighting the wavelength-dependent curve 10 of the longitudinal chromatic aberration of the optical element 4 with the wavelength-dependent curve 11 of the pixel intensity values. Thus, the values ​​of these curves for the same wavelength can be multiplied with each other, for example, across the entire wavelength range.

[0072] The focus offset can then be determined as the average of the weighted curve. The weighted curve is shown as an example by a dashed line 13, the average of which over all wavelengths then forms the resulting focus offset.

[0073] Fig. 4 shows a schematic flow diagram of a method according to the invention for determining an object-dependent focus offset of an optical detection system 2 (see e.g. Fig. 2).

[0074] In a first sub-step of a first step S1, a first image of the object 3 is generated by detecting a first radiation reflected by the object 3, and in a further sub-step of this first step S1, the pixel-specific intensity values ​​for one or more pixels in the first image are determined. In a first sub-step of a second step S2, a second image of the object 3 is generated, and in a second sub-step, the pixel-specific intensity values ​​for the pixel(s) are determined. In a first sub-step of a third step S3, a third image of the object 3 is generated, wherein in a second sub-step, the pixel-specific intensity values ​​for the pixel(s) are again determined. Fig. 4 shows that this generation and determination can be carried out for a predetermined number n of wavelengths, wherein each of the images is generated with radiation of a different wavelength or with radiation from different wavelength ranges.

[0075] Then, in a determination step SB, the pixel-specific focus offset is determined at least as a function of the intensity values ​​and a predetermined chromatic longitudinal deviation CHL of the at least one optical element 4. This has already been explained above.

[0076] Fig. Figure 5 shows a schematic flow diagram of a method according to the invention for measuring an object 3 (see e.g. Fig. 2) with an optical coordinate measuring machine. In this step, a focus offset is determined. In this step, the Fig. 4 are carried out. In a measuring step MS, a measured value is then generated, whereby the measured value is determined depending on a set focus position of the optical detection system 2 of the coordinate measuring machine. The measuring step MS includes a correction step in which the focus position is corrected depending on the previously determined focus offset. Overall, therefore, a corrected measured value is determined. Fig. 5 shows that the focus offset is determined before the measured value is generated.

[0077] Fig. Figure 6a shows an exemplary standardized spectrum of the radiation emitted by an illumination device 7. This generates radiation with constant intensity for all wavelengths in a range from 400 nm to 700 nm. This radiation is radiated onto an object 3 through an optical element 8 in the illumination beam path. Fig. Figure 6b shows an example of a normalized spectrum of the radiation emitted by optical element 8 in the illumination beam path. It can be seen that wavelengths in the range of 400 nm to 500 nm were filtered out by optical element 8 in the illumination beam path. Fig. Figure 6c shows an example of a normalized spectrum of radiation reflected from a white standard. This corresponds to the radiation radiated by optical element 8 in the illumination beam path. Fig. Figure 6d shows an example of a normalized spectrum of radiation reflected and filtered from an object 3. It can be seen that a narrow wavelength range has been filtered out from the reflected radiation, with the intensity of the reflected radiation remaining constant in this filtered wavelength range. In particular, exactly one image can be generated with this filtered radiation.

[0078] Fig. Figure 6e shows an exemplary representation of a chromatic longitudinal deviation CHL of an optical element 4 in the imaging beam path over the wavelength λ in the range from 400 to 700 nm. Fig.Figure 6f shows an example of the superposition of the longitudinal chromatic aberration CHL with the reflected and filtered radiation. The mean value of the hatched area corresponds to the longitudinal chromatic aberration to be determined for the radiation with wavelengths from the filtered wavelength range. Thus, the longitudinal chromatic aberration CHL of the optical element 4 can be weighted with the spectrum of the filtered radiation, e.g., by multiplying the values ​​of the longitudinal chromatic aberration by the intensity value of the filtered spectrum for the same wavelength. The mean value of this weighted curve can then be determined as the effective longitudinal chromatic aberration to be determined for the radiation with wavelengths from the filtered wavelength range. List of reference symbols 1 device 2 optical detection system 3 Object 4 first optical element 5 optical detection device 6 Evaluation device 7 Lighting equipment 8 additional optical element 9a, 9b filter element 10 Course of longitudinal chromatic aberration 11 Intensity value curve 13 weighted course S1, Sc2, S3, SN steps SB determination step FAB focus offset determination step MS measurement step CHL longitudinal chromatic aberration λ wavelength

Claims

[1] Method for determining an object-dependent focus offset of an optical detection system (2) for imaging an object (3), which comprises at least one optical detection device (5) and at least one optical element (4) for imaging, comprising the steps: - generating a first image of the object (3) by detecting a first radiation reflected from the object (3), - generating at least one further image of the object (3) by detecting further radiation reflected from the object (3), wherein the first radiation and the further radiation have different wavelengths or comprise wavelengths from different wavelength ranges, - determining the pixel-specific intensity values ​​in the first and in the at least one further image for at least one pixel in the images, - Determining the pixel-specific focus offset at least as a function of the intensity values ​​and a predetermined chromatic longitudinal deviation (CHL) of the at least one optical element (4). [2] Method according to claim 1, characterized by that the radiation used to create the images is provided by radiation filtering. [3] Method according to claim 2, characterized by that the radiation filtering takes place in an illumination beam path and / or in an observation beam path. [4] Method according to claim 2 or 3, characterized by that the radiation used to generate the images is provided by filtering with at least one gradient filter or with at least one variable filter (9a, 9b). [5] Method according to one of the preceding claims, characterized bythat a weighted curve is determined by weighting a wavelength-dependent curve of the predetermined chromatic longitudinal deviation (CHL) with a wavelength-dependent curve of the determined intensity values, wherein the focus offset is determined as the mean value of the weighted curve. [6] Method according to one of the preceding claims, characterized by that the focus offset is additionally determined depending on a reference receiver spectrum. [7] Method according to one of the preceding claims, characterized by that a frame rate of the image capture device (5) corresponds to a capture rate of the images with different wavelengths or different wavelength ranges or an integer multiple of the capture rate. [8] Method for measuring an object (3) with an optical coordinate measuring machine, wherein a measured value is determined as a function of a focus position of an optical detection system (2) of the coordinate measuring machine, wherein a focus deviation is determined with a method according to one of claims 1 to 7, wherein a corrected measured value is determined as a function of the focus deviation. [9] Method according to claim 8, characterized by that the focus offset is determined before the measured value is generated. [10] Method according to one of claims 8 or 9, characterized by that a resulting focus offset is determined as a function of several pixel-specific focus offsets, whereby the corrected measured value is determined as a function of the resulting focus offset. [11] Device for determining an object-dependent focus offset of an optical detection system (2) for imaging an object (3), which comprises at least one optical detection device (5) and at least one optical element (4) for imaging, wherein the device (1) comprises an evaluation device (6) and means for generating images of the object (3) by detecting radiation reflected from the object (3) with different wavelengths or wavelengths from different wavelength ranges, wherein the device (1) is configured to carry out a method according to one of claims 1 to 7. [12] Device according to claim 11, characterized by that the device (1) comprises at least one filter element (9a, 9b) for filtering radiation, wherein the object (3) is exposed to the radiation filtered by the filter element or radiation reflected by the object (3) is filtered by the filter element (9a, 9b). [13] Coordinate measuring machine for optically measuring an object (3), comprising an optical detection system (2), a device (1) according to one of claims 11 or 12 and an evaluation device (6), wherein the coordinate measuring machine is configured to carry out a method according to one of claims 8 to 10.

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