DE102023205077A1 - Method and device for determining an object-dependent focus offset, method for measuring an object and coordinate measuring device - Google Patents

Abstract

The invention relates to a method and a 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, 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), as well as a method for measuring an object and a coordinate measuring device.

Description

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.

In coordinate measuring technology, optical methods are used, among other things, to measure an object. The object is usually illuminated by one or more light sources and the radiation reflected or scattered by the object is recorded by an optical detection device with an image sensor, whereby a generally two-dimensional image is generated. In most cases, optical elements are arranged in an observation beam path for this purpose, e.g. in the form of lenses.

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.

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. Different focus positions can be set for different wavelengths, e.g. using an autofocus function, which also generates different measured values in the previously explained method for generating measured values. 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. by appropriate measurement or calibration. This makes it possible to take into account the contribution to the longitudinal chromatic aberration generated by the specific optical element when determining measured values.

The problem, however, is that different objects can contribute to longitudinal chromatic aberration in different ways, since different objects reflect radiation differently, especially with different spectra. This in turn means that the focus offset depends on the object or test piece.

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

Known from the state of the art is the DE 10 2019 214 865 A1 which discloses an illumination system, an optical measuring device and a method for meteorological measurement of workpieces.

Also known is the 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 optoelectrical converter with downstream signal processing and evaluation circuit.

Also known is the 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.

Also known is the US 5,940,183 A which discloses a filter arrangement and a detection device utilizing a plurality of filters each having a unique beam wavelength.

Also known is the US 10,401,738 B2 , which refers generally to overlay metrology and more particularly to overlay metrology with multiple parameter configurations.

Also known is the DE 10 2010 064 387 which discloses an imaging system and an imaging method.

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 a reliable and precise determination of the focus offset. The technical problem is also to create a method for measuring an object with an optical coordinate measuring device and a coordinate measuring device that generate an accurate measurement value.

The solution to the technical problem is provided by the objects having the features of the independent claims. Further advantageous embodiments of the invention are set out in the subclaims.

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. with a detection device designed as a line sensor. The optical detection system also comprises at least one optical element for imaging, with the rays detected to generate the image passing through the optical element. Of course, the optical detection system can comprise several such optical elements. In particular, the optical detection system can comprise optical elements that are configured to set or change a focal point of the optical system. For this purpose, these optical elements can, for example, be movably mounted. Their position and/or orientation can be adjustable to change the focus point.

• In einem ersten Schritt erfolgt das Erzeugen eines ersten Abbilds eines Objekts durch Erfassen einer vom Objekt reflektierten ersten Strahlung. Das Objekt kann hierzu beleuchtet werden, insbesondere mit Strahlung aus einem vorbestimmten Spektrum, z.B. einem Weißlicht-Spektrum oder in einem Spektrum von sichtbaren Licht, aber auch mit einem davon verschiedenen Spektrum. Das Spektrum kann Wellenlängen von einer Minimalwellenlänge bis zu einer Maximalwellenlänge umfassen. Die Minimalwellenlänge kann in einem Bereich von 350 nm von 400 nm und die Maximalwellenlänge kann in einem Bereich von 700 nm bis 1000 nm liegen. Aber auch Teilspektren aus diesem Spektrum können zur Beleuchtung verwendet werden. Hierzu kann das optische Erfassungssystem mindestens eine Beleuchtungseinrichtung zur Beleuchtung des Objektes umfassen.

The procedure includes the following steps:

• In a first step, a first image of an object is generated by detecting a 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 include 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 include at least one lighting device for illuminating the object.

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 that the different wavelength ranges 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.

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 lighting 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 spectrum explained above.

The different images can be generated sequentially, whereby 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 different images can preferably be the same. However, it is also conceivable that these change, e.g. by a predetermined amount. The wavelengths of the filtered radiation, in particular the central wavelength, can be changed in such a way that the radiation filtered to generate the images covers the entire illumination spectrum or a selected sub-range thereof.

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

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 images generated and a predetermined chromatic longitudinal deviation 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.

Depending on this focus offset, a measured value can then be corrected, which is determined by an optical coordinate measuring machine with the optical detection system. The measured value can be determined by determining a focus value, which is set manually or by an autofocus function. This was explained above. 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 manner 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 several pixels and then a corrected measured value can be determined on the basis of this resulting focus offset. The method thus advantageously enables a quick and reliable determination of an object-dependent focus offset of an optical detection system, which in turn then enables a reliable and precise correction when generating measured values.

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 that extends from an illumination device to the object, and/or in an observation beam path that extends 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 different images, e.g. automatically. The radiation filtering advantageously results in reliable generation of the images with rays of different wavelengths.

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.

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. If at least one optical filter is arranged in the illumination beam path, it advantageously results that requirements for the properties of optical filters can be lower compared to an arrangement outside the illumination beam path, e.g. in the observation beam path, e.g. tolerances for target properties can be larger. If at least one optical filter element is arranged in the observation beam path, it advantageously results that thermal loading of the optical filter by the light source is reduced.

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.

A gradient filter can change spectral filter properties along a dimension. For example, a spectral passband can be spectrally shifted along a dimension for each physical point of an incident surface. 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 resulting band-pass properties of the filter arrangement. In this case, a first gradient filter can have high-pass properties and the other gradient filter can have low-pass properties.

Preferably, the at least one gradient filter or the plurality of gradient filters are each mounted so that they can move. 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, although there is no need for the complex replacement of the filter elements to generate 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, whereby radiation with different radiation properties can be provided by changing the position of the gradient filter.

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 surface of the radiation to be filtered. Linearly variable filters are particularly known, which change the spectral properties of the filter in the wavelength range for each point along an axis, in particular in a linear manner. Also known are circular filters, in which the spectral properties of the filter in the wavelength range are shifted for each physical point along a circular line. Variable filters can have low-pass, high-pass, band-pass or band-stop filter properties. The statements on gradient filters therefore also apply accordingly to variable filters.

The use of movable filters in particular advantageously results in an easily implemented generation of images, in particular without the need for pulsating illumination with different wavelengths, which can have unfavorable biocompatibility.

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

In a preferred embodiment, a weighted curve is determined by weighting a (previously known) wavelength-dependent curve of the predetermined chromatic longitudinal deviation 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 intensity value that can be generated.

The wavelength-dependent course 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 course can be determined for each pixel, whereby the pixel-specific focus offset is in turn determined as the mean value of the weighted course. The wavelength-dependent course 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.

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

It is thus possible to divide the spectrum reflected by the object, i.e. the object receiver spectrum, by this reference receiver spectrum, whereby the two spectra are different from one another. 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 it. The wavelength-dependent curve of the predetermined chromatic longitudinal deviation can then be weighted with the wavelength-dependent curve of the determined intensity values determined in this way.

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

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

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 error of the optical element.

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.

The additional consideration of the reference receiver spectrum advantageously results in a more precise determination of the focus offset.

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 60 Hz, for example, the generation rate can also be 60 Hz, whereby 60 images 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 with different wavelengths or with wavelengths from different wavelength ranges are generated in one second. In this case, for example, two images with the same wavelength or with the same wavelength ranges generated consecutively by the image capture device 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. Then the two subsequent images can be generated with a different wavelength or with a different wavelength range and also fused to determine a resulting image, e.g. by determining an average of the intensity values in the two images.

This advantageously results in a synchronization of the image rate and the generation rate and thus the determination of the object-dependent focus offset. For example, it is possible that in a predetermined time interval each image generated by the image capture device is generated with different radiation properties. However, it is also conceivable that in the time interval of predetermined duration several images, in particular a predetermined number of images, are generated with the same radiation properties of the incident radiation and a resulting image is determined from these, with remaining subsets of the generated images then being captured again with different radiation properties.

A method for measuring an object with an optical coordinate measuring device is also proposed. The optical coordinate measuring device comprises an optical detection system designed as previously explained for imaging the object. The optical coordinate measuring device can also comprise a positioning device for setting various positions and/or orientations of the optical detection system, in particular the optical detection device, in space. In the method, a measured value is determined depending on a focus position of the optical detection system of the coordinate measuring device. 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 depending on a position and/or orientation of the optical detection device or the optical detection system. Position and/or orientation information can, for example, be detected by a position detection device. Alternatively or cumulatively, at least one manipulated variable of the positioning device can be evaluated in order to determine the position and/or orientation information.

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 dependent on the focus offset. In particular, this can be determined with a corrected focus position. The focus position of the optical detection system taken into account for determining the measured value can thus be corrected, in particular by adding the focus offset to the focus position or subtracting it from this focus position and then determining the corrected measured value depending on the focus position corrected in this way. The focus position can in particular be determined as a focus value.

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

In a further embodiment, the focus offset is determined before the measurement value is generated. In particular, the focus offset can be determined in the time required for the measurement Processes can be integrated. For example, the focus offset can be determined during a so-called focus move, whereby the images required to determine the focus offset are generated during the focus move. The focus move 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, whereby in particular each pixel can be assigned a focus offset. The database can also be referred to as a focus correction map.

It is possible that the optical detection device is a multi-channel detection device, e.g. an RGB camera. In this case, the pixel-specific focus offset can be determined for each image channel. In this way, a color channel-dependent focus offset can be determined. However, it is also possible that the optical detection device is a monochromatic detection device.

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

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

In a further embodiment, a resulting focus offset is determined depending on several pixel-specific focus offsets, wherein the corrected measured value is determined depending on 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 previously explained, the resulting focus offset can be determined to correct the measured value. Of course, it is also possible for the pixels to be selected automatically.

A device is also proposed 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. The previously explained steps of generating a first image and at least one further image can be carried out by means of the means for generating images of the object. The device advantageously enables the reliable and precise determination of the focus offset.

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 by the object is filtered by the filter element. This and corresponding advantages have already been explained above.

Also 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, wherein the coordinate measuring machine is configured to carry out 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.

• 1 eine schematische Darstellung einer erfindungsgemäßen Vorrichtung,
• 2 eine schematische Darstellung einer erfindungsgemäßen Vorrichtung in einer weiteren Ausführungsform,
• 3 einen exemplarischen Verlauf einer chromatischen Längsabweichung eines optischen Elements sowie von Intensitätswerten eines Pixels über der Wellenlänge,
• 4 ein schematisches Flussdiagramm eines erfindungsgemäßen Verfahrens zur Bestimmung einer objektabhängigen Fokusablage,
• 5 ein schematisches Flussdiagramm eines erfindungsgemäßen Verfahrens zur Vermessung eines Objekts,
• 6a ein beispielhaftes Spektrum der von einer Beleuchtungseinrichtung emittierten Strahlung,
• 6b ein beispielhaftes Spektrum der durch ein optisches Element im Beleuchtungsstrahlengang gestrahlten Strahlung,
• 6c ein beispielhaftes Spektrum einer von einem Weiß-Normal reflektierten Strahlung unter Beleuchtung der von der Beleuchtungseinrichtung durch das optische Element im Beleuchtungsstrahlengang gestrahlten Strahlung,
• 6d ein beispielhaftes Spektrum einer von einem Objekt reflektierten und gefilterten Strahlung unter Beleuchtung der von der Beleuchtungseinrichtung durch das optische Element im Beleuchtungsstrahlengang gestrahlten Strahlung,
• 6e eine beispielhafte Darstellung einer chromatischen Längsabweichung eines optischen Elements im Abbildungsstrahlengang,
• 6f eine beispielhafte Überlagerung der chromatischen Längsabweichung mit der reflektierten und gefilterten Strahlung.

The invention is explained in more detail using exemplary embodiments. The figures show:

• 1 a schematic representation of a device according to the invention,
• 2 a schematic representation of a device according to the invention in a further embodiment,
• 3 an exemplary course of a chromatic longitudinal deviation of an optical element as well as of intensity values of a pixel over the wavelength,
• 4 a schematic flow diagram of a method according to the invention for determining an object-dependent focus offset,
• 5 a schematic flow diagram of a method according to the invention for measuring an object,
• 6a an exemplary spectrum of the radiation emitted by a lighting device,
• 6b an exemplary spectrum of the radiation emitted by an optical element in the illumination beam path,
• 6c an exemplary spectrum of radiation reflected from a white standard under illumination of the radiation emitted by the illumination device through the optical element in the illumination beam path,
• 6d an exemplary spectrum of radiation reflected and filtered by an object under illumination of the radiation emitted by the lighting device through the optical element in the illumination beam path,
• 6e an exemplary representation of a chromatic longitudinal deviation of an optical element in the imaging beam path,
• 6f an exemplary superposition of the longitudinal chromatic aberration with the reflected and filtered radiation.

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

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 also 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 also comprises an evaluation device 6, which can be designed as a computing device, in particular as a microcontroller, or integrated circuit or can comprise one of these. The optical detection device 5 and the evaluation device 6 are connected via a data and/or signal line.

Also shown is an illumination device 7 that can generate radiation to illuminate the object 3. The illumination device 7 can also 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 show 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 several 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.

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, and 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.

The 1 The optical detection system 2 shown and the evaluation device 6 enable the execution of the 4 explained method 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 have different wavelengths or comprise wavelengths from different wavelength ranges. For this purpose, for example, the lighting device 7 can be controlled in such a way that radiation with different radiation properties is generated to generate different images.

2 shows a schematic representation of a device 1 according to the invention in a further embodiment. In contrast to the device in 1 In the embodiment shown, 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, this observation beam path directs 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. As a further alternative, 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.

In the embodiment shown, 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 an incident location or section, i.e. radiation impinging on 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. As a whole, the filter elements 9a, 9b represent 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 bandpass filter provided.

To generate the different images of the object, at least one of the two filter elements 9a, 9b can be moved into different positions, which changes the properties of the bandpass filter (center frequency and passband) and thus also the spectral properties of the radiation arriving on the sensor surface of the optical detection device 5. For the sake of completeness, it should be added that the use of two linear filter elements 9a, 9b is a preferred embodiment, but 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 in order to provide the radiation generated to generate the images.

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 embodiment shown. It is also possible to use a circularly variable filter element which performs a rotational movement to change the filter properties.

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.

3 shows a schematic progression of the chromatic longitudinal deviation CHL over the wavelength λ in the range from 400 to 700 nm. This progression may depend on a zoom setting of the optical detection system 1. The solid line in 3 represents the chromatic longitudinal deviation CHL induced by the first optical element 4 for different wavelengths (curve 10). A dashed line schematically shows a standardized curve 11 of the intensity values for a pixel over different wavelengths. The curve 11 is created by - as explained above - generating images with different wavelengths. 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 to generate a large number of images. The intensity value of the corresponding pixel 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 fact that the object 3 reflects or scatters different wavelengths in different ways.

A resulting focus offset can then be determined by weighting the wavelength-dependent curve 10 of the longitudinal chromatic deviation of the optical element 4 with the wavelength-dependent curve 11 of the pixel intensity values. The values of these curves for the same wavelength can be multiplied with each other, e.g. over the entire wavelength range.

The focus offset can then be determined as the mean value of the weighted curve. The weighted curve is shown as an example by a dash-dot line 13, the mean value of which over all wavelengths then forms the resulting focus offset.

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. 2 ).

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. In 4 It is shown 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.

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.

5 shows a schematic flow diagram of a method according to the invention for measuring an object 3 (see e.g. 2 ) with an optical coordinate measuring machine. In this case, a focus offset is determined in a focus offset determination step FAB. In this determination step FAB, the 4 The steps shown 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, a corrected measured value is thus determined. In 5 It is shown that the focus offset is determined in time before the measured value is generated.

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 by an optical element 8 in the illumination beam path. 6b shows an example of a standardized spectrum of the radiation emitted by the 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 the optical element 8 in the illumination beam path. 6c shows an example of a standardized spectrum of radiation reflected from a white standard. This corresponds to the radiation emitted by the optical element 8 in the illumination beam path. 6d shows an example of a standardized spectrum of radiation reflected and filtered by an object 3. It can be seen that a narrow wavelength range was filtered out of the reflected radiation, whereby the intensity of the reflected radiation is constant in this filtered wavelength range. In particular, exactly one image can be generated with this filtered radiation.

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. 6f shows an example of the superposition of the longitudinal chromatic deviation CHL with the reflected and filtered radiation. The mean value of the hatched area corresponds to the longitudinal chromatic deviation to be determined for the radiation with wavelengths from the filtered wavelength range. The longitudinal chromatic deviation CHL of the optical element 4 can thus be weighted with the spectrum of the filtered radiation, e.g. by multiplying the values of the longitudinal chromatic deviation with 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 deviation 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 the 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

QUOTES INCLUDED IN THE DESCRIPTION

This list of documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 10 2019 214 865 A1 [0007]
• DE 37 43 584 A1 [0008]
• US 2021/0302632 A1 [0009]
• US 5,940,183 A [0010]
• US 10,401,738 B2 [0011]
• DE 10 2010 064 387 [0012]

Claims (13)

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 by the object (3), - generating at least one further image of the object (3) by detecting a further radiation reflected by 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). procedure according to claim 1 , characterized in that the radiation used to generate the images is provided by radiation filtering. procedure according to claim 2 , characterized in that the radiation filtering takes place in an illumination beam path and/or in an observation beam path. procedure according to claim 2 or 3 , characterized in that the radiation which serves to generate the images is provided by filtering with at least one gradient filter or with at least one variable filter (9a, 9b). Method according to one of the preceding claims, characterized in that 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. Method according to one of the preceding claims, characterized in that the focus offset is additionally determined as a function of a reference receiver spectrum. Method according to one of the preceding claims, characterized in that an image 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. 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 offset is determined using a method according to one of the Claims 1 until 7 is determined, whereby a corrected measured value is determined depending on the focus offset. procedure according to claim 8 , characterized in that the focus offset is determined in time before the measured value is generated. Method according to one of the Claims 8 or 9 , characterized in that 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. 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 the Claims 1 until 7 to carry out. device according to claim 11 , characterized in 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 from the object (3) is filtered by the filter element (9a, 9b). Coordinate measuring machine for optically measuring an object (3), comprising an optical detection system (2), a device (1) according to one of the Claims 11 or 12 and an evaluation device (6), wherein the coordinate measuring device is configured to perform a method according to one of the Claims 8 until 10 to carry out.

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