Patent Description:
Document <CIT> describes a surface property indexing apparatus which includes a measurement device that generates a plurality of captured images by capturing images of reflected light of illumination light on a surface of the measured object while selecting its wavelength, and an arithmetic processing apparatus that indexes a surface property of the measured object on the basis of the obtained captured images. The captured images are of the same wavelength of the reflected light that forms images in an image capturing device, and is of different reflection angles of the reflected light that forms images in the image capturing device in the direction corresponding to the longitudinal direction of the measured object in the captured image. The arithmetic processing apparatus reconstructs the generated captured images to generate a plurality of processing target images having a common wavelength of the reflected light and a common reflection angle of the reflected light and composed of pixels corresponding to the different view field positions of the measured object, and indexes the surface property of the measured object on the basis of the generated processing target images.

Document <CIT> describes an interferometer which can vary a direction of an incident beam and which enables more accurate measurement while avoiding eclipse includes an optical system adapted to form an interference beam from a beam from a surface to be measured, and a detected position varying member for varying a position to be detected of the interference beam in accordance with information on inclination of the surface to be measured.

Optical measurement devices are widely used for measuring the surface profile of objects and in particular for determining possible discrepancies between the actual surface profile and the intended surface profile of an object under investigation. For some objects, a high degree of precision is desired, for instance for determining the quality of the surface of optical elements, such as optical lenses, mirrors and/or prisms. In case, a discrepancy between the actual and the intended shape and/or surface profile is determined, the respective object may undergo a post-treatment, such as a further polishing treatment, to reduce the discrepancy and/or to align the actual shape and/or surface to the intended shape and/or surface. Interferometric techniques are often used for measuring the surface profile of objects, in particular of objects having a plane or spherical surface.

For measuring the surface profile of objects having a non-plane surface, such as free-form objects or aspherical lenses, the efforts are unlike higher. In some cases interferometric devices of different kinds may be used, such as Fizeau interferometers with computer generated holograms, stitching interferometry, and tilted wave interferometry. In other cases Shack-Hartmann sensors may be used for determining a wavefront to characterize the surface of the object. Other techniques may use 3D profilometers having optical or tactile measuring heads.

Another kind of measuring device for measuring a wavefront and characterizing the surface profile of an object based on an angular selective optical element is described in the document <CIT>. This technique overcomes several disadvantages of other techniques mentioned further above and is adjustable to various different shapes of the surface to be measured.

However, these commonly used techniques and devices often require a high degree of complexity and come along with significant manufacturing costs for the respective measurement devices. Moreover, these techniques often require a high skill level and training effort to enable an operator to use such devices.

In the light of this state of the art, one of the objects of the present disclosure is to provide a method and a device being configured to overcome the above-described disadvantages of the state of the art, respectively.

This problem is solved by a method and a device having the features of the respective independent claim. Optional features are described in the dependent claims and in the description.

In one aspect a method for characterizing a surface of an object is provided. The method comprises illuminating the surface of the object with a predetermined illumination light, wherein the illumination light comprises a predetermined angular distribution of collimated light waves. The method further comprises collecting the light waves reflected off the surface with an imaging assembly, and consecutively isolating collimated light waves having different angular ranges from the light waves reflected off the surface. Moreover, the method comprises detecting the isolated light waves and providing detector signal data for each of the consecutively isolated light waves with a detector arranged at a detecting distance from the imaging assembly. The method further comprises deriving a surface profile of the surface based on the detector signal data and the respective different angular ranges of the consecutively isolated collimated light waves.

In another aspect a device for characterizing a surface of an object is provided. The device comprises an illumination light source adapted to illuminate the surface of the object with a predetermined illumination light, wherein the illumination light comprises a predetermined angular distribution of collimated light waves. The device further comprises an imaging assembly adapted to collect the light waves reflected off the surface. Moreover, the device comprises means for consecutively isolating collimated light waves having different angular ranges from the light waves reflected off the object, and a detector arranged at a detecting distance from the imaging assembly, wherein the detector is adapted to detect the isolated light waves and to provide detector signal data for each of the consecutively isolated light waves. In addition, the device comprises a control unit adapted to derive a surface profile of the surface based on the detector signal data and the respective different angular ranges of the consecutively isolated collimated light waves.

In another aspect a method for characterizing a wavefront of a light wave reflected from a surface of an object is provided. The method comprises providing an imaging assembly comprising a first lens having a first focal length and a second lens having a second focal length, wherein the first lens and the second lens share a common focal plane within the imaging assembly. The method further comprises providing a pinhole aperture arranged in a pinhole plane, wherein the pinhole plane is overlapping with or adjacent to the common focal plane. Moreover, the method comprises arranging the imaging assembly at a working distance of the first lens from the object essentially corresponding to the first focal length and collecting the light wave emitted and/or reflected from the object. In addition the method comprises arranging the pinhole aperture consecutively at multiple predetermined positions in the pinhole plane, and detecting the light wave collected by the imaging assembly and providing detector signal data for each of the consecutive positions of the pinhole aperture in the pinhole plane with a detector arranged at a detecting distance from the second lens essentially corresponding to the second focal length. The method further comprises deriving the wavefront of the light wave reflected from the surface of the object based on the detector signal data and the corresponding predetermined positions of the pinhole aperture in the pinhole plane.

In another aspect a device for characterizing a surface of an object is provided. The device comprises an imaging assembly comprising a first lens having a first focal length and a second lens having a second focal length, wherein the first lens and the second lens share a common focal plane within the imaging assembly and wherein the imaging assembly is adapted to collect a light wave reflected from the object at a working distance of the first lens from the object essentially corresponding to the first focal length. The device further comprises a pinhole aperture arranged in a pinhole plane, wherein the pinhole plane is parallel to the common focal plane of the first lens and the second lens and overlapping with or adjacent to the common focal plane, wherein the pinhole aperture is arrangeable at multiple predetermined positions in the pinhole plane. Moreover, the device comprises a detector arranged at a detecting distance from the second lens essentially corresponding to the second focal length, wherein the detector is configured to detect the light wave collected by the imaging assembly and to provide detector signal data. In addition, the device comprises a control unit configured to arrange the pinhole aperture consecutively at multiple predetermined positions in the pinhole plane, to receive the detector signal data for each of the consecutive positions of the pinhole aperture in the pinhole plane, and to derive the wavefront of the light wave emitted and/or reflected from the object based on the detector signal data and the corresponding predetermined positions of the pinhole aperture in the pinhole plane.

The device may be configured to carry out the method. Therefore, the description given with respect to the method applies mutatis mutandis to the device and vice versa.

Characterizing the surface of an object means that a surface profile of the surface is determined. This may include determining a spatially resolved angular distribution of the surface, i.e. a local slope of the surface, and/or a spatially resolved height profile of the surface.

A predetermined angular distribution of collimated light waves means that the illumination light may be described as a superposition of multiple collimated waves having different wave vectors, wherein the wave vectors have a predetermined angular distribution. In particular, the illumination light may have a specific predetermined relation of its intensity and its wave vector or propagation direction. Likewise, the illumination light source may have a specific predetermined relation of its emission intensity and its emission angle. This may be achieved for instance by an angular selective optical element, such as an optical interference filter. The relation of the intensity relative to the wave vector and/or the propagation direction may follow a predetermined mathematical function, such as for instance a Gaussian curve having its maximum at an angle of <NUM>° with respect to the optical axis of the illumination light source. The mathematical function or any other defined relation of the intensity relative to the wave vector or the propagation direction may be used for deriving the surface profile of the surface based on the different angular ranges of the consecutively isolated collimated light waves.

Isolating collimated light waves having different angular ranges means that out of the collimated light waves of the predetermined angular distribution only a subset of collimated light having wave vectors within a particular angular range is selected. The isolation may be carried out by transmitting only the collimated light wave having the specified angular range and blocking or reflecting the remaining collimated light waves. "Consecutively isolating" means that the light waves having different angular ranges are isolated in a sequential manner, i.e. one after the other.

The control unit may comprise a computing unit. For instance, the control unit may comprise a processing unit, such as a CPU, and a memory for storing program code and/or program data. For example, the control unit may consist of or comprise a microcontroller, a personal computer, a mobile phone, and/or a tablet computer. The control unit may be connected via a data communication connection with the detector and optionally with an illumination light source and/or a moving device for arranging the pinhole aperture within the pinhole plane. Moreover, the control unit may be connected with or may comprise one or more interfaces for communicating with an operator, such as a display for providing visual information and/or speakers for providing acoustic information to an operator, and/or a keyboard and/or a computer mouse and/or any other input device for receiving commands and/or information from the operator.

The disclosure provides the advantage that a method and a device are provided allowing a facilitated measurement of a surface profile of a surface of an object. The requirements for an operator to carry out the method are moderate and, hence, the method may be used by a wide range of operators without a need of extensive technical skills and training.

Moreover, the disclosure provides the advantage that the device for characterizing (e.g., measuring) surface profiles of objects can be manufactured with moderate effort and without the need of expensive optical and non-optical components. Consequently, the disclosure allows providing the device at low cost and hence to apply such techniques in cost sensitive fields, which have no access to such measurements with conventional devices and techniques.

The predetermined angular distribution of collimated light waves of the illumination light may have its maximum intensity at a propagation angle parallel to an optical axis of an illumination light source. This specified angular distribution may be considered when deriving the surface profile.

The imaging assembly may be adapted to image the surface of the object to the detector. This may allow characterizing the surface profile at a high spatial resolution.

The method may be adapted such that the imaging assembly comprises or is configured as an imaging lens assembly with a first lens having a first focal length and a second lens having a second focal length, wherein the first lens and the second lens share a common focal plane within the imaging lens assembly. Moreover, the method may be adapted such that a pinhole aperture is provided for consecutively isolating the collimated light waves having different angular ranges from the light waves reflected off the surface, wherein the pinhole aperture is arranged in a pinhole plane. The pinhole plane is overlapping with or adjacent to the common focal plane. The method may be further adapted such that the imaging assembly is arranged at a working distance of the first lens from the surface of the object essentially corresponding to the first focal length to collect the light waves reflected off the object. The step of consecutively isolating the collimated light waves having different angular ranges from the light waves reflected off the surface may comprise arranging the pinhole aperture consecutively at multiple predetermined positions in the pinhole plane, wherein each of the predetermined pinhole positions corresponds to one of the isolated collimated light waves having different angular ranges.

Accordingly, the device may be adapted such that the imaging assembly comprises or is configured as an imaging lens assembly with a first lens having a first focal length and a second lens having a second focal length, wherein the first lens and the second lens share a common focal plane within the imaging lens assembly and wherein the imaging lens assembly is adapted to collect light waves reflected off the surface at a working distance of the first lens from the object essentially corresponding to the first focal length. Moreover, the means for consecutively isolating collimated light waves having different angular ranges may comprise a pinhole aperture arranged in a pinhole plane, wherein the pinhole plane is overlapping with or adjacent to the common focal plane. The pinhole aperture is arrangeable at multiple predetermined positions in the pinhole plane, wherein each of the predetermined positions corresponds to a predetermined angular range of one of the isolated collimated light waves. The detector may be arranged at a detecting distance from the second lens essentially corresponding to the second focal length, wherein the detector is configured to detect the light wave collected by the imaging lens assembly and to provide detector signal data. The control unit may be configured to arrange the pinhole aperture consecutively at multiple predetermined positions in the pinhole plane, to receive the detector signal data for each of the consecutive positions of the pinhole aperture in the pinhole plane, and to derive a surface profile of the surface based on the detector signal data and the corresponding predetermined positions of the pinhole aperture in the pinhole plane.

In general, characterizing a surface may include characterizing a wavefront of a light wave reflected off said surface. Characterizing a wavefront of a light wave means that the wavefront of the light wave is measured and/or that information about a spatial distribution of a direction of the wave vector of the light wave is retrieved. Determining a wavefront may, thus, serve the purpose of retrieving information about a surface of an object reflecting said light wave. The wavefront being reflected off the object means that the wavefront characterizes a light wave propagating from the object after being reflected at the object. Likewise, a wavefront of a light wave transmitted through an object and in particular a surface of the object can be characterized for characterizing the surface of the object. The object may be illuminated with a light wave provided by an external light source, i.e. an illumination light source, such that the light wave having the wavefront to be determined is reflected at and/or transmitted through the surface of the object.

The first lens and the second lens sharing a common focal plane within the imaging lens assembly means that the first lens and the second lens are arranged on a common optical axis such that a distance between the first lens and the second lens equals the sum of the first focal length and the second focal length. The imaging lens assembly may be formed as a fixed assembly of the first lens and the second lens and may comprise further optical and/or non-optical elements, such as a housing.

The pinhole aperture is an aperture transmitting light waves propagating through the opening of the pinhole aperture and blocking, absorbing, reflecting and/or scattering light waves impinging on the pinhole aperture beyond the opening of the aperture. For instance, the pinhole aperture may comprise or be provided as a hole provided in an opaque element, such as a metal plate, which is capable of effectively blocking impinging light waves. The pinhole plane is a plane, in which a transversal extension of the pinhole aperture extends. The pinhole plane may be perpendicular to an optical axis of the pinhole aperture. The pinhole plane may be perpendicular to the optical axis of the imaging lens assembly, i.e. the optical axes of the first lens and the second lens. In other words, the pinhole plane may be parallel to the common focal plane of the first lens and the second lens. Alternatively, the pinhole plane may be tilted with respect to the common focal plane, such as by an angle in the range of ±<NUM>° or less. The pinhole plane may overlap and be identical to the common focal plane or may be adjacent to the common focal plane. Adjacent to the common focal plane means that a distance between the common focal plane and the pinhole plane is not more than <NUM>% of the shorter focal length of the first focal length and the second focal length.

A working distance of the imaging lens assembly and/or imaging assembly from the object is a distance suitable and/or recommended for the use of the imaging lens assembly and/or imaging assembly for determining the wavefront of the light wave emitted and/or reflected from the object. When using a telecentric imaging lens assembly, the working distance may essentially correspond to the first focal length. The working distance essentially corresponding to the first focal length means that a possible deviation of the working distance from the first focal length is not more than <NUM>% of the first focal length. Likewise, the detector being arranged from the imaging lens assembly at a distance from the second lens essentially corresponding to the second focal length, which is referred to as the detecting distance, means that a possible deviation of the distance from the second focal length is not more than <NUM>% of the second focal length.

The illumination light may be configured to illuminate at least the area of the surface of the object, which is to be characterized. Optionally the illumination light may exhibit a plane wavefront and/or a continuum of plane waveforms having a continuum of different propagation angles. Alternatively or additionally the illumination light may have a predetermined wavefront, which is well characterized in order to allow retrieving alterations of the wavefront imposed by a reflection of the illumination light off the surface of the object under investigation. For instance, the illumination light may be provided by an illumination light source which may comprise a collimator lens assembly collimating at least partly the illumination light waves. Alternatively or additionally the illumination light may propagate through an angular-selective optical element having a predetermined angular transmission profile in order to provide the illumination light impinging at a predetermined and optionally well characterized angular distribution onto the surface of the object. Hence, the illumination light may comprise a predetermined angular distribution of collimated light waves. Such an angular-selective optical element may for instance comprise or consist of one or more metal interference filters (MIF). Alternatively or additionally, the angular-selective optical element may comprise or consist of one or more dielectric mirrors. The illumination light wave may have a size and a shape that are suitable for entirely and simultaneously illuminating the surface of the object or at least the part of the surface that is to be characterized. For achieving this, the illumination light source may have a lateral extension being at least similar or larger than the (part of the) surface of the object to be characterized. Accordingly, the device may further comprise an illumination light source adapted to provide collimated light waves having a predetermined angular distribution for illuminating at least the surface of the object to be characterized.

The illumination light source may have a transversal extension, i.e. an extension in the dimensions perpendicular to the emission direction, which is at least <NUM>% and optionally at least <NUM>% of the respective dimensions of the part of the surface of the object to be characterized. Optionally, the illumination light source may have a transversal extension which is of the same extension or of a larger extension than the surface of the object to be characterized. The illumination light source may offer a homogeneous brightness over its emission surface. For instance, the illumination light source may comprise an OLED panel and/or a LED panel optionally using an edge-lighting configuration and/or any other light source commonly used for providing a homogeneous lighting, such as a homogeneous background lighting for a LCD display. For instance, the illumination light source may comprise a fiber and a collimator, wherein the fiber emits a divergent light wave which is then at least partly collimated by the collimator. The collimator may have an extension in the dimensions perpendicular to the emission direction, which is at least of the same extension as the part of the surface of the object to be characterized.

A beam splitter may be provided between the illumination light source and the surface of the object to be characterized allowing an illumination of the object with illumination light emitted by the illumination light source and reflecting the light wave(s) reflected off the surface to be characterized of the object into the imaging assembly.

In particular, the illumination light source may comprise an angular-selective optical element transmitting only light waves propagating in a predetermined angular range for isolating collimated light waves having a particular angular range of the wave vectors reflected off the surface. Transmitting in this context may optionally involve one or more reflections off an angular-selective mirror. The illumination light source, thus, provides light waves comprising a continuum of plane wavefronts having a predetermined angular distribution. In an optional embodiment, the continuum of plane light waves may be centered regarding its propagation direction at an optical axis of the illumination light source perpendicular to an emission surface which may be perpendicular to the surface to be characterized of the object.

The detector may comprise a two-dimensional pixel array with multiple pixels. This allows measuring the surface profile of the object with a spatial resolution defined by the pixel density of the pixel array and the ratio between the area of the measured surface and the image surface at the detector. Accordingly, the detector signal data may comprise a detected signal intensity for at least some and optionally for all of the pixels of the two-dimensional pixel array. Thus, each pixel of the pixel array may provide the detector signal data regarding a small portion of the surface of the object to be measured. In particular, the two-dimensional pixel array may be or comprise a CCD array and/or a CMOS array.

Deriving, i.e. characterizing, a surface profile of the surface of the object may comprise determining for each of the pixels of the two-dimensional pixel array the one position out of the multiple positions of the pinhole aperture in the pinhole plane resulting in the maximum detected signal intensity for the respective pixel. Accordingly, the pinhole aperture may be arranged consecutively at multiple different positions in the pinhole plane and transmitting only a small part of the light wave propagating in the imaging assembly, wherein the transmitted part of the light wave depends on the position of the pinhole aperture in the pinhole plane. Each of the multiple positions of the pinhole aperture in the pinhole plane corresponds to a specific angle or angular range of the light wave entering the imaging assembly, as only a light wave entering the imaging assembly under this specific angle or angular range propagates through the position of the pinhole aperture in the pinhole plane which is transmitted by the pinhole aperture. Consequently, arranging the pinhole aperture at specific predetermined positions in the pinhole plane allows selectively isolating light waves entering the imaging assembly at a specific and predetermined angle or angular range correlated with the position of the pinhole aperture in the pinhole plane.

For a surface of the object to be characterized being a non-flat surface, off which the illumination light is reflected, the light wave will propagate into different directions, depending on the angle or slope of the surface at different parts of the surface. The term "non-flat surface" means that the averaged shape of the surface to be characterized is not a plane but any other shape differing from a plane. For instance, a non-flat surface may be a spherical, cylindrical, parabolic or a nonspherical free-form surface. The reflection angle depends on the angle of incidence of the illumination light with respect to the actual surface normal at the position, at which the light hits the surface to be characterized. Hence, for different parts of the surface having different angles, the emitted and/or reflected light wave will propagate through different positions in the pinhole plane before impinging on the detector after passing the imaging assembly at the pixel being correlated with the part of the surface, which emitted and/or reflected the part of the light wave. Hence, by arranging the pinhole aperture at different positions in the pinhole plane, different parts of the light wave propagating through different positions in the pinhole plane may be selected and isolated, while other parts of the light waves may be blocked by the pinhole aperture. When determining the wave front of a light wave reflected off the surface of the object, the surface of the object may be illuminated by illumination light provided with a specific and predetermined angular distribution, which may be achieved by an angular-selective optical filer element. Consequently, the different parts of the surface will generate the maximum signal intensity at the respective pixel at the detector when the pinhole aperture is arranged at different positions in the pinhole plane, depending on the angle or slope of the surface to be characterized at the respective position of the surface. This angular resolution achieved by arranging the pinhole aperture in different positions in the pinhole plane may be used for sampling the angular distribution of the light wave, which corresponds to the shape of its wavefront, emitted and/or reflected from the surface. In particular, this may be achieved by simply arranging the pinhole aperture consecutively in different positions in the pinhole plane and retrieving from the detector for each measurement at each position respective signal data, to determine which pixel and accordingly which correlated part of the surface exhibits its maximum signal strength at the respective position of the pinhole aperture. Accordingly, the method may comprise deriving for each of the pixels an angle or angular range of the wavefront of the light wave emitted from the object at a position corresponding to the respective pixel and/or an angle of the surface of the object at the position corresponding to the respective pixel defined by the position of the pinhole aperture in the pinhole plane resulting in the maximum detected signal intensity for the respective pixel.

Recording and evaluating the detector signal data may be carried out simultaneously for all pixels. This has the benefit that only one scan of the pinhole aperture through all predetermined positions in the pinhole plane may be sufficient for characterizing the wavefront and, thus, the surface to be characterized. Accordingly, the step of determining for each of the pixels the one position out of the multiple positions of the pinhole aperture in the pinhole plane resulting in the maximum detected signal intensity for the respective pixel and the step of deriving for each of the pixels an angle or angular range of the light wave emitted and/or reflected from the object at a position corresponding to the respective pixel and/or an angle of the surface of the object at the position corresponding to the respective pixel defined by the position of the pinhole aperture in the pinhole plane resulting in the maximum detected signal intensity for the respective pixel may be carried out simultaneously.

The imaging assembly may comprise an imaging lens assembly, which may comprise or consist of a telecentric lens assembly. In particular, the imaging lens assembly may satisfy the condition of bilateral telecentricity. This may require introducing an aperture at the common focal plane of both lenses and arranging the imaging lens assembly such that a working distance of the first lens from the object to be measured is equal or close to the first focal length. A telecentric lens assembly provides the advantage that the light wave collected by the image lens assembly has a large beam waist within the imaging lens assembly and in particular in the pinhole plane within the imaging lens assembly. The beam waist may depend on the numerical aperture of the image lens assembly. This allows sampling the wave front of the light wave with the pinhole aperture in the pinhole plane with a high resolution, which translates in a high angular resolution of the obtained measurement.

The extension of an opening of the pinhole aperture in a direction within the pinhole plane may be in a range from <NUM> to <NUM>. This may provide a sufficient angular resolution of the obtained measurement combined with the light wave transmitted through the pinhole aperture being suitable for a sufficient signal-to-noise ratio of the detector signal data. Said opening may have for instance a round, elliptic, or polygonal shape. A round cross-sectional shape of the opening of the pinhole aperture may be beneficial for avoiding undesired distortions of the transmitted part of the wavefront.

The first lens of the imaging lens assembly may have an aperture of at least <NUM> and/or not more than <NUM>. In particular when using a telecentric imaging lens assembly, the aperture of the first lens of the imaging lens assembly may be equal to or larger than the area of the surface of the object to be measured simultaneously. Hence, a larger aperture of the first lens may offer a larger measurement area, which can be covered by the measurement.

Alternatively or additionally to moving a pinhole aperture to multiple predetermined positions in the pinhole plane, the step of consecutively isolating collimated light waves having different angular ranges from the light waves reflected off the surface may comprise rotating and/or turning the object with respect to the imaging assembly. This may be achieved by using a movable and/or rotatable mechanical support for the object.

Alternatively or additionally the consecutively isolating the collimated light waves having different angular ranges may comprise varying the predetermined angular distribution of the collimated light waves of the illumination light, such that an angle of a maximum intensity of the illumination light is varied. This may for instance be achieved by tilting an angular selective optical element.

Alternatively or additionally the consecutively isolating the collimated light waves having different angular ranges may comprise varying a position of an illumination light source providing the illumination light with respect to the surface. This may be achieved by using a movable mechanical support for the illumination light source.

Alternatively or additionally the consecutively isolating the collimated light waves having different angular ranges may comprise varying a position of the imaging assembly with respect to the surface. This may be achieved by using a movable mechanical support for the imaging assembly.

Alternatively or additionally the consecutively isolating the collimated light waves having different angular ranges may comprise varying a position and/or an orientation of an optical component guiding the illumination light source onto the surface with respect to the surface. This may be achieved by moving a beam splitter arranged between the illumination light source and the object.

Accordingly, the means for consecutively isolating collimated light having different angular ranges from the light waves reflected off the object may comprise a device for rotating and/or turning the object with respect to the imaging assembly, and/or varying the predetermined angular distribution of the collimated light waves of the illumination light, such that an angle of a maximum intensity of the illumination light is varied, such as by varying an angular orientation of an angular selective optical element. Alternatively or additionally the device may comprise a device for varying a position of an illumination light source providing the illumination light with respect to the surface, and/or a device for varying a position of the imaging assembly with respect to the surface, and/or a device for varying a position and/or an orientation of an optical component guiding the illumination light source onto the surface, such as a beam splitter, with respect to the surface.

It is understood by a person skilled in the art that the above-described features and the features in the following description and figures are not only disclosed in the explicitly disclosed embodiments and combinations, but that also other technically feasible combinations as well as the isolated features are comprised by the disclosure. In the following, several optional embodiments and specific examples are described with reference to the figures for illustrating the disclosure without limiting the disclosure to the described embodiments.

Further optional embodiments and features will be illustrated in the following with reference to the drawings.

In the drawings the same reference signs are used for corresponding or similar features in different drawings.

<FIG> depicts in a schematic view a device <NUM> according to an optional embodiment for characterizing a surface 12a of an object <NUM>.

The device <NUM> comprises an imaging assembly <NUM>, which is configured as an imaging lens assembly comprising a first lens <NUM> having a first focal length and a second lens <NUM> having a second focal length. The imaging assembly <NUM> may be provided as a closed assembly integrating all parts in a common housing, as indicated by the dashed frame. Alternatively, the imaging assembly <NUM> may be set up by individual components being separate from each other. The first lens <NUM> and the second lens <NUM> are arranged such that their optical axes are identical and such that the distance between the first lens <NUM> and the second lens <NUM> equals the sum of the first focal length and the second focal length. Accordingly, the first lens <NUM> and the second lens <NUM> share a common focal plane <NUM>, which is indicated by a dotted line. As can be seen, the second focal length is shorter than the first focal length. According to the presented embodiment, the first lens <NUM> may have an aperture of <NUM> while the second lens may <NUM> have an aperture of about <NUM>.

The device <NUM> further comprises means for consecutively isolating collimated light waves having different angular ranges from the light waves reflected off the surface of the an object to be characterized. Said means comprise a pinhole aperture <NUM>, which is arranged within the imaging assembly <NUM> between the first lens <NUM> and the second lens <NUM> and optionally in the common focal plane <NUM>. According to other embodiments, the pinhole aperture <NUM> may be arranged apart from the common focal plane <NUM>, for instance in a distance range of ± <NUM>% of the second focal length from the common focal plane <NUM>. The pinhole aperture <NUM> is movable within a pinhole plane and can be arranged at multiple different positions in the pinhole plane, wherein the pinhole plane may be parallel to the common focal plane <NUM> and may overlap with the common focal plane <NUM> when the pinhole aperture <NUM> is arranged in the common focal plane <NUM>. However, the pinhole plane does not necessarily need be fully parallel to the common focal plane <NUM>. Instead, the pinhole plane may be oriented to exhibit a small angle with respect to the common focal plane <NUM>, such as for instance ±<NUM>°.

Moreover, the device <NUM> comprises a detector <NUM> arranged behind (viewed in the direction of light propagation) the imaging assembly <NUM>, i.e. behind (viewed in the direction of light propagation) the second lens <NUM>. The detector <NUM> comprises a two-dimensional array of pixels in order to detect a spatial intensity distribution of the light wave impinging on the detector <NUM>. For example, the detector <NUM> may be or comprise a CCD detector and/or a CMOS detector. Between the imaging assembly <NUM> and the detector <NUM> one or more optional optical filter elements <NUM> may be arranged to filter and/or attenuate the light wave(s) before impinging on the detector <NUM>.

The device <NUM> is adapted such that the imaging assembly <NUM> images the light waves reflected off an upper surface 12a of the object <NUM> to a detection plane of the detector <NUM>. According to the presented embodiment, the imaging assembly <NUM> may be formed as a bilateral telecentric imaging assembly <NUM>, wherein a working distance of the first lens <NUM> from the surface 12a of the object <NUM> along the optical axis (which may be deflected by one or more mirrors and/or beam splitters) of the imaging assembly <NUM> equals the first focal length and a distance of the detection plane of the detector <NUM> from the second lens <NUM> equals the second focal length. Hence, a light wave reflected off the surface 12a of the object <NUM> is at least partly collected by the imaging assembly <NUM> and imaged to the detector <NUM>. When using an imaging assembly <NUM> having a telecentric configuration, the aperture of the first lens <NUM> has to be at least of the same extension as the area of the surface 12a to be measured.

In order to measure a light wave reflected off the surface 12a and, thus, to characterize a surface 12a of an object <NUM>, an illumination light source <NUM> is provided for illuminating the object <NUM>. The illumination light source <NUM> may comprise several components, such as an extended light emitter <NUM>, a collimating lens assembly <NUM> comprising one or more optical lenses and an angular-selective optical element <NUM>. The extended light emitter <NUM> offers a homogeneous emission of light over an extended area perpendicular to the emission axis, which may for instance extend over an area having at least <NUM>% of the size of the surface area of the object <NUM> to be measured. For instance, the extended light emitter <NUM> may comprise or consist of an OLED panel and/or an edge-lighting LED panel, which may have a spatial extension perpendicular to the optical axis of the illumination light source, i.e. the emission axis, which is referred to as the emission axis, being equal to or larger than the surface of the object to be determined. Alternatively or additionally a smaller light emitter may be used in combination with a collimator lens. The light emitter may comprise an optical fiber. The collimating lens assembly <NUM> may be adapted to at least partly collimate the light waves emitted by the light emitter <NUM> towards the emission axis. The angular-selective optical element <NUM> has an angle-dependent transmission profile, which may follow a Gaussian function having its maximum at a <NUM>° incidence angle. Hence, the angular-selective optical element <NUM> allows providing an illumination light having a predetermined angular distribution having its maximum at an angle of <NUM>°. The illumination light may, thus, be described as a continuum of plane wavefronts having an angular distribution defined by the angular-selective optical element <NUM>. The angular-selective optical element <NUM> may, for example, comprise an angular-selective optical metal interference filter.

In order to allow illuminating the object <NUM> and measuring the reflected light wave at the same time, the device <NUM> comprises a beam splitter <NUM> arranged between the illumination light source <NUM> and the object <NUM>. The beam splitter <NUM> is adapted to transmit at least partly the illumination light from the illumination light source <NUM> towards the object <NUM> and to reflect the light wave reflected from the surface 12a towards the imaging assembly <NUM>. According to the presented embodiment, the optical axis of the illumination light source <NUM> and the optical axis of the imaging assembly <NUM> are oriented under an angle of <NUM>° with respect to each other and the beam splitter <NUM> is arranged at a <NUM>° angle in between.

Moreover, the device <NUM> comprises a control unit <NUM>, which exhibits one or more communication connections with the detector <NUM>, the movable pinhole aperture <NUM> and the illumination light source <NUM>. The control unit <NUM> according to the presented embodiment is configured to arrange the pinhole aperture <NUM> consecutively at multiple predetermined positions in the pinhole plane, as well as to receive the detector signal data for each of the consecutive positions of the pinhole aperture <NUM> in the pinhole plane. The control unit <NUM> may then derive the surface profile of the surface 12a of the object <NUM> based on the detector signal data and the corresponding predetermined positions of the pinhole aperture <NUM> in the pinhole plane 22a (see <FIG>), which correspond to respective light waves having different angular ranges of the light waves reflected off the surface 12a.

The boundary lines <NUM> and <NUM> indicate a visualization of the boundaries of light waves transmitted through and collected by the device <NUM>. As such, the boundary lines <NUM> and <NUM> indicate the maximum area of the surface 12a to be measured with the device <NUM> in one configuration, i.e. without moving the object <NUM> with respect to the device <NUM> or vice versa.

Line <NUM> indicates an arbitrary light ray being part of the illumination light and visualizing a beam path through the illumination light source <NUM>, the beam splitter <NUM>, the reflection off the surface 12a, the deflection by the beam splitter <NUM> and the imaging to the detector <NUM> by the imaging assembly <NUM>. The exemplarily presented light ray is emitted at such an angle by the illumination light source <NUM> that it satisfies an angular condition, i.e. its wave vector is in a suitable angular range, to be transmitted through the pinhole aperture <NUM> in the pinhole plane <NUM>, while many other light rays (not shown) being emitted and/or reflected at a different angle from the surface 12a may not satisfy this angular condition and consequently are blocked by the pinhole aperture <NUM>. Thus, the pinhole aperture <NUM> and its position in the pinhole plane allow selecting light rays emitted and/or reflected in a predetermined specific angular range and, hence, to isolate a predetermined specific range of plane wavefronts from the continuum of plane wavefronts provided as illumination light.

A more detailed illustration of the working principle of the device and a method for characterizing a wavefront emitted and/or reflected from an object <NUM> is presented in the following with reference to <FIG>. These figures present a simplified schematic illustration of the device <NUM> discussed above with reference to <FIG>, wherein the illumination of the object <NUM> is omitted. Accordingly, the illumination light source <NUM> and the beam splitter <NUM> are not shown in <FIG>. However, it is understood by a person skilled in the art that the working principle may be applied to measuring light waves emitted and reflected from the surface 12a alike.

<FIG> identifies the components of the device <NUM> and illustrates their relative arrangement. As explained with reference to <FIG> above, the device <NUM> comprises an imaging assembly <NUM> comprising a first lens <NUM> and a second lens <NUM>. The device <NUM> further comprises a detector <NUM>. The object <NUM> having a surface 12a is illustrated on the left-hand side.

The first lens <NUM> has an aperture 16a having at least the same extension as the area of the surface 12a to be measured. The first lens <NUM> further has a first focal length f1 and the second lens <NUM> has a second focal length f2. The imaging assembly <NUM> is arranged such that the first lens <NUM> is arranged at a working distance from the surface 12a which equals the first focal length f1. The distance between the first lens <NUM> and the second lens <NUM> equals the sum of the first focal length f1 and the second focal length f2. The detector <NUM> is arranged behind the imaging assembly <NUM> at a detecting distance from the second lens <NUM> which equals the second focal length f2. Furthermore, a pinhole aperture may be introduced in the common focal plane of the first lens <NUM> and the second lens <NUM> (see <FIG>). Accordingly, the device <NUM> is configured and arranged to satisfy a bilateral telecentricity condition.

<FIG> illustrates that not all light rays collected by the first lens <NUM> will be transmitted through the second lens <NUM>. However, as illustrated in <FIG>, all light rays that are transmitted also through the second lens <NUM> and originating in the same point on the surface 12a will be focused to the same point at the detector <NUM>. The boundary lines <NUM> and <NUM> in <FIG> indicate the boundary rays, which can be collected and imaged by the imaging assembly <NUM>. It is apparent from the boundary rays <NUM> and <NUM> that the aperture 18a of the second lens <NUM> limits the total numerical aperture of the imaging assembly <NUM>. Even if the first lens <NUM> could collect further light rays having a larger angle with respect to the optical axis <NUM> than the boundary light rays <NUM> and <NUM>, they would not be transmitted through the second lens <NUM>.

<FIG> depicts the light rays <NUM> originating in a different point on the surface 12a. Also all light rays <NUM>, which originate in this point and are within an angular range, i.e. an angular cone, with respect to the optical axis <NUM> to be imaged by the imaging assembly <NUM> are focused onto the same spot at the detector <NUM>. Hence, all light rays originating in the same point on the surface 12a and being imaged by the imaging assembly <NUM> are focused onto the same point at the detector <NUM>.

So far, the angular acceptance range of the imaging assembly <NUM> is very large, as all light rays originating in different points on the surface 12a in a large angular cone are collected and imaged by the imaging assembly <NUM>. The detector signal data retrieved by the detector <NUM> does not allow a differentiation between different angles of the imaged light rays with respect to the optical axis <NUM>.

Angular information can, however, be retrieved by using the pinhole aperture <NUM>. <FIG> depicts the device <NUM> having a pinhole aperture <NUM> inserted in a pinhole plane 22a indicated by a dotted line. According to the presented embodiment the pinhole plane 22a overlaps with the common focal plane <NUM> (see <FIG>). The pinhole aperture <NUM> is movable and may be arranged at multiple different positions in the pinhole plane 22a, wherein each of the different positions corresponds to a different angular range of the light waves imaged to the detector <NUM> by the imaging assembly. According to the configuration presented in <FIG>, the pinhole aperture <NUM> is arranged at a position intersecting with the optical axis <NUM> of the imaging assembly <NUM>, i.e. in a centered position. The center light rays <NUM> represent the boundary light rays of a cone of light rays originating in a point at the surface 12a of the object <NUM>, which are able to pass the pinhole aperture <NUM>, while light rays originating in the same point at the surface 12a having a larger angle with respect to the optical axis <NUM> are blocked by the pinhole aperture <NUM>.

Compared to the light rays <NUM> originating in the very same point at the surface 12a shown in <FIG>, the light cone is significantly smaller and, thus, the smaller angular range transmitted through the pinhole aperture <NUM> results in a significant limitation of the angular range imaged to the detector <NUM>. This not only applies to light rays originating in a point at the surface 12a intersecting the optical axis <NUM>. As visualized by the light ray <NUM> at a different point at the surface 12a in an outer region of the surface 12a, the same applies to light rays originating in off-axis points of the surface 12a. Consequently, the pinhole aperture <NUM> located at a specific position in the pinhole plane 22a in or close to the common focal plane <NUM> allows adjusting the angle or angular range of the light waves to be imaged by the imaging assembly <NUM> to the detector <NUM>.

<FIG> further illustrates the case in which the pinhole aperture <NUM> is arranged in a centered position at the optical axis <NUM> resulting in all light rays <NUM> parallel to the optical axis <NUM> originating in all points of the measured surface 12a being transmitted and imaged by the imaging assembly <NUM> onto the detector <NUM>. Each point at the surface 12a is imaged to a corresponding point at the detector <NUM>. The light rays <NUM> represent a light wave having a plane wavefront perpendicular to the optical axis <NUM>. Hence, when arranging the pinhole aperture <NUM> in a centered position at the optical axis <NUM>, the detector <NUM> will receive a maximum signal at these points, which represent an image of points at the surface 12a emitting (or reflecting) a light ray parallel to the optical axis <NUM> given that the illumination light has its maximum intensity at <NUM>° with respect to the emission axis. From this detector signal data the surface profile of the surface 12a at these points may be derived. For instance, when illuminated with illumination light having a plane wavefront propagating parallel to the surface normal of the surface 12a, the sensor signal data may indicate that the surface 12a is perpendicular to the optical axis <NUM> at the points in which the light rays result in a maximized signal.

<FIG> depicts a case in which the pinhole aperture <NUM> is arranged in a position further apart from the optical axis <NUM>. In this position, the pinhole aperture <NUM> transmits only such light waves <NUM>, which propagate at an angle different from <NUM>° with respect to the optical axis <NUM>, for instance at an angle of <NUM>°. This corresponds to a plane wavefront propagating at said angle of <NUM>°. Only for this wavefront propagating at said angle and, depending on the size of an opening of the pinhole aperture corresponding to an angular range around said propagation direction, the light rays <NUM> are transmitted and imaged by the imaging assembly <NUM> to the detector <NUM>. Consequently, in this arrangement of the pinhole aperture <NUM>, those points of the surface 12a will provoke a maximum signal at the detector <NUM>, which emit (or reflect) the light rays at the given angle with respect to the optical axis <NUM>, given that the illumination light has its maximum intensity at <NUM>° with respect to the emission axis.

Hence, a specific propagation angle or angular range can be attributed to each specific position of the pinhole aperture <NUM> in the pinhole plane 22a. In the same manner, a specific propagation angle or angular range can be attributed to each of the multiple positions of the pinhole aperture <NUM> in the pinhole plane 22a. Consequently, by arranging the pinhole aperture <NUM> at different positions in the pinhole plane 22a an angular selectivity of the signal measured by the detector <NUM> can be achieved. This allows measuring the angular distribution of the light waves reflected off the surface 12a and, hence, allows characterizing a surface profile of the surface 12a of the object <NUM>, given that the illumination light has its maximum intensity at a known angle with respect to the emission axis.

For retrieving detailed information on the wavefront and/or the surface profile of the surface 12a, a detailed angular resolution may be achieved by arranging the pinhole aperture <NUM> in multiple specific and predetermined positions in the pinhole plane 22a and, thus, isolate collimate light waves having different angular ranges from the light waves reflected off the surface 12a.

In the following an exemplary process for characterizing a surface 12a and in particular deriving a surface profile of the surface 12a based on the detector signal data and the respective different angular ranges of the consecutively isolated collimated light waves based on the multiple positions of the pinhole aperture <NUM> und the pinhole plane 22a is schematically described for one point at the surface 12a.

<FIG> schematically depicts an illumination light source <NUM> having a light emitter <NUM> and a collimating lens assembly <NUM>. The light emitter <NUM> has a lateral extension d perpendicular to the emission axis <NUM>. The collimating lens assembly <NUM> comprises a lens having a focal length f, a clear aperture D and is arranged along the emission axis <NUM> at a distance equal to its focal length f. Due to the light emitter <NUM> having a lateral extension d, the collimated light waves behind the collimating lens assembly <NUM> have a divergence with respect to the emission axis <NUM>. The full divergence angle is indicated as Θ and the half divergence angle as O/<NUM>. The maximum divergence angle may be calculated from the radius r of the light emitter <NUM>, which corresponds to d/<NUM> and the focal length f of the collimating lens assembly <NUM>: <MAT>.

The maximum divergence angle Θ is a measure for the degree of collimation.

According to the disclosure, the maximum divergence angle Θ of the illumination light source <NUM> shall be larger than the maximum acceptance angle of the imaging assembly <NUM>.

As described above, the illumination light source <NUM> is adapted to offer a specific relation between the emission intensity and the divergence angle, i.e. between the angular distribution of the emitted collimated light waves and their intensity. This may be achieved with an angular selective optical filter, such as an interference filter, which offers a transmissivity decreasing for increasing divergence angles.

In the discussed example, the collimating lens assembly <NUM> is assumed to have a focal length f of <NUM>, the full divergence angle Θ is assumed to be <NUM>°, the radius r of the lateral extension d of the light emitter <NUM> is assumed to be <NUM>,<NUM> and the angular selective optical element is chosen to have a transmissivity decreasing with increasing divergence following the mathematical function <NUM>-|α|<NUM>, wherein α indicates the propagation angle of the light wave with respect to the optical axis of the angular selective optical element.

<FIG> schematically indicates the intensity of the illumination light impinging on an arbitrary point of the surface 12a in dependence of the angle. The horizontal axis indicates the angle in degree and the vertical axis the normalized relative intensity. As can be seen, the angular intensity distribution has its maximum value at an angle of zero degree with respect to the emission axis <NUM> and the intensity decreases for angles deviating from the emission axis <NUM> reaching zero at about +<NUM>° and -<NUM>°.

Characterizing the surface includes characterizing the angular slope of the surface at each points on the surface 12a to be characterized. Accordingly, the slope is to be determined. The relative intensity of the light reflected off a specific point on the surface 12a strongly depends on the slope of the surface 12a at said point, which, hence, changes the relative angular intensity distribution as shown in <FIG>. In <FIG> also the horizontal axis indicates the angle in degree and the vertical axis indicates the normalized relative intensity. In contrast to the impinging intensity of the illumination light, as shown in <FIG>, the maximum intensity of the reflected light is at an angle of about -<NUM> ° and drops to zero at about -<NUM>° and <NUM>°.

The reflected light wave is collected by the imaging assembly <NUM> and imaged to the detector <NUM>. The light wave reflected off each specific point at the surface 12a is imaged to a specific point at the detector <NUM>. As the acceptance angle of the imaging assembly <NUM> is smaller than the full divergence angle Θ of the illumination light source <NUM> including the angular selective optical element, only light waves of a smaller angular range are imaged by the imaging assembly <NUM> to the detector <NUM>. The imaging assembly <NUM> may have a pinhole aperture <NUM> providing an acceptance angle of <NUM>,<NUM>°. When the pinhole aperture <NUM> is positioned at a center position at the optical axis of the imaging assembly <NUM>, the intensity of the reflected light wave outside an angular range from -<NUM>,<NUM>° to +<NUM>,<NUM>° is blocked by the pinhole aperture <NUM>. The intensity of the light wave after the pinhole aperture <NUM> is depicted in <FIG>, wherein the horizontal axis indicates the angle in degree and the vertical intensity the relative intensity. As can be seen, the transmitted intensity is very low compared to the total intensity of the light wave reflected at the specific point of the surface 12a (see <FIG>). The pixel of the detector <NUM> detecting said imaged light wave transmitted through the pinhole aperture <NUM> integrates the detected light wave and, thus, provides a detector signal data for said pixel being proportional to the area under the graph in <FIG>. In the presented case this corresponds to a value of <NUM>,<NUM> for the detector signal data. This value can be plotted in a respective graph, as shown in <FIG>, which indicates the angle in degree at the horizontal axis and the relative (integrated) intensity of the detector signal data at the vertical axis. The angle can be converted in a lateral displacement of the pinhole aperture <NUM> in millimeters.

The pinhole aperture <NUM> may then be consecutively arranged at multiple different positions in the pinhole plane 22a and respective detector signal data may be taken for the very same pixel (and every other pixel likewise) at each of the multiple positions. According to the presented example, all positions are located along one dimension. Each position of the pinhole aperture <NUM> in the pinhole plane 22a corresponds to a specific angular range of the light wave reflected off the surface 12a. As the size of the pinhole aperture <NUM> remains constant, the acceptance angle also stays constant. However, for the different positions the transmitted angular range varies. This allows measuring the transmitted intensity for multiple different angular ranges by arranging the pinhole aperture <NUM> at multiple different positions, as shown in <FIG>. Again, the horizontal axis indicates the angle in degree and the vertical axis the relative (integrated) intensity of the detector signal data. These data points can be interpolated, as shown in <FIG> and the maximum may be determined, as depicted in <FIG>.

In the presented case, the interpolation and determination of the maximum results in a determined angle of -<NUM> ,<NUM>°, which corresponds to a slope of the surface 12a at the characterized point of half of the angle, which is about - <NUM>,<NUM>° at a preset value of <NUM>,<NUM>°. The deviation between the measured value and the present value may be reduced by using a larger number of interpolation points. The interpolation function does not use the original function (intensity over angle) but merely requires that there is a maximum. The maximum shall be well pronounced, for which the angular selective optical element is used. With a more complex interpolation the function "intensity over angle" may support the interpolation function.

The presented simplified explanation relates to one single measurement point and one sensor pixel in one dimension. The method may be carried out for all measurement points at the surface 12a and all sensor pixels for both dimensions X and Y. From the slopes X and Y for all measurement points the surface profile of the surface 12a can be derived.

<FIG> schematically shows an exemplary grid-like layout of multiple positions <NUM> in the pinhole plane 22a, at which the pinhole aperture <NUM> may be arranged during the characterization of the surface 12a. According to an optional embodiment, the pinhole aperture <NUM> may be consecutively arranged in all of the multiple positions <NUM> defined in the layout. The positions <NUM> of the layout are all arranged in the pinhole plane 22a. The numbers given on the axes are provided in arbitrary units, wherein the +<NUM> and -<NUM> may relate to the maximum deflection from the optical axis, while the position <NUM> at the coordinates (<NUM>;<NUM>) may be located at the optical axis of the imaging assembly <NUM>. However, also other configurations of arrangements are possible. Other configurations may for instance vary in terms of number, pattern and/or spacing of the defined positions. Moreover, the pattern does not necessarily be regular and/or symmetric. It may be sufficient to provide the positions <NUM> in a well-known manner. The spacing of the positions <NUM> may vary depending on the size of an opening of the pinhole aperture <NUM> and the numeric aperture of the imaging assembly <NUM>.

<FIG> illustrates the positions of the pinhole aperture <NUM> of <FIG> together with a measure of the signal intensity obtained for an arbitrary pixel of the detector <NUM>, which represents a point in the image plane of the imaging assembly <NUM>. The intensity information is coded in a relative brightness of the spots <NUM>, wherein brighter spots indicate a higher signal strength than darker spots. On the righthand side a signal strength scale is provided in arbitrary units. A measurement may comprise arranging the pinhole aperture <NUM> in each of the predetermined positions <NUM> and registering the signal strength of each of the detector's <NUM> multiple pixels. Thus, for each of the pixels an information corresponding to the signal strength map as shown in <FIG> may be retrieved.

The retrieved values for the signal strength may be interpolated (as described above) to achieve a continuous function of the signal strength distribution, as exemplarily shown in <FIG>. This may facilitate determining the maximum of the signal strength detected for the respective pixels among all the signals measured at different pinhole positions <NUM>. The position of the determined maximum <NUM> then provides the information, at which of the pinhole positions <NUM> the respective pixel or point at the detector <NUM> has its maximum signal intensity, which allows calculating the propagation angle of the light wave originating the correlated point in the object plane, i.e. at the surface 12a. This determination can be carried out for each pixel of the detector <NUM> independently based on only one measurement run, in which the pinhole aperture <NUM> is arranged in each position <NUM> only once. Needless to say that several measurement runs and averaging may be carried out for improving a quality of the obtained signal and information.

Claim 1:
Method for characterizing a surface (12a) of an object (<NUM>), the method comprising:
- illuminating the surface (12a) of the object (<NUM>) with a predetermined illumination light, wherein the illumination light comprises a predetermined angular distribution of collimated light waves;
- collecting the light waves reflected off the surface (12a) with an imaging assembly (<NUM>);
- consecutively isolating collimated light waves having different angular ranges from the light waves reflected off the surface (12a), wherein the light waves having different angular ranges are isolated in a sequential manner;
- detecting the isolated light waves and providing detector signal data for each of the consecutively isolated light waves with a detector (<NUM>) arranged at a detecting distance from the imaging assembly (<NUM>); and
- deriving a surface profile of the surface (12a) based on the detector signal data and the respective different angular ranges of the consecutively isolated collimated light waves.