Optical measurement device and method

Abstract

The present invention relates to the field of optical measuring devices, more particularly to displacement sensors, 3D sensors for measuring the position and / or shape or thickness of a measured object. The measurement light is projected onto the measurement object after being filtered by the Fabry-Perot filter such that, at each point in the measurement plane, the filtered measurement light has a locally unique wavelength or combination of wavelengths in at least one direction in the measurement plane. Measurement light reflected from the surface of the measurement object is also filtered by a Fabry-Perot filter in order to filter out measurement light not reflected from the intersection of the measurement object and the measurement plane.

Description

TECHNICAL FIELD

[0001] The present invention relates to the field of optical measurement devices, more specifically to displacement sensors, 3D sensors for measuring the position and / or shape or thickness of a measurement object. BACKGROUND

[0002] In many displacement and 3D profile sensors known in the art, light is projected onto a measurement object and the light reflected from the surface of the measurement object is measured in order to determine the shape of the object. In many such devices, the angle of incidence of the light on the image sensor is offset from zero. Image sensors such as CCDs and APSs are not designed to operate at such angles and therefore their performance is degraded. In devices based on triangulation, the angle of incidence can be reduced by increasing the triangulation angle; however, this strategy is limited by physical constraints of the system, for example, it is not possible to increase the triangulation angle to 90 degrees because the sensor would occupy the same space as the measurement object. Furthermore, increasing the triangulation angle also has the undesired side effect of increasing the shadow of the reflected light due to height variations of the measurement object. The greater the triangulation angle, the smaller the height variations that cause a shadow. Furthermore, magnification factors that cause a further rotation of the intermediate image plane, if the image sensor is placed on the intermediate image plane, further increase the angle of incidence of the light on the image sensor, which essentially prevents the use of such magnification factors, which otherwise could be useful to improve the accuracy of the sensor. SUMMARY

[0003] A first aspect of the present invention relates to a sensor for measuring the displacement of a surface of a measurement object relative to the sensor. The sensor comprises:

[0004] • a light source configured to emit measurement light;

[0005] • at least one Fabry-Perot filter;

[0006] • first optics configured to focus the measurement light in a measurement plane and to focus the measurement light reflected from the measurement plane at infinity;

[0007] • a light sensor; and

[0008] • second optics for focusing the filtered reflected measurement light on the light sensor.

[0009] The measurement light emitted from the light source and incident in the measurement plane as well as the reflected measurement light from the measurement plane is filtered by the at least one Fabry-Perot filter such that at least a portion of the measurement light reflected from outside the measurement plane is filtered out from the reflected measurement light incident on the light sensor.

[0010] An illumination axis extends from the light source to the measurement plane and a measurement axis extends from the measurement plane to the light sensor, and a co-axial portion of the illumination axis and a co-axial portion of the measurement axis can be co-axial adjacent to the measurement plane.

[0011] The at least one Fabry-Perot filter can be located on the co-axial portion and can be tilted with respect to the co-axial portion.

[0012] The at least one Fabry-Perot filter can include two Fabry-Perot filters, a first Fabry-Perot filter of the two Fabry-Perot filters positioned on the illumination axis outside the co-axial portion of the illumination axis, a second Fabry-Perot filter of the two Fabry-Perot filters can be positioned on the measurement axis outside the co-axial portion of the measurement axis. An angle of the first Fabry-Perot filter with respect to the illumination axis can be equal to an angle of the second Fabry-Perot filter with respect to the measurement axis.

[0013] The measurement plane can be located within a focal plane of the first optics.

[0014] The light sensor can be located within a focal plane of the second optics.

[0015] The sensor can further include a beam splitter or a beam splitting aperture located between the light source and the at least one Fabry-Perot filter, such that at least a portion of the measurement light reflected from the measurement plane is transmitted or reflected toward the light sensor.

[0016] The first optics can include a first optical subset, a diffraction grating, and a second optical subset. The diffraction grating can be positioned in a focal plane of the first optical subset, and the measurement light diffracted from the diffraction grating can be focused in the measurement plane by the second optical subset.

[0017] The measurement plane and the diffraction grating can be tilted with respect to a lens plane of the second optical subset according to the Scheimpflug principle.

[0018] The reflected measurement light from the measurement plane can be focused on the diffraction grating by the second optical subset.

[0019] The diffraction grating can be a first diffraction grating, and the first optics can further include a mirror reflector, a second diffraction grating, and a third optical subset. The measurement light from the light source can be incident on a first side of the measurement plane, the reflected measurement light received from a second side of the measurement plane by the second optical subset can be focused on the second diffraction grating by the second optical subset, the second diffraction grating can be positioned in a focal plane of the third optical subset, such that the third optical subset focuses the measurement light diffracted from the second diffraction grating to infinity, and

[0020] The mirror reflector can be configured to reflect reflected measurement light received from a second side of the measurement plane onto the second diffraction grating or to reflect measurement light diffracted from the first diffraction grating onto the second optical subset.

[0021] The diffraction grating can be a first diffraction grating and the first optical device can further comprise a second diffraction grating, a beamsplitter and combiner, a first reflective surface and a second reflective surface, and a third optical subset. Measurement light from the light source can be incident on the beamsplitter and combiner such that a portion of the measurement light is transmitted by the beamsplitter and combiner and a portion of the measurement light is reflected by the beamsplitter and combiner, the measurement light transmitted by the beamsplitter and combiner can be focused by the first optical subset onto the first diffraction grating, the measurement light diffracted by the first diffraction grating can be reflected from the first reflective surface such that it enters the second optical subset and is focused in the measurement plane. The second diffraction grating can be positioned in a focal plane of the third optical subset, the measurement light reflected by the beamsplitter and combiner can be focused by the third optical subset onto the second diffraction grating, and the measurement light diffracted by the second diffraction grating can be reflected from the second reflective surface such that it enters the second optical subset and is focused in the measurement plane.

[0022] Reflected measurement light from a first side of the measurement plane can be reflected by the first reflective surface onto the first diffraction grating and focused by the second optical subset on the first diffraction grating. Reflected measurement light from a second side of the measurement plane can be reflected by the second reflective surface onto the second diffraction grating and focused by the second optical subset on the second diffraction grating. Reflected measurement light diffracted by the first diffraction grating can be focused at infinity by the first optical subset, reflected measurement light diffracted by the second diffraction grating can be focused at infinity by the third optical subset, and the reflected measurement light diffracted by the first diffraction grating and the reflected measurement light diffracted by the second diffraction grating can be combined by the beamsplitter and combiner such that the combined reflected measurement light is incident on the at least one Fabry-Perot filter.

[0023] When the sensor is in use, the distance from the light sensor to the surface of the measurement object is determined by measuring the position of one or more local intensity maxima of the light received at the light sensor.

[0024] A second aspect of the present invention relates to a method. The method comprises positioning a measurement object at a first position relative to the above-mentioned sensor such that a surface of the measurement object intersects the measurement plane and the light intensity received by the light sensor.

[0025] The method can further comprise repositioning the measurement object from the first position to a second position relative to the sensor, the change in position of the measurement object being defined by a first displacement vector, and measuring the light intensity received by the light sensor.

[0026] The method can further comprise determining the displacement of a first set of one or more points on the surface of the measurement object by identifying the position of one or more light intensity peaks measured by the light sensor when the measurement object is in the first position.

[0027] The method can further comprise determining the displacement of a second set of one or more points on the surface of the measurement object by identifying the position of one or more light intensity peaks measured by the light sensor when the measurement object is in the second position.

[0028] The method can further comprise combining the displacement of the first set of one or more points with the displacement of the second set of one or more points and the first displacement vector to generate a three-dimensional model of the measurement object.

[0029] The method can further comprise determining the thickness of a transparent layer of the measurement object by calculating the distance between at least two different light intensity peaks on the light sensor.

[0030] A third aspect of the present invention relates to the use of the above-mentioned sensor for measuring the displacement of a surface of a measurement object relative to the sensor, measuring the profile of a measurement object, measuring the three-dimensional shape of a measurement object and / or measuring the thickness of a transparent layer of a measurement object. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1 Schematic diagram of a first optical sensor employing a beam splitter.

[0032] Figure 2 Schematic diagram of a first optical sensor employing a beam-splitting aperture.

[0033] Figure 3 Schematic diagram of a second optical sensor employing a beam splitter.

[0034] Figure 4 Schematic diagram of a second optical sensor employing a beam-splitting aperture.

[0035] Figure 5 Schematic diagram of a third optical sensor.

[0036] Figure 6 Schematic diagram of a fourth optical sensor. DETAILED DESCRIPTION

[0037] The present invention relates to apparatuses, systems and methods for measuring displacement of an object relative to a sensor. Such displacement measurements can be used to determine the position, shape and / or thickness of a measured object or layers thereof. In the sensor of the present invention, measurement light is filtered using a Fabry-Perot filter before being projected onto the measured object, such that at each point in the measurement plane intersecting the measured object, the measurement light has a locally unique wavelength or combination of wavelengths in the direction of the measurement. Measurement light reflected from the surface of the measured object is then filtered again by the Fabry-Perot filter to filter out or attenuate measurement light that has not been reflected from the measurement plane. In this context, "filtering out" or "attenuating" does not mean that the measurement light reflected from outside the measurement plane is completely removed, but only that its intensity is reduced. As explained below, a high-finesse Fabry-Perot filter can more effectively filter out measurement light that has not been reflected from the measurement plane, which can enable more accurate measurements, but the present invention is still effective even with one or more relatively low-finesse Fabry-Perot filters.

[0038] The basic operation of the sensor and method for measuring displacement of an object relative to a sensor is described in relation to claim 1, but the basic principles of using a Fabry-Perot filter as described above are the same in all embodiments of the present invention.

[0039] Figure 1 The sensor 100 in FIG. 1 comprises a light source 101 configured to emit polychromatic measurement light. The emitted measurement light has a relatively narrow bandwidth, or can emit light across a wide spectrum, provided that the spectrum is wide enough to ensure that light can pass through the first Fabry-Perot filter 104 (explained in more detail below) at a range of angles. Although the term "measurement light" is used throughout this specification, it will be understood that the apparatus is not limited to electromagnetic radiation in the visible wavelength range, but can also or alternatively include infrared, ultraviolet or other wavelengths depending on the particular application.

[0040] The measurement light emitted by the light source 101 is incident on the first Fabry-Perot filter 104. The light source 101 is preferably a diffuse area light source that emits light at a range of angles from every point on its surface. In this way, when the measurement light is incident on the first Fabry-Perot filter 104, it is incident at a range of angles. The light source 101 can be a single light source or a plurality of light sources. The light source 101 can emit light across the entire area, or can comprise a series of parallel lines of light emission.

[0041] Herein, the term "Fabry-Perot filter" preferably refers to an etalon with a fixed distance between its reflectors, although other types of Fabry-Perot filters, such as tunable interference filters or interferometers with a tunable distance between reflectors, may also be used. The wavelength of light transmitted by a Fabry-Perot filter is defined by the distance l between the reflectors, the refractive index n of the material between the reflectors, and the angle of incidence θ of the light on the Fabry-Perot filter. A transmission peak occurs when the optical path distance 2nlcosθ of the light reflected between the reflectors is an integer multiple of the wavelength λ of the incident light. Therefore, for a fixed refractive index n and a fixed distance l between the reflectors, the wavelength of light transmitted by the Fabry-Perot interferometer depends on the angle of incidence θ. Therefore, when measurement light is incident on the first Fabry-Perot filter 104, it is incident at a range of wavelengths, so that some light incident on the first Fabry-Perot filter 104 at different angles is not completely filtered by the first Fabry-Perot filter 104.

[0042] Since the incident angle of the measuring light on the Fabry-Perot filter and the exit angle of the filtered light are substantially the same, all parallel lights filtered by the first Fabry-Perot filter 104 have the same wavelength or wavelength combination of light.

[0043] like Figure 1 As shown, the normal vector of the first Fabry-Perot filter 104 is offset by an angle 131 from an illumination axis 121 extending from the light source 101 to the measurement plane 152 .

[0044] The filtered light is focused into the measurement plane 152 by the first optical device 105. Figure 1 In the embodiment of the present invention, the first optical device 105 is a lens, but as Figure 3 As depicted, for example, the first optical device may alternatively be a composite optical device (such as a lens assembly). In all cases, the functionality of the first optical device is the same: focusing light filtered by the first Fabry-Perot filter into the measurement plane. Because all parallel light is focused at the same point in the focal plane of the first optical device (i.e., measurement plane 152), and because all parallel light has the same wavelength or wavelength combination due to filtering by the Fabry-Perot filter, all measurement light focused at a given point in measurement plane 152 has the same wavelength or wavelength combination.

[0045] The measurement light filtered by the first Fabry-Perot filter 104 and focused by the first optical device 105 is reflected from the surface of the measurement object 151. The intensity of the reflected light is maximum at the point where the surface of the measurement object 151 intersects the measurement plane 152, i.e. where the measurement light is focused on the surface of the measurement object 151. The measurement light is typically scattered from the surface of the measurement object 151, i.e. diffusely reflected. Some of this reflected measurement light is reflected back towards the first optical device 105. Thus, the measurement light reflected from the measurement plane 152 is focused at infinity by the first optical device 105. In other words, all measurement light reflected from a given point on the measurement plane 152 propagates in parallel after passing through the first lens 105.

[0046] This reflected measurement light focused at infinity by the first optical device 105 is incident on the Fabry-Perot filter 104 again on the opposite side in the opposite direction to the unfiltered measurement light emitted by the light source 101. The Fabry-Perot filter 104 filters out measurement light reflected from any point not within the measurement plane 152. Since all light focused at a given point in the measurement plane 152 has the same wavelength or combination of wavelengths, the light rays reflected from the measurement plane 152 and propagating in parallel from the first optical device 105 towards the Fabry-Perot filter 104 have the same wavelength or combination of wavelengths. Furthermore, the angle at which the measurement light reflected from the measurement plane propagates is equal to the angle at which light of the same wavelength propagates after the first filtering by the Fabry-Perot filter 104. Thus, the light reflected from the measurement plane 152 incident on the Fabry-Perot filter 104 can pass through the second Fabry-Perot filter, whereas light reflected by other parts of the measurement object 151 not intersecting the measurement plane 152 will typically not propagate at an angle suitable to pass through the Fabry-Perot filter 104 and is filtered out by the Fabry-Perot filter 104.

[0047] This arrangement is particularly effective when the wavelength or combination of wavelengths of light focused at each point in the measurement plane is locally unique in at least one direction in the measurement plane. In other words, the wavelength or combination of wavelengths of light focused at each point in the measurement is unique in points along at least one axis lying in the measurement plane. Preferably, this axis is parallel to the z-axis shown in the accompanying drawings. In other words, at each point in the measurement plane having the same y-coordinate, light of a different wavelength or combination of wavelengths is in focus.

[0048] The performance of the optical sensor 100 for measuring the height of the measurement object 152 (i.e., the position on the z-axis at each y-coordinate) is improved when the wavelength or combination of wavelengths of the light focused at each point in the measurement plane 152 is unique among the points having the same y-coordinate. Therefore, preferably, the angle 131 of the Fabry-Perot filter 104 relative to the illumination axis 131 and relative to the total used angular range is set so that at each point in the measurement plane, the wavelength or combination of wavelengths of the focused light is unique among the points having the same y-coordinate. The Fabry-Perot filter 104 is preferably tilted relative to the illumination axis 121 by rotating the Fabry-Perot filter around an axis parallel to the measurement plane and perpendicular to the illumination axis 121. Figure 1 In the drawings, this is the Y axis. The offset angle 131 is measured in the ZX plane, as Figure 1 shown.

[0049] The input angle (ie, incident angle) of light on the first Fabry-Perot filter 102 may also be limited to only positive or negative angles to avoid light of the same wavelength or wavelength combination being transmitted at two different input angles.

[0050] Since the illumination axis 121 and the measurement axis 122 are coaxial in the region between the Fabry-Perot filter 104 and the measurement plane 152, Figure 1 In the embodiment of FIG1 , a beam splitter 103 is used to redirect the reflected measurement light from the coaxial region 123 toward the light sensor 107. Thus, the filtered reflected measurement light exiting the Fabry-Perot filter 104 is incident on the beam splitter 103 and reflected toward the light sensor 107. The sensor lens 106 focuses the reflected measurement light received from the Fabry-Perot filter 104 into the sensor plane, i.e., into the focal plane of the sensor lens 106. Similarly, because the incident angle of the reflected measurement light on the Fabry-Perot filter 104 and the exit angle of the reflected measurement light passing through the Fabry-Perot filter 104 are the same, the rays of reflected measurement light from a single point on the measurement plane 152 have the same wavelength or wavelength combination, propagate in parallel toward the sensor lens 106, and are focused on the same point in the sensor plane.

[0051] The light sensor 107 (e.g., an image sensor) is positioned in the sensor plane so that its active surface is aligned with the sensor plane and the filtered reflected measurement light is focused on the active surface of the light sensor. The light sensor can be an image sensor such as a CCD (charge coupled device), a CMOS (complementary metal oxide semiconductor), or another APS (active pixel sensor) or a line scan camera.

[0052] By measuring the intensity of the light received at each point (e.g., each pixel) on the image sensor, the shape of the measurement object 151 at its intersection with the measurement plane 152 can be determined. In particular, the measurement light reflected from the intersection of the surface of the measurement object 151 with the measurement plane 152 produces a local maximum in intensity on the surface of the light sensor 107. The position of the local intensity maximum on the light sensor 107 can be used to determine the distance from the light sensor 107 to each point on the surface of the measurement object 151 from which the measurement light is focused and reflected (i.e., at the intersection of the measurement plane 152 with the measurement object 151), since each point (e.g., pixel) on the light sensor 107 corresponds to a single point in the measurement plane 152 and the position of the measurement plane 152 relative to the light sensor 107 is known. The position of the measurement plane 152 relative to the light sensor 107 is determined by the optical properties of the optical elements located between the measurement plane 152 and the light sensor 107. For example, in the sensor 100, the position of the measurement plane 152 relative to the sensor 107 is determined by the focal length of the first optical device 105 and the angle of the lens plane of the first optical device 105 relative to the coaxial portion 123 of the illumination axis and the measurement axis, the angle of the coaxial portion 123 of the illumination axis and the measurement axis relative to the surface of the beam splitter 103, the focal length of the sensor lens 106, and the angle of the lens plane of the sensor lens 106 relative to the measurement axis 122. This list is not exhaustive.

[0053] The image sensor can be physically or logically divided into regions, e.g., individual pixels or groups of pixels, each of which is sensitive to only a single wavelength or combination of wavelengths or narrow range of wavelengths or combinations of wavelengths.

[0054] The sensor 100 can include a light source lens 102 that receives light from the light source 101. In this case, the light source 101 can be positioned outside the focal plane of the light source lens 102 so that the measurement light is incident at multiple angles at each point on the Fabry-Perot filter 104 and so that potential small non-uniformities, such as gaps between different individual light source elements, do not result in non-illuminated points on the measurement plane. This can enable the use of smaller or less diffuse light sources, e.g., by placing the light source outside the focal plane of another lens. If the light source is smaller, the angular range of the light source to the condenser lens must be larger in order to achieve the same angular and power distribution in the first Fabry-Perot filter 102.

[0055] The bandwidth of the light source 101 can be limited based on the offset (tilt) angle 131 of the Fabry-Perot filter 104 and the input angle range of the Fabry-Perot filter 104 to ensure local uniqueness of the wavelength or combination of wavelengths focused in the measurement plane and / or the sensor plane

[0056] The limitation of the light source bandwidth can be achieved by using a suitable narrowband light source, such as an LED, or by additional filtering of the light emitted from the wideband light source 101 before it is incident in the measurement plane 152.

[0057] Alternatively, in the case of using a wideband light source, the measurement light can be filtered to the appropriate wavelength range at any point between the light source 101 and the sensor 107 to ensure the local uniqueness of the wavelength or combination of wavelengths focused in the sensor plane.

[0058] In the sensor 100 of Figure 1 The Fabry-Perot filter 104 is located on the coaxial portion 123 of the illumination axis 121 and the measurement axis 122. This enables the use of the same Fabry-Perot filter to filter the measurement light from the light source 101 and the reflected measurement light from the measurement plane 152, and ensures that the angle of inclination of the Fabry-Perot filter is the same for the measurement light from both directions. However, the single Fabry-Perot filter 104 can be replaced by two separate Fabry-Perot filters located on non-coaxial portions of the illumination axis 121 and the measurement axis 122. That is, a first Fabry filter can be positioned on the illumination axis 121 outside the coaxial portion 123 of the illumination axis, and a second Fabry-Perot filter can be positioned on the measurement axis 122 outside the coaxial portion 123 of the measurement axis. In this case, the angle of the first Fabry-Perot filter relative to the illumination axis 121 is equal to the angle of inclination of the second Fabry-Perot filter relative to the measurement axis 122.

[0059] In the case of two Fabry-Perot filters described above, the angle of inclination of each Fabry-Perot filter can be tuned so that the measurement light reflected from the measurement plane 152 can pass through the second Fabry-Perot filter after being filtered by the first Fabry-Perot filter. One or both of the angles of inclination can be adjusted to ensure the maximum transmission of light passing through the second Fabry-Perot filter positioned between the measurement plane and the light sensor 107. Alternatively, in the case of using a Fabry-Perot interferometer or interference filter with a tunable distance between the reflectors, this can be used in addition to or instead of adjusting the relative angles of the Fabry-Perot filters to ensure the correct performance of the device. The use of fixed etalons is preferred because the angle tuning needs to be performed only once, typically during the manufacturing process. As an example, the angle tuning of one or both Fabry-Perot filters can be achieved by adjusting the screws of the angle of the Fabry-Perot filters.

[0060] The reflectivity of the reflective surface within the Fabry-Perot filter determines the width of the Fabry-Perot filter's transmission peak in the frequency (or wavelength) domain. Fabry-Perot filters with narrow transmission peaks (i.e., high Q factors) are considered to have high finesse. The use of high-fineness Fabry-Perot filters improves the accuracy of the sensor of the present invention because the Fabry-Perot filter provides improved filtering of measurement light reflected from outside the measurement plane, resulting in a narrower intensity peak at the light sensor.

[0061] However, in embodiments employing two separate Fabry-Perot filters, the narrower transmission peaks of the Fabry-Perot filters require more precise alignment of the filters to ensure the desired overlap of the transmission peaks. Therefore, as described above, the ability to precisely tune the angle of the Fabry-Perot filters also allows the use of high-finesse Fabry-Perot filters.

[0062] Although Figure 1 The sensor 100 uses a beam splitter 103 to direct the reflected measurement light toward the light sensor 107, but any component or arrangement that allows unfiltered measurement light from the light source to reach the Fabry-Perot filter and diverts the reflected measurement light toward the light sensor may be used. Figure 2 In the example, the beam splitting aperture 201 is used instead of Figure 1 beam splitter 103 . Figure 2 A second optical sensor 200 is depicted, which is similar to that described above with respect to Figure 1 The displacement sensor 100 operates in substantially the same manner as described. Equivalent features of the two devices are indicated by similar reference numerals, for example 101 and 201 both represent light sources as described above. Figure 2 The shaded area shown in indicates a space in which reflected measurement light propagates from the measurement plane 252 to the light sensor 207 , as opposed to an open area in which light propagates from the light source 201 toward the measurement plane 252 .

[0063] It should also be understood that Figure 1 and Figure 2 In the two embodiments depicted in , the arrangements of the illumination axis 121 and the measurement axis 122 can be interchanged so that unfiltered measurement light is reflected from the beam splitter 103 or the beam splitting aperture 203 and the reflected measurement light propagates directly to the sensor lens 106 , 206 and the light sensor 107 , 207 .

[0064] Figure 3 A further sensor 300 is depicted, corresponding to Figure 1sensor 100, where the first optics 105 of the sensor 100 is replaced by first optics 305 in the sensor 300. Equivalent features of the two devices are denoted by similar reference numerals, e.g. 101 and 301 both denote the light source as described above. The first optics 305 of the sensor 300 is a compound optics comprising a diffraction grating 308, a first optical subset 309 and a second optical subset 310. The diffraction grating 308 is positioned in the focal plane of the first optical subset 309 and the measurement light diffracted from the diffraction grating 308 is focused in a measurement plane 352 by the second optical subset 310. The measurement plane 352 and the diffraction grating 308 are tilted with respect to the lens plane of the second optical subset 310 according to the Scheimpflug law. The use of a diffraction grating in this arrangement allows to change the angle of the measurement plane 352 with respect to the coaxial part 323 of the measurement axis.

[0065] The first optical subset 309 can be a simple lens or a compound optics as shown in Figure 3 One function of the first optical subset 309 is to focus the measurement light filtered by the Fabry-Perot filter 304 onto the surface of the diffraction grating 308. Since all parallel rays of light exiting the Fabry-Perot filter 304 have the same wavelength combination, all measurement light focused by the first optical subset 309 onto each point on the surface of the diffraction grating 308 has the same wavelength or wavelength combination.

[0066] The diffraction angle θ m of light from a diffraction grating is determined by the ruling or slit pitch d (also referred to as ruling or slit spacing) of the diffraction grating, the wavelength λ of the incident light and the angle of incidence θ i -sinθ m ) = ±mλ according to the grating equation, where n is the refractive index of the medium between the diffraction grating and the incident light i The angle of incidence θ i and the diffraction angle θ m are defined in opposite directions with respect to a plane that extends parallel to the rulings or slits of the diffraction grating and perpendicular to the planar surface of the diffraction grating, also referred to as grating normal.

[0067] The filtered measurement light is diffracted from the surface of the diffraction grating 308 towards the second optical subset 310. The second optical subset 310 focuses the light diffracted from the surface of the diffraction grating 308 in the measurement plane 352. In other words, for the optical system consisting of the diffraction grating 308, the second optical subset 310 and the measurement plane 352, the diffraction grating 308 is located in the object plane, the lens plane is defined by the second optical subset 310 and the measurement plane 352 is located in the image plane defined by the Scheimpflug law.

[0068] ​The measurement light reflected from the surface of the measurement object 352 is received by the second optical subset 310 and focused onto the surface of the diffraction grating 308. The reflected measurement light diffracted by the diffraction grating 308 is then received by the first optical subset 309, which focuses the reflected measurement light focused on the surface of the diffraction grating 308 at infinity.

[0069] Since the diffraction angle of light from the diffraction grating 308 is not necessarily the same as the angle of incidence, by choosing a suitable diffraction grating and wavelength range of the measurement light, the angle of the measurement plane 352 relative to the coaxial portion 323 of the illumination axis can be changed. In the case of a sensor of the present application being used to measure the three-dimensional shape of a measurement object, the measurement object is moved through a measurement plane parallel to the X-axis shown in the drawings so as to sample its surface at a plurality of X-positions. For a sensor 100 and 200 in which the measurement plane is perpendicular to the coaxial portion 123 of the illumination axis, the coaxial portions 123 of the illumination and measurement axes must be tilted relative to the Z-axis in order for the measurement object to intersect the measurement plane 152, 252 at different Z-positions. This means that large variations in the height of the measurement object will cause a shadow of part of the surface of the measurement object that will never be measured. In contrast, in a sensor 300 in which the coaxial portions 323 of the illumination and measurement axes can be arranged parallel to the Z-axis, high aspect ratio features (i.e. features with large variations in height on the surface of the measurement object) will not cause a shadow. Figure 1 Figure 2 For a sensor 100 and 200 in which the measurement plane is perpendicular to the coaxial portion 123 of the illumination axis, the coaxial portions 123 of the illumination and measurement axes must be tilted relative to the Z-axis in order for the measurement object to intersect the measurement plane 152, 252 at different Z-positions. This means that large variations in the height of the measurement object will cause a shadow of part of the surface of the measurement object that will never be measured. In contrast, in a sensor 300 in which the coaxial portions 323 of the illumination and measurement axes can be arranged parallel to the Z-axis, high aspect ratio features (i.e. features with large variations in height on the surface of the measurement object) will not cause a shadow. Figure 3

[0070] Figure 3 The diffraction grating 308 is depicted as a reflective diffraction grating, but a transmissive diffraction grating could also be used.

[0071] Figure 4 An alternative sensor 400 is depicted in which the beam splitter 303 of the sensor 300 is replaced by a beam splitting aperture 403. Equivalent features of the two devices are denoted by similar reference numerals, e.g. 301 and 401 both denote the light source as described above. Figure 4 The beam splitting aperture 403 of the sensor 400 functions in the same way as described above in relation to the beam splitting aperture 203 of the sensor 200. Figure 2

[0072] Figure 5 A further sensor 500 is shown in which separate Fabry-Perot filters are provided on each of the illumination and measurement axes as described above in relation to the sensor 300. Figure 1 Equivalent features of the sensors 100 and 500 are denoted by similar reference numerals, e.g. 101 and 501 both denote the light source as described above.

[0073] ​​​As in sensors 300 and 400, first optics 505 of sensor 500 is a compound optical system as opposed to a simple lens as shown in Figure 1 The illumination mode of sensor 500 functions in substantially the same way as in sensors 300 and 400. First optical subset 509 can be a simple lens or a compound optic as shown in Figure 5 The function of first optical subset 509 is to focus the measurement light filtered by Fabry-Perot filter 504 onto the surface of diffraction grating 508. Since all parallel rays of light exiting Fabry-Perot filter 504 have the same wavelength combination, all measurement light focused by first optical subset 509 onto each point on the surface of diffraction grating 508 has the same wavelength or wavelength combination. The filtered measurement light is diffracted from the surface of diffraction grating 508 towards second optical subset 510. Second optical subset 510 focuses the light diffracted from the surface of diffraction grating 508 in measurement plane 552. In other words, for the optical system consisting of diffraction grating 508, second optical subset 510, and measurement plane 552, diffraction grating 508 is located in the object plane, the lens plane is defined by second optical subset 510, and measurement plane 552 is located in the image plane as defined by the Scheimpflug principle.

[0074] Measurement light reflected from measurement plane 552 is received by second optical subset 510 and focused onto second diffraction grating 511 via specular reflector 514. Specular reflector 514 acts as a beam-splitting aperture, allowing measurement light in the illumination mode to enter second optical subset 510 while diverting reflected measurement light exiting second optical subset 510 towards second diffraction grating 511. The reflected measurement light diffracted by second diffraction grating 511 is then received by third optical subset 512, which focuses the reflected measurement light focused on the surface of second diffraction grating 511 at infinity.

[0075] In an alternative arrangement, specular reflector 514 can instead be positioned such that light diffracted from first diffraction grating 508 is reflected by specular reflector 514 onto second optical subset 510.

[0076] Reflected measurement light exiting third optical subset 512 is incident on second Fabry-Perot filter 516, which is arranged at the same angle of inclination relative to the measurement axis extending from second diffraction grating 511 to photosensor 507 as first Fabry-Perot filter 504 is relative to the illumination axis extending from light source 501 to first diffraction grating 508. In this way, second Fabry-Perot filter 516 filters out or attenuates measurement light reflected from any point not within measurement plane 552.

[0077] As in sensors 300 and 400, first optics 505 of sensor 500 is a compound optical system as opposed to a simple lens as shown in Figure 5As shown, the measurement plane 552 can be aligned with the surface of the first diffraction grating 508, i.e. the measurement plane 552 and the surface of the diffraction grating 508 are parallel. Furthermore, the measurement plane 552 is aligned with the Z-axis. This arrangement prevents most shadowing of the measurement light by high aspect ratio features on the surface of the measurement object 551, but this is not essential.

[0078] Figure 6 The further sensor 600 depicted in Fig. 6 corresponds to Figures 1 to 4 The sensor 100, 200, 300 and 400 depicted in Figs. 1 to 5, as it employs a single Fabry-Perot filter 604 located on the coaxial portion of the illumination and measurement axes. However, similar to the sensors 100, 200, 300 and 400, it will be appreciated that the single Fabry-Perot filter 604 can be replaced by separate Fabry-Perot filters located on the illumination and measurement axes outside the coaxial region. Like all the other sensors depicted in the above figures, the sensor 600 is configured to measure the intensity of the measurement light reflected by the measurement object 551. Figure 1 Like the sensors 100, 200, 300 and 400, the sensor 600 is configured to measure the intensity of the measurement light reflected by the measurement object 551. The sensor 600 is configured to measure the intensity of the measurement light reflected by the measurement object 551 in a similar manner to the sensors 100, 200, 300 and 400, as described above.

[0079] In the sensor 600, the first optical train 605 comprises a first optical subset 609, a first diffraction grating 608, a second optical subset 610, a second diffraction grating 611 and a third optical subset 612, a second diffraction grating (611). The first optical train 605 further comprises a beam splitter and combiner 613, a first reflective surface 614 and a second reflective surface 615. The first reflective surface 614 and the second reflective surface 615 are specular reflectors, and can be part of a single component having a plurality of reflective surfaces as shown, or can be separate reflectors. Figure 6 The first optical train 605 further comprises a first optical path 616 and a second optical path 617. The first optical path 616 extends from the beam splitter and combiner 613 to the first diffraction grating 608, and the second optical path 617 extends from the beam splitter and combiner 613 to the third optical subset 612.

[0080] Measurement light from the light source 601 and filtered by the Fabry-Perot filter 604 is incident on the beam splitter and combiner 613, a portion of the measurement light is transmitted by the beam splitter and combiner 613 along a first optical path towards the first optical subset 609, and a portion of the measurement light is reflected by the beam splitter and combiner along a second optical path towards the third optical subset 612.

[0081] The measurement light transmitted by the beam splitter and combiner 613 along the first optical path is focused by the first optical subset 609 onto the first diffraction grating 608. The measurement light diffracted by the first diffraction grating 608 is then reflected from the first reflective surface 614 such that it enters the second optical subset 610 and is focused on the measurement plane 652. The first diffraction grating 608, the first reflective surface 614, the second optical subset 610, and the measurement plane 652 are arranged according to the Scheimpflug principle such that the first diffraction grating 608 is located in the object plane, the lens plane is defined by the second optical subset 610, and the measurement plane 652 is located in the image plane.

[0082] The measurement light reflected by the beam splitter and combiner 613 along the second optical path is focused by the third optical subset 612 onto the second diffraction grating 611. The measurement light diffracted by the second diffraction grating 611 is then reflected from the second reflective surface 615 such that it enters the second optical subset 610 and is focused on the measurement plane 652. The second diffraction grating 611, the second reflective surface 615, the second optical subset 610, and the measurement plane 652 are arranged according to the Scheimpflug principle such that the second diffraction grating 611 is located in the object plane, the lens plane is defined by the second optical subset 610, and the measurement plane 652 is located in the image plane.

[0083] When the measurement light is reflected from the measurement plane, the measurement light is reflected back toward the second optical subset 610. The reflected measurement light that exits the second optical subset 610 and is reflected from the first reflective surface 614 is focused by the second optical subset 610 onto the surface of the first diffraction grating 608. The reflected measurement light that exits the second optical subset 610 and is reflected from the second reflective surface 615 is focused by the second optical subset onto the surface of the second diffraction grating 611.

[0084] The reflected measurement light focused onto the first diffraction grating 608 is diffracted from the first diffraction grating 608 and into the first optical subset 609, which focuses the reflected measurement light from the measurement plane at infinity. Similarly, the reflected measurement light focused onto the second diffraction grating 611 is diffracted from the second diffraction grating 611 and into the third optical subset 612, which focuses the reflected measurement light from the measurement plane at infinity.

[0085] The measurement light reflected from the measurement plane and focused by the first optical subset 609 and the third optical subset 612 is combined by the beam splitter and combiner 613 and propagated toward the Fabry-Perot filter 604. The reflected measurement light incident on the Fabry-Perot filter 604 is filtered by the Fabry-Perot filter such that only the measurement light reflected from the measurement plane 652 is transmitted.

[0086] The other components of the sensor 600 work in the same manner as described above with respect to Figures 1 to 4 the sensor 500.

[0087] Importantly, the first and second optical paths have the same path length, as the measurement light is split and recombined after reflection from the measurement plane 652.

[0088] Any of the above displacement sensors can be used as a three-dimensional sensor for measuring the three-dimensional shape of a measurement object. By imaging the measurement light projected onto the measurement object at multiple locations on the measurement object, a three-dimensional model of the measurement object can be constructed. In practice, the displacement measurements are repeated or continuous as the measurement object is moved through the measurement plane, which can be achieved by moving the sensor relative to the stationary measurement object or by moving the measurement object relative to the sensor, for example on a conveyor belt. Each measurement can be seen as a profile of a cross-sectional slice of the measurement object, and the three-dimensional shape of the object can be reconstructed from these profile measurements by combining them with the known displacement between each measurement.

[0089] The sensor can also be used for multi-layer measurements, for example for measuring the thickness of a transparent film. Reflection of the measurement light from the surface of each layer of the transparent film produces distinguishable intensity peaks, and when the refractive index of the layers is known, the thickness can be calculated based on the distance between two subsequent peaks on the light sensor.

[0090] Furthermore, in case the light sensor is an image sensor such as a CCD or an APS such as a CMOS sensor, the light sensor can also capture regular 2D images of the surface of the measurement object while measuring the displacement as described above.

Claims

1. A sensor for measuring the displacement of a surface of an object relative to the sensor, the sensor comprising: - a light source (101, 201, 301, 401, 501, 601) configured to emit measurement light; - at least one Fabry-Perot filter (104, 204, 304, 404, 504, 512, 604, 612); - a first optical device (105, 205, 305, 405, 505, 605) configured to focus the measurement light in a measurement plane and to focus the measurement light reflected from the measurement plane at infinity; - a light sensor (107, 207, 307, 407, 507, 607); and - a second optical device (106, 206, 306, 406, 506, 606) for focusing the filtered reflected measurement light onto the light sensor; The measurement light emitted from the light source and incident in the measurement plane and the reflected measurement light from the measurement plane are filtered by the at least one Fabry-Perot filter, so that at least a portion of the measurement light reflected from outside the measurement plane is filtered out from the reflected measurement light incident on the light sensor.

2. A sensor according to any preceding claim, wherein an illumination axis (121, 221, 321, 421, 521, 621) extends from the light source to the measurement plane, and a measurement axis (122, 222, 322, 422, 522, 622) extends from the measurement plane to the light sensor, and wherein a coaxial portion of the illumination axis and a coaxial portion of the measurement axis (123, 223, 323, 423, 523, 623) are coaxial adjacent to the measurement plane.

3. The sensor according to any preceding claim, wherein the at least one Fabry-Perot filter (104, 204, 304, 404) is located on the coaxial portion and is tilted relative to the coaxial portion.

4. The sensor of claim 2 , wherein the at least one Fabry-Perot filter ( 504 , 512 , 604 , 612 ) comprises two Fabry-Perot filters, a first Fabry-Perot filter ( 504 , 604 ) being positioned on the illumination axis outside the coaxial portion of the illumination axis, a second Fabry-Perot filter ( 512 , 612 ) being positioned on the measurement axis outside the coaxial portion of the measurement axis, and wherein the angle of the first Fabry-Perot filter relative to the illumination axis is equal to the angle of the second Fabry-Perot filter relative to the measurement axis.

5. The sensor according to any of the preceding claims, wherein the measurement plane (152, 252, 352, 452, 552, 652) is located in the focal plane of the first optics (105, 205, 305, 405, 505, 605).

6. The sensor according to any preceding claim, wherein the light sensor (107, 207, 307, 407, 507, 607) is located in a focal plane of the second optics (106, 206, 306, 406, 506, 606).

7. A sensor according to any preceding claim, wherein the sensor further comprises a beam splitter (103, 303, 603) or a beam splitting aperture (203, 403) located between the light source and at least one Fabry-Perot filter, so that at least a portion of the measurement light reflected from the measurement plane is transmitted or reflected towards the light sensor.

8. A sensor according to any preceding claim, wherein the first optical device (305, 405, 505, 605) comprises a first optical subset (309, 409, 509, 609), a diffraction grating (308, 408, 508, 608) and a second optical subset (310, 410, 510, 610), wherein the diffraction grating is positioned in a focal plane of the first optical subset, and wherein measurement light diffracted from the diffraction grating is focused in the measurement plane by the second optical subset.

9. The sensor of claim 8, wherein the measurement plane (352, 452, 552, 652) and the diffraction grating (308, 408, 508, 608) are tilted relative to the lens plane of the second optical subset (310, 410, 510, 610) according to the principle of Scham's law.

10. The sensor according to claim 8 or 9, wherein reflected measurement light from the measurement plane (352, 452) is focused by the second optical subset (310, 410) onto the diffraction grating.

11. The sensor of claim 8 or 9, wherein the diffraction grating is a first diffraction grating (508), and the first optical device (505) further comprises a specular reflector (514), a second diffraction grating (511), and a third optical subset (512), wherein: The measurement light from the light source (501) is incident on a first side of the measurement plane (552); the reflected measurement light received by the second optical subset from the second side of the measurement plane is focused by the second optical subset onto the second diffraction grating; the second diffraction grating being positioned in a focal plane of the third optical subset such that the third optical subset focuses the measurement light diffracted from the second diffraction grating at infinity; and The specular reflector is configured to reflect reflected measurement light received from a second side of the measurement plane onto the second diffraction grating, or to reflect measurement light diffracted from the first diffraction grating onto the second optical subset.

12. The sensor according to claim 8 or 9, wherein the diffraction grating is a first diffraction grating (608), and the first optical device (605) further comprises a second diffraction grating (611), a beam splitter and combiner (613), a first reflective surface (614) and a second reflective surface (615), and a third optical subset (612), wherein: The measurement light from the light source (601) is incident on the beam splitter and the beam combiner, so that a portion of the measurement light is transmitted by the beam splitter and the beam combiner, and a portion of the measurement light is reflected by the beam splitter and the beam combiner; The measurement light transmitted by the beam splitter and the beam combiner is focused onto the first diffraction grating by the first optical subset; The measurement light diffracted by the first diffraction grating is reflected from the first reflection surface so that it enters the second optical subset (610) and is focused in the measurement plane (652); The second diffraction grating is positioned in the focal plane of the third optical subset, and the measurement light reflected by the beam splitter and the beam combiner is focused by the third optical subset onto the second diffraction grating; The measurement light diffracted by the second diffraction grating is reflected from the second reflection surface so that it enters the second optical subset and is focused in the measurement plane.

13. The sensor according to claim 12, wherein: Reflected measurement light from a first side of the measurement plane (652) is reflected by the first reflection surface (614) onto the first diffraction grating (608) and is focused on the first diffraction grating by the second optical subset (610); Reflected measurement light from the second side of the measurement plane is reflected by the second reflection surface (615) onto the second diffraction grating (611), and is focused on the second diffraction grating by the second optical subset; The reflected measurement light diffracted by the first diffraction grating is focused at infinity by the first optical subset (609); The reflected measurement light diffracted by the second diffraction grating is focused at infinity by the third optical subset (612); and The reflected measurement light diffracted by the first diffraction grating and the reflected measurement light diffracted by the second diffraction grating are combined by the beam splitter and combiner (613) so that the combined reflected measurement light is incident on the at least one Fabry-Perot filter (604).

14. A sensor according to any preceding claim, wherein when the sensor is in use, the distance from the light sensor to the surface of the measurement object is determined by measuring the position of one or more local intensity maxima of light received at the light sensor.

15. A method comprising: positioning the measurement object at a first position relative to a sensor according to any preceding claim such that the surface of the measurement object intersects the measurement plane; as well as The intensity of light received by the light sensor is measured.

16. The method according to claim 15, wherein the method further comprises: The measurement object is repositioned from the first position to a second position relative to the sensor, wherein The change in position of the measurement object is defined by a first displacement vector; as well as The intensity of the light received by the light sensor is measured.

17. The method of claim 15 or 16, wherein the method further comprises determining the displacement of a first set of one or more points on the surface of the measurement object by identifying the positions of one or more light intensity peaks measured by the light sensor when the measurement object is in the first position.

18. The method of claim 17, wherein the method further comprises determining the displacement of a second set of one or more points on the surface of the measurement object by identifying the positions of one or more light intensity peaks measured by the light sensor when the measurement object is in the second position.

19. The method of claim 18, wherein the method further comprises combining the displacements of the first set of one or more points with the displacements of the second set of one or more points and the first displacement vector to generate a three-dimensional model of the measurement object. 20 . The method of claim 16 , further comprising determining a thickness of the transparent layer of the measurement object by calculating a distance between at least two different light intensity peaks on the light sensor.

21. Use of the sensor according to any one of claims 1 to 14 for measuring the displacement of a surface of a measurement object relative to the sensor, measuring the contour of the measurement object, measuring the three-dimensional shape of the measurement object and / or measuring the thickness of a transparent layer of the measurement object.

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