CHROMATIC CONFOCAL MEASURING DEVICE WITH ONE CAMERA

Description

TECHNICAL FIELD

[0001] This application relates generally to the field of optical inspection and control of a part, and more specifically to the measurement of a height and / or point orientation of a surface of a sample using an optical device. STATE OF THE ART

[0002] Currently, optical comparator-type measuring devices exist, comprising an optical pen that allows for point-by-point, non-contact measurement of the height, thickness, and / or local orientation of a sample surface. The diameter of the light beam at the sample surface (generally referred to as the "spot") is typically between 1.5 µm and 50 µm, depending on the chosen objective.

[0003] Such comparators can be based on the principle of chromatic confocal coding, in which a light, usually polychromatic, is focused onto the surface to be studied by a light pen comprising an axially chromatic objective. The spectral distribution of the light beam is then analyzed to identify the wavelength of the light source for which a sharp image is obtained on the surface, thus allowing the distance between the objective and the surface to be deduced.

[0004] Some of these optical comparators require visualization of the measurement point on the surface of the sample being studied. However, when the measurement is performed with a chromatic confocal lens, the measurement point is represented by a very small spot of light which, moreover, may not be visible depending on the type of surface being measured. Furthermore, the surface being studied is itself very small and may require optical assistance to correctly position the measurement spot at the desired location on the surface.

[0005] It has therefore been proposed to use cameras to correctly position the optical pen relative to the surface being studied. For example, it has been suggested to mount the optical pen and camera on a robot configured to successively place the camera and then the optical pen above the surface to be studied, in order to adjust the position of the optical pen relative to the surface. However, this solution requires a motorized measuring station, which entails a significant cost and drastically increases the overall size of the installation.

[0006] It has also been proposed to place the camera on the side of the optical pen. However, in the case of rough surfaces, the measurement area may be obscured by uneven terrain.

[0007] Finally, it has been proposed to deflect part of the optical beam, using a beam splitter cube placed between the light source and the lens, towards a camera positioned near the optical pen. US patent 2017 / 0010096 A1 discloses a similar solution with a beam splitter comprising a circular reflective area surrounded by a transparent annular area. Alternatively, a camera is placed directly in the middle of the measurement beam upstream of the chromatic confocal lens. However, the image obtained through the chromatic confocal lens is only sharp for a specific wavelength, which corresponds to the wavelength focused on the surface to be measured. Furthermore, since the optical pens are optimized at the center of the field, the resulting image contains significant aberrations that make image analysis difficult. The system is also bulky and expensive. SUMMARY OF THE INVENTION

[0008] One objective of this application is therefore to propose a measurement device, for example of a height and / or thickness, of a surface of a sample allowing to visualize in a simple, efficient and precise way the point of measurement on the surface to be studied, which is of a reasonable cost and size.

[0009] To this end, a chromatic confocal measurement device for a parameter of a sample surface is proposed, according to a first aspect, the device comprising: a light source configured to generate a light beam; a light pen comprising an axial chromatic lens configured to apply the light beam to the sample surface, the light pen having an input end connected to the light source and an output end configured to be placed near the sample surface; and a camera fixed near the output end of the light pen, between the axial chromatic lens and the sample surface, such that part of the light beam from the light source and exiting the axial chromatic lens is blocked by the camera at the output end of the light pen and another part of the light beam bypasses the camera and reaches the sample surface.

[0010] Some preferred but not limiting characteristics of the measuring device are as follows, taken individually or in combination: The optical pencil comprises a lens placed near the exit end of the optical pencil; a first surface, corresponding to the orthogonal projection of the lens onto a first plane that is perpendicular to an axis of propagation of the light beam through the lens, being larger than a second surface, corresponding to the orthogonal projection of the camera onto the first plane, such that the camera only partially obstructs the optical field of the lens; the first surface is at least twice as large as the second surface, preferably at least four times as large; a through-passage is formed in the lens and the camera is housed at least partially in the through-passage; the lens exhibits axial chromatic aberration and forms part of the axially chromatic objective;the lens is achromatic and is placed between the axially chromatic objective and the camera, a through-pass being formed in the achromatic lens and the camera being housed at least partially in the through-pass; the camera is placed between the optical pen and the surface to be measured; the measuring device further includes an additional camera, fixed near the exit end of the optical pen, between the axially chromatic objective and the surface of the sample such that part of the light beam from the light source and exiting the axially chromatic objective is also blocked by the additional camera at the exit end of the optical pen and the other part of the light beam also bypasses the additional camera and reaches the surface of the sample; a depth of field of the camera is between 2 mm and 100 mm;The measuring device further includes a light fixed to the optical pen and configured to illuminate the surface of the sample; the light is integrated into the camera or is added and fixed to the optical pen near the exit end; and / or the measuring device further includes a spectral analysis system configured to determine a spectral distribution of the light beam reflected by the surface of the sample. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Other features, purposes, and advantages of the present invention will become more apparent upon reading the detailed description that follows, and with reference to the accompanying drawings, which are given by way of non-limiting examples and on which: There figure 1 is a schematic view of a first example of an embodiment of an optical pen or a measuring device according to one embodiment of the invention; The figure 2is a schematic view of a second example of an embodiment of a light pen or measuring device according to one embodiment of the invention; The figure 3 is a schematic view of a third example of an embodiment of a light pen or measuring device according to an embodiment of the invention; The figure 4 is a schematic view of a fourth example of an embodiment of a light pen or a measuring device according to an embodiment of the invention; and The figure 5 illustrates in a very schematic way a measuring device conforming to an embodiment of the invention and comprising a fifth example of an optical pencil.

[0012] Across all figures, similar elements bear identical references. DETAILED DESCRIPTION OF A METHOD OF IMPLEMENTATION

[0013] A chromatic confocal measurement device 1 according to the invention comprises a light source 2 configured to generate a light beam 3, an optical pen 4 comprising an axially chromatic lens 5 configured to apply the light beam 3 to the surface 6 of the sample, and a camera 7. The measurement device 1 further comprises a spectral analysis system 8 such as a spectrograph 8 configured to determine a spectral distribution of the light beam 3 reflected after its passage through the optical pen 4, as well as signal processing means 9 for analyzing this spectral distribution in order to calculate the axial position of the surface 6.

[0014] The optical pencil 4 has an inlet end 10 connected to the light source 2 and an outlet end 11 configured to be placed near the surface 6 of the sample. The axial chromatic objective 5 is housed within the optical pencil 4 and comprises a series of lenses exhibiting known axial chromatic aberration, arranged within the objective 5 such that their optical axes are coaxial. Optionally, the optical pencil 4 may further include a window 12 (i.e., a flat protective plate, free of chromatic aberrations) mounted at its outlet end 11, downstream of the axial chromatic objective 5 (upstream and forward being defined according to the direction of propagation of the incident light in the optical pencil 4), i.e., between the objective 5 and the surface 6 of the sample.

[0015] The light source 2, the optical pen 4, and the spectrograph 8 can be connected via at least one optical fiber 13. The light source 2 and the spectrograph 8 can be housed in an optronic enclosure, which can be connected to the optical pen 4 via at least one optical fiber 13 and to the processing means 9 via a cable. If necessary, the optical pen 4 and / or the processing means 9 can also be housed in the optronic enclosure. Note that, in this case, the optical fiber 13 is optional.

[0016] The light source 2 is configured to generate a light beam 3, preferably polychromatic (as opposed to a monochromatic light source 2 such as a laser). The use of a polychromatic light beam 3 allows for chromatic confocal measurements and, in particular, provides axial chromatic aberration along the measurement axis, thus enabling distance measurement without any movement of the measuring device 1. The light source 2 may, in particular, be white light, for example, one or more light-emitting diodes.

[0017] Furthermore, light beam 3 has low coherence (spatial and temporal), in contrast to the laser beam which is spatially and temporally coherent.

[0018] In a first embodiment, the measuring device 1 is a "spot sensor". Since the light source 2 is polychromatic, the optical pen 4 forms a set of monochromatic images of the light source 2 (or, more precisely, of the end of the optical fiber 13, which acts as a pinhole). These images define a line segment of observation in space, each image of the source being defined by its wavelength and its focal distance relative to the optical pen 4. This same optical pen 4 also collects the backscattered light beam 3 to form, on the end of the optical fiber 13, which acts as a spatial filter hole, a common polychromatic image of all the monochromatic images. This polychromatic image is composed of the wavelengths of the different interfaces encountered by the light beam 3 in the sample.

[0019] The polychromatic image is then transmitted, via optical fiber 13, to the spectrograph 8. The spectrum measured there then shows one or more peaks corresponding to the interfaces encountered by the light beam 3 when the sample is placed inside the space covered by the chromaticism ( i.e. the line segment of observation). For example, if the sample includes an opaque surface 6, there is only one wavelength of the light source 2 for which a clear image is obtained on the surface 6. The signal processing means 9 then make it possible to deduce the distance between the optical pencil 4 and the surface 6 from the wavelength identified by the spectrograph 8.

[0020] In a second embodiment, the measuring device 1 is a "line sensor" and allows the simultaneous measurement of a set of points aligned along a line. Unlike the point sensor, the line sensor comprises a series of optical fibers 13 configured to guide the light generated by the light source 2 to the inlet of the optical pen 4 and, in return, from the optical pen 4 to the spectrograph 8. More precisely, the ends of these optical fibers 13 are spatially arranged to define a lateral measurement field (in particular, a line). In one embodiment, the light beams emitted by each of these optical fibers 13 then propagate through the optical pen 4 via a splitter to the surface 6, where they are dispersed along the optical axis.Similar to the point sensor, each perfectly focused wavelength for each point of the lateral field is reflected by the surface 6 of the sample, propagates in the opposite direction in the optical pencil 4 and is guided, via the separator and optical fibers 13, to the entrance of the spectrograph 8. The spectrograph 8 then includes a photodetector in order to visualize the spectra corresponding to each measurement point of the line.

[0021] In a third embodiment, the device is a "multipoint sensor." This is a sensor in which several light sources are arranged in an ordered manner, but not necessarily aligned as in the case of a line sensor. For example, these might be three points in a triangle, four points in a square or rectangle, N points distributed on a circle, and, more generally, N points distributed according to a geometric pattern. These point sources can be the ends of optical fibers 13, each originating from a single-point controller, or the ends of optical fiber bundles 13 in which the points are not necessarily aligned.

[0022] To allow visualization of the surface 6 under study, and in particular the measurement point, the camera 7 is fixed near the output end 11 of the optical pen 4, between a downstream portion of the axial chromatic lens 5 and the sample surface 6. Thus, part of the light beam 3 from the light source 2 and exiting the axial chromatic lens 5 is blocked by the camera 7 at the output end 11 of the optical pen 4, while another part of the light beam 3 bypasses the camera 7 and reaches the sample surface 6. The size of the camera 7 is therefore chosen so that it only partially blocks the light beam 3, thus allowing simultaneous measurement by the measuring device 1.

[0023] Since camera 7 is positioned downstream of the axially chromatic lens 5, the image it produces is sharp and undistorted by the aberrations of the optical pen 4 or the axial chromatic aberration of the lens 5. Furthermore, because the measuring device 1 operates on the principle of chromatic confocal coding, partially blocking the light beam 3 does not prevent measurement or significantly degrade the performance of the measuring device 1. On the contrary, partially blocking the light beam 3 reduces certain optical aberrations and improves measurement accuracy on transparent objects with significant thicknesses or pronounced shapes (such as thick transparent tubes). Thus, the presence of camera 7 only reduces the light intensity of the light beam 3, which does not prevent the measuring device 1 from performing the measurement.Finally, with the camera 7 placed in the light beam 3 of the optical pencil 4, its field of vision 21 is optimally positioned with respect to the measurement sport.

[0024] The camera 7 can be fixed inside the optical pencil 4 or outside the optical pencil 4, on its exit end 11.

[0025] Camera 7 can be centered on the X propagation axis of the light beam 3 ( Figures 1 , 3 And 4 Alternatively, the camera 7 can be offset with respect to the X propagation axis of the light beam 3 ( figure 2 ), and where appropriate be positioned so that its line of sight is inclined relative to the X propagation axis ( figure 5 ).

[0026] In order to avoid completely blocking the light beam 3 and to allow the measurement to be performed, the cross-section of the light beam 3 occupied by the camera 7 is smaller than the total cross-section of the light beam 3 exiting the optical pen 4. For this reason, the surface S1 (seen in cross-section on the figure 1 The area of ​​the lens 14 furthest downstream of the optical pencil 4, which can correspond to the lens 14 furthest downstream of the axial chromatic objective 5 or to the port 12 of the optical pencil 4, is larger than the area of ​​the camera 7. By area S1 of the lens 14, we will understand here the orthogonal projection of the lens 14 into a first plane P1 which is perpendicular to the propagation axis X of the light beam 3 through this lens 14. Similarly, by area S2 of the camera 7, we will understand here the orthogonal projection of the camera 7 into this first plane P1.

[0027] Camera 7 is chosen so that its surface area S2 is less than that of lens 14, preferably at least twice as small, for example at least four times smaller in order to limit the loss of light signal during measurement.

[0028] Camera 7 can therefore be a miniature camera 7. This type of camera 7 is notably used in the field of medical endoscopy. Typically, there are cameras 7 whose surface area (as defined above) is between approximately 3.0 mm² and 30.0 mm² (i.e., a diameter between approximately 1.0 mm and approximately 3.1 mm if the camera 7 is essentially tubular).

[0029] An example of a camera 7 suitable for use in the measuring device 1 is marketed by MIKROP, which uses an Ominivision sensor (reference OV6946) with the following characteristics: resolution 400x400 pixels; optical size: 1 / 18"; progressive scanning mode; frame rate 160 Kpixel (400x400): 30 fps; dimensions: 950 µm x 940 µm. A camera 7 marketed by MIKROP that incorporates this sensor includes a lens with the following characteristics: an aperture between 5.80 and 6.40; a field of view between 90° and 140°; a diameter of 1.50 mm; a length of 8.9 mm; a minimum working distance of 2 mm and a depth of field between 2 mm and 100 mm.

[0030] The use of miniature cameras 7 allows for the maintenance of conventional dimensions for the optical pencil 4 and the lenses of the axial chromatic objective 5. In particular, it is not necessary to use lenses larger than those typically used in current chromatic confocal measuring devices. Typically, the lens 14 can have a diameter of approximately 3 mm to 10 mm in small-diameter devices. Generally speaking, the diameter of the lens 14 is preferably at least twice that of the camera.

[0031] Preferably, the camera 7 has a depth of field suitable for capturing sharp images across the entire working range of the measuring device 1. This type of camera 7 is therefore generally without a manual or automatic focusing device ("autofocus"). The working range of the measuring device 1 (covering all distance measurements that can be performed by the measuring device 1) can be from 2 mm to 100 mm. The camera 7 is therefore chosen to have a depth of field covering at least the range from 2 mm to 100 mm.

[0032] Note that the device may include several cameras 7 fixed near the output end 11 of the optical sensor (see for example figure 2) in order to obtain images from different viewpoints, or even to obtain stereoscopic vision allowing the creation of a three-dimensional image of the sample. Each of the cameras 7 is then placed in the light beam 3 so as to partially obstruct this beam, while allowing a portion of this beam to pass through in order to reach the surface 6 of the sample. The sum of the areas S2 of the cameras 7 (as defined above) therefore remains less than the area S1 of the lens 14. Preferably, the sum of the areas S2 of the cameras 7 is less than half the area S1 of the lens 14, and preferably less than a quarter of the area S1 of the lens 14.

[0033] In a first embodiment, the camera 7 is fixed on the output end 11 of the optical sensor.

[0034] For example, the optical pencil 4 may include an achromatic lens positioned at the exit end 11 of the optical pencil 4. The achromatic lens may, for example, correspond to the window 12 of the optical pencil 4. The camera 7 may then be attached, for example by gluing, to one of the faces of the achromatic lens 12, for example the downstream face (exit face of the incident beam) (see, for example, figure 4 ).

[0035] Alternatively, the camera 7 can be fixed on an achromatic lens which is attached and fixed to the output end 11 of the optical pencil 4, for example by screwing or snapping.

[0036] This first embodiment has the advantage of being simple to implement and does not require modification of the optical pencil 4, apart from taking into account the possible addition of the achromatic lens (which has an impact on the optical path of the light beam 3) in the optical calculation performed by the processing means 9. On the other hand, placing the camera 7 outside the optical pencil 4 reduces the available working distance.

[0037] In a second embodiment, a through-hole 19 is formed in one of the lenses of the optical pen 4, and the camera 7 is housed at least partially within this through-hole. The lens is therefore drilled to accommodate the camera 7, which reduces the axial size of the measuring device 1 and, in particular, does not affect its working distance.

[0038] The lens that is drilled can correspond to the lens 14 furthest downstream of the axial chromatic objective 5, that is to say the last lens of the objective 5 which is closest to the exit end 11 of the optical pencil 4. The choice of this lens 14 makes it possible to prevent the axial chromatic objective 5 from distorting the image obtained by the camera 7.

[0039] Alternatively, the lens that is drilled can correspond to an achromatic lens placed downstream of the axial chromatic objective 5, typically the port 12 of the optical pencil 4.

[0040] According to yet another variant, both the lens 14 furthest downstream of the axial chromatic lens 5 and an achromatic lens, typically the porthole 12, can be drilled to receive the camera 7.

[0041] In practice, the choice of the lens or lenses 12, 14 with holes depends on the configuration of the measuring device 1, the size of the camera 7 along the direction of the propagation axis X of the light beam 3 and the desired working distance. Indeed, when the distance between the most downstream lens 14 of the objective 5 and the port 12 is less than the length of the camera 7, and when it is desired to maximize the working distance, the lens 14 and the port 12 can be drilled to fully accommodate the camera 7. Conversely, when this distance is less than the length of the camera 7, or when the available working distance is adjustable, it is possible to drill only the port 12. Finally, it may also be possible to drill only the lens 14 or the two most downstream lenses when the optical pencil 4 lacks a port 12 or when the configuration of the objective 5 allows the camera 7 to be housed in these two downstream lenses.

[0042] It should be noted that the lens(es) 12, 14 can also be drilled to, where appropriate, allow the passage of power and / or data transmission cables 20 from the camera 7 to the processing means 9. Alternatively, an opening can be formed in the body of the optical pencil 4 to allow the exit of the cable(s) 20 from the camera 7 to the processing means 9.

[0043] The lens 12, 14 can be drilled by all known means, for example by optical coring, typically by diamond machining, or by trepanning.

[0044] The spectrograph 8 includes an optical sensor configured to measure the total energy (in joules) of the light beam 3 reflected from the sample surface 6 through the projection lens 5 during an integration interval (in seconds). The signal processing means 9 include a processing unit 15, for example, a computer or server equipped with processing means, adapted to perform the chromatic confocal measurement procedure. The processing unit 15 may, for example, include memory in which the code instructions for performing the chromatic confocal measurement procedure are stored, and a computer such as a processor, microprocessor, microcontroller, etc., configured to execute these instructions. The equipment also includes control means (touchscreen, keyboard, mouse, buttons, etc.).

[0045] The camera 7 can be connected, for example by cable, to the processing means 9 or to dedicated processing means 9. The processing means 9 include in particular an acquisition system 16 configured to receive the images captured by the camera 7 and display them on a display device 17 such as a screen in order to allow a user to view in real time the position of the measurement point on the surface 6.

[0046] Since the camera 7 partially blocks the light beam 3, the measuring device 1 can also include a light 18 mounted on the optical pen 4 and configured to illuminate the surface 6 of the sample. Illuminating the surface 6 of the sample improves image quality, particularly when ambient lighting is insufficient or when the measuring device 1 blocks ambient light.

[0047] The light 18 can be integrated into the camera 7. For example, the camera 7 can incorporate a ring of light-emitting diodes mounted around its optics.

[0048] Alternatively, the light 18 can be brought and fixed onto the optical pencil 4 near the output end 11 (see figure 5 ). For example, the measuring device 1 may include an annular ring comprising light-emitting diodes which is attached and fixed around its output end 11.

[0049] It should be noted that the optical principle of chromatic confocal microscopy is inherently insensitive to ambient light. In particular, the measurement accuracy and sensitivity of the measuring device 1 are generally unaffected by the presence of illumination from light 18. The measuring device 1 can therefore be used normally without being affected by illumination.

[0050] If necessary, when the surface 6 to be measured is highly reflective, the presence of the light 18 may interfere with the measurement. In this case, the processing unit can be configured to modulate the illumination of the light 18 (particularly when it includes addressable LEDs) in order to synchronize the measurement and the illumination by offsetting them temporally.

Claims

1. A chromatic confocal measurement device (1) for measuring a parameter of a surface (6) of a sample, the device comprising: - a light source (2) configured to generate a light beam (3); - a light pen (4) comprising an objective with axial chromatism (5) configured to apply the light beam (3) on the surface (6) of the sample, the light pen (4) having an input end (10) connected to the light source (2) and an output end (11) configured to be placed close to the surface (6) of the sample; and - a camera (7) fixed close to the output end (11) of the light pen (4), between the objective with axial chromatism (5) and the surface (6) of the sample, such that a portion of the light beam (3) originating from the light source (2) and emerging from the objective with axial chromatism (5) is blocked by the camera (7) at the output end (11) of the light pen (4) and such that another portion of the light beam (3) is not blocked by the camera (7) and reaches the surface (6) of the sample.

2. The measurement device (1) according to claim 1, wherein the light pen (4) comprises a lens (12, 14) placed close to the output end (11) of the light pen (4), a first area (S1), corresponding to the orthogonal projection of the lens (12, 14) in a first plane (P1) which is perpendicular to an axis of propagation (X) of the light beam (3) through the lens (12, 14), being larger than a second area (S2), corresponding to the orthogonal projection of the camera (7) in the first plane (P1), such that the camera (7) only partially blocks the optical field of the lens (12, 14).

3. The measurement device (1) according to claim 2, wherein the first area (S1) is at least twice larger than the second area (S2), preferably at least four times larger.

4. The measurement device (1) according to one of claims 2 and 3, wherein a through-passage (19) is formed in the lens (12, 14), and the camera (7) is at least partially housed in the through-passage (19).

5. The measurement device (1) according to claim 4, wherein the lens (14) has an axial chromatism and forms part of the objective with axial chromatism (5).

6. The measurement device (1) according to one of claims 2 to 4, wherein the lens (12) is achromatic and is placed between the objective with axial chromatism (5) and the camera (7), a through-passage (19) being formed in the achromatic lens (14) and the camera (7) being at least partially housed in the through-passage (19).

7. The measurement device (1) according to one of claims 1 to 3, wherein the camera (7) is placed between the light pen (4) and the surface (6) to be measured.

8. The measurement device (1) according to one of claims 1 to 7, further comprising an additional camera (7), fixed close to the output end (11) of the light pen (4), between the lens with axial chromatism (5) and the surface (6) of the sample, such that a portion of the light beam (3) originating from the light source (2) and emerging from the objective with axial chromatism (5) is also blocked by the additional camera (7) at the output end (11) of the light pen (4) and such that the other portion of the light beam (3) likewise bypasses the additional camera (7) and reaches the surface (6) of the sample.

9. The measurement device (1) according to one of claims 1 to 8, wherein a depth of field of the camera (7) is between 2 mm and 100 mm.

10. The measurement device (1) according to one of claims 1 to 9, further comprising a light (18) fixed on the measurement device (1) and configured to illuminate the surface (6) of the sample.

11. The measurement device (1) according to claim 10, wherein the light (18) is incorporated to the camera (7) or is attached and fixed on the light pen (4) close to the output end (11).

12. The measurement device (1) according to one of claims 1 to 11, further comprising a spectral analysis system (8) configured for determining a spectral distribution (8) of the light beam (3) reflected by the surface (6) of the sample.