Profilometer for optical surfaces

Abstract

The invention relates to a profilometer (1) for an optical surface (So), comprising a frame (2), distance sensors (6) mounted on a sensor support (3) to each measure a distance along a measurement axis (9) locally substantially normal to a mounting surface (5) of the sensor support (3), and an optical element support (10) for holding an optical surface (So) of an optical element (Eo) to be measured opposite the distance sensors (6) and the sensor support (3), the mounting surface (5) being curved to be complementary to a curved reference surface of a reference optical element held opposite the sensor support (3), the optical surface (So) of the optical element (Eo) to be measured being theoretically identical to the reference surface of the reference optical element. FIGURE IN ABRIDGED DIAGRAM: Fig. 1

Description

Title of the invention: Profilometer for optical surfaces

[0001] The present invention relates to the field of optics and, more particularly, to the field of dimensional control of optical surfaces of large optical elements, such as telescope mirrors. BACKGROUND OF THE INVENTION

[0002] A profilometer is a machine that allows the determination of a surface profile, that is to say a shape of the latter in a reference frame which depends on the mechanical elements of the surface whose profile we wish to determine, and to detect shape defects in relation to an arbitrary reference shape.

[0003] For frosted optical surfaces, it is known to use a three-dimensional measuring machine. Such a machine is effective when the shape defects are sufficiently large, typically greater than several microns or even tens of microns.

[0004] However, the accuracy of the measurements is generally considered insufficient for optical elements.

[0005] Moreover, such machines also have the disadvantage of being relatively slow.

[0006] For reflective optical surfaces, it is possible to use an interferometric measuring device allowing measurements with an accuracy greater than 100 nm in effective value, or root mean square, also referred to by the acronym RMS for "Root Mean Square" in English.

[0007] However, interferometric measuring devices do not have a dynamic range that allows them to measure shape defects whose dimensions exceed a few microns PtV, acronym for "Peak To Valley" in English.

[0008] Moreover, interferometric measuring devices are not suitable for measuring the shape of frosted optical surfaces, unlike three-dimensional measuring machines. SUBJECT OF THE INVENTION

[0009] The invention is intended in particular to remedy at least in part the aforementioned disadvantages. Summary of the invention

[0010] To this end, the invention relates to a profilometer comprising a frame, distance sensors mounted on a sensor support to each measure a distance along a measurement direction locally substantially normal to a mounting surface of the sensor support, and an optical element support to maintain a surface optics of an optical element to be measured in relation to the distance sensors and the sensor support.

[0011] The mounting surface is curved to be complementary to a curved reference surface of a reference optical element which would be held opposite the sensor support, the surface of the optical element to be measured being theoretically identical to the reference surface of the reference optical element.

[0012] It is therefore understood that the optical element to be measured can be substantially similar to the reference surface of the reference optical element, in particular by a few millimeters, without this impacting a measurement and the performance of the profilometer.

[0013] The term "substantially normal" means that the measuring axis forms an angle of 90 degrees to the mounting surface, to within 1 degree, preferably to within 0.5 degrees.

[0014] Since the mounting surface is complementary to the reference surface and each distance sensor measures a distance along a measurement direction locally substantially normal to the mounting surface, the distance sensors measure the distance along a direction theoretically perpendicular to the surface to be measured.

[0015] The profilometer will therefore measure a difference between the reference surface and the optical surface to be measured. Such an arrangement provides precise measurements and limits the influence of mounting defects on the accuracy of the measurements.

[0016] Advantageously, the distance sensors are distributed on the mounting surface in a quasi-random arrangement. Preferably, the quasi-random arrangement is a Hammersley-type arrangement.

[0017] Such an arrangement guarantees obtaining a complete profile with good accuracy of surface measurement and immunity to the loss of a distance sensor.

[0018] According to a first particular feature of the invention, the support of the optical element is supported by a positioner, in particular a positioner with six axes of mobility.

[0019] Such an arrangement makes it possible to guarantee minimum forces exerted on the optical element to be measured and to maintain the optical element to be measured in a particularly stable manner.

[0020] In addition, the positioner allows the optical surface of the optical element to be measured to be positioned precisely in relation to the sensor support, which allows for better measurement accuracy.

[0021] According to another particular feature of the invention, the sensor support is fixed and the support for the optical element to be measured is mobile, in particular mounted on a mobile carriage, between a measurement position, in which the optical element support is arranged opposite the sensor support, and a loading position / unloading in which the optical element carrier is moved away from the sensor carrier to allow loading and / or unloading of the optical element to be measured onto the optical element carrier.

[0022] Displacement necessarily causes stress on the displaced optical element to be measured. By keeping the sensor support stationary, the risk of such stresses disrupting the positioning of the distance sensors or damaging the sensor support and distance sensor assembly is limited.

[0023] Advantageously, the sensor support is suspended from the frame by fasteners, in particular by three fasteners advantageously arranged equidistant from each other. The fasteners respectively comprise two connecting rods having first ends articulated to the frame at widely spaced points and second ends articulated to the sensor support at closely spaced points.

[0024] This allows for differential expansion of the sensor support and the frame. Furthermore, this is also likely to limit the stresses exerted on the sensor support by the system for attaching the sensor support to the frame.

[0025] Other features and advantages of the invention will become apparent from the following description of a particular, non-limiting embodiment of the invention. Brief description of the drawings

[0026] Reference will be made to the attached drawings, among which:

[0027] [Fig. 1] [Fig. 1] is a schematic perspective view of a profilometer according to the invention;

[0028] [Fig.2] [Fig.2] is a schematic front view of the profilometer according to the invention;

[0029] [Fig.3] [Fig.3] is a schematic view from below of the sensor arrangement of the profilometer according to the invention;

[0030] [Fig.4] [Fig.4] is a partial schematic view of a sensor support in section according to a cross-sectional plan; and

[0031] [Fig. 5] [Fig. 5] is a schematic elevation view of a fastening device for the sensor support on frame 2. DETAILED DESCRIPTION OF THE INVENTION

[0032] With reference to the figures, the invention is here described in application to the measurement of the optical surface So of an optical element Eo.

[0033] The optical element Eo may include a mirror M, which may include a reflective surface, forming the optical surface So and a fixing structure F for the mirror M.

[0034] The optical element Eo is known in itself and will not be further detailed here.

[0035] The optical element Eo can be manufactured using a profilometer 1 in such a way that the optical surface So is identical to a reference surface up to manufacturing tolerances.

[0036] However, the optical element Eo can be manufactured using the profilometer 1 according to an arbitrarily defined shape provided that it is substantially similar to the reference surface of the reference optical element, in particular by a few millimeters, without this impacting a measurement and performance of the profilometer.

[0037] The reference surface may, in particular, be a reference optical element, for example defined in a digital model established by a computer-aided design system. The digitally generated reference surface corresponds to that to which the optical surface So of an optical element Eo to be measured must be compared.

[0038] The reference surface, in particular materialized by a physical optical element Eo, allows for a precise position of the distance sensors and a precise determination of the shape defect.

[0039] The profilometer of the invention is designed to verify whether the optical surface So is actually identical to the reference surface within manufacturing tolerances or whether errors have resulted in shape defects of the surface So.

[0040] Shape defects are positive reliefs, protruding from the reference surface, or negative reliefs, recessed from the reference surface.

[0041] As shown in the figures, the profilometer 1 comprises a frame 2, in particular made, as shown in figures 1 and 2, in the form of a gantry comprising two lateral frames 2.1, 2.2 and an upper frame 2.3, connecting the two lateral frames 2.1,2.2.

[0042] More specifically, the side frames 2.1, 2.2 have a lower part fixed to the ground, such that the side frames 2.1, 2.2 extend vertically parallel to each other and the upper frame 2.3 rests on an upper part of the side frames 2.1, 2.2 which thus support the upper frame 2.3.

[0043] According to a particular embodiment, the upper frame 2.3 may comprise, as shown in Figures 1 and 2, two parallel longitudinal beams, having ends fixed to the lateral frames 2.1, 2.2 to connect the upper parts of the lateral frames 2.1, 2.2 to each other, and two transverse beams, extending at a distance from the ends of the longitudinal beams, forming a trapezoid with the two longitudinal beams.

[0044] A sensor support 3 is suspended under the upper frame 2.3 of the frame 2.

[0045] The sensor support 3 comprises, according to the embodiment shown, a plate having : - an upper surface 4, also called the rear surface 4, forming a surface of the connection between the sensor support 3 and the frame 2, - a lower surface 5, also called the front surface 5, forming a mounting surface.

[0046] The sensor support 3 is suitable for being suspended from the frame 2, in particular by means of three fasteners 7, advantageously arranged at equidistant intervals from each other and preferably arranged symmetrically around a central axis 8 of the sensor support 3.

[0047] Each fastener 7 comprises: - a base 7.1, elongated and fixed under the upper frame 2.3 of the frame 2, and - two connecting rods 7.2, each comprising a first articulated end to the base 7.1, in particular by means of ball joints, and a second end articulated to a block 7.3 fixed to the upper surface 4 of the sensor support 3, in particular by means of ball joints.

[0048] The first ends of the connecting rods 7.2 are connected to the ends of the base 7.1 and the second ends of the connecting rods 7.2 are connected to the same block 7.3, thus forming a triangular arrangement.

[0049] The connecting rods 7.2 therefore have first ends fixed to the frame 2 at widely spaced points and second ends fixed to the sensor support 3 at closely spaced points, in particular so close as to be coincident or substantially coincident.

[0050] The connecting rods 7.2 have adjustment possibilities relative to the frame 2, in order to adjust the horizontality and / or the height of the upper surface 4.

[0051] In a first embodiment, the connecting rods 7.2 are not adjustable. The first ends of the connecting rods 7.2 are wedged in various positions relative to the frame 2, in particular relative to the upper frame 2.3, especially at the level of the base 7.1, to adjust a horizontality and / or a height of the upper surface 4.

[0052] In a second embodiment, alternative or combined, the connecting rods 7.2 are able to be adjusted in length in order to be able to adjust the horizontality and / or the height of the upper surface 4.

[0053] Advantageously, the lower surface 5 has been machined to be complementary to a reference surface. In particular, a curvature of the lower surface is concave if the reference surface is convex, and vice versa.

[0054] In the case of mirror M, the reference surface is concave and the lower surface 5 is convex.

[0055] Advantageously, the reference surface is curved in order to limit the angle with respect to the normal to the optical element Eo to be measured.

[0056] Thus, the fact that the curvature of the lower surface 5 is complementary to the reference surface is only a consequence of the adaptation of the normals.

[0057] Alternatively, it is possible to calibrate the profilometer with a flat reference surface.

[0058] The sensor support 3 is drilled with holes each receiving a distance sensor 6, such that each distance sensor 6 has an active part 6.1 extending outward from the lower surface 5, forming the mounting surface of the distance sensors 6.

[0059] The distance sensor 6 has a rear part 6.2 arranged in holes drilled in the lower surface 5 oriented with the upper surface 4.

[0060] The rear part 6.2 of the distance sensor 6 includes a linking means for connecting the distance sensor 6 to a signal processing unit, here electronic, from the distance sensors 6.

[0061] The linking means may be an electrical cable or an optical fiber or any medium suitable for transmitting information collected by the distance sensor 6 to the signal processing unit.

[0062] The processing unit, not shown in the figures, is programmed in a way known in itself to calculate a shape of the optical surface So from the signals provided by the distance sensors 6.

[0063] The distance sensors 6 can be probes or non-contact sensors, such as, in particular, ultrasonic sensors, laser sensors or white light sensors.

[0064] The distance sensor 6 is positioned in the hole drilled in the lower surface 5 to have a measuring axis 9, in particular capable of corresponding to an axis of the hole drilled in the lower surface 5.

[0065] The measuring axis 9 is locally normal to the lower surface 5 of the sensor support 3, so that the distance sensors 6 each measure a distance along a measuring direction locally normal to the lower surface 5.

[0066] The distance sensors 6 are distributed on the lower surface 5 in a quasi-random arrangement, here a Hammersley-type arrangement. Such an arrangement is described in the document "On the efficiency of certain quasi-random sequences of points in evaluating multi-dimensional integrals", JH HALTON, Nu-merische Mathematik, Volume 2, pages 84-90, 1960.

[0067] The positioning of each of the distance sensors 6, transposed into cylindrical coordinates, will follow the following law:

[0068] R = R0*V(xh)

[0069] 0 = 2*n*vh

[0070] In which: - R is the distance from the distance sensor 6 to the center of the lower surface 5, - Ro is the measured optical surface radius So, also called the radius of pupil of the arrangement consisting of all 6 distance sensors, in particular equal to approximately 760 mm, - xh and vh are the coordinates of the Hammersley points, as defined in the document referenced above, and - 0 is the angle of the line passing through the center and along which the sensor The point at a distance of 6 is placed at a distance R from the center of the lower surface 5.

[0071] The number of 6-way distance sensors installed is chosen according to the spatial frequencies to be measured. The minimum number of 6-way distance sensors to be installed is 5, but preferably a larger number of 6-way distance sensors will be used, for example between 15 and 75 6-way distance sensors, in particular between 19 and 72 6-way distance sensors, especially 19 or 72 6-way distance sensors.

[0072] The profilometer 1 further includes an optical element support 10, suitable for ensuring that the optical surface So of the optical element Eo to be measured is held in relation to the distance sensors 6 and the sensor support 3.

[0073] The optical element support 10 is supported by a positioner 11, in particular a positioner 11 with six axes of mobility.

[0074] In the embodiment shown in [Fig. 1], the optical element support 10 is movable. Indeed, the positioner 11 can be mounted on a carriage capable of moving between: • a measuring position, in which the optical element support 10 is positioned opposite the sensor support 3, and • a loading / unloading position, in which the optical element support 10 is moved away from the sensor support 3 to allow loading and / or unloading of the optical element Eo onto the optical element support 10.

[0075] According to the embodiment example in [Fig.1], the trolley on which the positioner 11 is mounted rests on rails 12.

[0076] An actuator 13, in particular a telescopic actuator 13, enables the movement of the carriage, and consequently of the positioner 11 and the optical element support 10, along the rails 12, between the measurement position and the loading / unloading position. Preferably, the frame 2 is fixed.

[0077] In the embodiment shown in [Fig.2], the positioner 11 is fixed in a measuring position, in which the optical element support 10 is arranged opposite the sensor support 3.

[0078] The positioner 11, in particular the six-axis mobility positioner 11, allows the optical surface So of the optical element Eo to be measured to be positioned precisely in relation to the sensor support 3.

[0079] The lower surface 5 has been machined to be complementary to the reference surface such that if the optical surface So of the optical element Eo If the optical element support 10 is held opposite the sensor support 3 and is exactly identical to the reference surface, the distance sensors 6 will all measure the same distance. However, if the optical surface So has a defect, at least one of the distance sensors 6 will measure a distance different from that measured by the other distance sensors 6.

[0080] The profilometer 1 according to the invention therefore makes it possible to quickly measure the optical surface So of a series of identical optical elements Eo.

[0081] The profilometer 1 according to the invention also allows the optical surface So of optical elements Eo of another series to be measured, provided that the variation between the series is within a certain range of tolerances, in particular a maximum of + / - 2.5 mm of the initial reference surface.

[0082] If the variation between the series does not fall within the tolerance range, the optical surface So of the optical elements Eo of this other series being significantly different from the optical surface So of the optical elements Eo of the previous series, i.e. corresponding to a reference surface different from that used to machine the optical surface of the optical elements of the previous series, it is necessary to replace the sensor support used for the previous series with a sensor support having a mounting surface complementary to the reference surface of the new series.

[0083] Of course, the invention is not limited to the embodiment described but encompasses any variant falling within the scope of the invention as defined by the claims.

[0084] In particular, the profilometer 1 may have a different structure from that described.

[0085] Thus, each of the distance sensors 6 can be mounted to move in translation on a socket itself mounted on the sensor support 3 by a ball joint allowing the socket and therefore the distance sensor 6 to be oriented in space.

[0086] It is understood that the adjustment of the socket in orientation and the adjustment in translation of the distance sensor 6 constitute means arranged to modify the curvature of the mounting surface, which is, here for example, defined by the active parts of the distance sensors 6.

[0087] The sensor support 3 can alternatively be formed of a lattice comprising connecting rods 7.2, in particular connecting rods 7.2 articulated with each other, and / or of adjustable length to allow modification of the shape of the mounting surface to adapt it to different optical elements to be measured. Alternatively, the frame can be movable and the optical element support fixed.

[0088] The connecting rods 7.2 can be attached directly to the frame 2 and / or to the sensor support 3.

[0089] The sensor support 3 can be fixed to the frame 2 by means of other fixing methods than that described. Furthermore, the sensor support 3 may not be suspended from the frame 2 but rest on it.

[0090] The frame 2 may have a different shape than that described, in particular a cradle shape on which the sensor support 3 rests.

[0091] The optical element support 10 can be mounted on a positioner 11 comprising fewer than six axes of mobility, or even on a chassis without axes of mobility.

[0092] The distance sensors 6 mounted on the same sensor support 3 can be of different types.

Claims

Claims

1. Profilometer (1) for optical surface (So), comprising • a frame (2), • distance sensors (6), mounted on a sensor support (3) for each measuring a distance along a measurement axis (9) locally substantially normal to a mounting surface (5) of the sensor support (3), and • an optical element support (10) for holding an optical surface (So) of an optical element (Eo) to be measured opposite the distance sensors (6) and the sensor support (3), the mounting surface (5) being curved to be complementary to a curved reference surface of a reference optical element which would be held opposite the sensor support (3), the optical surface (So) of the optical element (Eo) to be measured being theoretically identical to the reference surface of the reference optical element.

2. A profilometer (1) according to claim 1, wherein the distance sensors (6) are distributed over the mounting surface (5) in a quasi-random arrangement.

3. Profilometer (1) of claim 2, wherein the quasi-random arrangement is a Hammersley type arrangement.

4. Profilometer (1) according to any one of the preceding claims, wherein the optical element support (10) is supported by a positioner (11), in particular a positioner (11) with six axes of mobility.

5. Profilometer (1) according to any one of the preceding claims, wherein the sensor support (3) is fixed and the optical element support (10) is movable between: • a measuring position, in which the optical element support (10) is arranged opposite the sensor support (3), and • a loading / unloading position, in which the optical element support (10) is spaced from the sensor support (3) in order to allow loading and / or unloading of the optical element (Eo) to be measured onto the optical element support (10).

6. Profilometer (1) according to any one of the preceding claims, in which the sensor support (3) is suspended from the frame (2) by fasteners (7), in particular by three fasteners (7) advantageously arranged equidistant from each other.

7. Profilometer (1) according to the preceding claim, in which the attachments (7) respectively comprise two connecting rods (7.2) having first ends articulated to the frame (2) at spaced points and second ends articulated to the sensor support (3) at close points.

8. Profilometer (1) according to any one of the preceding claims, wherein the distance sensors (6) comprise feelers and / or non-contact sensors.

9. A profilometer (1) according to any preceding claim, wherein means are arranged to modify a curvature of the mounting surface (5).

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