Measuring device and method

Abstract

Measuring device for measuring at least one property of an optical element, wherein the device comprises: - a frame; - a supporting device arranged for supporting the optical element on the frame; - a rotating body arranged to rotate relative to the frame around a rotation axis; - a measurement system arranged to measure the relative position of the rotating body and the frame in at least one measurement plane; - a probe arranged on the rotating body for measuring the at least one property of the optical element, wherein the probe is located at a position out of the at least one measurement plane.

Description

[0001] MEASURING DEVICE AND METHOD

[0002] The present invention relates to a measuring device and method, in particular for measuring optical elements.

[0003] Measuring devices, in particular for measuring optical elements, may comprise a movable measuring probe which is moved against, or close to, the surface to be measured. To determine the position of the part of the surface that is measured, the location of the measurement probe has to be determined with respect to a reference, such as the frame of the measuring device. In measuring devices which possess the accuracy required for measuring high-accuracy optical elements, the position of the measuring probe is typically determined by means of a plurality of interferometers, in particular displacement measuring laser interferometers. These devices thus require a very large and accurately built frame, for example made from pieces of granite, as well as relatively large reference mirrors to accommodate said interferometers, which leads to high costs. Moreover, the degree of accuracy provided by such measuring devices is not always required. In addition, it is often required to use multiple different measuring probes to measure an optical element, requiring the probe to be changed throughout the process, and requiring recalibration of the measuring device.

[0004] It is a goal of the present invention to at least partially alleviate at least some the above mentioned problems. This goal is at least partially met by a measuring device according to appended claim 1. More specifically, this goal is at least partially met by a measuring device for measuring at least one property of an optical element, wherein the device comprises: a frame; a supporting device arranged for supporting the optical element on the frame; a rotating body arranged to rotate relative to the frame around a rotation axis; a measurement system arranged to measure the relative position of the rotating body and the frame in at least one measurement plane; a probe arranged on the rotating body for measuring the at least one property of the optical element, wherein the probe is located at a position out of the at least one measurement plane.

[0005] The probe being located at a position out of the measurement plane allows the probe to rotate without interfering with the measurement of its location. This beneficially allows multiple probes to be mounted on the rotating body, allowing the use of multiple probes during measurement, without requiring probes to be removed and replaced, and without requiring recalibration after another probe is selected.

[0006] Preferably, the probe extends in a direction radially outwardly from the rotation axis. A tip of the probe is more preferably directed away from the rotation axis. This allows at least part of the measurement system to be arranged close to the probe, while still allowing clearance for the optical element. This further allows the use of a plurality of probes, as will be discussed further below. More preferably, the probe extends radially at a nonzero distance from the rotating body, in particular at a nonzero distance from the rotation axis. The rotation axis is preferably internal to the rotating body. More preferably, the at least one probe extends radially outwardly from the rotating body.

[0007] Preferably, the measurement system is arranged to measure the relative position of the rotating body and the frame in at least a volume delineated by a plane in which the probe rotates, a cylindrical surface extending parallel to the rotation axis and delineated by a (possibly hypothetical) full rotation of the tip of the probe, and a plane extending perpendicular to the rotation axis and intersecting a part of the frame extending within said cylindrical surface that is closest to the plane in which the probe rotates. This allows the location of the rotating body, in particular the probe, relative to the frame to be measured close to the probe without interfering with the optical element, as the optical element is preferably located substantially outside said cylindrical surface.

[0008] Preferably, the measurement plane and the probe extend at a mutual distance seen along the rotation axis of the rotating body. This allows the rotating body to be a relatively simple longitudinal body. In addition, the measurement system can thus be arranged close to the probe without interfering with the probe, and approximately in-line with the rotation axis, allowing for a more compact measurement system and frame.

[0009] Preferably, the probe extends from the rotating body in a probe plane substantially perpendicular to the rotation axis of the rotating body, wherein the probe plane extends parallel to and at a distance from the measurement plane. This provides for a relatively simple system, as well as preferably preventing spatial interference between the probe and the at least one measurement plane.

[0010] Preferably, the rotating body comprises a plurality of probes, preferably each extending in the probe plane. This allows multiple probes to be used during measurement, without requiring replacing probes. The probes extending in the same plane also prevents the location of the probe that is used for measurements relative to the measuring plane from changing when the rotating body is rotated to use a different probe. As mentioned above, the probes preferably extend in a direction radially outwardly from the rotation axis.

[0011] Preferably, the rotating body extends along a longitudinal axis, wherein the longitudinal axis preferably aligns with the rotation axis. Preferably, the measurement system is arranged to determine an angular deviation of the longitudinal axis of the rotating body from the rotation axis. The rotation axis is thus taken as a reference axis. Due to possible error motion (play) in bearings that rotatably support the rotating body, the rotating body, in particular the longitudinal axis of the rotating body, may develop an angular deviation relative to the rotation axis, which results in a deviation of the actual location of the probe relative to the intended location. The measurement system being arranged to determine this angular deviation thus allows for compensating for this angular deviation to increase the accuracy of the measurement device, in particular the accuracy of the measurement system in determining the location of the measurement probe relative to the frame.

[0012] Preferably, the measurement system is arranged to measure the relative position of the rotating body and the frame in at least two measurement planes, extending parallel to and at a nonzero distance from each other and wherein the probe is located at a position out of the at least two measurement planes. The position of the rotating body relative to the frame is thus measured in at least two different locations, allowing for for example a rotation of the rotating body around an axis other than the rotation axis to be determined, in particular the aforementioned possible angular deviation of the longitudinal axis of the rotating body relative to the rotation axis. This allows the measurement system to also measure an angular displacement of the rotating body relative to the frame around axes extending obliquely relative to the rotation axis, and / or a torsional displacement in the rotating body.

[0013] Preferably, the measurement system is arranged to measure the relative rotation of the rotating body and the frame. This allows the rotating body to rotate, for instance to allow another probe to be used and / or measure from a different direction without for example requiring a recalibration of the measuring device.

[0014] Preferably, the rotating body is further arranged to translate with respect to the frame, wherein the measurement system is arranged to measure a translational position of the rotating body with respect to the frame. Preferably, the frame comprises a rotating body guide arranged to support the rotating body in at least one lateral direction, wherein the rotating body is arranged to be directly supported by the rotating body guide. The movement of the rotating body is thus directly constrained in at least one lateral direction.

[0015] Preferably, the rotating body guide comprises a planar surface, and the rotating body comprises at least one bearing, preferably an air bearing, arranged to support the rotating body on said planar surface. An air bearing inherently allows the rotating body to translate across the planar surface, and additionally allows a rotation of the rotating body around an axis perpendicular to the planar surface. Preferably, the planar surface extends perpendicular to the rotation axis. This allows for the movement of the rotating body, and thus the one or more probes, to be constrained with high accuracy in a translational direction, in particular parallel to the rotation axis. It is thus not required to additionally measure the position of the rotating body relative to the frame in the direction parallel to the rotation axis. Preferably, the rotating body guide comprises a slab of stone, such as for example granite or other suitable materials, which comprises said planar surface.

[0016] Preferably, the measuring device further comprises a movable stage arranged to rotatably support the rotating body on the frame, wherein the measurement system is arranged to measure the relative rotation between the movable stage and the rotating body in the measurement plane, wherein the movable stage is arranged to translate relative to the frame in a first lateral direction, wherein the rotation axis extends substantially perpendicular to the first lateral direction, wherein the measurement system is arranged to measure the relative position of the movable stage and the frame.

[0017] Preferably, the rotating body is movably supported in the movable stage in the lateral direction parallel to the rotation axis. The rotating body is thus, in a particularly beneficial embodiment, directly supported by the rotating body guide in a third lateral direction, which is the lateral direction perpendicular to the first lateral direction and parallel to the rotation axis. Thus, the position of the rotating body, and thus the one or more probes, is constrained with sufficient accuracy in the third lateral direction, eliminating the requirement of measuring the position of the rotating body relative to the frame in the third lateral direction. Since the rotating body is not supported in the movable stage in the third direction, errors otherwise resulting from error motion in bearings supporting the rotating body in the movable stage in the third direction, as well as error motion in bearings supporting the movable stage in the third direction, are prevented. This is also referred to as ‘unstacking’. Optionally, the movable stage comprises biasing means to bias the rotating body towards the rotating body guide. It is thus ensured that the at least one bearing, preferably air bearing, supporting the rotating body on the rotating body guide remains in contact with the rotating body guide.

[0018] This provides for a compact measuring device, wherein the measuring probe that is arranged on the rotating body possesses at least two degrees of freedom. By measuring the relative position of the movable stage relative to the frame, in particular in the first lateral direction, a location, in particular in the first lateral direction, of the rotating body and thus the probe relative to the frame can be determined. In addition, by measuring the relative rotation, in particular around the rotation axis, of the rotating body relative to the movable stage the relative rotation, in particular around the rotation axis, of the rotating body relative to the frame can be determined.

[0019] Preferably, the measurement system is arranged to measure the relative position of the movable stage and the frame in the at least two measurement planes. Thus, the translational position of the movable stage and the frame is measured in two locations, preferably along the rotation axis, which allows the measurement system to measure an angular deviation of the movable stage around the axis extending perpendicular to the rotation axis and the first lateral direction. As the measurement probe extends at a nonzero distance from the measurement planes, said angular deviation affects the position of the probe in the first direction. Thus, measuring in said two measurement planes allows said deviation to be determined and compensated for. As the rotating body is mounted in the movable stage, the translational position of the movable stage relative to the frame is indicative of the translational position of the rotating body relative to the frame.

[0020] Preferably, the measurement system is further arranged to measure the relative rotational position of the rotating body and the movable stage in a measurement plane. As the movable stage is fixed relative to the rotation axis of the rotating body, the relative rotational position of the rotating body and the movable frame is indicative of the relative rotational position of the rotating body relative to the frame.

[0021] Preferably, the measurement system is further arranged to measure a lateral position of the rotating body relative to the movable stage. The rotating body is preferably supported in the movable stage with bearings. These bearings may however allow an amount of lateral play of the rotating body relative to the movable stage, as well as longitudinal play. The measurement system being arranged to measure a lateral position of the rotating body relative to the movable stage thus allows any lateral displacement resulting from this play to be determined and thus compensated for. In a preferred embodiment, the measurement system is arranged to measure the lateral position of the rotating body relative to the movable stage in two locations extending at a mutual distance along the rotation axis. This allows the measurement system to determine an angular deviation of the longitudinal axis relative to the rotation axis. This thus also allows a deviation of the location of the probe due to play in the bearings relative to the intended location of the probe to be determined and compensated for.

[0022] Additionally or alternatively, the rotating body is supported in the movable stage by at least two rotary bearings extending at a nonzero mutual distance from each other along the rotation axis, wherein one of the at least two rotary bearings is a rotary bearing with a higher tolerance than another of the at least two rotary bearings. The measurement system may then be arranged to measure the lateral position of the rotating body relative to the movable stage in one location, extending at a nonzero distance along the rotation axis from the bearing with a higher tolerance. It may then be assumed that the rotating body does not laterally deviate relative to the movable stage where it is supported by the bearing with a higher tolerance. By then measuring the lateral position of the rotating body relative to the movable stage in said location at a nonzero distance from the bearing with a higher tolerance, an angular deviation of the rotating body, in particular the longitudinal axis of the rotating body relative to the rotation axis, may then be determined. This may result, as also a high tolerance bearing will allow a degree of play (albeit very small), in a lower measurement accuracy of the location of the rotating body relative to the movable stage and thus the location of the probe relative to the frame than in an embodiment where the lateral position of the rotating body relative to the movable stage is measured in at least two locations extending at a nonzero mutual distance along the rotation axis, but the accuracy may be acceptable for certain applications. This results in a simpler and cheaper measurement device.

[0023] In a particularly advantageous embodiment the position of the measurement probe is thus determined by measuring the translational position of the movable stage in the first direction relative to the frame in at least two measurement planes, and by measuring the relative position of the rotating body and the movable stage in at least one measurement plane, preferably in two measurement planes. Preferably, the movable stage is fixed relative to the frame in all directions except the first direction, and the rotating body is fixed relative to the movable stage in all directions except the rotational direction around the rotation axis. Thus, the three dimensional position of the at least one measurement probe may be determined. Preferably, the measurement system comprises at least one encoder system. Encoder systems are cheaper and simpler than interferometers as used in the prior art. The fact that the one or more probes are located out of the at least one, preferably at least two, measurement planes allows the use of the at least one encoder system. This leads to a particularly relatively simple and affordable measuring device.

[0024] Preferably, the measurement system comprises at least one linear encoder arranged to measure a lateral position of the probe relative to the frame. More preferably, the at least one linear encoder is arranged to measure the lateral position of the movable stage relative to the frame, which is indicative of the lateral position of the probe relative to the frame.

[0025] Preferably, the measurement system comprises a first linear encoder arranged to measure the lateral position of the movable stage relative to the frame in a first measurement plane of the at least two measurement planes, and a second linear encoder arranged to measure the lateral position of the movable stage relative to the frame in another measurement plane of the at least two measurement planes. Thus, the position of the movable stage relative to the frame is measured in at least two locations, providing the above mentioned benefits. It will be appreciated that the use of said one or more linear encoders is also possible and beneficial without the rotating body being rotatable, i.e. wherein the probe, optionally together with the movable stage, is arranged to translate along one or more lateral directions, and linear encoders are provided to measure the position of the probe relative to the frame along the one or more lateral directions.

[0026] Preferably, the measurement system comprises a rotary encoder for measuring the rotation of the rotating body relative to the frame, wherein the rotary encoder comprises a circular scale and a sensor. A rotary encoder is also relatively simple, and the fact that the one or more measuring probes extend out of the at least one, preferably at least two, measurement planes allows the use of one or more rotary encoders. It will be appreciated that the use of rotary encoders is also possible and beneficial in embodiments of the measuring device wherein the rotating body is not arranged to translate relative to the frame.

[0027] Preferably, at least one rotary encoder is arranged for measuring the rotational position around the rotation axis of the rotating body relative to the movable stage. This provides the above mentioned benefits. In particular, as the movable stage is preferably fixed relative to the frame around the rotation axis, the rotational position of the rotating body relative to the movable stage is indicative for the said position relative to the frame. Preferably, the rotary scale of at least one encoder is mounted on the rotating body, and the one or more sensors are mounted on the movable stage.

[0028] Preferably, the rotary encoder comprises at least a first sensor and a second sensor, wherein the first sensor is mounted along a first radial emanating from the centre of the rotary scale, and wherein the second sensor is mounted along a second radial emanating from the centre of the rotary scale, wherein the first radial and second radial extend at a nonzero mutual angle, preferably at a mutual angle of approximately 90°. This allows the at least two sensors to also register a translational displacement of the rotary scale relative to the sensors in the plane of the rotary scale, in particular in a plane perpendicular to the rotation axis. Thus, a translational deviation of the rotating body relative to the frame, in particular relative to the movable stage, may be registered by the one or more rotary encoders, as explained above and providing for example the above mentioned benefits. Other mutual angles may also provide said benefits, for example 120°.

[0029] Preferably, the rotary encoder comprises at least a third sensor, wherein the third sensor is mounted along a third radial emanating from the centre of the rotary scale, wherein the third radial extends at a nonzero angle relative to either the first radial or the second radial, preferably at an angle of approximately 90° relative to either the first radial or the second radial. This allows a translational movement along the two perpendicular directions in the plane of the rotary scale, in particular in a plane perpendicular to the rotating body, to be determined. These movements may then also be accounted (compensated) for to increase the accuracy of the measuring device. Other mutual angles may also provide said benefits, for example 120°. If three sensors are used, the sensors then extend at 120° mutual angles.

[0030] Additionally or alternatively, other sensors may be used, such as capacitive sensors or interferometers may be used to determine one or more of the rotational and lateral position of the rotating body relative to the movable stage and / or the frame, and the lateral position of the movable stage relative to the frame.

[0031] Preferably, the probe and the supporting device are movable with respect to each other in at least two, preferably three, degrees of freedom, for measuring the surface of the optical element supported by the supporting device, wherein the probe is movable in a first degree of freedom and is fixed along a second degree of freedom, wherein the supporting device is movable in the second degree of freedom and fixed in the first degree of freedom. Thus, the probe nor the supporting device preferably translate in more than one direction. This removes the need for multiple stages that each move in a different direction to move the probe and supporting device relative to each other, removing the inaccuracy resulting from such arrangements (unstacking). It will be appreciated that this also applies for measuring devices without a rotating body and / or rotating probe, i.e. in measuring devices wherein the probe is not arranged to rotate relative to the frame. Thus, the probe may be (uniquely) movable along a first degree of freedom, while the supporting device is movable along the second degree of freedom. This also provides the aforementioned unstacking and associated benefits.

[0032] Preferably, the supporting device is movable relative to the frame in a second direction, perpendicular to the first lateral direction.

[0033] Preferably, the frame comprises first guides to guide the movable stage in the first direction and to constrain the movement of the movable stage in all other directions, and wherein the frame comprises second guides to guide the supporting device in the second direction and constrain the movement in of the supporting device in all other directions. This prevents the movement of the movable stage in all directions except a translational movement in the first direction, and the movement of the supporting device in all directions except a translational movement in the second direction.

[0034] Preferably, the supporting device comprises a rotating stage which is arranged to rotate relative to the frame and the supporting device. Preferably, the rotating stage supporting device is arranged to rotate around an axis parallel to the second direction. The aforementioned translational movement of the movable stage and the supporting device along respectively the first and second directions, along with said rotational movement of the rotating stage allows the one or more probes to reach and measure substantially the entire surface of the optical element that faces the one or more probes. In addition, a translational movement of the movable stage or the supporting device has no impact on the position on the other of the movable stage and supporting device, increasing the accuracy of the measurement device (the aforementioned unstacking). The rotating stage brings the total degrees of freedom of the probe relative to an optical element supported on the rotating stage to four: the rotation axes of the rotating body and the rotating stage, the translation of the movable stage in the first direction and the translation of the supporting device in the second direction.

[0035] Preferably, the first direction extends parallel to the horizontal plane, and wherein the second direction extends in the vertical direction. This provides a user friendly measuring device, and allows gravity to aid in the supporting of the optical element in the supporting device. Another aspect relates to a method for measuring at least one property of an optical element using a device according to any of the preceding claims.

[0036] The present invention is further illustrated by the following Figures, which show a preferred embodiment of the device according to the invention, and are not intended to limit the scope of the invention in any way, wherein: figure 1 shows an overview of a measuring device; figure 2 shows a view of part of the measuring device; figure 3 shows a perspective view of the rotating body; figure 4 shows a perspective view of the measuring device; and figure 5 shows a schematic overview of a rotary encoder and a linear encoder.

[0037] Figure 1 shows an overview of a measuring device 1. Probes, in particular measurement probes 61- 64 are mounted on a rotating body 5, in particular on a probe holder section 52 of the rotating body 5. The rotating body 5, in particular a shaft section 51 of the rotating body is rotatably supported in a movable stage 3. The movable stage 3 is supported on a frame 2. The rotating body 5 is able to rotate around probe rotation axis Rp, and the movable stage 3 translates in direction Tp. Linear guide 81 supports the movable stage 3 in the direction parallel to axis Rp, and linear guides 82, 83 support the movable stage 3 in the vertical direction. The rotating body 5, as well as the probes 61- 64 thus posses one translational degree of freedom and one rotational degree of freedom relative to the frame 2. The probes 61-64 are arranged to measure the optical element in a probe plane P, which extends perpendicular to the probe rotation axis Rp.

[0038] The frame 2 comprises a vertical planar surface 81, in particular a rotating body guide 81, which is abutted by a bearing, in particular an air bearing 810 to support the rotating body 5 in the third direction, parallel to the axis Rp, and permit movement of the rotating body 5 in the first direction Tp. It is appreciated that the air bearing 810 does preferably not physically contact the surface 81, but remains at a constant distance to that surface by means of an air cushion provided by the air bearing 810. Linear guides 82, 83 and associated bearings (not shown) mounted on the movable stage 3 support the movable stage 3 in the vertical direction, perpendicular to Rp and Tp, and parallel to Ts. The linear guides 82, 83 may additionally serve to support the movable stage 3 in a direction parallel to the axis Rp. Thus, guides 82, 83, and the respective associated aforementioned bearings, provide the first guides to guide the movable stage in the first direction Tp.

[0039] Linear position sensors, in particular linear encoders 43, 44 measure the translational position of the movable stage 3 relative to the frame 2 in measurement planes Ml and M4, which extend parallel to the probe plane P, and at respective distances DI and D4. Consequently, the translational position of the movable stage 3 relative to the frame 2 can be determined, as well as an angular deviation of the movable stage 3 relative to the frame 2 in the plane parallel to Rp and Tp. The linear encoders 43, 44 each comprise an elongated scale 431, 441 mounted on the frame 2 and a sensor 430, 440 mounted on the movable stage 3.

[0040] Rotary position sensors, in particular rotary encoders 41, 42, measure the rotational position of the rotating body 5 relative to the movable stage 3 in respective measurement planes M2, M3. The measurement planes M2 and M3 extend at respective distances D2, D3 from the probe plane P. In addition, the rotary encoders 41, 42 measure the translational position of the rotating body 5 relative to the movable stage. The rotary encoder 41 may extend closer to the probe plane P than is currently shown. In other words, distance D2 may be smaller than currently shown, and in particular smaller than DI. At least one of the probes 61-64 extends from the rotation axis Rp with a distance Dp, which is larger than the radius of the rotary encoder De. As the optical element usually extends outside of the cylinder extending around rotation axis Rp with radius Dp, the ‘interior’ of said cylinder provides room for the rotary encoder 41 to extend close to the probe plane P, as long as De remains smaller than Dp.

[0041] The optical element E is supported on a supporting device 7, which is arranged to translate in direction Ts. The supporting device 7 is shown partially behind the probe holder 52. The supporting device 7 comprises a rotating stage 71 on which the optical element E is supported, which rotates around axis Rs. The rotating stage 71 thus possesses two degrees of freedom: a translation in direction Ts and a rotation around axis Rs. Rotation axis Rp and the measurement plane P coincide in a plane perpendicular to the rotation axis Rp. Thus, at least the entire upwards facing surface of the optical element E may be contacted by a probe 61-64 by translating the supporting device 7 in direction Ts and rotating the rotating stage 71 around axis Rs, and by translating the probe 61-64 in direction Tp.

[0042] Figure 2 shows a view of part of the measuring device 1 , with the movable stage 3 made translucent. The rotary encoder 41 comprises rotary scale 410, as well as three sensors 411 (not shown), 412, 413. The rotary encoder 42 comprises rotary scale 420, as well as three sensors 421 (not shown), 422, 423. Rotary scales 410, 420 are mounted to the rotating body, and sensors 411, 412, 413, 421, 422, 423 are mounted on the movable stage 3. Bearings 53, 54 rotatably support the rotating body 5 in the movable stage 3, and allow translational movement of the rotating body 5 along the rotation axis Rp. Figure 3 shows a perspective view of the rotating body 5. The rotating body 5, in particular the probe holder 52 comprises four probes 61-64. The probe used for measuring the optical element E can thus easily be changed by rotating the rotating body around axis Rp.

[0043] Figure 4 shows a perspective view of the measuring device 1. The frame 2 comprises linear guides 85, 86 which form the second guides mentioned above, and guide the supporting device in direction Ts along vertical face 84, and restrain the supporting device 7 in all other directions. The supporting device comprises a rotating stage 71 which holds the optical element E and is able to rotate around rotation axis Rs. The axis Rs as well as the probe plane P (shown as an axis in figure 3) extend in the same plane, which is perpendicular to Rp.

[0044] Figure 5 shows a schematic overview of rotary encoder 41 and liner encoder 43. Rotary scale 410 is connected to the rotating body 5, and sensors 411, 412, 413, 430 to the movable stage 3. Linear scale 431 is mounted on the frame 2. Sensor 430 measures its translation relative to the scale 431 in direction Y2, which is representative for a displacement of the movable stage 3 relative to the frame 2 in direction Ts in plane M2. Sensors 411, 412, 413 measure displacement of the rotary scale 410 in respective directions Zl, Yl, Z2. Thus, an angular displacement cp of the rotating body 5 in measurement plane Ml is proportional to (Zl - Yl - Z2) / 3. In addition, a vertical displacement of the rotating body 5 relative to the movable stage 3 in measurement plane Ml is equal to (Zl + Z2) / 2, and a displacement of the rotating body 5 relative to the movable stage 3 in direction Ts in measurement plane Ml is approximately equal to Yl - (Zl - Z2) / 2.

Claims

Claims1. Measuring device for measuring at least one property of an optical element, wherein the device comprises: a frame; a supporting device arranged for supporting the optical element on the frame; a rotating body arranged to rotate relative to the frame around a rotation axis; a measurement system arranged to measure the relative position of the rotating body and the frame in at least one measurement plane; a probe arranged on the rotating body for measuring the at least one property of the optical element, wherein the probe is located at a position out of the at least one measurement plane, and wherein the probe extends radially outwardly from the rotation axis.

2. Measuring device according to claim 1, wherein the measurement plane and the probe extend at a mutual distance seen along the rotation axis of the rotating body.

3. Measuring device according to claim 1 or 2, wherein the probe extends from the rotating body in a probe plane substantially perpendicular to the rotation axis of the rotating body, wherein the probe plane extends parallel to and at a distance from the at least one measurement plane.

4. Measuring device according to any of the preceding claims, wherein the rotating body comprises a plurality of probes each extending in the probe plane.

5. Measuring device according to any of the preceding claims, wherein the measurement system is arranged to measure the relative position of the rotating body and the frame in at least two measurement planes, extending parallel to and at a distance from each other and wherein the probe is located at a position out of the at least two measurement planes.

6. Measuring device according to any of the preceding claims, wherein the measurement system is arranged to measure the relative rotation of the rotating body and the frame.

7. Measuring device according to any of the preceding claims, wherein the rotating body is further arranged to translate with respect to the frame, wherein the measurement system is arranged to measure a translational position of the rotating body with respect to the frame.

8. Measuring device according to any of the preceding claims, wherein the measuring device further comprises a movable stage arranged to rotatably support the rotating body on the frame, wherein the measurement system is arranged to measure the relative rotation between the movable stage and the rotating body in the measurement plane, wherein the movable stage is arranged to translate relative to the frame in a first lateral direction, wherein the rotation axis extends substantially perpendicular to the first lateral direction, wherein the measurement system is arranged to measure the relative position of the movable stage and the frame.

9. Measuring device according to at least claims 5 and 8, wherein the measurement system is arranged to measure the relative position of the movable stage and the frame in the at least two measurement planes.

10. Measuring device according to at least claim 8, wherein the measurement system is further arranged to measure the relative rotational position of the rotating body and the movable stage in a measurement plane.

11. Measuring device according to any of the preceding claims, wherein the measurement system comprises at least one encoder system.

12. Measuring device according to at least claim 5 and 11, wherein the measurement system comprises a first linear encoder arranged to measure the lateral position of the movable stage relative to the frame in a first measurement plane of the at least two measurement planes, and a second linear encoder arranged to measure the lateral position of the movable stage relative to the frame in another measurement plane of the at least two measurement planes.

13. Measuring device according to at least claim 11, wherein the measurement system comprises a rotary encoder for measuring the rotation of the rotating body relative to the frame, wherein the rotary encoder comprises a circular scale and a sensor.

14. Measuring device according to at least claim 8 and 13, wherein at least one rotary encoder is arranged for measuring the rotational position around the rotation axis of the rotating body relative to the movable stage.

15. Measuring device according to at least claim 13, wherein the rotary encoder comprises at least a first sensor and a second sensor, wherein the first sensor is mounted along a first radial emanating from the centre of the rotary scale, and wherein the second sensor is mounted along a second radial emanating from the centre of the rotary scale, wherein the first radial and second radial extend at a nonzero mutual angle.

16. Measuring device according to claim 15, wherein the rotary encoder comprises at least a third sensor, wherein the third sensor is mounted along a third radial emanating from the centre of the rotary scale, wherein the third radial extends at a nonzero angle relative to either the first radial or the second radial.

17. Measuring device according to any of the preceding claims, wherein the probe and the supporting device are movable with respect to each other in at least two degrees of freedom, for measuring the surface of the optical element supported by the supporting device, wherein the probe is movable in a first degree of freedom and is fixed along a second degree of freedom, wherein the supporting device is movable in the second degree of freedom and fixed in the first degree of freedom.

18. Measuring device according to claim 17, wherein the supporting device is movable relative to the frame in a second direction, perpendicular to the first lateral direction.

19. Measuring device according to claim 18, wherein the frame comprises first guides to guide the movable stage in the first direction and to constrain the movement of the movable stage in all other directions, and wherein the frame comprises second guides to guide the supporting device in the second direction and constrain the movement of the supporting device in all other directions.

20. Measuring device according to any of the preceding claims, wherein the supporting device comprises a rotating stage which is arranged to rotate relative to the frame and the supporting device.

21. Measuring device according to at least claim 18 and 20, wherein the rotating stage is arranged to rotate around an axis parallel to the second direction.

22. Measuring device according to any of the preceding claims 18 - 21, wherein the first direction extends parallel to the horizontal plane, and wherein the second direction extends in the vertical direction.

23. Method for measuring at least one property of an optical element using a device according to any of the preceding claims.

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