Interferometric measuring device
A method of measuring a surface of an optical element and an interferometric measuring device for measuring a surface or profile of the optical element. The optical element having a first surface and a second surface opposite the first surface. The method includes defining at least a first measurement point, a second measurement point and a third measurement point on a measurement surface of the optical element being one of the first surface and the second surface, measuring a first position of the first measurement point by directing a measurement beam from a measurement head onto the first measurement point and by detecting a measurement beam portion reflected at the first measurement point, subsequently measuring at least a second position of the second measurement point and a third position of the third measurement point by directing the measurement beam onto the second measurement point and onto the third measurement point and by detecting a measurement beam portion reflected at the second measurement point and the third measurement point, respectively, and determining at least one of a decenter and a tilt of the measurement surface relative to a reference axis on the basis of at least the first position, the second position and the third position.
1 . A method for interferometrically measuring a surface or a profile of an optical element having a first surface and a second surface opposite the first surface, the method comprising:
defining at least a first measurement point, a second measurement point and a third measurement point on a measurement surface of the optical element, the measurement surface being one of the first surface and the second surface;
measuring a first position of the first measurement point by directing a measurement beam from a measurement head onto the first measurement point, receiving by the measurement head a first measurement beam portion reflected at the first measurement point and detecting the first measurement beam portion by a detector optically coupled to the measurement head;
subsequently measuring at least a second position of the second measurement point and a third position of the third measurement point by directing the measurement beam onto the second measurement point and directing the measurement beam onto the third measurement point, receiving by the measurement head a second measurement beam portion reflected at the second measurement point and detecting a third measurement beam portion reflected at the third measurement point, respectively, and the second measurement beam portion and the third beam portion by the detector, respectively; and
determining at least one of a decenter and a tilt of the measurement surface relative to a reference axis on the basis of at least the first position, the second position and the third position;
wherein one of the first surface and the second surface of the optical element facing towards the measurement head and the other one of the first surface and the second surface facing away from the measurement head is the measurement surface, wherein the first position, the second position and the third position is measured by directing the measurement beam onto a first target point, a second target point and a third target point, wherein the first target point, the second target point and the third target point are located on the one of the first surface and the second surface facing towards the measurement head, and wherein the measurement beam propagates through a medium of the optical element, and
wherein any of the first, second and third measurement beams propagating from the respective first, second and third target points towards the first, second and third measurement points, respectively, hit the measurement surface substantially perpendicularly.
2 . The method according to claim 1 , wherein measuring at least one of the first position, the second position and the third position comprises focusing the measurement beam onto at least one of the first measurement point, the second measurement point and the third measurement point and detecting the respective measurement beam portion reflected at the at least one of the first measurement point, the second measurement point and the third measurement point, respectively.
3 . The method according to claim 1 , wherein a position of the first target point, the second target point and the third target point on the one of the first surface and the second surface facing towards the measurement head is determined on the basis of:
a refractive index of the medium of the optical element;
an angle of incidence of the measurement beam on the first target point, the second target point or the third target point; and
a local surface profile of at least one of the first surface and the second surface in the region of the first measurement point, the second measurement point or the third measurement point and/or the respective first target point, the second target point or the third target point.
4 . The method according to claim 1 , wherein a first measurement path and a second measurement path are defined on the measurement surface, wherein at least two of the first measurement point, the second measurement point and the third measurement point are located on the first measurement path and wherein at least one of the first measurement point, the second measurement point and the third measurement point is located on the second measurement path.
5 . The method according to claim 4 , wherein at least one of the first measurement path and the second measurement path is a closed measurement path.
6 . The method according to claim 4 , wherein the first measurement path and the second measurement path are concentric with regards to an optical axis of the optical element or with regard to the reference axis.
7 . The method according to claim 1 , wherein the optical element is attached to a mount arranged on a measurement stage, the measurement stage being rotatable about an axis of rotation, wherein at least one of a radial position and an orientation of the mount relative to the axis of rotation is adjusted to minimize at least one of the decenter and the tilt of the optical element.
8 . The method according to claim 1 , wherein in a first measurement procedure at least one of the decenter and the tilt of one of the first surface and the second surface is determined and wherein in a second measurement procedure at least one of the decenter and the tilt of the other one of the first surface and the second surface is determined.
9 . The method according to claim 1 , wherein measuring the first, second and third positions of the respective first, second and third measurement points is conducted by measuring a path difference of the reflected measurement beam portion compared to a reference beam.
10 . The method according to claim 1 , wherein the detector is optically coupled to the measurement head via an optical circulator.
11 . A non-transitory computer readable storage medium comprising a computer program comprising instructions, the instructions capable of being executed by a processor associated with an interferometric measuring device that measures a surface or a profile of an optical element having a first surface and a second surface opposite the first surface,
wherein an interferometric measuring device, upon its processor executing the instructions, carries out the following:
defines at least a first measurement point, a second measurement point and a third measurement point on a measurement surface of the optical element, the measurement surface being one of the first surface and the second surface;
measures a first position of the first measurement point by directing a measurement beam from a measurement head onto the first measurement point, by receiving via the measurement head a first measurement beam portion reflected at the first measurement point, and by detecting the first measurement beam portion by a detector optically coupled to the measurement head;
subsequently measures at least a second position of the second measurement point and a third position of the third measurement point by directing the measurement beam onto the second measurement point and directing the measurement beam onto the third measurement point, by receiving via the measurement head a second measurement beam portion reflected at the second measurement point and a third measurement beam portion reflected at the third measurement point, respectively, and by detecting the second measurement beam portion and the third measurement beam portion by the detector, respectively; and
determines at least one of a decenter and a tilt of the measurement surface relative to a reference axis on the basis of at least the first position, the second position and the third position;
wherein one of the first surface and the second surface of the optical element facing towards the measurement head and the other one of the first surface and the second surface facing away from the measurement head is the measurement surface, wherein the first position, the second position and the third position is measured by directing the measurement beam onto a first target point, a second target point and a third target point, wherein the first target point, the second target point and the third target point are located on the one of the first surface and the second surface facing towards the measurement head, and wherein the measurement beam propagates through a medium of the optical element, and
wherein any of the first, second and third measurement beams propagating from the respective first, second and third target points towards the first, second and third measurement points, respectively, hit the measurement surface substantially perpendicularly.
12 . The non-transitory computer readable storage medium according to claim 11 , wherein measuring the first, second and third positions of the respective first, second and third measurement points is conducted by measuring a path difference of the reflected measurement beam portion compared to a reference beam.
13 . The non-transitory computer readable storage medium according to claim 11 , wherein the detector is optically coupled to the measurement head via an optical circulator.
14 . An interferometric measuring device for measuring a surface or a profile of an optical element having a first surface and a second surface opposite the first surface, the measuring device comprising:
a light source for emitting a measurement beam;
a mount to fix the optical element;
a measurement head variably movable relative to the mount and optically coupled to the light source for directing a measurement beam onto a predefined first measurement point, onto a predefined second measurement point and onto a predefined third measurement point of the measurement surface, wherein the measurement head is further configured to receive a first measurement beam portion reflected at the first measurement point, a second measurement beam portion reflected at the second measurement point, and a third measurement beam portion reflected at the third measurement point, respectively;
a detector optically coupled to the measurement head for detecting the measurement beam portion reflected at least at the first measurement point, the second measurement point and the third measurement point;
a signal analyzer connected to the detector for determining at least a first position of the first measurement point, a second position of the second measurement point and a third position of the third measurement point, and for determining at least one of a decenter and a tilt of the measurement surface relative to a reference axis on the basis of at least the first position, the second position and the third position;
wherein the measurement head is positioned relative to the optical element to direct the measurement beam onto at least a first target point, a second target point and a third target point on one of a first surface and a second surface of the optical element opposite to the measurement surface and facing towards the measurement head, wherein each of the first target point, the second target point and the third target point correlates with one of the first measurement point, the second measurement point and the third measurement point, and
wherein the measurement head is positioned such that the measurement beam entering a medium of the optical element at the first target point, at the second target point and at the third target point hits the measurement surface substantially perpendicularly and is internally retroreflected at the first measurement point, the second measurement point and the third measurement point, respectively.
15 . The interferometric measuring device according to claim 14 , wherein the mount is arranged on a rotary measurement stage defining the reference axis and wherein at least one of a radial position of the mount and an orientation of the mount relative to the reference axis is adjustable.
16 . The interferometric measuring device according to claim 14 , further comprising a controller operable to adjust at least one of a radial position of the mount and an orientation of the mount relative to the reference axis on the basis of at least one of the decenter and the tilt of the measurement surface.
17 . The interferometric measuring device according to claim 14 , further comprising a measurement head controller operable to move and/or to align the measurement head relative to the mount.
18 . The interferometric measuring device according to claim 14 , further comprising a reference beam, wherein a path difference is defined between the reflected measurement beam portion and the reference beam for measuring the first, second and third positions.
19 . The interferometric measuring device according to claim 14 , wherein the detector is optically coupled to the measurement head via an optical circulator.