Production method and measurement method
A method for measuring a surface shape of an optical element, wherein the optical element has a main body with a substrate and a reflective surface, and wherein at least one cooling channel for receiving a coolant is formed in the substrate, comprising: a) recording a cooling channel pressure, b) recording a measurement environment pressure, c) determining a pressure difference based on the cooling channel pressure and the measurement environment pressure, d) comparing the pressure difference with a predetermined target pressure difference, e) monitoring for a deviation between the pressure difference and the target pressure difference, wherein, if a deviation greater than a predetermined limit value is detected, the cooling channel pressure is adapted in such a way that the deviation becomes less than or equal to the predetermined limit value, and f) measuring the surface shape if the deviation is less than or equal to the predetermined limit value.
1. A method for measuring a surface shape of an optical element in a predetermined measurement environment, wherein the optical element comprises a main body with a substrate and a reflective surface, and wherein at least one cooling channel for receiving a coolant is formed in the substrate,
the method comprising:
a) recording a cooling channel pressure,
b) recording a measurement environment pressure,
c) determining an actual pressure difference based on the cooling channel pressure and the measurement environment pressure,
d) comparing the actual pressure difference with a predetermined target pressure difference,
e) monitoring for a deviation between the actual pressure difference and the target pressure difference, wherein, if a deviation greater than a predetermined limit value is detected, the cooling channel pressure is adapted in such a way that the deviation becomes less than or equal to the predetermined limit value, and
f) measuring the surface shape if the deviation is less than or equal to the predetermined limit value.
2. The method of claim 1 , wherein the cooling channel pressure is adapted so that the deviation is less than 10 mbar.
3. The method of claim 2 , wherein the cooling channel pressure is adapted so that the deviation is less than 1 mbar.
4. The method of claim 1 , further comprising feeding a liquid or gaseous coolant into the cooling channel, wherein a pressure on the coolant is increased or decreased in order to adapt the cooling channel pressure.
5. The method of claim 4 , wherein the pressure on the coolant is increased or decreased hydraulically or pneumatically.
6. The method of claim 4 , wherein feeding the coolant comprises feeding the coolant such that the coolant flows through the cooling channel at a predetermined flow speed.
7. The method as of claim 4 , wherein the coolant comprises a refractive index is substantially equal to the refractive index of the substrate of the optical element.
8. The method of claim 1 , wherein the target pressure difference is determined based upon a predetermined target measurement environment pressure and a predetermined target cooling channel pressure, wherein the target measurement environment pressure is at least 0.01 mbar and at most 0.20 mbar and the target cooling channel pressure is at least 200 mbar and at most 10 000 mbar.
9. The method of claim 8 , wherein the predetermined target measurement environment pressure is 0.05 mbar and the predetermined target cooling channel pressure is 500 mbar.
10. A measurement apparatus for examining a surface shape of an optical element, wherein the optical element has a main body with a substrate and a reflective surface, and wherein at least one cooling channel for receiving a coolant is formed in the substrate, the measurement apparatus comprising:
i) a measurement light source,
ii) an interferometer with which an examination of at least a partial surface of a surface of the optical element is carried out by interferometric superimposition of a test wave, which is obtained from measurement light produced by the measurement light source and is directed onto the optical element, and a reference wave;
iii) at least one controllable coolant reservoir for storing coolant, and
iv) a control device, which is configured to carry out the method as claimed in claim 1 .
11. The measurement apparatus of claim 10 , wherein the coolant has a refractive index that is substantially equal to a refractive index of the substrate of the optical element.
12. The measurement apparatus of claim 10 , wherein the coolant is a solution of an inorganic or organic substance in water.
13. The measurement apparatus of claim 12 , wherein the inorganic or organic substance forms a homogeneous phase when mixed with water.