Evaluation of the deformation value of an optical flat under gravity

Abstract

The flatness of an optical surface can be evaluated using a Fizeau interferometer. There is strong demand for ensuring that the measurement uncertainty of flatness is of nanometer order over a measurement range of 300 mm or more; however, the measurement range and measurement uncertainty of flatness at the National Metrology Institute of Japan (NMIJ) are 300 mm and 10 nm, respectively. In a Fizeau flatness interferometer, the gap distance between the reference flat and the specimen is measured. To obtain the absolute profile of the specimen, the absolute profile of the reference flat should be measured in advance. The three-flat test is one of the methods used to measure the absolute profile of a reference flat. The reference flat, however, deforms under the force of gravity, and its absolute deformation value cannot be determined by the three-flat test. The deformation value of the reference flat can be corrected by the finite element method (FEM) analysis; however, it is difficult to ensure the validity of the analysis and there is a large uncertainty component of the Fizeau flatness interferometer. To verify the FEM analysis, we developed a scanning deflectometric profiler (SDP) that does not require a reference flat and can directly measure a profile. We calibrated an optical flat using a Fizeau flatness interferometer and the SDP. Finally, the deformation value of the reference flat under the force of gravity was evaluated by comparing the measurement results.

Approaching sub-50 nanoradian measurements by reducing the saw-tooth deviation of the autocollimator in the Nano-Optic-Measuring Machine

“…Such research is underway at several institutes: The Diamond-NOM of the Diamond Light Source Ltd., UK [3], the Alba-NOM of the Spanish Synchrotron Radiation Facility, Spain [4], and the APS-NOM of the Advanced Photon Source (APS), USA [5,6]. Also under development is the longtrace profiler of the Advanced Light Source (ALS), USA [7], the MHPP-LTP-NOM of Brookhaven National Laboratory (BNL), USA [8], the Zeeko Scanning Penta-prism Profiler SPP 1600 at Zeeko Ltd., UK [9], and the Scanning deflectometric profiler (SDP) of the National Metrology Institute of Japan [10].…”

High-lateral-resolution scanning deflectometric profiler using a commercially available autocollimator

“…A three-flat test is typically used to determine the absolute profile of the reference flat in the Fizeau interferometer. However, the accuracy of the three-flat test is limited by gravitational effects [1]. In addition, Fizeau interferometry is not suitable for the measurement of large specimens, such as a bar mirror of more than 1 m in length.…”