Measuring Aspheric Lenses
ROCHESTER, NY, May 14, 2009 – In a discussion on aspheric lenses at Optifab 2009, Piotr Szwaykowski of Engineering Synthesis Design Inc. compared a newly developed single-measurement Fizeau interferometer with conventional interferometric stitching methods and mechanical stylus profiles to see how well each method measured aspheric surfaces.
The new interferometer is based on the Fizeau configuration and uses a sub-Nyquist CCD camera as a detector. The camera design allow the instrument to record very dense fringe patterns of up to 4 fringes/pixel, enabling processing of interferograms with hundreds of fringes in the field of view.
The tests showed good correlation and also highlighted the strengths and weaknesses of each method. “All of the measurements came with lateral scale,” Szwaykowski said.
As a result of the comparison, Szwaykowski and his colleagues came to the conclusion that the measurements of a set of aspheric lenses using the three interferometers generally produced the same results for all three instruments. An adjusted aperture kept the comparisons the same, he said.
The difference between the measurements can be accredited to the disparities in the measurement of the radius of curvatures coupled with the aspheric surfaces. “When you do the measurements with the different systems you start seeing a difference,” Szwaykowski said.
Amanda D. Francoeur