Electro-optic products sometimes require sapphire plate lenses of high quality that are highly accurate in terms of flatness and parallelism and are in a scratch-free condition. This is difficult to achieve by conventional methods (for example, with a general polishing machine, single-sided polishing machine, etc.). In addition, the preparation of the laps and the lapping process are inconvenient and time consuming. We have designed a new method for lapping sapphire flat lenses that is based on the mechanism of the correction of a double-sided lapping machine's lapping plate and the measurement of highly accurate parallelism by means of lateral shearing interference. This method can accurately and efficiently lap extremely hard sapphire flat lenses.
"Highly accurate flatness and parallelism in the manufacture of thin sapphire flat lenses," Optical Engineering 33(2), (1 February 1994). https://doi.org/10.1117/12.152031