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Absolute testing of flats using four data sets

Recent interest in using optical systems at shorter and shorter wavelengths for applications like UV photolithography and synchrotron focussing mirrors has put increasing pressure on optical metrologists to improve absolute methods of testing. Because optical metrologist do not have the luxury of possessing measuring instruments that are many times more accurate than what they want to measure, they are forced to make differential measurements between "reference" surfaces and the surface under test. However, even the (often) flat reference surface figure is not known to the accuracy now required by these new applications. This need for a more accurate knowledge of the reference surfaces has created a wide interest in the absolute calibration of flats.