Parasitic reflection separation deflectometry based on harmonic analysis

Abstract

In the measurement of transparent planar element using deflectometry, the fringe patterns reflected off the double surfaces of transparent planar element are superimposed on the camera, causing the phenomenon of parasitic reflection. The parasitic reflection makes it difficult for traditional deflectometry to accurately obtain the front surface shape. Previous solutions suffer from inability to reliably measure the element with thickness less than 10 mm. In practice, modulated by the nonlinear response, the signal captured by the camera contains higher-order harmonics, whose frequency difference is larger than that of the fundamental. Therefore, a novel method based on harmonic analysis is proposed. This parasitic issue of thin transparent element can be eliminated by analyzing the harmonic component in power spectrum. The surface shape of element with thickness of 5 mm is measured in experiment. The measurement error is around 80 nm in RMS with a diameter of 52 mm.