High-speed double-slit interferometer using a position sensitive detector for in-line thickness variation measurement of glass panels

We present a high-speed double-slit interferometer (HS-DSI) for in-line thickness measurement of large-sized glass panels. Because the HS-DSI has a differential and common-path configuration, it is robust to external disturbance and vibration of glass panels. For the in-line measurement, the interferometer employs a position sensitive detector (PSD) to detect an interference fringe shift with high speed, which corresponds to the differential thickness variation. We present the method and the results of theoretical analysis on the performance of the HS-DSI in respect of sensitivity and nonlinearity error to determine the optimal design parameter values, and the optimal design strategy. A calibration method using a cylindrical lens is also proposed. In the comparison with an interferometric thickness measurement system, the deviations were less than 70 nm and 15 nm for thickness profiles and their waviness components, respectively. When a glass plate was translated with speed of 300 mm/s, the measurement repeatability was less than 0.4 nm for the waviness component of the thickness profile.