Interferometer is a powerful tool for optical surface measurement, including figure and roughness, due to its nanometer accuracy and non-contact manner. Traditional phase-shifting interferometry (PSI) is much sensitive to environmental vibration that impairs its application in measurement in workshop or on machine. Based on the iterative algorithm that is tolerant to phase-shifting error caused by vibration, two interferometers are developed to measure the optical surface figure and roughness respectively. A laser interferometer, of which the aperture size is 150mm, has been built to measure the surface figure. Practical test demonstrates that the laser interferometer achieves accuracy better than 5nm under vibration of 0.4 micron-amplitude over a large frequency range, 0-35Hz. And an interferometric microscope has been proposed to measure the surface roughness and verified to be effective. The measuring area of the microscope depends on the employed interference objective, and a typical value is about 1 squared millimeter. The error of measured roughness (Sq) under vibration, 0.4 micron-amplitude and over 0-20Hz frequency range, is less than 0.5nm. The developed method and instruments could be applied to optical surface measurement in vibration. The study relaxes the requirement of interferometers on environment and predicates an in-workshop or on-machine solution for optical surface measurement.
In complex phase-shift interferometry, an adaptive phase algorithm based on complex vibration environment is proposed. The phase shift amount calculation of two or more interferometric images could be achieved by self-adaptive phase-shift algorithm (SPA), a closed-loop feedback system is constructed to adaptively correct the characteristic phase shift amount in which the phase shift step length is a set value, thereby realizing self-correction of the phase shift error. According to the actual examinations, the single-step phase shift error is less than π/50, and PV and RMS values of plane objects were obtained respectively better than λ/20 and λ/100. Additionally, the feasibility of the algorithm and the self-adaptive correction system for complex vibration environment were verified. The results show that the proposed method achieves high-speed and high-precision phase-shift extraction, greatly reducing the phase shift error caused by vibration interference, and the overall operation of measurement process is simple relatively.
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