Out-of-plane displacement measurement by means of endoscopic moiré interferometry

Abstract

An endoscopic moiré technique is proposed for measuring out-of-plane displacements in difficult to reach places. The Ronchi grid is projected onto the tilted object with one endoscope with a 0° viewing angle. The object with the projected grid is imaged by a second endoscope with a 30° viewing angle onto a charge-coupled device. The captured images are stored in a PC, and are used to calculate the out-of-plane displacement of the object with a phase stepping technique. A computer generated grating method is used instead of a physical phase-shift device in the optical setup. This allows designing a set of three reference grids with profiles closely similar to the projected grating. The technique is robust against problems associated with the temporal shifting method, such as nonlinear phase shift and noise. To test the feasibility of the technique the measurement of out-of-plane displacements of about 35 μm of a latex membrane under deformation is demonstrated. The advantages and disadvantages are discussed.