Measurement of a microphone membrane deflection profile using an optical fibre and wedge fringe projection

A new technique for determining the deformation of the membrane in a microphone is described. Using an optical fibre and an optical wedge a sinusoidal fringe pattern with a pitch of the order of micrometres can be readily generated. Due to the laser interference in a perfect common path the proposed fringe projector is compact and the stable fringe pattern from the optical wedge can be projected onto the test surface. A four frame phase-shifting technique is employed for phase measurement. The phase shifting is carried out by translating the optical fibre mounted on a translation stage. The fringe pattern projected onto the test surface is captured by a CCD camera mounted on a long distance microscope. As demonstrated by the experimental results the proposed set-up is capable of measuring deformations of the membrane of the microphone to sub-micrometre levels.