Membrane shaping by low coherence speckle interferometry for pressure measurement

In this work we developed a novel pressure measurement technique based on the deformation evaluation of a membrane submitted to a pressure differential. The deformed membrane shape was determined by low-coherence speckle interferometry. In this method, a tunable diode laser at 660 nm emitting simultaneously two or more longitudinal modes illuminates the optical setup. The resulting speckled low spatial frequency interferogram of the image corresponding to the membrane shape was evaluated by conventional 4-stepping and phase unwrapping analyses. The sensitivity of the measurement process was controlled by tuning the laser with the help of the Littman-Metcalf arrangement using a 2380 lines/mm reflective diffraction grating, which provided a tunable range of 3 nm. The 0.420 mm thick aluminum membrane was submitted to pressure values from 0 to 90 kPa and a curve of the maximum membrane deformation as a function of the pressure was obtained. The experimental results were compared with the ones obtained by a numerical algorithm.