Phase-Shifting Electronic Speckle Pattern Interferometry (ESPI) with Unresolved Speckle

Electronic speckle pattern interferometry (ESPI) is usually realized with resolved speckle. In that case the mean speckle size approximately equals the pixel size of the CCD or CMOS sensor. This is achieved by using a small aperture of the objective which simultaneously reduces the intensity of the object light. If the laser power is limited and large objects are to be illuminated, the aperture must be opened in order to ensure a sufficient exposure of the sensor. Thereby the speckle become smaller and every pixel integrates over several speckle (up to some 10). However, even in this case speckle interferometry is possible, as shown theoretically by Lehmann (see Opt. Commun. 128 (1996) 325-340). We are presenting experimental results which were gained for different phase shifting techniques under variation of the speckle size and which are analyzed with respect to the signal-to-noise ratio in the resulting mod-2pi phase maps.