Projected fringes and speckle reduction
When fringes are projected on an object their appearance, as seen from an angle, will change according to the shape of the object. The shape of the object can then be calculated from the phase of the disturbed fringe pattern.
This means that it is of importance to have sharp fringes all over the object, hence we need fringes with a large depth of focus. A large depth of focus can be achieved if the projected light is coherent and the fringes are produced by interference. In the imaging, however, we don’t want any coherence since this will give a speckle pattern which will complicate the detection of the fringes and thus also the phase.
In order to reduce the error in the phase we need to suppress the speckles. This can be done by moving the aperture in the Fourier plane and in that way obtain a sum of many independent speckle patterns on the detector.