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A modified approach for collimation testing using Lau interferometry

A modified approach for the collimation testing of an incoherent optical beam using the Lau effect coupled with moiré readout is proposed. In the experimental set-up a white-light incoherent source illuminates a coarse grating G1. The collimating lens is mounted on a translation stage, so that the grating G1 can be adjusted to be at the ‘in-focus’, ‘at-focus’ or ‘out-of-focus’ position. Grating G2 and grating G3 are mounted on a precision rotational stage and are so positioned that the lines of the gratings make a small but equal and opposite angle with the vertical. The inclination angle of the resulting moiré fringe demonstrates the quality of collimation of optical beam. The proposed technique can also be conveniently used for measurement of the focal length of a collimating lens, at a very low cost.