Shashi Prakash

A low cost, accurate and relatively simple technique based on the Lau effect coupled with Moiré readout for measuring the focal length of collimating lenses is demonstrated. The method is based on defocusing the collimating lens and measuring the corresponding variation in the inclination angle of the Moiré fringes. Details of theoretical background, experimental arrangement and error analysis are presented. Experimental results of the measurements are in close agreement with the true values.

We present an easy, simple, and inexpensive technique for checking the quality of the collimation of optical beams using the Lau effect combined with moiré readout. The experimental arrangement consists of a modified Lau-based interferometer in which a white-light incoherent source illuminates a set of two gratings. A collimating lens is placed between the two gratings such that the self-images of the second grating are formed. The third grating is positioned at one of the self-imaging planes forming moiré fringes. The type of the moiré fringe demonstrates the quality of collimation of the optical beam. The necessary theoretical background is presented and the results of our experimental investigation are reported. The technique can also be used for accurate determination of the focal length of a collimating lens using low-cost components.

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 G_1. 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.

A technique for a tilt-angle measurement of reflecting objects based on the Lau interferometry coupled with the moiré readout has been proposed. A white-light incoherent source illuminates a set of two gratings, resulting in the generation of the Fresnel image due to the Lau effect. The Fresnel image is projected onto a reflecting object. The image reflected from the object is superimposed onto an identical grating, which results in the formation of a moiré fringe pattern. The inclination angle of moiré fringes is a function of tilt angle of the object. Theory and experimental arrangement of the proposed technique is presented and results of the investigation are reported.