Improved collimation testing using Talbot interferometry.

Several schemes for checking collimation of a light beam based on the self-imaging (Talbot effect) combined with the moire technique have been described. When a grating is illuminated by a collimated beam of monochromatic light, exact self-images of the grating are formed at certain welldefined planes perpendicular to the direction of the beam. The moire pattern is observed by placing an identical second grating at one of these planes. In the event of the illuminat­ ing beam being noncollimated, the spatial frequency of the self-image is different from that of the grating itself. This change in frequency is detected by a change in the moire pattern. This is the basic principle of collimation testing by this method. We present a variation of this technique which has a twofold increase in the sensitivity and does not require a separate reference. A grating illuminated by a collimated beam produces selfimages at several distances from the grating given by