ABSTRACT
A simple method for attaining alignment of a target and parallelism between a plane of reference and a target is developed by using as reference planes diffraction gratings of a convenient pattern. Coherent and monochromatic light, emitted from a laser, illuminates the. target, which is either a cross slit or an annular aperture. The reference grating is placed at a distance from the target. The principal maxima created by the light passing through the target and the grating form distorted shapes of the target on the plane of the grating. Alignment can be achieved by displacing the grating in its plane to form a symmetric pattern of the (+/-m)-order principal maximum image of the target. The distance between the target and the reference grating, as well as the angle subtended by their planes, can be accurately estimated by the size and the distortion of the symmetric (+/-m)-order principal maximum image. The apparatus used was very simple, and its flexibility can be enhanced by using a series of amplitude gratings of different line frequencies. Furthermore, automated detection and scanning of the spectrum of the grating increase considerably the accuracy of the method.
