Varifocal moiré zone plates for straightness measurement.

A Fresnel zone plate of variable size can be formed as a moiré pattern between a suitable pair of identical grids. The focusing power may be varied over a considerable range by relative translation of the grids if their line sbapejs are determined by a combination of cubic and linear functions. Elliptical and hyperbolic patterns are also obtainable. If both grids ae constructed in phase reversal form, the irradiance in the diffraction image due to the zone plate is enhanced substantially. The device may then be used with photoelectric detection to monitor the straightness of a continuous linear motion. A possible application to field widening in interference spectroscopy is also suggested.