A light beam generated by two laser-illuminated circular apertures in a plane screen and two properly adjusted phase-shifting glass plates exhibits a central irradiance minimum that makes it favorable for alignment applications. The far-field transverse irradiance distribution in such a beam is here determined theoretically and experimentally to facilitate appraisal of its suitability for this purpose. Criteria aiding optimization of the beam and factors that degrade the beam and produce alignment errors are discussed. Alignment accuracy depends upon correspondence of a reference beam to a mathematically ideal, perfectly symmetrical shape. It is shown that a beam closely approaching the ideal can be easily produced by this method. Alignment accuracy is then found to be limited chiefly by air turbulence and stray light, which in the experimental investigation limited precision to ± 10 /µm at 8.2 m.
B. G. Whitford, "Two-Aperture Method of Producing Light Beams with Odd-Symmetry for Alignment and Straightness Measurements," Appl. Opt. 11, 2069-2074 (1972)