By stepping opaque rectangles which are closely adjacent, the gap between edges defines a single slit which by coherent illumination produces a diffraction pattern on film. This diffractogram recording is improved by an attenuator of decreasing density away from the axis. The method applies to step and repeat error measurement, as well as mask and copy line size evaluation.
Robert E. Lewis, Robert E. Lewis,
"Diffractogram Step And Repeatability Testing", Proc. SPIE 0055, Technological Advances in Micro and Submicro Photofabrication Imagery, (1 March 1974); doi: 10.1117/12.954247; https://doi.org/10.1117/12.954247