Slotted aperture filter masks can be used to "tune" a photographic system to an integral multiple of a fundamental grating frequency. Such a procedure can be used in moire photography for multiplying sensitivity and for obtaining improved rendering of fringe patterns. Depth of field is increased, and a camera lens of poorer quality than is normally required for high resolution photography of moire gratings is adequate. Also gained is flexibility in optical data processing of moire photographs and in the use of two-dimensional grid and dot specimen arrays.
"Slotted Apertures for Multiplying Grating Frequencies and Sharpening Fringe Patterns in Moire Photography," Optical Engineering 15(6), 156578 (1 December 1976). https://doi.org/10.1117/12.7972044