Fizeau digital interferometry with a diffraction-generated spherical wave for testing focusing optics

A sufficiently small, reflective spherical particle at the focus of a test lens can replace the spherical autostigmatizing mirror in Fizeau digital interferometry, which is used for testing focusing optics. This small particle serves as a secondary coherent point source in the test arm and generates by diffraction a spherical wave that is used to produce the test beam. The relationships between the diffraction-generated wave and the size and shape of the diffracting element are discussed and also used to properly select the diffracting element. Experimental results confirm that the new technique has high accuracy.