Measurement of long focal lengths with a double-grating interferometer

A method for measuring long focal lengths using a double-grating interferometer is proposed. The intensity distribution of a double-grating interferometer illuminated by a spherical beam is derived with diffraction theory. A tiny rotation angle is set between the two gratings, and a high-contrast interference pattern is produced by the adjacent diffracted orders. The angular change in the fringes after insertion of a test lens is a measure of the focal length. The uncertainty due to aberration of the collimated beam was analyzed by measuring a series of lenses. The relative deviations are less than 0.1%.