Reflective grating interferometer for measuring the focal length of a lens by digital moire effect

A new technique is proposed, based on digital moire effects,for measuring the focal length of a lens. The test lens is illuminated by a variable pitch interference pattern produced by an interferometer which makes use of a reflective diffraction grating, Moire fringes are generated by digitally adding the magnified images produced by the lens on the original interference pattern. The focal length of the lens can be determined by counting the number of moire fringes. This technique is suitable for measuring both long and short focal length lenses. The focal lengths of both positive and negative lenses were experimentally measured.