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Simple digital technique for high-accuracy measurement of focal length based on Fresnel diffraction from a phase wedge

An accurate and simple technique based on Fresnel diffraction of light from a phase wedge is described to measure the effective focal lengths (EFLs) of optical imaging systems. When the neighbourhood of a transparent wedge is illuminated by a monochromatic parallel beam of light, the Fresnel diffraction occurs as a result of sharp change in phase at the wedge boundary. The wedge with small apex angle imposes a linearly varying phase change that causes periodic diffraction fringes in the direction parallel to the wedge boundary. The EFL is determined by measuring the fringe spacing of the Fresnel diffraction both before and after installing the test lens in the setup. The method is easy to apply and it is less sensitive to vibration compared to interferometric methods. It covers a wide range of both positive and negative focal lengths. Experimental results for several different lenses are quite consistent with the values indicated by the lens manufacturer.