Interferometry of wave fronts reflected off conical surfaces requires new interferometric schemes. The pencil beam interferometer is proposed as a metrology tool for conical as well as for spherical optical surfaces. The instrument employs two narrow pencil beams which scan the optical surface to be measured. An electronic fringe position readout system, which measures the location of the fringes that develop through interference of the pencil beams, provides high accuracy surface information. Metrology on a waxicon indicates a measurement precision of ±6 nm. Due to the differential nature of the instrument, alignment requirements are relatively modest.
© 1983 Optical Society of America
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