The authors were presented with the task of measuring the focal length of a large, 9055mm focal length lens (at 633nm) to a precision of two parts in 104. One method for testing the focal length of a lens (with lesser precision) involves transmitting the beam of a phase-shifting interferometer (PSI) through the lens, and retro-reflecting the resulting converging (or diverging) beam from a spherical mirror back through the lens to the PSI. For the mirror position giving a null fringe in the PSI, knowledge of the mirror radius of curvature and its separation from the lens vertex determines the back focal distance (BFD) of the lens. Further knowledge of the lens figure may be used to determine its effective focal length.

© 2002 Optical Society of America

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