Precise focal-length measurement technique with a reflective Fresnel-zone hologram.

A new technique for precise focal-length measurement with a hologram is presented. This technique is widely applicable and is particularly useful for measuring large, slow lenses. In diffraction, the Fresnel-zone plate hologram emulates the reflective properties of a convex spherical mirror for use during transmission null tests of an optic by use of a phase-shifting interferometer. The hologram is written lithographically and therefore offers a higher degree of precision at a lower cost than its spherical mirror counterpart. A hologram offers the additional benefit of easy characterization by use of the same interferometer employed in examining the test optic. Better than +/-0.01% precision is achieved during measurement of a 9-m focal-length lens by use of a 150-mm aperture interferometer.