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Turned edges and interferograms, an example

By Eric Jensen

There have been discussions regarding interferometry testing and evaluation, and whether interferometry is an adequate tool for testing for a turned edge.

The below images show what a 1/2 wave turned edge looks like in an interferogram. This particular sample is a lens, but the result applies also to mirrors, since the interferometer measures the wavefront as it converges toward final focus. It thus does not differentiate between lens or mirror.

The images clearly show that the lines bend upwards at the edges. This is the turned edge.

There is no diffraction ring along the edge of the interferogram, the condition of the entire optic, including the edge, is revealed without difficulty.

Two images are included to show the difference a change in the number of lines makes. The interferogram with the fewest lines has increased sensitivity, with more visibly curving lines. The edge fault is however still quite visible in image also in the other interferogram.

Vev-ansvarlig: Hilde Birch
Sist endret: 16. oktober 2002.