March 14, 2007, Rochester, NY--QED Technologies has commercially introduced an aspheric version of its SSI (Subaperture Stitching Interferometry) tool, called the SSI-A. Both the SSI and the SSI-A can measure the surface shape of optical surfaces much larger than the aperture of the tool by taking many smaller overlapping measurements and then stitching them together in software.
For aspheres, the technique is especially important. Traditional interferometric techniques for characterizing aspheres fall into two categories: 1) for very mild aspheres, an ordinary interferometer can be used; and 2) for moderate to severe aspheres, an interferometer plus null optics (which change the aspheric wavefront to a spherical one for test) are required. The disadvantage for method 1) is that if the asphere departs from sphericity in any substantial way, the interference fringes get too close together for accurate measurement. For method 2), the required null optics can become quite expensive and difficult to fabricate and work with.
What the SSI-A technique fills is the very important middle ground--the characterization of moderately strong aspheres (see http://www.laserfocusworld.com/articles/283870 and http://www.laserfocusworld.com/articles/279859). No null optics are required, and the SSI-A can measure aspheric surfaces that depart significantly from sphericity.
In operation, the SSI-A automatically selects an appropriate transmission sphere and then steps through an automated process to complete the measurement. The SSI-A magnifies the subaperture measurements over the full aperture of the asphere, so the subsequent stitching process and lattice design result in superior lateral resolution and overall accuracy.
"This one metrology tool can measure flats, spheres and aspheres," said Marc Tricard, QED Technologies' director of business development. "The SSI-A provides a practical solution to optics system designers for them to finally enable creative, cost-effective use of aspheres as design elements."