Metrology FAQs


Do I need separate machines to measure different geometries?


No. For the first time, the SSI metrology systems provide automated, high precision metrology for both spherical (including large clear aperture (CA) and high numerical aperture (NA) parts) and aspherical optics (including aspheres with high departures from best fit spheres)—an unprecedented achievement in the optics industry. The ASI offers a complete metrology solution that will meet the needs of advanced optics manufacturers.


Why is SSI Metrology better than conventional metrology?

Traditional measurement technologies are unable to characterize next generation optics, such as the aspheres and steep convex optics, causing significant challenges in manufacturing these optics and limiting their widespread use.

The goal of any metrology system is to obtain precise measurements that can characterize the surface of a lens. In an optics shop, interferometer is the name given to an instrument used for measuring either the surface of an optic or the wavefront transmitted through an optical system. Over the past 25 years, phase-measuring interferometry has proven to be a precise, accurate, and flexible metrology technology essential to the manufacture of precision optics. Traditional full-aperture interferometers and transmission optics can provide adequate metrology for many surfaces; however, a standard 4” or 6” aperture system with a set of standard transmission spheres leaves many coverage gaps for both large convex and high numerical aperture parts.

The capability gap is considerable. In fact, many optics under 50 mm in diameter cannot be measured with a standard 4” interferometer. Larger interferometers and transmission spheres capable of covering these surfaces are available only at great expense by custom order, with long lead times or by self-manufacture. The costs of implementing a traditional metrology solution for large optics are so great, they are prohibitive. The problem is compounded for aspheric surfaces.

There are various ways to test aspheres, but most of them are expensive, time-consuming, difficult-to-use and difficult-to-qualify. Null lenses and computer generated holograms (CGHs) are the de-facto workaround solution. But they come with several serious drawbacks. They are expensive. A new reference optic is needed for every design change. There are very long lead-times associated with manufacturing null lenses and CGHs. Qualifying the accuracy of the null corrector is a challenge unto itself (the Hubble Space Telescope is perhaps the most infamous example of an aspheric null test gone bad). And finally manufacturers end up with obsolete inventories of null lenses to track and store.

Although new products have started to chip away at the problem, a standard method for asphere metrology does not currently exist, particularly one that can reliably and repeatably measure very steep aspheres (aspheric departures along the lines of 1000 waves of departure from best fit spheres). Subaperture stitching helped to alleviate this problem because it allows division of the wavefront into smaller pieces whose fringes can be individually resolved. QED commercialized this capability, introducing the SSI-A® in 2006 for testing mild aspheres (up to ~100 waves of departure from best-fit sphere) without null-correcting optics.

This was a significant advance for flexible asphere testing, but of course limiting departures to 100 waves does not provide a robust enough solution for many manufacturers. QED has further refined the asphere stitching process to extend capability to ~200 waves departure from best-fit sphere, pushing the capability to its useful limit. Extending it further reduces the accuracy and comes with a significant cost to cycle time.

As a result, fabrication of high quality, large, and particularly aspheric optics is seriously hampered by the lack of accurate, and affordable, metrology. The ASI provides a revolutionary technology—for the first time, enabling the automatic capture of precise metrology data for large (up to 280mm in diameter) and/or strongly curved (concave and convex) parts and/or even steeply aspheric surfaces (up to 1000 waves of departure).

The ASI provides several advantages over conventional technologies:
  • The process is almost entirely automatic, making it user-friendly, deterministic and repeatable.
  • Its unique software characterization even provides an estimate of measurement quality.
  • It is capable of measuring large clear aperture and high numerical aperture parts.
  • It is able to test larger aspheric departure (up to 1000 waves) than integrated instruments.
  • It has improved resolution over traditional, full aperture, instruments.
  • It boosts accuracy by automatically calibrating systematic instrument errors.

With the introduction of the Aspheric Stitching Interferometer (ASI®), QED has introduced a break-through metrology product that will help to redefine optics manufacturing capabilities.

Are the SSI / ASI  processes patented?


Yes, there are fundamental method, apparatus, and process patents protecting SSI metrology.

Can I get a demonstration of the SSI® process on my production parts?


QED has a fully equipped applications laboratory and welcomes the opportunity to provide prospective customers with a complete demonstration on production parts—just call, fax, or This e-mail address is being protected from spambots. You need JavaScript enabled to view it. your request.