Metrology Products Featured Article

Introducing ASI^® Metrology:

A metrology powerhouse

For the first time, optics manufacturers can measure a wide range of steep, complex aspheres with up to 1000 waves of departure, without relying on dedicated, expensive null-optics. QED's new Aspheric Stitching Interferometer (ASI™) system has been designed to accurately test mild to steep aspheres with one fast, easy-to-use, accurate system. This breakthrough capability makes new things possible for optics systems designers and manufacturers alike.

Aspheric Stitching Interferometer (ASI^®) with Variable Optical Null (VON™) technology.

The ASI^® is a critical metrology innovation that will have an immediate impact on the applications and industries that currently are forced to accept high precision spherical optics in system design instead of reaping the inherent benefits that aspheric optics can offer. Markets such as the lithography, semiconductor, aerospace, measurement instrumentation, laser, medical equipment, scanning devices, military systems, multispectral systems and illumination systems markets will be positively impacted by the widespread availability of affordable, high precision, high departure aspheric optics.

Aspheric surfaces enable better optical systems: better imaging, lighter weight, smaller packaging, and better overall performance. While aspheres are pervasive in many markets, limitations in aspheric metrology have prevented their widespread use in high precision visible applications.

Standard interferometers are limited in their ability to measure aspheres with even mild departure. Aspheric interferometry has traditionally required the use of dedicated null lenses or computer generated holograms (CGHs) that must be specifically designed for each different aspheric surface. This approach requires a custom-made null element for each asphere prescription adding significant expense and long lead times. Alternate approaches such as stylus profilometry are prohibitively slow, inaccurate, and only able to provide low resolution results.

QED's new ASI^® removes these limitations and takes asphere metrology to the next level of performance. This metrology system extends the capability of subaperture stitching interferometry to measure high-departure aspheres with a departure from best fit sphere of 1000 waves without the use of dedicated null lenses or complicated set ups.

To achieve this, QED Technologies® developed Variable Optical Null™ technology.  Because the ASI^® is powered by QED's powerful Subaperture Stitching Interferometry (SSI®), the surface of an optic is measured by first examining smaller subsections, or subapertures, of the surfaces and then stitching together the results into a cohesive, full-aperture picture. Because the SSI™ only considers a smaller portion of the lens at a time, the fringes from mild aspheric departures can be resolved. However, with more severe aspheric departures, the fringe density of each subaperture is too great. The ASI's Variable Optical Null solves this problem; it is an advanced opto-mechanical subsystem that generates a wavefront that closely matches the surface of each aspheric subaperture being measured.

This technique produces an interferogram with reduced fringe density as compared with one produced by using a normal spherical wavefront.  In this way, significantly more aspheric departure within each subaperture can be tolerated, and larger and therefore fewer subaperture measurements are required. This process happens quickly - it typically takes between 15-30 seconds for each subaperture measurement. A complex surface that required 30 subapertures could be measured in less than 10 minutes. It is also an automatic process; the ASI^® automatically selects and measures each subaperture without requiring user intervention.  Once the subapertures are measured, the surface profile of the asphere is reconstructed using advanced stitching algorithms.  The outcome: fast, full-aperture, high-resolution measurements of aspheric surfaces.

Like our SSI® metrology systems, the ASI^® uses advanced software and process automation in a 6-axis CNC motion platform to allow an operator with minimal training and skills the capability to produce full-aperture measurements for flats, spheres and aspheres up to 200 mm in diameter.

Until now, it was not possible to measure the full aperture of high departure aspheres with a commercially available system. Contact us if you would like to learn more about the ASI^® metrology system.