Measuring When Vibration Isolation is Impractical

When measuring small optics it is relatively easy to set up a measurement that is isolated from vibration and turbulence. An air table in a controlled lab is usually sufficient. But when you need to measure an optic with a long radius of curvature, or a large mirror, the challenges pile up. How do you isolate the system when the test piece is many meters from the measurement system? How do you mechanically couple the instrument and test optic? And, how do you account for air turbulence over a meters-long cavity?

PhaseCam measuring a 1-meter aspheric mirror with a Computer Generated Hologram (CGH). 

 With a conventional interferometer the solutions are complicated, expensive and inconvenient. Many test engineers are familiar with taking measurements late at night when street noise is at a minimum, and with the air conditioners off to reduce vibration and turbulence—all in an effort to get one reliable measurement.

Vibration-insensitive Dynamic Interferometry makes it possible to measure without vibration isolation or mechanical coupling between the instrument and the test optic. By acquiring all measurement data simultaneously dynamic interferometers, such as 4D's PhaseCam interferometers, reduce acquisition time to the point that vibration no longer impacts the measurement.

By averaging measurements the effects of air turbulence are removed from the data even when the measurement path is long.

Additional features of the PhaseCam family also aid in the measurement of large optics. The PhaseCam 6000, for example, has a fiber-coupled measurement head which makes it extremely easy to position and align the test optic, even atop a tall measurement tower with tight space constraints. Because all controls are motorized a single technician or engineer can set up and operate the system remotely. These same features also enable manufacturers to measure large optics in situ during polishing operations. Moving an optic—particularly a large, heavy, high-value optic—back and forth between a polishing station and metrology lab is time consuming and risky. Measuring optics in situ provides immediate process feedback while dramatically reducing the measurement cycle and the risk of potential damage.

 A PhaseCam in a tower measures a mirror blank in the polishing station below. Courtesy Glyndwr University.