Surface Error of Freeform and Aspheric Optics

Overview

Verifying the surface quality of non-spherical optics is easily accomplished with a Twyman-Green laser interferometer. This class of optics includes small- to meter-class mirrors, AR/VR optics, off-axis parabolas, telescope optics, illumination systems, and compound complex shapes.

PhaseCam Twyman-Green interferometers are excellent for measuring low-return systems and for measuring in high vibration environments.

To measure a freeform or aspheric optic, a Computer Generated Hologram (CGH) element is often used to match the test beam to the surface shape. The CGH can significantly reduce the power of the returned signal. PhaseCam Twyman-Green interferometers include a beam ratio adjustment to produce excellent fringe contrast even with low return systems.

With a traditional, phase-shifting interferometer, measurement data is acquired sequentially—a long enough process that vibration and air turbulence will degrade or even prevent measurement. The interferometer and all mounts must be situated on an air table to isolate the system from vibration.  Air isolation can be expensive—prohibitively expensive for very large optics or long measurement paths.

4D PhaseCam Twyman-Green interferometers feature Dynamic Interferometry^®, which enables measurement data to be acquired simultaneously. Because acquisition speed is so fast, vibration is no longer an issue, and an isolation table is not needed.

A Fizeau interferometer with a transmission sphere can also be used to measure an asphere or freeform optic. When it is necessary to launch a collimated beam into the CGH, a Fizeau laser interferometer is the preferred option.