Transmitted Wavefront Error of Afocal Systems

Overview

Measuring the transmitted wavefront error (TWE) of image relays, beam expanders, binoculars, telescopes, multi-element lens assemblies, and aerospace optical systems is typically completed with a Fizeau laser interferometer. The diagram below shows a typical setup.

An interferometer with a HeNe laser source can often be used to measure surface quality. However, depending on the optical coatings and/or the use case for the optic, it may be important to test TWE using an interferometer with a laser source close to the optic’s functional wavelength. AccuFiz Fizeau interferometers are available with a range of visible and IR wavelengths.

A beam expander enables measurement of flat optics larger than the instrument aperture. Beam expanders up to 600 mm are available for AccuFiz interferometers. The expanders include kinematic mounting which enables the instrument to be removed from the expander, used separately for smaller setups, then returned to the larger setup without realignment. 

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 AccuFiz Fizeau interferometers feature optional 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.