Transmitted Wavefront Error of Plano Optics

Overview

Measuring the transmitted wavefront error (TWE) of flat optics such as windows, reticles, and filter substrates is typically completed with a laser interferometer. The diagram below shows a typical setup using a Fizeau Interferometer, though testing can also be completed with a short coherence Fizeau Interferometer or a Twyman-Green interferometer.

An interferometer with a HeNe laser source can often be used to measure surface quality. However, depending on the optical coatings and/or application, it may be important to test TWE using an interferometer with a laser source close to the optic’s functional wavelength.

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 interferometers feature optional Dynamic Interferometry^®, which enables all measurement data to be acquired simultaneously. Vibration is no longer an issue, and an isolation table is not needed.