Transmitted Wavefront Error of a Finite Conjugate System

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 laser interferometer. The diagram below shows a typical setup using a Twyman-Green interferometer. A Fizeau interferometer can also be used.

Depending on the application, it may be important to test TWE using an interferometer with a laser source close to the optic’s functional wavelength. PhaseCam interferometers are available with a range of visible and IR wavelength sources.

Polarization control lets you match the energy in the test and reference beam to optimize fringe contrast. This feature lets you significantly improve the measurement noise floor and vertical resolution. A wide range of system reflectivities can be supported, and excellent contrast can be achieved, even in 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.