Transmitted Wavefront Error of Infinite Conjugate Lenses

Overview

Measuring the transmitted wavefront error (TWE) of infinite conjugate singlets, doublets, triplets, achromatic lenses, and compound lenses with diffractive or aspheric surfaces, 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 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.

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 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.