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Transmitted Wavefront Error of Afocal Systems

Optical Setups


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.


Measuring TWE with a Fizeau Interferometer

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Standard setup for measuring the transmitted wavefront error of an afocal system.

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.

Required Equipment

The diagram below shows the equipment needed to measure an afocal system of a diameter equal to or smaller than the aperture of the interferometer. Standard AccuFiz Fizeau interferometers are available in either 100mm or 150 mm apertures, with beam expanders up to 600 mm.
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Required Components
AccuFiz Interferometer More Information
Transmission Flat Specifications
2-Axis Base with Tip/Tilt Specifications
Bayonet, 3-Jaw, or Shelf Mount Specifications
Return Flat Specifications


Other Considerations

Measuring Larger Optics

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. 

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An AccuFiz 100 mm aperture interferometer (left) used with a 600 mm beam expander and motorized, large optic mounts.

Measuring in Vibration

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.

Want a convenient reference to this and other optical test setups? Download the Common Optical Testing Setups guide  (PDF).

Need Assistance with Measuring Transmitted Wavefront Error?

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