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Surface Quality and Flatness of Plano Optics

Measuring the surface quality of optical flats, windows, mirrors, and prisms is typically completed with a laser interferometer. The diagram below shows a typical setup using a Fizeau Laser Interferometer, though testing can also be completed with a Twyman-Green interferometer.

Standard setup for measuring the optical surface quality of a flat optic.

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 the surface quality using an interferometer with a laser source close to the optic’s functional wavelength.

Required Equipment

The diagram below shows the equipment needed to measure a flat 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.
Required Components
AccuFiz Interferometer More Information
Reference Flat Specifications
Bayonet Mount Specifications
2-Axis Base Specifications
3-Jaw Chuck – or – Specifications
Shelf Mount Specifications


Typical Measurement Procedure

For a typical setup the procedure is as follows:

  1. Install a bayonet mount on the aperture of the instrument.
  2. Mount a transmission flat (i.e., the reference optic) in the bayonet mount.
  3. While viewing the alignment screen in software, use the tip/tilt controls on the AccuFiz to align the reference optic.
  4.  Assemble a mount for the text optic using a 2-axis mount and either a 3-jaw chuck or shelf mount.
  5. Secure the test optic in its mount and place it in the beam path. Typically you’ll position the optic as close as possible to the intereferometer to minimize any error that can be introduced by air turbulence.
  6. While viewing the alignment screen in software, use the tip/tilt controls on the test mount to align the optic.
  7. While viewing live video from the interferometer, adjust the tip and tilt of the test mount to minimize the fringes.

The system is now aligned for measurement.

Measuring Larger Optics

A beam expander enables measurement of flat optics larger than the instrument aperture. Beam expanders up to 800mm 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. 

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

Measuring High Reflectors

For highly reflective optics the test beam returned to the system may be so bright that it washes out the interference fringes. In this case, a beam attenuator such as a pellicle can be inserted into the test path between the instrument and the test piece to reduce the return beam power and enable measurement.

A pellicle can be used to reduce the return beam power from a highly reflective flat.

Measuring in Vibration

In a typical measurement setup, the interferometer and all mounts are situated on an air table which isolates the system from measurement-degrading vibration. Air isolation can be expensive—prohibitively so 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.

Need Assistance with Measuring Flat Optics?

Want a convenient reference to this and other optical test setups? The Optical Setups poster is available as a PDF and also as a free poster.

Contact our applications staff today for expert opinions on optical measurement and on measurement equipment.

More Information on AccuFiz Fizeau Laser Interferometers


AccuFiz Specs

Configuration Fizeau interferometer
Acquisition Phase-shifting; optional vibration-insensitive Dynamic
Source Internal
Wavelength 632.8  nm or 532 nm
Aperture 100 mm (4 in) and 150 mm (6 in standard); optional to 800 mm (24 in)
Camera 1200 x 1200, or 2400 x 2400
Controls Remote control focus, zoom, pan, extended source size, measurement
Zoom Continuous 1–10X  with pan, calibrated at all zoom settings; 6X optical zoom optional
Software 4Sight™ Focus included
Size 100 mm models:  51.4 cm (20.2 in) long;
150 mm models: 76.2 cm (30 in) long
Reflectivities 1 – 100%

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