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Surface Error of Plano (Flat) Optics

Optical Setups


Verifying the surface quality of flat mirrors, windows, polished metal and glass, wafers, discs, and seals is typically accomplished with a laser interferometer. Both Fizeau or Twyman-Green interferometers have their advantages for measuring flat optics.


Measuring a Flat with a Fizeau Interferometer

With its large aperture, a Fizeau interferometer is ideal for measuring flat optics. With a beam expander, a Fizeau can readily measure plano optics up to 600 mm in diameter. 

plano surface error, flat surface error, optical setups fizeau

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

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 100 mm or 150 mm apertures.
plano surface error, flat surface error, optical setups fizeau
Required Components
AccuFiz Interferometer More Information
Reference Flat Specifications
2-Axis Base with tip/tilt Specifications
3-Jaw, Shelf or Bayonet Mount 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. 

fizeau interferometer for measuring large optics - accufiz 600 mm

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

Vertical Measurement

Vertical measurement can speed measurements, particularly for repeated measurements of the same type of component. The lightweight AccuFiz interferometer and lookdown stand make vertical measurement easy.

fizeau interferometer lookdown stand

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 to improve fringe contrast. An attenuation-coated transmission flat (available through 4D) can be used instead of a pellicle.

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

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. 

4D Fizeau interferometers feature optional Dynamic Interferometry®, which enables all 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).


Measuring Plano Optics with a Short Coherence Fizeau

The short coherence AccuFiz D Fizeau interferometer features an entirely on-axis design, making it particularly well suited for measuring flat optics at a considerable angle relative to the interferometer. With Dynamic Interferometry® standard, the AccuFiz D measures despite vibration, without isolation.

The required equipment and setup are similar to that of the AccuFiz, described above.

plano surface error, flat surface error, optical setups fizeau

Setup for measuring the optical surface quality of a flat optic with a short coherence Fizeau interferometer.


Measuring Plano Optics with a Twyman-Green Interferometer

A Twyman-Green interferometer is very compact, lightweight, vibration-insensitive system that can be used with an off-axis parabola to measure very large flat surfaces.

Need Assistance with Measuring Flat Optics?

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

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