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Surface Error of Convex Optical Surfaces

Optical Setups


Verifying the surface quality of convex lenses, mirrors, domes, and telescope secondary optics is typically accomplished with a Fizeau Laser Interferometer. A short coherence laser interferometer can also be used, and it is the preferred option for measuring thin domes.


Measure Convex Surface Error with a Fizeau Interferometer

convex optics surface error, measure convex optics

Standard setup for measuring the surface quality of a convex optical surface.

Required Equipment

The diagram below shows the equipment needed to measure a convex optic with a standard AccuFiz Fizeau interferometer.
convex optics surface error, measure convex optics
Want a convenient reference to this and other optical test setups? Download the Common Optical Testing Setups guide (PDF).
Required Components
AccuFiz Interferometer More Information
Transmission Sphere Specifications
5-Axis Base Specifications
Bayonet or 3-Jaw Mount Specifications


Other Considerations

Vertical Measurement

A vertical configuration 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. A beam attenuator such as a pellicle can be inserted into the test path to improve beam contrast. A reference optic with attenuation coating can also be used.

A pellicle improves fringe contrast with a highly reflective optic.

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 interferometers feature optional Dynamic Interferometry®, which enables all measurement data to be acquired simultaneously. Vibration is no longer an issue, and an isolation table is not needed.

Download the Transmission Sphere Selection Guide

Learn how to choose the correct transmission sphere for your application.


Measuring Convex Surface Error with a Short Coherence Fizeau

Transparent domes can be challenging to measure because both surfaces of the dome reflect, creating a complex interference pattern. A short coherence AccuFiz D interferometer allows you to isolate each surface to measure independently.

measure transparent domes with a fizeau interferometer

Measuring Transparent Domes

Learn about measuring thin, transparent domes in this application note.

measure transparent domes with a fizeau interferometer

Need Assistance with Measuring Convex Surfaces?

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

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