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Surface Error of Concave Optical Surfaces

Optical Setups

Overview

Verifying the surface quality of concave optical surfaces such as lenses, mirrors, off-axis parabolas, and telescope optics is typically accomplished with either a Fizeau Laser Interferometer or a Twyman-Green Interferometer.

RECOMMENDED METHOD

Measure Concave Surface Error with a Fizeau Interferometer

concave optics surface error, measure convex optics

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

Required Equipment

The diagram below shows the equipment needed to measure a concave optic with a standard AccuFiz Fizeau interferometer.
concave optical surface, optical mounts for Fizeau interferometer

Typical setup for measuring concave surface error.

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 test 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.

Transmission Sphere Selection Guide

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

ALTERNATE METHOD

Measure Concave Surface Error with a Short Coherence Fizeau Interferometer

The AccuFiz D short coherence Fizeau interferometer features an entirely on-axis design for excellent accuracy, particularly on highly curved surfaces. Dynamic Interferometry®, standard on the AccuFiz D, enables measurement without vibration isolation over cavity lengths up to 2 meters. 

short coherence fizeau interferometer, accufiz d ,4d technology

AccuFiz D short coherence Fizeau laser interferometer.

ALTERNATE METHOD

Measure Concave Surface Error with a Twyman-Green Interferometer

concave optics surface error, measure convex optics, twyman-green interferometer

Standard setup for measuring the surface quality of a concave optical surface with a Twyman-Green interferometer.

A Twyman-Green laser interferometer is also well suited for verifying the surface quality of concave optical surfaces. It is the preferred method for verifying the surface quality of large optics.

Dynamic Interferometry, standard on PhaseCam Twyman-Green interferometers, enables long path measurements without vibration isolation—a substantial cost savings for meter-class optics.

Need Assistance with Measuring Concave Optics?

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

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