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Concave Radius of Curvature

Optical Setups


Verifying the radius of curvature of concave optical surfaces such as mirrors, lenses, and transparent domes is typically accomplished with either a Fizeau interferometer or a short coherence Fizeau interferometer.

Measure Concave Radius of Curvature with a Fizeau Interferometer

Calculating Radius of Curvature requires two measurements: one at the cateye position (Location 1) and one a the confocal position (Location 2).

concave radius of curvature, optical setup, fizeau interferometer

Standard setup for measuring concave radius of curvature.

Required Equipment

The diagram below shows the equipment needed to measure the radius of curvature of a concave optic. Standard AccuFiz Fizeau interferometers are available in either 100mm or 150 mm apertures.
concave radius of curvature, optical setup, fizeau interferometer
Required Components
AccuFiz Interferometer More Information
Transmission Sphere Specifications
5-Axis Base Specifications
Bayonet, 3-Jaw, or Shelf Mount Specifications
Digital Radius Slide Specifications


4D Technology Digital Radius Slides provide axial positioning resolution down to 10 nm. Vertical and horizontal slides work with 4D Technology’s 4Sight Analysis Software for easy, extremely accurate Radius of Curvature measurement.

concave radius of curvature, optical setup, fizeau interferometer

Other Considerations

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 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).

Measure Concave Radius of Curvature 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 such as transparent domes. 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.

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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