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Surface Error of Freeform and Aspheric Optics

Optical Setups


Verifying the surface quality of non-spherical optics is easily accomplished with a Twyman-Green laser interferometer. This class of optics includes small- to meter-class mirrors, AR/VR optics, off-axis parabolas, telescope optics, illumination systems, and compound complex shapes.


Measure with a Twyman-Green Interferometer

PhaseCam Twyman-Green interferometers are excellent for measuring low-return systems and for measuring in high vibration environments.

freeform and aspheric optics surface error, measure convex optics, twyman-green interferometer

Measuring the surface quality of a freeform optical surface with a Twyman-Green interferometer.

Want a convenient reference to this and other optical test setups? Download the Common Optical Testing Setups guide (PDF).

Required Equipment

A 5-axis stand makes it easy to position and adjust a PhaseCam Twyman-Green interferometer. A range of diverging optics are available to match the test setup.
The CGH and its mount are typically customer-provided.
freeform and aspheric optics surface error, measure convex optics, twyman-green interferometer
Required Components
Twyman-Green Interferometer More Information
Diverging Optic Specifications
5-Axis Stand Specifications
Dovetail Mount Specifications
Bayonet, 3-Jaw, or Shelf Mount Specifications
2-Axis Base Specifications


Other Considerations

Measuring Low-Return Systems

To measure a freeform or aspheric optic, a Computer Generated Hologram (CGH) element is often used to match the test beam to the surface shape. The CGH can significantly reduce the power of the returned signal. PhaseCam Twyman-Green interferometers include a beam ratio adjustment to produce excellent fringe contrast even with low return systems.

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 PhaseCam Twyman-Green interferometers feature 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.


Measure with a Fizeau Interferometer

A Fizeau interferometer with a transmission sphere can also be used to measure an asphere or freeform optic. When it is necessary to launch a collimated beam into the CGH, a Fizeau laser interferometer is the preferred option.

Need Assistance with Measuring Large Concave Optics or Parabolas?

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

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