What is a Twyman-Green Interferometer?
A Twyman-Green interferometer is a laser interferometer configuration used for measuring the surface shape and transmitted wavefront quality of optical components, assemblies, systems, and optical-grade mechanical surfaces.
In a Twyman-Green laser interferometer, a source beam is collimated, then divided by a beam splitter. The “reference beam” is directed to a high-quality reference mirror. The “test beam” is directed to the test surface. The test wavefront can be shaped to match the concave/convex curvature of the test surface or system by using a diverging lens or beam expander.
Twyman-Green Interferometers
Measure Surface Shape
When the two reflected beams pass back through the beam splitter, they recombine, forming an interference pattern. This pattern is due to changes in phase caused by the differences between the test surface and “perfect” reference surface. The sensor records this interference pattern as a single acquisition frame.
Phase shifting interferometry combines several frames of data to measure the heights of all points on the test surface to accurately quantify the surface shape. Several methods are available to acquire multiple frames. The typical method introduces small, systematic phase shifts by moving the reference mirror between frames. This method can be described as a “frame rate” limited technique.
Dynamic Interferometry:
Measure Despite Vibration
4D Technology’s Twyman-Green interferometers employ a different approach. These instruments use a polarization-based Dynamic Interferometry® technique to collect four frames of data simultaneously. A quarter-wave plate is introduced into both the test and reference beams, imparting different polarization states.
An element introduced before the camera separates the data into four phases which can be imaged simultaneously by the sensor.
The advantage of the Dynamic Interferometry technique is that it is exposure limited. With Dynamic Interferometry, acquisition times are as short as a few microseconds. This incredibly quick acquisition freezes vibration and turbulence, producing excellent measurements even in noisy environments.
4D Technology pioneered the development of Dynamic Interferometry for single frame, quantitative phase measurement.
Advantages of a Twyman-Green Interferometer
Twyman-Green interferometers have several key advantages over other types of interferometers.
Measure in Difficult Locations
The compact optical design makes for a small measurement head, which is easy to position in tight spaces. The laser source can be coupled to the head via a fiber optic cable, which moves heat-generating elements away from the optics. The combined result: easy positioning in challenging locations.
Measure Without an Expensive Reference
With a Fizeau interferometer, expensive, high quality reference transmission (flat or spherical) optics are required. The Twyman-Green interferometer does not require this external reference optic.
No Calibration Required
With no mechanical phase-shifting as in a temporal interferometers, fewer parts, and no delicate high-voltage PZT actuators, a Twyman-Green PhaseCam is always accurate.
Measure Low Return Systems or Highly Reflective Surfaces
Polarization control lets you match the energy in the test and reference beam to optimize fringe contrast. This feature lets you significantly improve the measurement noise floor and vertical resolution. A wide range of test setup reflectivities can be supported, and excellent contrast can be achieved, even in low return setups.
Measure in Difficult Locations
With a dynamic PhaseCam interferometer, vibration isolation is not required. This advantage is particularly critical when measuring large optics, or when measuring over long paths, where isolation systems would be prohibitively expensive. The interferometer does not even need to be coupled to the test optic, further simplifying setup.
Operate Remotely
Motorized control of all functions means that a PhaseCam Twyman-Green can be operated remotely in measurement towers, vacuum chambers, environmental chambers and other challenging locations.
High Resolution
The inherently on-axis design of the Twyman-Green interferometer means high resolution and accuracy is built into PhaseCams.
Applications for Dynamic Twyman-Green Interferometers
Twyman-Green interferometers are particularly well suited for measuring:
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large, concave optics
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slow f/# concave components
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focal optical systems
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aspheric optics
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low-return setups
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in environmental chambers and pressure vessels
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over long measurement paths
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in measurement towers or other hard-to-access locations
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inside polishing equipment for in situ process feedback
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moving systems, such as flexible membranes, adaptive optics, and actuating/spinning components.
4D Technology’s Range of Twyman-Green Laser Interferometers
4D’s Twyman-Green PhaseCam laser interferometers from 4D Technology have continually led the industry for high performance and innovation.
With available wavelengths from visible through 3.39 microns, industry-leading analysis software and a range of high-quality accessories, 4D Twyman-Green laser interferometers offer incredible value and flexibility.
4D Twyman-Green Laser Interferometers
PhaseCam 6110
A 4MP sensor, fiber-coupled measurement head and motorized controls make the incredibly small PhaseCam 6110 ideal for long path measurements and remote mounting. Use it for remote mounting in measurement towers or pressure vessels, and other challenging test setups.
PhaseCam 6010
With a tiny, easy-to-position, fiber-coupled measurement head, 4MPx camera and motorized controls, the PhaseCam 6010 is ideal for on-machine metrology, remote mounting in measurement towers or pressure vessels, and other challenging environments.
PhaseCam 4030
The PhaseCam 4030 features a 4 MPx camera, simple manual controls and vibration immunity. Flexible and low cost for the lab or shop floor.
PhaseCam NIR
Lightweight and vibration-insensitive for surface and wavefront measurements at near infrared (1.053, 1.064, 1.3 and 1.55 micron) wavelengths.
PhaseCam SWIR
Compact, lightweight and vibration-insensitive for accurate surface and wavefront measurements at 1.55 micron wavelengths.
PhaseCam MWIR
Accurate surface and wavefront measurements at 3.39 micron wavelength. This vibration-immune system operates in dynamic mode for demanding environments such as clean rooms, or in traditional, temporal phase-shifting as well. Ideal for quality verification of IR optics, focal optical systems and afocal components.