Twyman-Green Interferometers from 4D Technology

A Twyman-Green interferometer can be described as a laser interferometer configuration that is used to measure the transmitted wavefront quality and surface shape of optical-grade mechanical surfaces, optical components, systems, and assemblies.

A source beam is collimated in a Twyman-Green laser interferometer and subsequently split by a beam splitter. The “reference beam” is guided toward a high-quality reference mirror, and the “test beam” is guided toward the test surface. By using a beam expander or diverging lens, the test wavefront can be molded to correspond with the convex/concave curvature of the test system or surface.

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Twyman-Green Interferometers Measure Surface Shape

The two reflected beams recombine when they pass back via the beam splitter, creating an interference pattern. This interference pattern is caused by changes in phase that, in turn, is induced by the variations between the “perfect” reference surface and the test surface. Using 4D Technology's patented Dynamic Interferometry® technique, this interference pattern can be recorded by the sensor as a single acquisition frame.

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Phase shifting interferometry integrates a number of data frames to quantify the heights of all points present on the test surface to precisely measure the shape of the surface. There are many techniques available to achieve multiple frames. The standard technique introduces systematic and minute phase shifts by shifting the reference mirror between the frames. This technique can be defined as a “frame rate” limited method.

Dynamic Interferometry: Measure Despite Vibration

The Twyman-Green interferometers offered by 4D Technology use a completely different method. Using a polarization-based Dynamic Interferometry^® method, these instruments gather four phases of data from one frame. When a quarter-wave plate is introduced into the reference beam and the test beam, it imparts different states of polarization.

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When an element is introduced before the camera, the data is separated into four phases that can be imaged concurrently by the sensor.

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Rather than being frame rate limited, the Dynamic Interferometry method is exposure limited, which is an added advantage. Dynamic Interferometry enables short acquisition times of just a few microseconds. This extraordinarily rapid acquisition freezes turbulence and vibration, yielding exceptional measurements even in noisy settings.

4D Technology was the first to develop the Dynamic Interferometry for quantitative, single-frame phase measurement.

Advantages of a Twyman-Green Interferometer

Twyman-Green interferometers offer a number of major benefits when compared to other kinds of interferometers.

Measure in Difficult Locations

The compact optical design of the Twyman-Green interferometers accommodates a tiny measurement head, which can be easily positioned in constricted spaces. The laser source can be attached to the head through a fiber optic cable, which shifts heat-producing elements away from the optics. The collective result is effortless positioning in difficult areas.

Measure Despite Vibration, Without Isolation

A dynamic PhaseCam interferometer eliminates the need for vibration isolation. This benefit is specifically crucial when quantifying bulk optics, or when quantifying across long paths, where isolation systems would prove very costly. The interferometer no longer has to be attached to the test optic, which further simplifies the setup.

Measure Without an Expensive Reference

High-quality and expensive reference transmission (spherical or flat) optics are needed in the case of a Fizeau interferometer. On the other hand, the Twyman-Green interferometer eliminates the need for this external reference optic.

Operate Remotely

Motorized control of all functions allows a PhaseCam Twyman-Green to be used remotely in environmental chambers, vacuum chambers, measurement towers, and other complicated locations.

No Calibration Required

A Twyman-Green PhaseCam is always precise because it contains fewer parts, does not include any fragile high-voltage PZT actuators, and does not have mechanical phase-shifting as in temporal interferometers.

High Resolution

The Twyman-Green interferometer has an intrinsically on-axis design that makes sure that PhaseCams offers both precision and high resolution.

Measure Highly Reflective Surfaces or Low Return Systems

Polarization control allows users to match the energy in the test beam and the reference beam to improve fringe contrast. This characteristic allows users to considerably enhance the measurement of vertical resolution and noise floor. Additionally, a broad range of test setup reflectivities can be supported, and exceptional contrast can be realized, even in the case of low return setups.

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Applications for Dynamic Twyman-Green Interferometers

Twyman-Green interferometers are specifically suitable for quantifying:

Slow f/# concave components

Massive, concave optics

Aspheric optics

Focal optical systems

In pressure vessels and environmental chambers

Low-return setups

In measurement towers or other locations that are difficult to access

Over extended measurement paths

Moving systems, like spinning/actuating components, adaptive optics, and flexible membranes

Within polishing equipment for in situ process feedback

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4D Technology’s Range of Twyman-Green Laser Interferometers

4D Technology’s Twyman-Green PhaseCam laser interferometers have constantly led the industry for innovation and high performance.

4D Twyman-Green laser interferometers provide excellent flexibility and value, thanks to industry-leading analysis software, available wavelengths ranging from visible through 3.39 µm, and an array of high-quality accessories.

4D Twyman-Green Laser Interferometers

PhaseCam 6100 Twyman-Green Laser Interferometers

Thanks to motorized controls and an easy-to-position, fiber-coupled measurement head, the exceptionally compact PhaseCam 6100 Twyman-Green Laser Interferometeris perfect for long-path measurements. The system can be used for remote mounting in pressure vessels, measurement towers, and other difficult test setups.

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PhaseCam 6000 Twyman-Green Laser Interferometers

Motorized controls as well as a small, easy-to-position, fiber-coupled measurement head make the PhaseCam 6000 perfect for remote mounting in pressure vessels or measurement towers, on-machine metrology, and other difficult settings.

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PhaseCam 4020 Twyman-Green Laser Interferometers

The PhaseCam 4020 is resistant to vibration and features easy manual controls. It is affordable and flexible for the shop floor or the laboratory.

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PhaseCam NIR Laser Interferometers

These interferometers are lightweight and vibration-resistant for wavefront and surface measurements at near-infrared wavelengths of 1.053, 1.064, 1.3and 1.55 µm.

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PhaseCam SWIR Laser Interferometers

These interferometers are lightweight, compact, and vibration-insensitive for precise wavefront and surface measurements at wavelengths of 1.55 µm.

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PhaseCam IR Laser Interferometers

These interferometers offer precise wavefront and surface measurements at a wavelength of 3.39 µm. The dual-mode systems work in vibration-resistant mode for challenging environments, like clean rooms, or in conventional, temporal phase-shifting. The PhaseCam IR laser interferometers are perfect for quality verification of afocal components, focal optical systems, and IR optics.

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PhaseCam 4020HP High Power Laser Interferometers

A high-power, 25-mW HeNe laser helps quantify low reflectivity (<1%) setups, like quantifying aspheres through Computer Generated Holograms (CGHs).

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