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Long Path Measurements


Measuring optical performance down to the smallest of measurements over extremely long distances, is no easy task. This is due to the unavoidable presence of air-turbulence and vibration.

Typical Long Path Measurements

For this type of measurement, the interferometer may need to be positioned far from the test piece – in some cases tens of meters away. Such a large space between the test and reference optics, could lead to a large amount of excess motion due to vibration and air turbulence.  This may prevent precise measurements with conventional interferometric systems.

4D Technology’s Long Path Measurement

Figure 1 – James Webb Space Telescope. Long Path Testing with PhaseCam Dynamic Interferometer.

4D Technology’s PhaseCam and AccuFiz product lines address the need for precise measurement under long pathlengths by effectively “freezing” the excess motion with nanometer resolution. Extended averaging techniques are then used to remove the errors from air-turbulence and achieve nanometer level resolution over meters of distance.

The James Webb Space Telescope (see Figure 1: James Webb Space Telescope – Long Path Testing) is a perfect example of how 4D Technology uses Long Path-Measurement to accurately measure all-around optical performance of a telescopic imaging system.

Figure 2 – Off-Axis parabola testing of large optic, using fold mirrors. Note that hoods aren’t necessary.


4D’s PhaseCam dynamic laser interferometers are used by the world’s premier large optics manufacturers and telescope integrators to ensure excellent performance after installation. Their ability to measure optics despite vibration and to thrive in environments with significant turbulence makes PhaseCam laser interferometers perfectly adapted to long stand-off measurements without vibration isolation.

Figure 3 – Cryogenic and ambient shape measurements of James Webb mirror segments.

Credit: NASA

Learn More about Dynamic Interferometers for Long Path Measurements

Added Technical Resources

Measuring Large Mirrors With Secondary and Tertiary Support Structures

Challenges of measuring a mirror or system that is partially obscured by structural members. This app note demonstrates an original method of acquiring contiguous data of the surface(s).


Measuring Transmitted Wavefront Error

A technique for vibration-insensitive measurement of transmitted wavefront error (TWE) improves repeatability by more than 7X.

Products for Measuring Long Path Optics


Vibration-immune metrology for convex and aspheric optics, from millimeters to meters in diameter.

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