in situ polishing metrology

Accurate shape and roughness feedback guides grinding and polishing operations  for both small and large optics. Moving smaller optics between polishing centers and a metrology lab may not prove difficult. With large and very large optics, however, these trips can be time-consuming and extremely risky.

One answer is to install measurement equipment directly into polishing systems, which eliminates material handling and fixturing time. The challenge for in situ polishing metrology, however, is overcoming the air flow, vibration and noise associated with such equipment. The measurement systems must also be very compact, and robust enough to stand up to the vibration and debris in this challenging application.

Instantaneous measurement using 4D’s dynamic interferometers and optical profilers makes it possible to measure surface shape (PhaseCam) and surface roughness (NanoCam HD) despite vibration. Both systems have been incorporated directly into polishing equipment, providing immediate feedback for the polishing process on many of the world’s premier telescope and space optics projects.


Homogeneity is a measurement of how much a specific glass optic’s refractive index deviates across its substance. Using our dynamic, short coherence interferometers, you can measure homogeneity more easily with three, not four, measurements. Plane-parallel flats without wedge can also be measured.

Parallelism (Wedge)

Parallelism is a measurement of how well aligned the two faces of a flat are to each other. For many optics, a wedge angle between the front and back surface is deliberately created to prevent the two surfaces from interfering. Both parallelism and wedge can be measured on 4D interferometers. Parallelism is particularly easy, using our short coherence source.

Added Technical Resources

High Throughput Measurement Speeds Production of Large Mirrors

This article discusses the challenges of measuring large optics during polishing, how Dynamic Interferometry
has been applied to overcome these challenges, and the benefits to optics manufacturers of employing the

Products for In Situ Polishing Metrology

NanoCam HD

NanoCam HD provides shape and roughness feedback throughout the polishing process.


4D PhaseCam Twyman-Green interferometers are the industry choice for measuring concave spherical optics, from several millimeters to tens of meters in diameter.

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