Measuring Prisms and Corner Cubes

Overview

Measuring a prism or corner cube is challenging because every facet reflects some light back to the measurement system. The result is a complex interferogram which is difficult or impossible to interpret.

A short coherence Fizeau interferometer such as the AccuFiz D isolates the fringes from each face, allowing you to:

• verify the surface quality of the faces
• calculate the dihedral angle errors and retroreflection errors, and
• measure the transmitted wavefront error in single or double pass.

Measurement of solid, hollow, and porro prisms can be readily measured; more complex geometries (e.g., pentaprisms) may require different test arrangements.

For highly reflective optics the test beam returned to the system may be so bright that it washes out the interference fringes. In this case, a beam attenuator such as a pellicle can be inserted into the test path to improve fringe contrast. An attenuation-coated transmission flat (available through 4D) can be used instead of a pellicle.

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.

The AccuFiz D interferometer features Dynamic Interferometry^®, which enables all measurement data to be acquired simultaneously. Because acquisition speed is so fast, vibration is no longer an issue, and vibration isolation is not needed.