Surface Error of Concave Optical Surfaces

Overview

Verifying the surface quality of concave optical surfaces such as lenses, mirrors, off-axis parabolas, and telescope optics is typically accomplished with either a Fizeau Laser Interferometer or a Twyman-Green Interferometer.

A vertical configuration can speed measurements, particularly for repeated measurements of the same type of component. The lightweight AccuFiz interferometer and lookdown stand make vertical measurement easy.

For highly reflective optics the test beam returned to the system may be so bright that it washes out the interference fringes. A beam attenuator such as a pellicle can be inserted into the test path to improve beam contrast. A reference optic with attenuation coating can also be used.

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.

4D AccuFiz interferometers feature optional Dynamic Interferometry^®, which enables all measurement data to be acquired simultaneously. Vibration is no longer an issue, and an isolation table is not needed.

The AccuFiz D short coherence Fizeau interferometer features an entirely on-axis design for excellent accuracy, particularly on highly curved surfaces. Dynamic Interferometry^®, standard on the AccuFiz D, enables measurement without vibration isolation over cavity lengths up to 2 meters.