Contact-free Interferometric Inspection of Optical Components (Metrology)

6.13.
To be able to measure precisely the curvature or flatness of optical components is
essential for control of optical glass-working. The interferometric instrument developed for
this purpose utilises the measuring ray path arranged vertically below the top of a vibration
damped table. The interferogram is made visible via a video system. The entire interferometer
arrangement is enclosed in a box in order to protect the fringe pattern from environmental
influences. The test pieces to be tested are placed on circular knife-edges made of plastic
materials or steel to ensure highly reproducible location and each part need not be adjusted
separately. It utilises He-Ne laser emitting at 632.8 nm. The beam is expanded by a factor-4
Zoom collimator so as to produce a plane wave of 25 mm diameter. The beam can be further
expanded to 100 mm diameter through a 4X beam a expander. A number of high-performance
measuring objectives are needed to adapt to various kinds of test objects. The quality of the
reference surface of the rear-most lens faces (which may have reflectance of 4%) determines
the measuring accuracy of the overall system. Objectives can be changed simply by taking
away one and placing the other in its position on the table top.
For focussing arrangements to move the measuring objective are made. Radius of test

pieces to a measuring uncertainty of less than 1 \im can be measured. The visual evaluation
of the fringe pattern is facilitated by overlaying a square grid on the VDU picture.