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Conventional Lens Fabrication and Spherical Surfaces 123

5.4 Testing
Now that the spherical surface has been generated and polished, how
to check it? Again, a simple and practical way is to use a “bell,” as
illustrated in Fig. 5.5. A linear dial indicator is placed inside a bell that
has an opening with a known diameter. The bell with indicator is first
placed on a flat to zero the indicator. When it is placed on a part, a sag
reading is made. The absolute value may be used to evaluate the
radius of curvature, or a reading from a standard may be used for
comparison. Concave surfaces may be evaluated in a similar manner.
The method is fast and effective for many small lenses.
Many lens fabrication operations use test plates fabricated from
optical glass for a specific radius of curvature needed for many parts.
The curvature is tested by placing the part in light contact with the test
plate under a light source with a specific light emission. Interference
fringes results are counted in number and shape to evaluate the fin-
ished surface. Others have an instrument capable of measuring the

0.0000

Bell

Flat
Standard Part

FIGURE 5.5 Mechanical spherometer for measuring radius of curvature.