Tips for Designing
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A successful lens design succeeds not only in the creation of a working
model, but also manufacturing, assembly, testing and implementation.
Occasionally, a lens may appear to succeed in conception, but
fail in one of the subsequent phases of manufacturing, assembly, or
testing. For this reason, it is imperative to recognize the nuances of
optical manufacturing, paying careful attention to the statistical assumption
of models and manufacturing practicality. Designers must
consider the individual lens element geometry, the assembly setup,
and the tolerancing models when creating an optic from scratch.
Design software digitizes the optical planning process and o ers tools
to ease the detailed and time-consuming procedure; however, one
must be aware that most optical software does not always warn of
or prevent the user from creating physically impossible or di cult to
manufacture solutions. Remember, optical software is just a tool and
the user must pay careful attention to review the outputs.
One common technique optical manufacturers use when machining
glass or other materials to the nal speci cation is to oversize the
lens diameter early in the fabrication process. Anticipating the need
to oversize the diameter of the lens, designers need to ensure that the
edge thickness is not too small and not too sharp, such that the edge
would be prone to damage. It is suggested to keep the edge thickness
above ~0,7 mm, at a diameter 1 mm larger than the intended nal
diameter of the lens.
The Karow or Z-factor, which measures the ability for a lens to center itself
automatically between bell chucks, also known as bell clamps, is given by:
Z = D¹ + D²
D, D are the bell chuck diameters (commonly equal to the lens clear
aperture diameter). R, R are the radii of curvature for the rst and
second surfaces. Convex and concave surfaces, respectively, have
positive and negative radii (Figure 1).
Figure 1: The left lens Karow factor (Z = 2,5) is greater than the
right lens (Z = 0,4). As such, the left lens would be easier to center
via automated bell-chucking while the right would be more di cult.
Lenses with a Karow factor greater than 0,56 will automatically center
well via automated bell-chucking; those with a Karow factor less than
0,56 may not automatically center and will need to be centered manually.
This is a time intensive process and therefore more expensive.
Lenses with nearly concentric radii are di cult to center since a large
amount of material must be removed to correct for surface-to-surface
relative decentering. To ensure that a lens can be centered, concentricity
(r) should be greater than 2 mm as a rule of thumb:
r = |R¹| – |R²| – CT
Where CT is the center thickness of the lens (Figure 2).
Figure 2: This meniscus lens has radii that are nearly concentric.
Ensure that r is greater than 2 mm so that the lens can be centered.
Lenses with hemispherical (radii of curvature less than or equal
to 0,7 times the diameter) or near at (a sag equal to or less than
100 m) surfaces should also be avoided if possible, as this is also
di cult to manufacture.