Tech Briefs Magazine - May 2024 - PIT-18

plier's global footprint to ensure they
can service localized R&D efforts and
larger, global production facilities.
Precision motion control is a fundamental
aspect of many cutting-edge photonic
processes, and it deserves ample consideration.
Whether you are manufacturing, inspecting,
aligning or bonding, it is essential
to have a fundamental understanding of
precision motion principles to effectively
engage with suppliers and choose one who
will maximize your effectiveness. An ideal
motion supplier will navigate your precision
motion control journey alongside you
as a partner who is invested in your longterm
success.
This article was written by Brian M.
Fink, Product Manager, Aerotech, Inc.,
(Pittsburgh, PA). For more information,
visit www.aerotech.com.
References
1 Slocum, A. (1992). Precision Machine Design.
Society of Manufacturing Engineers.
Properly Tolerancing Aspheric Lenses to Balance Cost and Performance
W
hen developing an optic, the striving
for optimal performance can
often be met with the challenge of managing
costs. Aspheric lenses, with their
complex surface profiles, require some
additional expertise to maintain this
balance between precision and manufacturing
expenses. Understanding
how to effectively tolerance aspheres is
an incredibly useful communication
tool to get the most out of your lens. In
this article, we will provide some context
behind asphere tolerances, exploring
the factors that influence cost
and performance.
Importance of Tolerancing
Optical Components
Tolerances play a crucial role in optical
design, as they constrain the amount
of optical path difference induced
during fabrication and influence manufacturing
cost. Balancing as-built performance
with cost is essential, and tolerancing
is a way of communicating this
with your optical manufacturer.
Knowing how much deviation can afford
in order to keep that performance
that you need is a balancing act. Tolerancing
charts provide a valuable starting
point (see Figure 1).
While it is not advisable to simply work
down a column in a tolerancing chart, categories
like those shown in Figure 1 give
an idea of the attributes you need to tolerance.
As you move from left to right across
these charts you will understand what values
can make your system more expensive.
While it might be obvious that tighter tolerances
will be more difficult to manufacture
and thus more expensive, it is useful
to be aware of the relationships between
certain specifications.
Mechanical Dimensions
Whether discussing spherical or
aspheric lenses, certain parameters remain
consistent in tolerance considerations:
diameter, center thickness (CT),
overall height, and sag. Achieving very
tight thickness tolerances becomes more
challenging when combined with stringent
cosmetic requirements, fractional
wavelength irregularity, or soft optical
materials. Fabricators typically aim to hit
the upper limit of thickness tolerances,
as once you remove too much material
you cannot add more CT. Tight thickness
tolerances will reduce yield, subsequently
driving up prices.
Thin edges should generally be avoided
whenever possible due to their susceptibility
to chipping, which can result in surface
defects or even catastrophic failure. Moreover,
they can pose difficulties during processing
and measurement, making them
less desirable for optical components.
It is crucial to avoid compounding conflicting
dimensions in tolerance specifications.
For instance, overall height depends
on center thickness and sag, so
tolerancing all three simultaneously may
lead to unnecessary complications. Instead,
prioritize tolerancing what is essential,
particularly if certain dimensions are
controlled for mounting features.
Aspect ratio, the relationship between
diameter and center thickness, holds significant
importance in mechanical dimension
considerations. A high aspect ratio can
result in glass flexing during fabrication,
making it challenging to handle. The ideal
aspect ratio for precision optics is typically
around 6:1, to help avoid the production of
thin, wafer-like optics that are difficult to
manage during fabrication.
For optimal tolerancing, involve an
optomechanical designer early in the
process. Their expertise can help identify
mounting surfaces and establish lens
spacing, ensuring that all lens tolerances
align with the requirements of your optomechanical
assembly.
Figure 1. Sample asphere manufacturing tolerances. (Image: Edmund Optics)
18
Photonics & Imaging Technology, May 2024
http://www.aerotech.com
Tech Briefs Magazine - May 2024

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