Tech Briefs Magazine - May 2024 - PIT-19

Do not forget that you and your fabricator
will need some room to work - this is
where clear aperture comes into play. Beyond
the clear aperture, additional room is
allocated for coating margins, polishing
edge effects, and bevels. This is particularly
important for aspheric lenses, which are typically
processed oversized to accommodate
tool runoff. Additionally, strong high-order
terms can pose problems outside of the
clear aperture. Even after manufacturing,
the area outside of the clear aperture may
be used for mounting purposes - do not pay
for more of the surface than you need!
One final note: make sure to tolerance
your clear aperture with linear units. In
most tolerance charts clear aperture will be
described as a percentage of the surface,
but this is just to scale with size. These tolerance
charts are recommendations for a
wide audience, but on your official drawing
be sure to use linear units.
Figure 2. Mechanical dimensions of a plano-convex lens. (Image: Edmund Optics)
Radius and Power
For spherical optics, the power tolerance
serves as a means to control the radius
by governing increments of sag change.
This radius is derived from the difference
in sag measured from a test plate. Additionally,
a linear radius tolerance is employed,
which dictates radius in linear units relative
to the nominal radius. This measurement
is commonly conducted using a distance
measuring interferometer.
While a spherical radius can be toleranced
using fringes of power or linear radius
tolerance, aspheric lenses should only
be toleranced with a linear radius tolerance.
Fringes of power assume your lens to
have a constant radius of curvature.
This is a measurement of surface deviation
from the nominal surface. Data is supplied
either interferometrically or via profilometry
as a 3D surface map or 2D line
scan. Do not tolerance the conic constant
or aspheric terms in the asphere equation.
Use radius and irregularity tolerance to
control permissible form error.
MSF errors are periodic ripples that
degrade surface quality. These errors often
arise due to residual imperfections
left by sub-aperture tooling during the
fabrication
process. Aspheric
lenses,
with their complex surface profiles, are
particularly susceptible to these mid-spatial
frequency errors. Post-processing
techniques may be necessary to mitigate
these surface irregularities and ensure
optimal optical performance.
One effective approach for tolerancing
mid-spatial frequency errors is through
slope error. Slope error focuses on measurPhotonics
& Imaging Technology, May 2024
ing the angles of perturbation over short
integration lengths, thereby limiting and
controlling the magnitude of these
mid-spatial frequency errors.
Wedge
Wedge directly influences the image formation
and propagation through optical
systems. Ideally, the optical and mechanical
axes should align perfectly. The centration
process for spheres allows for post-processing
to correct (with some limitation) centration.
In contrast, aspheric lenses require
precise control during fabrication to ensure
proper centering. In many cases,
aspheric lenses are approximated as spherical
for wedge measurements (e.g. via
beam deviation). However, this approach
19
Lower Power
Upper Power Limit
Upper Radius
Lower Radius Limit
Figure 3. Depiction of the lower and upper limits for both the power and radius tolerances of a lens, illustrating
that their directions are opposite. The upper power limit is in the same direction as the lower
radius limit. (Image: Edmund Optics)

Tech Briefs Magazine - May 2024

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