Tech Briefs Magazine - May 2024 - PIT-20

Nominal surface,
described with
aspheric equation
Actual surface,
deviations from nominal
controlled with radius and
irregularity tolerances
Figure 4. Illustration of the surface irregularity of an aspheric lens exaggerated for demonstrative purpose. (Image: Edmund Optics)
S1
S2
How Can I Tolerance a Most
Cost-Effective Asphere?
Aspheres
inherently
Optical Axis
incur higher
S1
S2
Center of
Curvature
Center of
Curvature
Mechanical Axis
S1
S2
Asphere Axis
Tilt
Lateral
Displacement
Mechanical Axis
costs compared to spherical lenses. They
require small tools for polishing (and often
grinding, too) and can only be processed
one lens at a time. Beyond that,
there are several factors that can drive
the cost of aspheres: surface form error,
local slope, cosmetics, material selection,
tilt and decenter, and overall geometry.
These specifications and tolerances
determine the manufacturing process,
with tighter tolerances requiring different
equipment, further impacting costs.
While it is challenging to pinpoint
the most influential factors on cost, a
lot of what we discussed here is which of
these tolerances will cause the fabricator
to have longer process time and lower
yields, which will greatly influence
cost. Therefore, early collaboration between
designers and manufacturers is
crucial to determine specifications and
tolerances that minimize costs while
maximizing performance for your specific
project.
Figure 5. The wedge of an asphere is the displacement between the optical and mechanical axes. (Image:
Edmund Optics)
overlooks lateral displacement (or decenter)
as it does not account for the unique
geometry of the aspheric surface. To accurately
characterize wedge in aspheric lenses,
it is essential to consider both the
aspheric tilt and the lateral displacement of
the aspheric surface.
Cosmetics (Scratch-Dig)
Surface quality is often the primary cost
driver for optical components, whether
they are spherical or aspheric. Often confused
with
surface
roughness,
surface
quality describes the localized, nonperiodic
defects across an optical surface rather
than the overall texture. Interestingly, cosmetic
defects may have minimal impact
on system performance and a surface
quality of 60-40 is acceptable for most applications.
However, exceptions arise in
20
shorter wavelength systems or high-power/intensity
optical systems, where tighter
surface quality specifications are necessary.
Our recommendation for tolerancing
surface quality is to utilize the ANSI/
OEOSC/ISO 10110-7:2017 standard for
specifying scratch-dig. It includes both dimensional
and comparison inspection
specifications. In this, dimensional specifications
measure digs by a defined area
and scratches by a defined width. The
comparison method is the more traditional
visual inspection method (similar to
MIL-13830B) that evaluates scratches via
brightness and digs via 10X grade in micrometers.
It is important to note that in
the comparison method, designers need
to specify the comparison standard to ensure
consistency and accuracy in evaluating
surface quality.
While most aspheres can be toleranced
similarly to spherical components,
there are differences to consider. Key
considerations
include vertex
radius,
conic constant, aspheric coefficients, sag
table, form error (including vertex radius
and irregularity), mid-spatial frequency
errors (tolerance slope plus integration

http://www.edmundoptics.com

Tech Briefs Magazine - May 2024

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