Medical Design Briefs - August 2021 - 46

* If quality control does not need to be
tightly held.
Once product volumes rise above
500 per year or the need for more compact
instrument form factors is critical,
the cost, size, and performance can be
optimized through investing in custom
optical design. Through a custom supply
chain, including development of
suitable metrology and inspection
steps, quality and consistency is easily
controlled.
■ Examples
1
0.9
0.8
0.7
0.6
MTF 0.5
0.4
0.3
0.2
0.1
20
40
60
80
100
120
Spatial frequency, Ip/mm
140
160
180
200
Diffractiom limited
Design (on-axis)
Measure (on-axis)
Fig. 3 - Lens sharpness measurement. The dotted black line represents diffraction limit for a high magnification
lens, blue represents the nominal design and orange represents the measured performance.
understanding of how the final image(s)
will be used. Typically, when the imaging
objective has greater resolution than
the image sensor, a digital sampling
artifact called aliasing can occur. When
edges or patterns are imaged, artifacts
appear in the final image. To avoid this,
the most optimal combination of objective
lens and image sensor is when the
resolution of the image sensor is at least
2X greater than that of the objective
lens or configured for Nyquist sampling.
That is, the sampling frequency f
= d/2, where d is the image space spatial
frequency of smallest feature size to
be detected. For most imaging systems,
no greater image sensor resolution is
needed. When the final image needs to
be zoomed or manipulated, greater
than 2X sampling is beneficial. Finally,
the optical resolution of a lens is often
46
Intro
Cov
compared to the diffraction limit, or
the physical resolution limit that is
defined by the diameter of the lens
aperture size and focal length of the
lens. How sharp the image is, depends
on how close to the diffraction limit the
lens performs at.
■ Custom vs. Off-the-Shelf
Most existing off-the-shelf microscopy
components and optics are interchangeable
on larger bench top systems. They
are made to reasonably high precision
and good consistency of quality. Off-theshelf
components are best suited in the
following situations:
* For proof of concept and early prototypes.
*
When cost and size is not critical.
* If product volumes are relatively low
(under 200-500 per year).
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ToC
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A
The degree of magnification in cell
microscopy systems generally fall into two
primary categories, macro imaging, where
the magnification is under 4X and high
magnification at 10X, or greater. For
macro imaging, the resolution is limited
to approximately 1-2 mm and individual
cells cannot be fully resolved in most
microscope configurations. In these applications,
only aggregate information about
the cell sample is needed to perform a
measurement. In high magnification optical
microscopy, where submicron resolution
can reach the optical resolution limit
of 200 nm, individual cells, or even subcellular
features can be resolved. High
magnification microscopy can be used for
cell counting, cell morphology measurements
and cell differentiation and editing
using lasers. Furthermore, in some
advanced techniques such as superresolution
imaging, the physical limits of
optical resolution can be overcome,
enabling sub-50 nm resolution, which is
ideal for next-generation spatial sequencing
within cells.
■ Conclusion
Developing the right product for the
use case and achieving the required
image quality requires understanding
and balance of the design parameters.
Often, an off-the-shelf lens is a suitable
option for product development and initial
product release. Once product volumes
increase or unique performance is
required, a custom lens design should be
considered. When engaging with a
design firm for a custom lens design, providing
documentation of the requirements
and commercial aspects of the
product help to streamline the process
and reduce the risks of arriving at a
design that does not suit the application.
This article was written by Neil Anderson,
Vice President of Sales and Marketing for Gray
Optics, Portland, ME. For more information,
visit http://info.hotims.com/79416-344.
Medical Design Briefs, August 2021
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Medical Design Briefs - August 2021

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