Medical Design Briefs - April 2022 - 44

Optimizing Illumination for qPCR Diagnostics - Design
Considerations
A complete optical path
minimizes signal crosstalk
between detection
channels.
Gray Optics
Portland, ME
Dichroic Mirror
Optics and photonics technologies are
enabling modern-day medical products
for both diagnostic testing and treatment.
In diagnostics, one of the most ubiquitous
and impactful tools is the polymerase
chain reaction (PCR) instrument, which
plays a pivotal role in diagnosing infectious
diseases across various end applications
and has played a central role in
understanding the spread of SARS-Cov-2,
the coronavirus that causes COVID-19.
One of the primary optical methods
for rapid diagnostic testing is quantitative
polymerase chain reaction (qPCR),
which utilizes fluorescence for detection.
Here, disease molecules of interest are
labeled with fluorescent dyes using antibody
chemistry, illuminated with specific
wavelengths of light (excitation/absorption
wavelength), and then emit redshifted
fluorescence light (emission
wavelength). PCR instruments rely on
carefully designed optics, optical filters,
and photodetectors to enable accurate
and reliable testing. The strength of the
emission wavelength allows the PCR
screening system to determine whether
the molecules of interest are present and
subsequently indicate a positive or negative
sample. Figure 1 shows the layout of
a typical qPCR diagnostic system with the
subsystems consisting of the illumination
source, illumination optics, excitation filters,
collection optics, emission filters,
and detection optics.
Given the bandwidth of the excitation
wavelengths for the tagged disease molecules
and potentially low emission signal,
high signal-to-noise (SNR) detection
in the system is critical to test success.
Thus, the success of qPCR as an
accurate and reliable diagnostic test
method of low sample volumes is related
to the design and selection of the light
source that matches the fluorophores
used in the test assay and yields high
signal-to-noise detection.
Therefore, instrument developers must
consider the illumination system design,
44
Cov
Detector
Detection Optics
Emission Filter
Illumination Optics
Excititation Filter
LED Light Source
Imaging Optics
Sample
Fig. 1 - The layout of a typical qPCR diagnostic system with the subsystems consisting of the illumination
source, illumination optics, excitation filters, collection optics, emission filters, and detection optics.
LED
Collecting Optics
LED Emitter
h0
Ѳ
Collected LED
Emission
NA = sinѲ0
Ѳ1
h1
NA1= sinѲ1
Total LED
Emission
Fig. 2 - A graphical representation of the Lagrange invariant.
including light source selection and delivery
optics design, to achieve this goal.
LEDs are an increasingly popular choice,
over lamps and lasers, for the illumination
source due to their cost and widespread
availability, making them easily accessible.
However, due to LED's unique properwww.medicaldesignbriefs.com
ToC
ties,
specific design considerations are
required compared to other light sources.
These design considerations evolve for
multi-channel qPCR instruments, which
can run multiple tests using multiple fluorophore
labels from the same sample. It is
necessary to design the complete optical
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Medical Design Briefs - April 2022

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