Tech Briefs Magazine - August 2022 - 14

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Imaging filters maximize image contrast in the DUV-MWIR spectral regions and require high
durability for applications in military/aerospace markets from airborne imaging and sensing to
satellite optical instrumentation. New uses are emerging in drone-based crop monitoring allowing
detailed health data down to the individual plant. Omega Optical Holdings' companies design and
manufacture precision optical filters, coatings, diffractive optics, and infrared components to enable
mission-critical applications in a wide range of industries, including life sciences, environmental
monitoring, aerospace, defense, and more. In this interview, Sarah Locknar, Ph.D., Marketing
Manager, Omega Optical, discusses latest trends and technologies in hyperspectral imaging.
Tech Briefs: Hyperspectral imaging has grown increasingly
popular over the past decade in industrial applications. What
have been the key growth drivers for hyperspectral imaging
in environment and military?
Sarah Locknar: We have entered an era
of big data and AI/machine learning in
everything. It's no longer good enough
to monitor a single wavelength or a
handful of wavelengths. The machine
learning algorithms can use the entire
spectral signature to interpret an image
whereas in the past, discrete algorithms
and human interpretations could only
handle a few well-resolved colors at a
time. Of course, there is a training period where the humans and
AI systems work together, but the AI results are quite remarkable.
Tech Briefs: Applications must be carefully considered when
selecting a hyperspectral imaging system. In many cases,
choosing the highest spectral resolution is not the best
option, so how do engineers select the right optical filter for
each application?
Locknar: It depends on the application. If you are looking for
very narrow gas lines in a spectrum, you really do need that
high degree of spectral resolution, whereas if you're looking at
chlorophyll in plants, which has a relatively broad reflectance
spectral signal, the spectral resolution is much less important.
There are also other factors to consider, such as the signal
strength and lighting conditions of the field-of-view.
Tech Briefs: Diffraction gratings have been used in various
spectrometric devices for hundreds of years. What are the
best practices in gratings for hyperspectral imaging?
Locknar: Omega's sister company, Optometrics, has been manufacturing
gratings for 57 years. There are a variety of factors
that go into choosing the grating, but spectral separation and
response are probably most important, followed by order separation
and size. The diffraction orders occur at specific angles to
the surface of the grating, so the overlap of the orders (if any) is
constant at all distances. This can be a problem in both diffractive
and refractive gratings. Order sorting filters are used to eliminate
any spectral crosstalk of higher orders into the lower order signals.
Insufficient order separation leads to artifacts in the image
or measurement. In miniaturized systems, the order-sorting
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filter is often placed directly above or even on the detector itself.
Gratings are the best choice for low-light level imaging.
Tech Briefs: Are linear variable filters commonly used in hyperspectral
imaging?
Locknar: Linear variable filters (LVFs) are becoming popular as
a low-complexity and low-weight alternative to grating-based
systems. They are most used in a band-pass format that is applied
directly on top of or in very close proximity to the detector array
or camera. A series of different wavelength bands is projected
onto each row of pixels in the camera. Depending on the gradient
of the bandpass, the resolution of the spectrum will vary. The best
spectral resolution is seen with shallow gradients. For instance, a
gradient of 5 nm per mm will give a better spectral resolution than
one with 50 nm/mm. This type of LVF-based hyperspectral system
is best suited to situations with high signal levels.
Tech Briefs: What are some of the innovations Omega Optical
has applied to this technology?
Locknar: Omega has adapted its plasma-assisted reactive sputtering
machines to produce the LVFs in high volume. We have
also developed specialized measurement routines to plot out
the position versus wavelength parameters on the part. You
might imagine that accurately measuring the spectral response
of LVF filters at various positions is not trivial. Optometrics has
developed a proprietary replication method that enables them to
supply surface relief transmission gratings offering hyperspectral
imaging camera designers a more robust grating, which is better
suited toward the physical and environmental demands of handheld
and drone applications, at a reasonable price and in volume.
Tech Briefs: What are the top trends that will continue to drive
innovation in hyperspectral imaging over the next five years?
Locknar: Big data and AI will continue to drive the market,
but there are other things such as miniaturization that are
equally important. The size and weight become important in
applications from drone-based imaging to endoscopy and medicine.
As the cameras become smaller, they will be integrated
into all sorts of things we haven't even thought of yet! These
mini-systems will need all the same functionality as table-top
models including lots of filters and gratings to ensure the
spectral separation and performance of the larger systems.
To learn more visit: www.omega-optical.com.
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Tech Briefs, August 2022

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Tech Briefs Magazine - August 2022

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