Multispectral Imaging in the SWIR with Optical Filters

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Outline●●●●●●Introduction
SWIROptical Filters
Multispectral Imaging
Choosing the Right Optical filters
Examples

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Introduction● Everything that has color has a spectral signature.
● When light interacts with an object, that object either reflects,absorbs, transmits, or re-emits incident light energy.
● The amount of reflection, absorption, etc., varies with wavelengthand this variation depends on the wavelengths that constitute theincident light, and the nature of the object.
● Some objects, like this apple, reflect the red wavelengths from thesun’s visible rays, but absorb the rest.
● What we see as color is due to the apple’s reflectance spectrum
that is heavily weighted to the red.
● In fact, this apple reflects light energy beyond the red, into theinfrared, but our eyes cannot see this IR light.

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Introduction•

Some objects of interest have a relatively boring reflectance spectrum in the visible
wavelengths, but more interesting spectral signatures in the SWIR (short-waveinfrared).

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Introduction•

Some objects of interest have a relatively boring reflectance spectrum in the visible
wavelengths, but more interesting spectral signatures in the SWIR (short-waveinfrared).

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SWIR (short-wave infrared)• Wavelength range past NIR from ~ 1um to 3um

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SWIR (short-wave infrared)• Wavelength range past NIR from ~ 1um to 3um• The range roughly follows the spectral response of the semiconductor detector indiumgallium-arsenide (InGaAs)

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SWIR (short-wave infrared)• Because most objects reflect some amount of SWIR light (as opposed to absorb it), theSWIR range can be very useful for a variety of imaging applications.

VisibleSWIR

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SWIR (short-wave infrared)• Because most objects reflect some amount of SWIR light (as opposed to absorb it), theSWIR range can be very useful for a variety of imaging applications.

Courtesy of Smart Vision Lights

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SWIR (short-wave infrared)
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•
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Agricultural monitoring
Waste management
Semiconductor inspection
Thermal Imaging
Food inspection/sorting
Life Sciences

Courtesy: "Why SWIR & InGaAs? Why is
it useful?" UTC Aerospace Systems

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Optical Filters• Transmit light of some wavelengths, and reject (or block) light of other wavelengths.
• They can be used in imaging devices to highlight the interesting spectral signatures ofobjects, increasing the image contrast.
• Give you the light you want, and reject the light you don’t want.
Incident White Light

Example: ‘red’ filter

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Transmitted Red LightA spectral plot shows % transmission as a functionof wavelength and represents the percentage ofenergy that gets through the filter -- what is notallowed to transmit is either absorbed orreflected. T + A + R = 100%
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Optical Filters
The most basic imaging system has a filter

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Multispectral Imaging• Multi-image capture, each of a particular color or spectral band.
• RGB camera provides images in three bands.
• Multispectral camera gives several discrete bands, while a
hyperspectral camera produces images over a continuous spectrum.
• Having these different channels allows much more information to beobtained.

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Variety of Approaches●The specifics of theapplication, including theenvironment in which theimaging must beaccomplished, determine
which approach is mostappropriate.

●Techniques that use thepassive component opticalfilter are often the simplest
and cheapest solution.

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Using Optical Filters for Multispectral

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For remote sensing applications, multispectral imaging using optical filters is a simple way to operationalize
imaging for precision agriculture (“smart farming”).
For machine vision/industrial applications, advantages include lower-cost cameras, the need for only one lightsource, and filters can be customized exactly for the situation for pass/fail outcomes.
Filters allow for better spectral separation to reduce crosstalk resulting in higher contrast (higher signal/noise) this in turn allows the robot or inspection line to move faster, and the inspection overall to be lower cost.
For systems with multiple LEDs, multiband filters can be used with strobing to save on expensive cameras (onlyone camera may be needed, and with no filter-wheel!).

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Choosing the Right Optical Filters
Filters can be customized
to place the band exactly
where you need it.

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Choosing the Right Optical Filters
Filters can be customized
to place the band exactly
where you need it.

Optical Filtertransmission
spectraSWIR LED
outputspectra

These filters remove the overlap present with the SWIR LEDs, increasing
image contrast by removing more unwanted light from each image.

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Choosing the Right Optical Filters
Optical filters made from sputtered
all-dielectric interference coatings
on glass have the highest level ofspectral performance and thedurability to handle extreme
conditions.

dielectric filtertransmission
spectracolored glass filtertransmission
spectraSWIR LED
outputspectra

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Machine Vision example
A multiband SWIR filter used withmultiple LED light sources (turnedon/off sequentially) in this application
helps identify the defective fruit aswell as potentially identify which wax
is on the pear (or if there is any wax
at all).

quad-band filter spectra
Multiple LEDs(strobed/sequentially)

The multiple bands give more
information by which the inspection
system can determine good vs. bad,wax vs. no wax, or wax type as theline moves quickly by.

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Remote Sensing example
Multispectral imaging with UAVs holding multiple low-costcameras, each with a filter, significantly lowers the cost andweight as compared with a hyperspectral camera, effectively
operationalizing precision agriculture.
●The simpler approach means more flights with less data
to crunch, and reduces the impact of a drone accident.

●Filters can be customized to highlight key absorption
features related to, for example, biomass and nitrogen
which are located in the SWIR range.

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Thank you for joining!

John Atkinson
Principal Product Engineer at Chroma Technology
Chroma.com
Sales@chroma.com

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Thank you for joining!

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