Selecting
the right multispectral method for your application
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Multispectral
imaging is a term that can be used to describe RGB images that we
are all accustomed to, however it's normally used as a term in machine
vision when selective spectral bands are combined to create an image
that highlights and contrasts the specific features of interest.
When deciding what method of multispectral imaging will suit your
application, it's not only the principal spectral wavelength bands
that are important. Other things to consider include whether bands
are to be captured at the same time, if you need pixel correlation
between bands and other technical aspects along with cost, ease-of-use
and overall functionality. When needing to capture pixel correlated
small details at the same time and at the maximum resolution possible,
prism based multispectral technology is an excellent choice.
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What
is the difference between hyperspectral imaging and multispectral
imaging?
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Hyperspectral
Imaging
- Used to capture full spectral signatures for spectral analysis
- Typically hundreds of wavebands captured
- Wavebands are equally sized with no gaps between them
- The principal wavebands of interest that contribute to the anylsis
are selected
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Multispectral
Imaging
- Used for targeted application requirements when principal wavebands
are known
- Typically 2-15 wavebands
- Wavebands are discretely positioned and sized
- Each waveband has a specific purpose for a given application
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Using
Multi-sensor dichroic prism camera to achieve multispectral imaging
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Using a multi-sensor
dichroic prism camera for multipspectral imaging empowers users
with a fast and compact solution. Prism based cameras for Multispectral
imaging use multiple sensors aligned to a common optical path.
This ensures simultaneous waveband capture with pixel correlation
between bands and so fully detailed images. Prism based cameras
are typically fine tuned to look at specific features and allow
users to identify what is required.
In the past we have typically seen multispectral imaging solutions
built with multiple cameras and so require image manipulation
to overlay wavebands causing a loss of sharpness in the resultant
multispectral image. They are larger and also typically cost more
than a prism based solution. Additionally, multi-sensor prism
based cameras with dichroic filters deliver strong spectral separation
with little crosstalk at full spectral intensity as well as high
frame rates.
Applications best suited to multi-sensor dichroic prism cameras
for multispectral imaging:
Fruit
and vegetable inspection/sorting systems: A multi sensor
dichroic prism camera with RGB plus NIR data is perfectly suited
to this due to its high spatial detail and precise spatial fusion.
Useful in identifying ripeness and deformities. Since nearly all
fruit and vegetable inspection systems involve some form of conveyance
or vehicle movement, a prism camera with instantaneous multi-waveband
capture and global shutter sensors remove all motion related artefacts.
Intelligent farming: Intelligent
farming systems typically operate when mounted to a continuously
moving vehicle. Dichroic prism cameras with global shutter, high
sensitivity and short exposure-times removes problems with motion
and vibration associated with agricultural vehicles in motion.
Today there is a growing demand for multispectral systems. Multispectral
systems can be tailored to work with a range of different applications.
In the past, multispectral prism cameras were manufactured with
standard wavebands and so suited a small number of applications.
More recent advancements in the manufacture of prism cameras now
allows for customised waveband centre wavelengths and band widths
and so can be tailored to a large variety of specific applications.
It is often the case that hyperspectral imaging is used to determine
the principal wavebands for an application but when deployed in
the field and in volume customised multispectral cameras save
time and cost.
Other applications include:
- Crop health, intelligent farming, environmental applications
- Surgical guidance, life sciences, forensics
- Electronics, recycling, scientific research
- Meat and poultry inspection
- Inspection of fruits, vegetables, nuts, grains, tea leaves
- Pharmaceuticals, cosmetics, packaging
- Currency inspection, passports, biometrics
- Fish farming
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A
comparison of the different multispectral techniques is described
below. Each has its own advantages and disadvantages.
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Two
separate camera method
The two separate camera method to achieve multispectral imaging
is perhaps one of the earliest methods used. For example, an RGB
camera with an IR cut filter and a monochrome camera with a IR pass
filter, create a colour and NIR combination.
Advantages:
- No specialised technology required
- Easy to source with lots of options
- Customisable waveband selection
Disadvantages:
- Difficult to achieve accurate pixel alignment/image fusion due
to optical parallax
- Users are forced to deal with two separate imaging setups to build
and maintain
Suitable applications:
Where the application can tolerate separate analysis of each waveband
and where separate pass/fail steps are acceptable
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Multispectral
filter array method
The Multispectral filter array method features a predetermined mosaic
pattern of spectral band pass filters on the sensor. The repeating
pattern might capture a few spectral bands in a single shot. Often
called "snapshot" multispectral.
Advantages:
- Single sensor/optical path, larger number of bands possible
- All bands are captured in a single snapshot
Disadvantages:
- Users will have to use demosaicing software methods to interpolate
waveband values other than those of the filter on a pixel. This
restricts spatial precision and blurs data.
- Has significant spectral crosstalk between wavebands and pixels
- Batch-to-batch consistency effects spectral accuracy
- Wavebands not customisable
Suitable applications
Where lower resolution is acceptable and in instances where
some spectral uncertainty is tolerable.
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Filter-wheel
camera method
The filter-wheel method addresses both the parallax issue with
the two camera method and the demosaicing issue with the multispectral
filter array method. This method puts a filter wheel either in front
or behind the lens to achieve multiple images of specific wavebands.
Advantages:
- Single sensor/optical path
- Full spatial resolution per spectral band
- Ability to customise filter set
Disadvantages:
- Sequential image capture results in a reduction of frame rates
and possibility of position shift between wavebands.
Suitable applications:
- Where targets are static
- Where different filter sets are needed for different targets
- Where lower Mean Time Between Failure (MTBF) is acceptable
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Two
cameras + a beam splitter method
Similar to the two camera method, this technique overcomes the
optical parallax issue by using a beam splitter. The beam splitter
gives each camera the same optical path to the target.
Advantages:
- Single optical path
- Full spatial resolution per spectral band
- Customisable waveband selection
Disadvantages:
- Large and expensive
- Loss of light intensity from split
Suitable applications:
- Where the target or the system is moving
- Where the system can be large/heavy
- Where illumination is plentiful
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Multi-Sensor,
Multi-path imaging
The multi-sensor method eliminates the need for a beam splitter
by simply using multiple sensors each with its own filter. This
gives full resolution and strong spectral separation to each band.
Because we give each sensor its own optical path, this method
reintroduces the issue of optical parallax and off-angle viewing.
This requires complex alignment algorithms.
Advantages:
- Simultaneous capture of wavebands
- Full resolution of each sensor
- Strong spectral separation and full intensity
- Compact all-in-one systems
- Customisable waveband selection
Disadvantages:
- Optical parallax and off-angle viewing. This requires complex
alignment algorithms that never produce perfect pixel correlation.
- Typically a higher cost
- Bulk and weight
Suitable applications:
- Where the target is flat and perpendicular to the camera
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Multi-sensor
dichroic prism camera for multispectral imaging
Unlike the multi-sensor, multi-path method we have eliminated
the need bulk and weight seen with multiple camera methods simply
by using multiple sensors in a single camera. Sensors are mounted
on prism faces and are precisely aligned to a common optical path.
Advantages:
- Common optical path
- Simultaneous capture of wavebands
- Full resolution of each sensor
- Strong spectral separation and spectral intensity
- High frame rates
- Lightweight, ultra-compact solution
- Customisable waveband selection
Disadvantages:
- Resolution and number of channels limited by current prism
designs
Suitable applications:
- Where both spectral and spatial precision are required including
precise fusion of multiple wavebands
- Where there is a large amount of target or camera motion
- Where there is angled viewing, uneven surfaces or varying object
sizes.
- Where light weight is important (e.g. drones)
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Need
a price or more application information on any of these multispectral
methods Adept Turnkey can help. Please
email Adept Turnkey or call our offices
Adept
Turnkey Pty Ltd is "The Machine Vision and Imaging Specialists"
and distributor of multispectral products in Australia and New Zealand.
To find out more about any multispectral product, please call Adept
Turnkey at Perth (08) 9242 5411 / Sydney (02) 9905 5551 / Melbourne
(03) 9384 1775 or contact us online.
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