A multispectral image is one that captures image data within specific wavelength ranges across the electromagnetic spectrum. The wavelengths may be separated by filters or detected via the use of instruments that are sensitive to particular wavelengths, including light from frequencies beyond the visible light range, i.e. infrared and ultra-violet. Spectral imaging can allow extraction of additional information the human eye fails to capture with its visible receptors for red, green and blue.
It was originally developed for military target identification and
reconnaissance. Early space-based imaging platforms incorporated
multispectral imaging technology to map details of the Earth related to coastal boundaries, vegetation, and landforms. Multispectral imaging has also found use in document and painting analysis.
Multispectral imaging measures light in a small number (typically 3 to 15) of spectral bands. Hyperspectral imaging is a special case of spectral imaging where often hundreds of contiguous spectral bands are available.
Applications
Military Target Tracking
Multispectral imaging measures light emission and is often used in detecting or tracking military targets. In 2003, researchers at the United States Army Research Laboratory and the Federal Laboratory Collaborative Technology Alliance reported a dual band multispectral imaging focal plane array (FPA). This FPA allowed researchers to look at two infrared (IR) planes at the same time.
Because mid-wave infrared (MWIR) and long wave infrared (LWIR)
technologies measure radiation inherent to the object and require no
external light source, they also are referred to as thermal imaging methods.
The brightness of the image produced by a thermal imager depends on the objects emissivity and temperature. Every material has an infrared signature that aids in the identification of the object. These signatures are less pronounced in hyperspectral systems (which image in many more bands than multispectral systems) and when exposed to wind and, more dramatically, to rain.
Sometimes the surface of the target may reflect infrared energy. This
reflection may misconstrue the true reading of the objects’ inherent
radiation.
Imaging systems that use MWIR technology function better with solar
reflections on the target's surface and produce more definitive images
of hot objects, such as engines, compared to LWIR technology. However, LWIR operates better in hazy environments like smoke or fog because less scattering occurs in the longer wavelengths.
Researchers claim that dual-band technologies combine these advantages
to provide more information from an image, particularly in the realm of
target tracking.
For nighttime target detection, thermal imaging outperformed
single-band multispectral imaging. Citation. Dual band MWIR and LWIR
technology resulted in better visualization during the nighttime than
MWIR alone. Citation Citation. The US Army reports that its dual band
LWIR/MWIR FPA demonstrated better visualizing of tactical vehicles than
MWIR alone after tracking them through both day and night.
Land Mine Detection
By
analyzing the emissivity of ground surfaces, multispectral imaging can
detect the presence of underground missiles. Surface and sub-surface
soil possess different physical and chemical properties that appear in
spectral analysis.
Disturbed soil has increased emissivity in the wavelength range of 8.5
to 9.5 micrometers while demonstrating no change in wavelengths greater
than 10 micrometers.
The US Army Research Laboratory's dual MWIR/LWIR FPA used "red" and
"blue" detectors to search for areas with enhanced emissivity. The red
detector acts as a backdrop, verifying realms of undisturbed soil areas,
as it is sensitive to the 10.4 micrometer wavelength. The blue detector
is sensitive to wavelengths of 9.3 micrometers. If the intensity of the
blue image changes when scanning, that region is likely disturbed. The scientists reported that fusing these two images increased detection capabilities.
Ballistic Missile Detection
Intercepting an intercontinental ballistic missile (ICBM) in its boost phase
requires imaging of the hard body as well as the rocket plumes. MWIR
presents a strong signal from highly heated objects including rocket
plumes, while LWIR produces emissions from the missile's body material.
The US Army Research Laboratory reported that with their dual-band
MWIR/LWIR technology, tracking of the Atlas 5 Evolved Expendable Launch
Vehicles, similar in design to ICBMs, picked up both the missile body
and plumage.
Space-based imaging
Most radiometers for remote sensing (RS) acquire multispectral images. Dividing the spectrum into many bands, multispectral is the opposite of panchromatic, which records only the total intensity of radiation falling on each pixel. Usually, Earth observation satellites have three or more radiometers.
Each acquires one digital image (in remote sensing, called a 'scene')
in a small spectral band. The bands are grouped into wavelength regions
based on the origin of the light and the interests of the researchers.
Weather Forecasting
Modern weather satellites produce imagery in a variety of spectra.
Multispectral imaging combines two to five spectral imaging bands of relatively large bandwidth into a single optical system. A multispectral system usually provides a combination of visible (0.4 to 0.7 µm), near infrared (NIR; 0.7 to 1 µm), short-wave infrared (SWIR; 1 to 1.7 µm), mid-wave infrared (MWIR; 3.5 to 5 µm) or long-wave infrared (LWIR; 8 to 12 µm) bands into a single system. — Valerie C. Coffey
In the case of Landsat satellites, several different band designations have been used, with as many as 11 bands (Landsat 8) comprising a multispectral image.
Spectral imaging
with a higher radiometric resolution
(involving hundreds or thousands of bands), finer spectral resolution
(involving smaller bands), or wider spectral coverage may be called hyperspectral or ultraspectral.
Documents and artworks
The technology has also assisted in the interpretation of ancient papyri, such as those found at Herculaneum,
by imaging the fragments in the infrared range (1000 nm). Often, the
text on the documents appears to the naked eye as black ink on black
paper. At 1000 nm, the difference in how paper and ink reflect infrared
light makes the text clearly readable. It has also been used to image
the Archimedes palimpsest
by imaging the parchment leaves in bandwidths from 365–870 nm, and then
using advanced digital image processing techniques to reveal the
undertext with Archimedes' work. Multispectral imaging has been used in a Mellon Foundation project at Yale University to compare inks in medieval English manuscripts.
Multispectral imaging can be employed for investigation of paintings and other works of art. The painting is irradiated by ultraviolet, visible and infrared
rays and the reflected radiation is recorded in a camera sensitive in
this regions of the spectrum. The image can also be registered using the
transmitted instead of reflected radiation. In special cases the
painting can be irradiated by UV, VIS or IR rays and the fluorescence of pigments or varnishes can be registered.
Multispectral imaging has also been used to examine
discolorations and stains on old books and manuscripts. Comparing the
"spectral fingerprint" of a stain to the characteristics of known
chemical substances can make it possible to identify the stain. This
technique has been used to examine medical and alchemical
texts, seeking hints about the activities of early chemists and the
possible chemical substances they may have used in their experiments.
Like a cook spilling flour or vinegar on a cookbook, an early chemist
might have left tangible evidence on the pages of the ingredients used
to make medicines.
Spectral bands
The wavelengths are approximate; exact values depend on the particular satellite's instruments:
- Blue, 450–515..520 nm, is used for atmosphere and deep water imaging, and can reach depths up to 150 feet (50 m) in clear water.
- Green, 515..520–590..600 nm, is used for imaging vegetation and deep water structures, up to 90 feet (30 m) in clear water.
- Red, 600..630–680..690 nm, is used for imaging man-made objects, in water up to 30 feet (9 m) deep, soil, and vegetation.
- Near infrared (NIR), 750–900 nm, is used primarily for imaging vegetation.
- Mid-infrared (MIR), 1550–1750 nm, is used for imaging vegetation, soil moisture content, and some forest fires.
- Far-infrared (FIR), 2080–2350 nm, is used for imaging soil, moisture, geological features, silicates, clays, and fires.
- Thermal infrared, 10400-12500 nm, uses emitted instead of reflected radiation to image geological structures, thermal differences in water currents, fires, and for night studies.
- Radar and related technologies are useful for mapping terrain and for detecting various objects.
Spectral band usage
For different purposes, different combinations of spectral bands can
be used. They are usually represented with red, green, and blue
channels. Mapping of bands to colors depends on the purpose of the image
and the personal preferences of the analysts. Thermal infrared is often
omitted from consideration due to poor spatial resolution, except for
special purposes.
- True-color uses only red, green, and blue channels, mapped to their respective colors. As a plain color photograph, it is good for analyzing man-made objects, and is easy to understand for beginner analysts.
- Green-red-infrared, where the blue channel is replaced with near infrared, is used for vegetation, which is highly reflective in near IR; it then shows as blue. This combination is often used to detect vegetation and camouflage.
- Blue-NIR-MIR, where the blue channel uses visible blue, green uses NIR (so vegetation stays green), and MIR is shown as red. Such images allow the water depth, vegetation coverage, soil moisture content, and the presence of fires to be seen, all in a single image.
Many other combinations are in use. NIR is often shown as red, causing vegetation-covered areas to appear red.
Classification
Unlike other Aerial photographic and satellite image interpretation
work, these multispectral images do not make it easy to identify
directly the feature type by visual inspection. Hence the remote sensing
data has to be classified first, followed by processing by various data
enhancement techniques so as to help the user to understand the
features that are present in the image.
Such classification is a complex task which involves rigorous
validation of the training samples depending on the classification
algorithm used. The techniques can be grouped mainly into two types.
- Supervised classification techniques
- Unsupervised classification techniques
Supervised classification makes use of training samples. Training samples are areas on the ground for which there is Ground truth, that is, what is there is known. The spectral signatures
of the training areas are used to search for similar signatures in the
remaining pixels of the image, and we will classify accordingly. This
use of training samples for classification is called supervised
classification. Expert knowledge is very important in this method since
the selection of the training samples and a biased selection can badly
affect the accuracy of classification. Popular techniques include the Maximum likelihood principle and Convolutional neural network. The Maximum likelihood principle calculates the probability of a pixel belonging to a class (i.e. feature) and allots the pixel to its most probable class. Newer Convolutional neural network based methods account for both spatial proximity and entire spectra to determine the most likely class.
In case of unsupervised classification
no prior knowledge is required for classifying the features of the
image. The natural clustering or grouping of the pixel values, i.e. the
gray levels of the pixels, are observed. Then a threshold is defined for
adopting the number of classes in the image. The finer the threshold
value, the more classes there will be. However, beyond a certain limit
the same class will be represented in different classes in the sense
that variation in the class is represented. After forming the clusters, ground truth
validation is done to identify the class the image pixel belongs to.
Thus in this unsupervised classification apriori information about the
classes is not required. One of the popular methods in unsupervised
classification is k-means clustering.
