Multi-spectral image
Encyclopedia
A multispectral image is one that captures image data at specific frequencies across the electromagnetic spectrum
. The wavelengths may be separated by filters
or by the use of instruments that are sensitive to particular wavelengths, including light from frequencies beyond the visible light range
, such as infrared
. Spectral imaging
can allow extraction of additional information the human eye fails to capture with its receptors for red, green and blue. It was originally developed for space-based imaging.
Multispectral images are the main type of images acquired by remote sensing
(RS) radiometer
s. 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, satellites have three or more radiometer
s (Landsat has seven). Each one acquires one digital image (in remote sensing, called a 'scene') in a small band of visible spectra, ranging from 0.7 µm to 0.4 µm, called red-green-blue (RGB) region, and going to infrared wavelengths of 0.7 µm to 10 or more µm, classified as near infrared (NIR), middle infrared (MIR) and far infrared (FIR or thermal). In the Landsat case, the seven scenes comprise a seven-band multispectral image. Spectral imaging
with more numerous bands, finer spectral resolution or wider spectral coverage may be called hyperspectral or ultraspectral.
This technology has also assisted in the interpretation of ancient papyri
, such as those found at Herculaneum
, by imaging the fragments in the infrared range (1000nm). Often, the text on the documents appears to be as black ink on black paper to the naked eye. At 1000nm, the difference in light reflectivity 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 of Archimedes work.
The availability of wavelengths for remote sensing and imaging is limited by the infrared window and the optical window
.
Many other combinations are in use. NIR is often shown as red, making vegetation-covered areas appear red.
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
. The wavelengths may be separated by filters
Filter (optics)
Optical filters are devices which selectively transmit light of different wavelengths, usually implemented as plane glass or plastic devices in the optical path which are either dyed in the mass or have interference coatings....
or by the use of instruments that are sensitive to particular wavelengths, including light from frequencies beyond the visible light range
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
, such as infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
. Spectral imaging
Spectral imaging
Spectral imaging is a branch of spectroscopy and of photography in which a complete spectrum or some spectral information is collected at every location in an image plane...
can allow extraction of additional information the human eye fails to capture with its receptors for red, green and blue. It was originally developed for space-based imaging.
Multispectral images are the main type of images acquired by remote sensing
Remote sensing
Remote sensing is the acquisition of information about an object or phenomenon, without making physical contact with the object. In modern usage, the term generally refers to the use of aerial sensor technologies to detect and classify objects on Earth by means of propagated signals Remote sensing...
(RS) radiometer
Radiometer
A radiometer is a device for measuring the radiant flux of electromagnetic radiation. Generally, the term radiometer denotes an infrared radiation detector, yet it also includes detectors operating on any electromagnetic wavelength....
s. Dividing the spectrum into many bands, multispectral is the opposite of panchromatic
Panchromatic
Panchromatic film is a type of black-and-white photographic film that is sensitive to all wavelengths of visible light. A panchromatic film therefore produces a realistic reproduction of a scene as it appears to the human eye. Almost all modern photographic film is panchromatic, but some types are...
, which records only the total intensity of radiation falling on each pixel
Pixel
In digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled....
. Usually, satellites have three or more radiometer
Radiometer
A radiometer is a device for measuring the radiant flux of electromagnetic radiation. Generally, the term radiometer denotes an infrared radiation detector, yet it also includes detectors operating on any electromagnetic wavelength....
s (Landsat has seven). Each one acquires one digital image (in remote sensing, called a 'scene') in a small band of visible spectra, ranging from 0.7 µm to 0.4 µm, called red-green-blue (RGB) region, and going to infrared wavelengths of 0.7 µm to 10 or more µm, classified as near infrared (NIR), middle infrared (MIR) and far infrared (FIR or thermal). In the Landsat case, the seven scenes comprise a seven-band multispectral image. Spectral imaging
Spectral imaging
Spectral imaging is a branch of spectroscopy and of photography in which a complete spectrum or some spectral information is collected at every location in an image plane...
with more numerous bands, finer spectral resolution or wider spectral coverage may be called hyperspectral or ultraspectral.
This technology has also assisted in the interpretation of ancient papyri
Herculaneum papyri
The Herculaneum papyri are more than 1,800 papyri found in Herculaneum in the 18th century, carbonized by the eruption of Mount Vesuvius in AD 79. After various methods of manipulation, a method was found to unroll and to read them....
, such as those found at Herculaneum
Herculaneum
Herculaneum was an ancient Roman town destroyed by volcanic pyroclastic flows in AD 79, located in the territory of the current commune of Ercolano, in the Italian region of Campania in the shadow of Mt...
, by imaging the fragments in the infrared range (1000nm). Often, the text on the documents appears to be as black ink on black paper to the naked eye. At 1000nm, the difference in light reflectivity makes the text clearly readable. It has also been used to image the Archimedes palimpsest
Archimedes Palimpsest
The Archimedes Palimpsest is a palimpsest on parchment in the form of a codex. It originally was a copy of an otherwise unknown work of the ancient mathematician, physicist, and engineer Archimedes of Syracuse and other authors, which was overwritten with a religious text.Archimedes lived in the...
by imaging the parchment leaves in bandwidths from 365-870 nm, and then using advanced digital image processing techniques to reveal the undertext of Archimedes work.
The availability of wavelengths for remote sensing and imaging is limited by the infrared window and the optical window
Optical window
The meaning of this term depends on the context:* In astronomy, the optical window is the optical portion of the electromagnetic spectrum that passes through the atmosphere all the way to the ground...
.
Spectral bands
The wavelengths are approximate; exact values depend on the particular satellite's instruments:- Blue, 450-515..520 nm, is used for atmospheric and deep water imaging, and can reach within 150 feet (45.7 m) deep in clear water.
- Green, 515..520-590..600 nm, is used for imaging of vegetation and deep water structures, up to 90 feet (27.4 m) in clear water.
- Red, 600..630-680..690 nm, is used for imaging of man-made objects, in water up to 30 feet (9 m) deep, soil, and vegetation.
- Near infrared, 750-900 nm, is used primarily for imaging of vegetation.
- Mid-infrared, 1550-1750 nm, is used for imaging vegetation, soil moisture content, and some forest fires.
- Mid-infrared, 2080-2350 nm, is used for imaging soil, moisture, geological features, silicates, clays, and fires.
- Thermal infrared, 10400-12500 nm, uses emitted radiation instead of reflected, for imaging of geological structures, thermal differences in water currents, fires, and for night studies.
- RadarRadarRadar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
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. Aa 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 for detection of 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 seeing the water depth, vegetation coverage, soil moisture content, and presence of fires, all in a single image.
Many other combinations are in use. NIR is often shown as red, making vegetation-covered areas appear red.
Multispectral data analysis software
- MicroMSIMicroMSIMicroMSI for Windows is a remote sensing imagery analysis program designed for use in introductory courses in remote sensing, developed by the National Geospatial-Intelligence Agency...
is endorsed by the NGANational Geospatial-Intelligence AgencyThe National Geospatial-Intelligence Agency is an agency of the federal government of the United States with the primary mission of collecting, analyzing and distributing geospatial intelligence in support of national security. NGA was formerly known as the National Imagery and Mapping Agency ...
. - OpticksOpticks (software)Opticks is an open source, remote sensing application that supports imagery, video , Synthetic Aperture Radar , multi-spectral, hyper-spectral, and other types of remote sensing data. Opticks is unlike other remote sensing applications because it treats imagery and video alike...
is an open-source remote sensing application.
See also
- Hyperspectral imagingHyperspectral imagingHyperspectral imaging collects and processes information from across the electromagnetic spectrum. Much as the human eye sees visible light in three bands , spectral imaging divides the spectrum into many more bands...
- Full spectral imagingFull Spectral ImagingFull spectral imaging is a form of Imaging spectroscopy and is the successor to Hyperspectral imaging. Full Spectral Imaging was developed to improve the capabilities of Earth remote sensing . Hyperspectral imaging acquires data as many contiguous spectral bands. Full Spectral Imaging acquires...
- Imaging spectroscopyImaging spectroscopyImaging spectroscopy is similar to color photography, but each pixel acquires many bands of light intensity data from the spectrum, instead of just the three bands of the RGB color model...
- Imaging spectrometerImaging spectrometerAn imaging spectrometer is an instrument used in hyperspectral imaging and imaging spectroscopy to acquire a spectrally-resolved image of an object or scene, often referred to as a datacube due to the three-dimensional representation of the data. Namely, two axes of the image corresponds to...
- Liquid Crystal Tunable FilterLiquid crystal tunable filterLiquid crystal tunable filters are solid-state optical filters that use electronically controlled liquid crystal elements to transmit a selectable wavelength of light and exclude others...
- Multispectral pattern recognitionMultispectral pattern recognitionMultispectral remote sensing is the collection and analysis of reflected, emitted, or back-scattered energy from an object or an area of interest in multiple bands of regions of the electromagnetic spectrum...
- Remote sensingRemote sensingRemote sensing is the acquisition of information about an object or phenomenon, without making physical contact with the object. In modern usage, the term generally refers to the use of aerial sensor technologies to detect and classify objects on Earth by means of propagated signals Remote sensing...
- Spy satelliteSpy satelliteA spy satellite is an Earth observation satellite or communications satellite deployed for military or intelligence applications....
- Satellite imagerySatellite imagerySatellite imagery consists of photographs of Earth or other planets made by means of artificial satellites.- History :The first images from space were taken on sub-orbital flights. The U.S-launched V-2 flight on October 24, 1946 took one image every 1.5 seconds...