Multi-spectral imaging is a technology originally developed for space-based imaging. Multi-spectral imaging can capture light from frequencies beyond the visible light range, such as infrared. The electromagnetic (EM spectrum is the range of all possible Electromagnetic radiation frequencies Infrared ( IR) radiation is Electromagnetic radiation whose Wavelength is longer than that of Visible light, but shorter than that of This can allow extraction of additional information that the human eye fails to capture with its receptors for red, green and blue.
Multi-spectral images are the main type of images acquired by Remote sensing (RS) radiometers. Remote sensing is the small or large-scale acquisition of information of an object or phenomenon by the use of either recording or real-time sensing device(s that is not in physical A radiometer is a device for measuring the radiant flux (power of Electromagnetic radiation. Multi-spectral is the opposite of panchromatic. Panchromatic film is a type of Black-and-white Photographic film that is sensitive to all Wavelengths of Visible light. Usually satellites have 3 to 7 or more radiometers (Landsat has 7). The Landsat program is the longest running enterprise for acquisition of imagery of Earth from space Each one acquires one digital image (in remote sensing, called a scene) in a small band of visible spectra, ranging 0. 7 µm to 0. A micrometre ( American spelling: micrometer; symbol µm) is one millionth of a Metre, or equivalently one thousandth of a Millimetre 4 µm, called red-green-blue (RGB) region, and going to infra-red wavelengths of 0. 7 µm to 10 or more µm, classified as NIR-Near InfraRed, MIR-Middle InfraRed and FIR-Far InfraRed or Thermal. In the Landsat case there are 7 scenes comprising a 7 band multi spectral image.
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). Herculaneum (in modern Italian Ercolano) is an ancient Roman town located in the territory of the current commune of Ercolano. 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.
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Spectral bands
The wavelengths are approximate; exact values depend on the particular satellite's instruments:
- Blue, 450-515. . 520 nm, used for atmospheric and deep water imaging. Can reach within 150 feet (46 m) deep in clear water.
- Green, 515. . 520-590. . 600 nm, used for imaging of vegetation and deep water structures, up to 90 feet (27 m) in clear water.
- Red, 600. . 630-680. . 690 nm, used for imaging of man-made objects, water up to 30 feet (9. 1 m) deep, soil, and vegetation.
- Near infrared, 750-900 nm, primarily for imaging of vegetation.
- Mid-infrared, 1550-1750 nm, for imaging vegetation and soil moisture content, and some forest fires. A wildfire, also known as a wildland fire, forest fire, brush fire, vegetation fire, grass fire, Peat fire,
- Mid-infrared, 2080-2350 nm, 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. wiki stranglesnakejpg|thumb|Thermographic image of a Snake held by a human]] Infrared Thermography, thermal imaging, or thermal video, is a type of
- Radar and related technologies, useful for mapping terrain and for detecting various objects. Radar is a system that uses electromagnetic waves to identify the range altitude direction or speed of both moving and fixed objects such as Aircraft, ships
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. A plain color photograph. Good for analyzing man-made objects. Easy to understand for beginner analysts.
- Green-red-infrared, where blue channel is replaced with near infrared. Vegetation, highly reflective in near IR, then shows as blue. This combination is often used for detection of vegetation and camouflage.
- Blue-nearIR-midIR, where blue channel uses visible blue, green uses near-infrared (so vegetation stays green), and mid-infrared 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. Near infrared is often shown as red, making vegetation covered areas appear red.
Multispectral Data Analysis Software
- MicroMSI endorsed by the NGA. From the NGA websiteMicroMSI for Windows is a remote sensing imagery analysis program designed for use in introductory courses in remote sensing
- Opticks - an open source remote sensing application. Opticks is an open source, Remote sensing application that supports imagery, video (motion imagery Synthetic Aperture Radar (SAR, multi-spectral
See also
References
- Harold Hough: Satellite Surveillance, Loompanics Unlimited, 1991, ISBN 1-55950-077-8
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