MULTI-SPECTRAL

A Multi-spectral 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. Multi-spectral imaging can allow extraction of additional information that the human eye fails to capture with its receptors for red, green and blue. It was originally developed for space-based imaging.
Multi-spectral images are the main type of images acquired by Remote sensing (RS) radiometers. Dividing the spectrum into many bands, multi-spectral is the opposite of panchromatic which records only the total intensity of radiation falling on each pixel. Usually satellites have 3 to 7 or more radiometers (Landsat has 7). Each one acquires one digital image (in remote sensing, called a scene) in a small band of visible spectra, ranging 0.7 µm to 0.4 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 the 7 scenes comprise a 7 band multi spectral image. Multispectral images with more numerous bands or finer spectral resolution or wider spectral coverage may be called "hyperspectral" or "ultra-spectral".
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 under text of Archimedes work.

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.
• 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.
• Radar and related technologies, useful for mapping terrain and for detecting various objects.
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