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) radiometers. 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 radiometers (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 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 (50 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 (30 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.
- 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. 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
- Hyperspectral imaging
- Full spectral imaging
- Imaging spectroscopy
- Imaging spectrometer
- Liquid Crystal Tunable Filter
- Multispectral pattern recognition
- Remote sensing
- Spy satellite
- Satellite imagery
- Harold Hough: Satellite Surveillance, Loompanics Unlimited, 1991, ISBN 1-55950-077-8
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