Most satellites today measure energy at many wavelengths. This is called multispectral imaging. Images taken at different wavelengths can be combined to make composite images by displaying the image for each wavelength as red, green, or blue in the final image. These composite images result in color patterns that can be used to identify surface features. Multi-spectral imaging can capture light from frequencies beyond the visible light range, such as infrared. This can allow extraction of additional information that the human eye fails to capture with its receptors for red, green and blue.
A multispectral image consists of several bands of data. For visual display, each band of the image may be displayed one band at a time as a grey scale image, or in combination of three bands at a time as a colour composite image. Interpretation of a multispectral colour composite image will require the knowledge of the spectral reflectance signature of the targets in the scene. In this case, the spectral information content of the image is utilized in the interpretation.
Panchromatic Image (0.52 to 0.90 μm) of Aligarh city and surrounding areas.
The following three images show the three bands of a multispectral image extracted from a LANDSAT multispectral scene at a ground resolution of 28.5 m. The area covered is the same as that shown in the above panchromatic image.
Band 2 (0.525 to 0.605 μm)
Band 3 (0.63 to 0.690 μm)
Note that both Band 2 (green) and Band 3 (red) bands look almost identical to the panchromatic image shown above.
Band 4 (0.75 to 0.90 μm)
In contrast, the vegetated areas now appear bright in the Band 4 (near infrared) band due to high reflectance of leaves in the near infrared wavelength region. Several shades of grey can be identified for the vegetated areas, corresponding to different types of vegetation. Water mass (both the river and the sea) appear dark in the Band 4 (near IR) band.
Multi-spectral images are the main type of images acquired by Remote sensing (RS) radiometers. Multi-spectral is the opposite of panchromatic. 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 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). 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.
(the wavelengths are approximate, exact values depend on the concrete satellite's sensors)
· 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.
Spectral band used in Multispectral Images
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.
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Department of Geology
Aligarh Muslim University, Aligarh - 202 002 (India)