Definition: Multispectral Imaging
Multispectral surveys image the earth in an average of ten wide bands over a wide spectral range. Multispectral sensors measure the electromagnetic spectrum in discrete, discontinuous bands (unlike the continuous hyperspectral image). Multispectral sensors are capable of relative material delineation. The thermal wavelength range of the multispectral survey senses heat energy from the Earth's surface. It can be used to sense surface temperature, including anomalies associated with active geothermal or volcanic systems. Both multispectral and hyperspectral remote sensing observations are available. This range can also be used to map mineralogy associate with common rock-forming silicates.
- 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, Earth observation satellites have three or more radiometers (Landsat has seven). Each acquires one digital image (in remote sensing, called a 'scene') in a small spectral band. The shortest is the visible band, ranging from 0.7 µm to 0.4 µm, called red-green-blue (RGB) region. The others are infrared with wavelengths from 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 (1000 nm). Often, the text on the documents appears to the naked eye as black ink on black paper. At 1000 nm, the difference in how paper and ink reflect infrared light 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 with Archimedes' work.Multispectral imaging can be employed for investigation of paintings and other works of art. The painting is irradiated by ultraviolet, visible and infrared rays and the reflected radiation is recorded in a camera sensitive in this regions of the spectrum. The image can also be registered using the transmitted instead of reflected radiation. In special cases the painting can be irradiated by UV, VIS or IR rays and the fluorescence of pigments or varnishes can be registered.The availability of wavelengths for remote sensing and imaging is limited by the infrared window and the optical window.