Diagnosis of pathologies in heritage
Introduction
A multispectral image is one that captures image data within specific wavelength ranges across the electromagnetic spectrum. Wavelengths can be separated by filters or by the use of instruments sensitive to particular wavelengths, including light of frequencies beyond the visible light range, such as infrared and ultraviolet. Obtaining spectral images can allow the extraction of additional information that the human eye does not capture with its red, green and blue receptors.
It was originally developed for images obtained from space[1] as well as in the application of document analysis and painting, with the first Landsat satellite missions around the 1970s, being the first to maturely exploit image capture of various bands.[2].
Multispectral imaging divides light into a small number (typically 3 to 15) of spectral bands. Hyperspectral imaging is a special case of spectral imaging where there are often hundreds of contiguous spectral bands.[3].
The technology that studies and treats multispectral capture is addressed by the branch of knowledge of remote sensing, associated with the study of the Earth's surface.
Origin
Thanks to the creation of the Landsat satellites, the application of techniques related to remote sensing was established in society early, specifically in the 1980s. The study of techniques to capture multispectral images became more prevalent until reaching its peak with the popularization of digital imaging at the beginning of the century.
Procedure
It is captured on board the satellite's multiband camera through a series of photoelectric detectors that transform radiance levels, coming from the Earth's surface, into numerical values called digital levels. The value of the level corresponding to a pixel will, in this way, be proportional to the intensity of radiance coming from the terrain. For this, analog-digital conversion devices are used.[4].
Within remote sensing sensors, specifically within the cameras used in this branch, the multispectral camera is the most special case. Up to 9 cameras can be used, although the usual number is 4 or 6.[5].
This digital image formation procedure is repeated for each of the bands, generating as a final result a three-dimensional matrix in which the digital level value of each pixel is located in a row, a column and a band. In this way the concept of multispectral image was coined.