HYPERSPECTRAL IMAGING

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INTRODUCTION


With the advent of new technologies, the field of remote sensing too has gained a new height. Hyperspectral imaging collects and processes information from across electromagnetic spectrum. The most significant recent breakthrough in remote sensing has been the development of hyperspectral sensors and software to analyze the resulting image data. Fifteen years ago only spectral remote sensing experts had access to hyperspectral images or software tools to take advantage of such images. Over the past decade hyperspectral image analysis has matured into one of the most powerful and fastest growing technologies in the field of remote sensing



HYPERSPECTRAL IMAGING- THE PROCESS
Hyper spectral sensors collect information as a set of 'images'. Each image represents a range of the electromagnetic spectrum and is also known as a spectral band.
These 'images' are then combined and form a three dimensional hyper spectral cube for processing and analysis. Hyper spectral cubes are generated from airborne sensors like the NASA’s Airborne Visible/Infrared Imaging Spectrometer (AVIRIS), or from satellites like NASA’s Hyperion.


SPECTRAL REFLECTANCE

In reflected – light spectroscopy, the fundamental property that we want to obtain is spectral reflectance, which is the ratio of reflected energy to incident energy as a function of wavelength. Reflectance varies with wavelength for most materials because energy at certain wavelengths is scattered or absorbed to different degrees. These reflectance variations are evident when we compare spectral reflectance curves for different materials. Pronounced downward deflections of the spectral curves mark the wavelength ranges for which the material selectively absorbs the incident energy. These features are commonly called absorption bands. The overall shape of a spectral curve and the position and strength of absorption bands in many cases can be used to identify and discriminate different materials.


IMAGING SPECTROMETERS



Hyper spectral images are produced by instruments called imaging spectrometers. The development of these complex sensors has involved the convergence of two related but distinct technologies: spectroscopy and remote imaging of Earth and Planetary surfaces.
Spectroscopy is the study of light that is emitted by or reflected from materials and its variation in energy with wavelength. As applied to the field of optical remote sensing, spectroscopy deals with the spectrum of sunlight that is diffusively reflected by materials at the earth surface.



MINERAL SPECTRA

In the inorganic materials like minerals, chemical composition and crystalline structure control the shape of the spectral and the presence and positions of specific absorption bands. Wavelength specific absorption may be caused by the presence of particular chemical ions or elements, the ionic charge of certain elements and the geometry of chemical bonds between elements, which is governed in part by the crystal structure.