Table of Contents
Understanding Hyperspectral Imagery
Spectral Data
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©Rob Whittington CMST 386 Section 6380 Spring Semester 2002 University of Maryland University College 10 July 2002
Spectral data are simply measurements of the electromagnetic energy that is reflected by objects. To better understand this reflected energy, let's take a closer look at the electromagnetic spectrum.
As most of us learned in our secondary school science classes, the sun continuously radiates enormous amounts of solar energy at wavelengths that cover the entire electromagnetic spectrum.
Waves in the electromagnetic spectrum vary in size from very long radio waves the size of buildings, to very short gamma rays smaller than the size of the nucleus of an atom (5). Once these energy waves enter the earth's atmosphere and strikes terrestrial objects, four things happen to that energy either singularly or in combination.
The "incident" energy may be (6):
- Transmitted through a transparent object such as air, water, or glass with a change in speed and direction.
- Partially or completely absorbed by an object; the amount of energy absorbed depending on the wavelength of the energy and the characteristics of the object. Blue tinted glass, for example, transmits energy with wavelengths of 425 to 490 nanometers and, at the same time, absorbs energy in the other wavelength bands.
- Scattered by being deflected in all directions, a common example being the scattering of sunlight as it traverses the atmosphere.
- Reflected by being returned from the surface of an object in an unchanged form except at an angle of reflection that is equal and opposite to the angle of incidence.
