Hyperspectral Imaging (HSI) is the process by which many (i.e., >50) narrow (e.g., 2-10 nm) samples of the electromagnetic spectrum are collected. With so many samples collected a representative spectrum is gathered. These data can be either collected over reflected (400-3300 nm) or emitted (>3300 nm) light, or both. The result of collecting all these inidividual spectra is a cube which can be visualized as in Figure 1.
Reflected light is light that originates externally and impinges upon material’s surface. This light is absorbed, transmitted, or reflected back. Since most light penetration is restricted to a few microns, reflected light contains information about the interactions of incoming photons with the surface material. Longer thermal infrared wavelengths (> 3300 nm) are dominated by emitted light. These are thermal photons emitted by materials with a temperature above absolute zero.
The wavelength of the light dictates the resolution capabilities of the measured light. Shorter wavelengths (< 1000 nm) interact with an atom’s electron cloud and absorbance features produced by this interaction are strongly dominated by intravalence charge transfer (IVCT). There are a number of diagnostic absorption features between 2000-2500 nm. Many of these are combinations and overtones from primary absorption features at wavelengths > 3300 nm. Thermal wavelengths (> 3300 nm) measure molecular bond properties, specifically bending, twisting, and stretching of molecules.
Mtn. Pass, California
Bayan Obo Mine, China