Spectral Properties of Materials

Abstract

Having laid the foundation of the physical mechanisms involved in scattering and absorption of light in the previous chapter, we now turn to the spectral reflectance and emittance properties of materials encountered in remote sensing using imaging spectroscopy. Here, we will qualitatively describe how those fundamental processes are observed in the spectra of common materials. Although we are not seeking to provide a comprehensive review of either the spectra or materials encountered in the practice and application of imaging spectroscopy, our goal is to provide a sufficient familiarity with both the principals and observations typically encountered and provide a basis for further analysis. Introduction The discipline of spectroscopy was created by Isaac Newton and has become an integral part of how the identity, physical structure, and environment of atoms, molecules, and solids are described. Its development goes back to the early seventeenth century when WilliamWollaston improved upon Newton's original spectrometer to show that the solar spectrum had gaps, which were further investigated by Joseph Fraunhofer, who created a catalog of what we now know as Fraunhofer lines. Joseph Foucault later identified two of the Fraunhofer lines as being from sodium emission. It is the description of this process of identification, using the unique spectral features characteristic of a particular material and captured in an imaging spectrometer measurement, that is the overarching goal of this book. In this chapter we will address the features themselves, while Chapter 4 is devoted to a description of how spectral measurements are performed using an imaging spectrometer, which is an evolution of the classical systems. The spectra are divided into those from reflectance, where the sun is an active source, or emittance, where the temperature and emissivity determine the spectral signature. The measured spectra will further be divided into those signatures that are due to organic materials, minerals, or are from man-made surfaces. The interaction of photons with the solid state structure of the surface materials introduces the features that are indicative of a particular substance. This interaction can be described at the macroscopic level through the classical theory based on Maxwell's equations; however, the interaction at the microscopic scale is between photons and the atoms that compose the solid and requires a quantum mechanical description.