Diffuse Reflectance Spectroscopy

Diffuse reflectance spectroscopy (tutorial) is now routinely used for the rapid non-destructive characterization of a wide range of materials (Davies and Giangiacomo, 2000). Spectral signatures of materials are defined by their reflectance, or absorbance, as a function of wavelength in the electromagnetic spectrum. Under controlled conditions, the signatures are due to electronic transitions of atoms and vibrational stretching and bending of structural groups of atoms that form molecules and/or crystals. Fundamental features in reflectance spectra occur at energy levels that allow molecules to rise to higher vibrational states. For example, the fundamental features related to various components of soil organic matter generally occur in the mid- to thermal-infrared range (2.5–25 µm), but their overtones (at one half, one third, one fourth etc. of the wavelength of the fundamental feature) occur in the near-infrared (0.7–1.0 µm) and short-wave infrared (1.0–2.5 µm) regions. Soil clay minerals have very distinct spectral signatures in the short-wave infrared region because of strong absorption of the overtones of SO42–, CO32– and OHand combinations of fundamental features of, for example, H2O and CO2 (Hunt, 1982; Clark, 1999). The visible (0.4–0.7 µm) region has been widely used for color determinations in soil and geological applications as well as in the identification of Fe oxides and hydroxides (Ben-Dor et al., 1999). Since the mid-1980s, developments in instrument technology and chemometrics (the application of mathematical and statistical techniques to chemical data) have led to the increased use of spectroscopy in the laboratory and field and from space platforms, notably in geological studies (Clark, 1999).

Recent research has demonstrated the ability of reflectance spectroscopy to provide non-destructive rapid prediction of soil physical, chemical and biological properties in the laboratory (Ben-Dor and Banin, 1995; Janik et al., 1998; Reeves et al., 1999). There has been some success with reflectance spectroscopy for sensing of soil organic matter in the field (Sudduth and Hummel, 1993), and for the discrimination of major soil types from satellite multi-spectral and aircraft hyperspectral data (Baumgardner et al., 1985; Coleman et al., 1993; Palacios-Orueta et al., 1999). Despite these indications of the potential of the technique, there are few examples of the application of reflectance spectroscopy for non-destructive assessment of soils (Janik et al., 1998; Myer, 1998). Although geological spectral libraries exist that include soil mineral spectra (e.g. Clark, 1999), there are few examples of soil spectral libraries that include a wide diversity of soils with information on physical, chemical, and biological properties (Ben-Dor et al., 1999; Malley et al., 2000; Chang, 2001). In particular there has been little focus on the development of soil spectral libraries for application to risk-based approaches to soil evaluation that explicitly consider uncertainty in predictions and interpretations of soil properties.