It was pointed out in Chapter 2 that the interaction of fields and waves with matter can be strongly dependent on the wavelength (or the frequency, or the photon energy for electromagnetic waves). Examples of wavelength-dependent physical parameters observed in subsurface imaging (the alpha parameters) are: velocity and refractive index (Sec. 2.2A), reflectance at boundaries between different media (Sec. 2.2B), absorption coefficient of atomic and molecular systems (Sec. 2.2A), scattering coefficient (Sec. 2.2C), fluorescence rate (Sec. 2.3A), and optical diffusion coefficient for turbid media (Sec. 2.2D). Wavelength dependence may obey a simple law (such as the λ-4 dependence of Rayleigh scattering) and may exhibit a complex pattern that uniquely identifies substances and chemicals (e.g., in the case of optical absorption). The concentration, or abundance, of certain substances, such as algae in water, oxygen in blood (see Fig. 2.2-4), or a specific molecular agent tagging some site in a cell, are important beta parameters whose spectral features can be used as identifiers of substances or conditions for functional imaging.

The penetration depth, which is crucial to subsurface imaging, is usually wavelength dependent, as exemplified in Fig. 6.0-1 for the propagation of electromagnetic waves in water and human skin. Waves at different wavelengths penetrate to different depths and therefore acquire images of different layers. Therefore, when a subsurface target is measured at multiple wavelengths, the wavelength dependence of the transmittance of the covering medium must be accounted for.