Chapter 7 introduced theoretical methods for calculating single scattering events of optical fields from individual particles, and illustrated examples of applications for measurement of individual cell properties by backscattering spectroscopy or by measurement of the scattering phase function. Chapters 9–12 will cover the concepts and theoretical framework for measurement of bulk tissue optical properties in the diffusion regime, where source-detector distances are large compared to 1/μ ′s (in most tissues, ≳0.5 cm), to enable application of the diffusion approximation to the radiative transport equation. Chapter 9, especially, rigorously addresses the parameter space for which the diffusion approximation to the transport equation is valid. In this chapter we cover the ground in between: the measurement of light that has scattered a few times, but not so many times that the light can be considered diffuse. The formalisms presented here, nonetheless, can be extended into the beginning of the diffusion regime, and may be advantageous where the P1 approximation holds (to be presented in Section 9.4). For the typical optical properties of tissue, this generally corresponds to distances less than 0.5 cm. Moreover, whereas the diffusion approximation also requires that the absorption coefficient be much lower than the reduced scattering coefficient, in the formalisms for short source-detector distances that are presented in this chapter that restriction is lifted. Consequently, the wavelengths typically used for short source-detector distances may span from the near-UV through the near infrared (NIR). In contrast, for applications in real tissue, the useful wavelength range for the diffusion regime is limited to the red-NIR wavelengths, where absorption by hemoglobin is minimal.

Given these considerations, it may seem confusing that the terms diffuse reflectance and diffuse reflectance spectroscopy (DRS) are commonly used to denote measurements of reflectance at any distance from the light source, including short distances. Such broad application of those terms may be partially justified by the fact that “diffuse” reflectance can be intended simply to distinguish between reflectance from a diffusing medium and single backscattering events or specular reflection (as from a mirror). DRS encompasses terms that more specifically relate to variations of the optical geometry and/or methods of the spectral analysis. These include elastic scattering spectroscopy (Bigio and Mourant, 1997), light scattering spectroscopy (Backman et al., 2001), differential pathlength spectroscopy (Amelink et al., 2004), and optical pharmacokinetics (Mourant et al., 1999) among others.