INDIRECT VERSUS DIRECT TECHNIQUES FOR THE MEASUREMENT OF THE OPTICAL PROPERTIES OF TISSUES

Abstract Knowledge of the optical absorption and scattering properties of tissues is required to calculate the spatial distribution of light in vivo for applications such as photodynamic therapy. In many soft tissues in the visible wavelength range the optical albedo is high, the scattering is highly forward peaked, and the mean free path between photon interactions is small. This regime presents severe technical difficulties in measuring the absorption and scattering coefficients. Two classes of technique may be used: 1. indirect methods in which, by applying specific models of light propagation, the properties are deduced from measurements of bulk tissue properties, and 2. direct, model-free measurements using optically thin tissue samples. These two approaches are described and the relative advantages and disadvantages discussed. Data for the optical properties of model soft tissues at 630 nm are presented to illustrate the applications of indirect and direct methods.