1. A method for calculating the scattering and absorption characteristics of tissue, the steps comprising: a) acquiring a set of diffuse reflectance measurements from the tissue; b) establishing a set of tissue optical parameters; c) calculating model reflectance values using the set of tissue optical parameters and a model which relates diffuse reflectance to tissue optical parameters; d) calculating the sum of squares error between the measured diffuse reflectance and the model reflectance values; e) repeatedly updating optical parameters and repeating steps c) and d) until the sum of squares error is substantially minimized; and f) calculating the scattering and absorption characteristics of the tissue from the updated optical parameters that yield a minimum sum of squares error.