Optical properties describe the physical interactions between electromagnetic radiation and matter dependent on the wavelength of the radiation. Incident power may be reflected, absorbed, scattered and/or transmitted. Reflection may consist of specular reflection and back scattering from inside the matter. The total amount of absorption and internal scattering may be expressed as extinction. According to the law of energy conservation, reflection, extinction and transmission add up to 1. For that reason, measuring optical properties of tissues must include at least two of these parameters or one of them must be excluded by the experimental design. Using a semi-infinite thickness of specimens, the influence of transmitted power may be neglected. According to the Kubelka-Munk-theory (Kubelka and Munk 1931, Kubelka 1948), the ratio of absorption and scattering can be calculated from reflection measurements assuming an ideally black background and a semi-infinite slice thickness (Blazek 1979). In reflection as well as in transmission measurements it has to be considered that the intensity of reflected and transmitted power depends on the solid angle of reflection or transmission (Hardy et al. 1956, Longini et al. 1968). This problem may be solved by using a two-beam spectral photometer with an integrating sphere as measuring instrument (Blazek 1975).