A non-invasive method to measure the optical properties of a turbid medium such as biological tissue is described. To determine the absorption and reduced scattering coefficient, the sample is illuminated with spatially intensity modulated light and the backscattered light is analysed. The spatial modulation of the backscattered light and the diffuse reflectivity of the sample are used to deduce its optical coefficients from a table generated by Monte Carlo simulations. The sensitivity of the method is analysed. Error influences on the method are investigated. The optical coefficients of human skin in vivo and of human bladder ex vivo are determined at 400, 500, 633, 700 nm and 633 nm respectively.