A precise method to determine the absorption and reduced scattering coefficients of turbid media from the spatial distribution of light has been developed. it allows in vitro local measurements on samples, and needs only one measurement performed by a linear CCD detector. The intensity profile of the scattered light is characterized by the maximum of the intensity and the full width at half maximum. These parameters have been related theoretically to the absorption and reduced scattering coefficients. The theoretical approach is based on Monte Carlo simulations, which are used to predict the intensity profile at the output surface. The boundary reflections and the source and detector characteristics have been taken into account. For a thickness lower than 6 transport mean free paths, significant differences have been found depending on whether a Mie or a Henyey-Greenstein phase function (with the same anisotropic factor) is used. This is of help in the determination of the validity range of the similarity relations. Very good agreement (error typically less than 5%, maximal 15%) has been found between simulations and experiments performed on microsphere suspensions.