A simple method for measuring the light propagation losses in thin films deposited on an transparent plane parallel substrate is described. It consists on measuring the scattered light intensity from a plane parallel waveguiding substrate on which the optical propagation loss measured thin film is deposited. The method is simple to set up and to operate. It allows to measure relatively large propagation losses. It is illustrated by measuring very large propagation losses in chromophore doped deoxyribonucleic - surfactant complex thin film deposited on a plane parallel glass plate. The method is particularly interesting for measuring large propagation losses.