Attenuation, scattering, and absorption coefficients of planar waveguides which contain light emitting areas are investigated by analysis of the lateral intensity distribution of the light scattered out from regions of the waveguides surrounding the active areas. Short attenuation lengths require an experimental method without the use of coupling prisms. This method was succesfully applied to a set of thin film electroluminescent devices. Additionally, the outcoupling efficiency of the devices was determined only using this optical method. i.e. independently of other efficiency measurements. It is shown that the separation of scattering from absorption processes allows a quantitative analysis of the influence of the preparation conditions on the optical device parameters.