There are some typical optical loss problems in the case of optical waveguides, though there are great differences between the fibres and planar waveguides. The optical losses can be significant especially in thin layers and this can remarkably decrease the sensitivity and increase the noise of the measurements when applied in sensors. The thickness of the sensitive oxide layers ranges from nanometers tip to several micrometers thus the thickness dependent transparency measured perpendicular to the surface is far from being sufficiently sensitive to characterise losses. In this work, efforts are made to characterize or visualise losses of the incoupled light propagating in the planar waveguide layer. An experimental setup was built for such measurements. The light is incoupled via a surface grating and the light scattered by various centres and scattered out from the layer is studied along the layer. Scattering or reflecting effect of the surface and interface roughness plays decisive role in thin planar waveguides. The scattered light, the position of the scattering centres, etc. are observed and detected by a CCD camera and the corresponding software allows also numerical evaluation of the detected picture. The method is described and first results are presented in this paper. (C) 2008 Elsevier B.V. All rights reserved.