interest and the contrast c 2 of the interference fringes in fig2 b may relate to the planar waveguide structure after exposure to a radiation absorbing chromophore fig3 illustrates the pattern of interference fringes measured in accordance with an embodiment of the process of the invention firstly the planar waveguide structure is irradiated with electromagnetic radiation in tm mode to produce first interference fringes fig3 a and with electromagnetic radiation in te mode to produce second interference fringes fig3 c the contrast 1 tm of the first interference fringes is measured and the contrast 1 te of the second interference fringes is measured secondly the planar waveguide structure is exposed to a radiation absorbing stimulus thirdly the planar waveguide structure is irradiated with electromagnetic radiation in tm mode to produce third interference fringes fig3 b and with electromagnetic radiation in te mode to produce fourth interference fringes fig3 d the contrast 2 tm of the third interference fringes is measured and the contrast 2 te of the fourth interference fringes is measured the distance of the radiation absorbing stimulus to the surface of the planar waveguide structure is proportional to 1 tm 2 tm 1 te 2 te fig4 illustrates the a calculated and b observed measured interference pattern of an embodiment of the method of the invention the quality of the observed image may be quantified using standard statistical analysis to identify optical aberrations or to select a subset of high quality fringes for subsequent analytical measurements a suitable equation for.