A simple technique is described for optically imaging the lateral distribution of normal forces exerted onto a flat surface. It is based on the detuning of a silicone rubber planar waveguide by the forces to be investigated. The method is demonstrated by imaging the contact line force of a sessile water droplet on the surface, with a force resolution better than &mgr;N. It is shown that the lateral resolution may be much better than the decay length of the waveguide modes used. ©1997 American Institute of Physics.