Sensing apparatus, consisting of a housing, including a medium having an elastic, transparent outer surface configured to contact a tissue within a living body and to deform in response to a force exerted by the body tissue on the surface. The apparatus has a radiation source contained in the housing and configured to direct radiation toward the surface, as well as a radiation detector contained in the housing and configured to sense the radiation that is reflected from the surface and to output a signal indicative of an amplitude of the reflected radiation. The apparatus further has a processor, which is coupled to measure a change in the signal due to deformation of the surface and, responsively to the change, to output an indication of the force.
