A light-guiding system comprising a plate-like light-emitting element provided with means for emitting light through at least part of its front surface. The electrometer includes an electrical amplifier to convert a current signal to a voltage signal. A plurality of leads extend from the housing, the leads being arranged in separate bundles of discrete leads, each bundle being associated with a particular site of the patient’s body. A first portion of the adhesive side is disposed at the perimeter of the lens forming a seal between the frame and the lens.