In the first three embodiments shown in FIGS. 2-6, 8-12, and 14-18, the substrate preferably is provided with an exterior covering (see FIG. 34) of a plastic material, for example urethane or silicone, and the emitters and detectors lie flush with the lower surface of the exterior covering, that is, the surface that faces the skin, so that the emitters and detectors lie on the skin.