an optical isolation element is provided on an optical sensor comprising a light source , at least one photodetector , and indicator material that emits light that is detected by the photodetector when optically excited by the light source . the optical isolation element limits the optical paths by which light may be transmitted by the light source , thereby limiting exposure of the excitation light source to regions of interest . the optical isolation element also limits the optical paths by which light may be transmitted to the photodetector , thereby limiting exposure of the photodetector to light from extraneous sources .