A spectrophotometric sensor assembly for non-invasive monitoring of a blood metabolite within a subject's body tissue is provided that includes a pad, a light source, and a light detector The light source is operative to emit light signals of a plurality of different wavelengths. The light detector is operative to detect light emitted by the light source and passed through the subject's body tissue. The light detector is at least partially enclosed in EMI shielding. In some embodiments, the light detector and EMI shielding are disposed in a detector housing that encloses the light detector and shielding. The housing is aligned with a detector aperture disposed in the pad.