An apparatus for measuring the blood oxygen level of a patient is disclosed. In one embodiment, the apparatus includes a housing having an interior surface of non-reflective material substantially surrounding an interior. A cooperating light source and detector are operatively positioned proximate the interior surface so as to emit and receive light projected within the interior of the housing. A material more conducive to light transmission, reflectance, or a combination thereof--as compared to the interior surface material--is positioned proximate the light source and/or detector.