An optical device includes a substrate, a light emitter, a light detector, a conductive structure, and an opaque material. The light emitter, the light detector and the conductive structure are disposed on a surface of the substrate and are electrically connected to traces on the surface of the substrate. The light emitter includes an emitting area facing the substrate. The light detector includes a receiving area facing the substrate. The light emitter emits light within a range of wavelengths, and the substrate passes the light emitted by the light emitter. The opaque material is disposed on the substrate, and absorbs or attenuates the light within the range of wavelengths.