A device and method for accurately performing quantitative diffuse optical spectroscopy on a sample includes a light source and a source optical fiber that is optically coupled to the light source. A diffuser material is interposed between the source optical fiber and the sample, the diffuser material comprising a high scattering, low absorption material. The diffuser material effectively increases the photon path length from the light source to the sample, which limits the depth of interrogation to superficial volumes despite the penetrating nature of the radiation typically used. A detector optical fiber is provided adjacent to or laterally disposed from the source optical fiber. The detector optical fiber is coupled to a detector which detects photons collected in the detector optical fiber. The detector optical fiber and the source optical fiber may be separated by a distance of less than 5 mm while still permitting the diffusion approximation to remain valid.