An optical apparatus for obtaining a reflectance spectrum includes a first means for generating a light, a second means for transferring and receiving the light on a substrate, a third means for collecting a diffusely reflected light, and a fourth means for separating the diffusely reflected light from a specular reflected light to obtain information about a concentration of a chromophore in the substrate. The second means is an optic probe made of Poly(methyl methacrylate) (PMMA) material including an inner rod and an outer rod, the inner rod is nested within the outer rod for collection and for illumination, the inner rod and the outer rod are coaxial, the inner rod is longer than the outer rod, the inner rod is isolated from the outer rod with a semi mirrored isolator, the reflected light is reflected from deep within the substrate by the inner rod.
