An apparatus for enhancing the selectivity for spectroscopic measurements of analytes in a turbid medium is described. In one example, spatial filters are used to select only certain radii from the medium to be imaged. This selection is accomplished by placing an optical obstruction on the surface of the medium or at an image plane of the surface later in the optical imaging system. In one implementation, this is achieved by placing a fiber bundle at an image plane of the collecting optical system and then using a spacer of appropriate size at the center of the fiber bundle to act as a central obstruction.