In one embodiment, an apparatus is provided for testing for the presence of analytes in a sample. The apparatus comprises a source of light directed at the bottom surface of a substrate to achieve total internal reflection and to generate an evanescent field. An array of capture elements is immobilized on the top surface of the substrate. The bottom surface of the substrate is configured as a sawtooth (in cross section), the “teeth” aligned with the rows or the columns in the array. The outer faces of the sawtooth “prisms” are non-parallel to the substrate top surface, and specific requirements are imposed on the prism light-entrance face, and the substrate thickness and refractive index.