A device for use in the detection of binding affinities comprises a planar waveguide (2) arranged on a substrate (3), and an optical coupler (4) for coupling coherent light (1) of a predetermined wavelength into the planar waveguide. The coherent light propagates through the planar waveguide (2) with an evanescent field (6) propagating along an outer surface (5) of the planar waveguide. The outer surface (5) of the planar waveguide comprises binding sites (7) thereon capable of binding target samples (8) to the binding sites (7) such that light of the evanescent field (6) is scattered by target samples (8) bound to the binding sites (7). The binding sites (7) are arranged along a plurality of predetermined lines (9) which are arranged such that the scattered light constructively interferes at a predetermined detection location with a difference in optical path length which is an integer multiple of the predetermined wavelength.
