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