A device for use in the detection of binding affinities, the device comprising a planar waveguide (2) arranged on a substrate (3), and further comprising an optical coupler (41) having a predetermined length for coupling coherent light (1) of a predetermined wavelength into the planar waveguide (2) such that a parallel beam of coherent light propagates through the planar waveguide (2) with an evanescent field (11) of the coherent light propagating along an outer surface (21) of the planar waveguide (2). The outer surface (21) of the planar waveguide (2) comprises binding sites thereon capable of binding target samples to the binding sites such that light of the evanescent field (11) is diffracted by target samples bound to the binding sites. The binding sites are arranged along a plurality of predetermined straight lines (7) running parallel to one another with a constant distance between adjacent straight lines. The predetermined straight lines (7) are arranged at an angle relative to the direction of propagation of the evanescent field (11) such that the coherent light (12) diffracted by the target samples bound to the binding sites impinges under a diffraction angle relative to the straight lines onto a further optical coupler (8) arranged in a portion (10) of the planar waveguide (2) outside the beam of coherent light propagating through the planar waveguide. The further optical coupler (8) couples the diffracted coherent light (13) out of the planar waveguide (2) such as to interfere at a predetermined detection location (9) with a difference in optical path length which is an integer multiple of the predetermined wavelength.