A device (1) for use in the detection of binding affinities comprises a planar waveguide (2) arranged on a substrate (22). The waveguide (2) has an outer surface (21) and a plurality of incoupling lines (31) for coupling a beam of coherent light into the waveguide (2) such that a parallel beam of coherent light (62) propagates along the waveguide (2). The incoupling lines (31) are curved and have an increasing distance between adjacent incoupling lines (31). A divergent beam of coherent light (61) of a predetermined wavelength is coupled into the waveguide (2) such that it propagates along the waveguide (2). A plurality of binding sites (51) is attached to the outer surface (21) along at least one further plurality of diffraction lines arranged in an outcoupling section of the waveguide (2). These diffraction lines comprise a plurality of curved outcoupling lines (41) having a decreasing distance between adjacent outcoupling lines. They decouple a diffracted portion of coherent light from the planar waveguide (2), and the decoupled portion of coherent light (63) converges into a predetermined second focal location (631).