Assay device

An optical detection unit for an assay device for quantitatively determining analyte concentration in a liquid sample comprises a planar emitter 2; planar detector 3; and lateral flow membrane 4 interposed between emitter and detector and comprising at least one test region 8,12; wherein the emitter 2 (detector 3) comprises an emission (absorption) layer 9,16 (10,15) of an organic electroluminescent (organic photovoltaic) material which comprises at least 15 emitter (detector) pixels; wherein each test region 8,12 is interposed between one emitter pixel and one detector pixel whereby to form an optical pathway from the emitter pixel to the respective detector pixel through at least a portion of the respective test region. Also claimed is a method comprising addressing in a predetermined sequence the emitter pixel and detector pixel forming the optical pathway through each test region.