An optical reader for an optical assay arrangement including a polymeric sample substrate having a reaction-site surface provided with protruding microstructures and at least one reaction-site area; a light source for illuminating the reaction-site area; and a detector device for detecting light emitted from the reaction-site area. The light source is arranged to inject exciting light rays into the polymeric sample substrate with a controlled angle of incidence such that the protruding microstructures guide the exciting light rays in the direction of the reaction-site area, and the detector device detects light emitted from the at least one reaction-site area.