A fluorescence reader for an optical assay arrangement includes a polymeric sample substrate having a reaction site-surface and a substrate surface. A light source is arranged to illuminate the reaction site-surface through the substrate surface, and a detector device is arranged to detect fluorescent light emitted from the reaction site-surface and transmitted through the substrate surface, the substrate surface provided, for example, as a light-collecting body to allow fluorescent light rays exceeding a critical emission angle for total internal reflection to escape the substrate and enter the body.