<p>Fluorescence reader (10) for an optical assay arrangement comprising a polymeric sample substrate (1) having a reaction site-surface and a substrate surface (3) , the fluorescence reader comprising a light source (5) arranged to illuminate the reaction site-surface through the substrate surface, and a detector device (6) arranged to detect fluorescent light emitted from said reaction site-surface and transmitted through the substrate surface, the substrate surface provided with total-internal-reflection suppressing means (15) .</p>
