on - column optical detection apparatus and method for use with capillary separation columns are shown . the apparatus and method minimize the adverse effects of light scattered from the walls of the column , thereby improving detection sensitivity and providing greater dynamic range . light from a conventional source is focussed onto a column . spatial filter means are positioned in front of the column to prevent at least some light from striking the column wall . another spatial filter is positioned in front of a light collecting means which gathers light emanating from the column . in a specific embodiment , a unique monolens design is shown comprising collecting and focussing lens formed as an integral unit with a bore for a capillary column .