A planar sample, particularly of the type used in biological laboratories for detection and sometimes analysis of two-dimensional arrays of proteins, nucleic acids, or other biological species, is illuminated by epi-illumination using optically filtered line lights that are arranged along opposing parallel sides of a rectangle in which the sample array resides, with two coaxial line lights on each side of the rectangle, and the two on any given side being separated by a gap whose optimal width depends on the wavelength band transmitted by the optical filter. Surprisingly, the gap eliminates the peak in intensity at the center of the sample area and the decrease that occurs from the center outward that would otherwise occur with a single continuous filtered line light, producing instead a substantially uniform intensity along the direction parallel to the line lights.