A method and apparatus for achieving uniform illumination in an electrophoresis apparatus is provided. Uniform illumination allows quantitative measurements of an electrophoresis gel to be made, thus increasing the information which can be obtained from an electrophoretic analysis. Uniform illumination is achieved by scanning the light source, preferably in a trans-illumination mode, across the sample gel in a direction perpendicular to the axis of the source. The light source is comprised of one or more light bulbs placed in a light tray. Variations in light intensity near the source end portions may be minimized using a variety of techniques including extended light bulbs, filters, reflectors, diffusers, or supplemental sources.