An internal drum plotter including apparatus for mounting a recording substrate in a generally cylindrical configuration about a longitudinal axis, a multi-beam light source assembly (14) disposed at a first location along the longitudinal axis, and including a plurality of light sources (16) which are separated from each other along an axis (18) perpendicular to the longitudinal axis (12), light directing apparatus operative to direct light received from the multi-beam light source assembly to the recording substrate and apparatus for transmitting light from said multi-beam light source assembly to said light directing apparatus substantially without inaccuracies which are functions of the period of rotation of the light directing apparatus about the longitudinal axis.