A printing apparatus 10 exposes an image onto a photosensitive medium 14, having a printhead 20 with a linear array of exposure sources 40, each exposure source 12 operable at a variable intensity. A shuttle mechanism 16 moves the printhead 20 over the photosensitive medium 14 in a reciprocating motion between one end of a carriage assembly 72 and the other. An encoder 38 is coupled to the shuttle mechanism (16) for providing an index encoder pulse 60 at each of a plurality of increments of position of the shuttle mechanism 16 along the carriage assembly 72. Exposure control logic calculates a shuttle velocity according to index signal timing and adjusts the variable intensity of each exposure source 12 according to the shuttle velocity.