Abstract: Apparatus for estimating LED printhead junction temperature as a function of an input data stream, and for automatically compensating the printhead drive signal to maintain proper printer calibration to provide junction-level compensation for efficiency loss of an LED as the LED warms due to thermal energy generation from an electrical drive current is effected by a plurality of selectable LED temperature-specific look up tables constructed for a plurality of different LED temperatures. The selected look up table produces an output for driving the LED when the selected look up table is accessed by the input data. A model of the thermal characteristics of the LED, its heat sink, and the structure attaching the LED to the heat sink is adapted to produce an estimate of the temperature of the LED as a function of the image data. An address decoder selects one of the plurality of look up tables to process the input data stream in accordance with the estimate produced by the model.