Temperature compensated continuous tone thermal printer

Claim 1

In a continuous tone thermal printing apparatus including a print head having a plurality of heating elements, one for each image pixel formed into a plurality of groups of heating elements, the heating elements of each group being simultaneously addressable in parallel N different times and selectively energizable when addressed with constant current pulses to provide N different possible temperature levels, the improvement comprising: (a) a shift register having one operative stage for each heating element; (b) a digital memory for holding a selectable number of dye density words, each such word being representative of the dye density level of a particular image pixel; (c) logic means responsive to the dye density words for sequentially storing N image data digital signals in the stages of the storage device in which the state of each stage corresponds to whether or not a corresponding heating element is to be selectively energized; (d) means for sequentially addressing the groups of heating elements each time one of the N image data digital signal is stored in the storage device with each group of heating elements being addressed N separate times; (e) means for producing a signal representative of the average temperature of the print head; and (f) means responsive to such average temperature signal for selectively energizing each heating element each time it is addressed and for adjusting the pulse width of the constant current pulse applied to the heating elements.