Pulse count modulation for thermal printing

Claim 1

In a continuous tone color thermal apparatus including a print head having a plurality of heating elements for printing a line at a time of image pixels, each heating element being selectively energized in response to a digital dye density word signal to apply heat to a carrier having dye to cause such dye to transfer to a receiver to form a dye image pixel of a line, each dye image pixel having a density which is a function of the value of its corresponding digital dye density word signal, the improvement comprising: (a) means responsive to significant digits of the digital dye density word signal to provide a selected number of constant current pulses having a first width (PCM); and (b) means responsive to at least two least significant digits of the digital density dye word signal to selectively provide at least two constant current pulses having second and third different widths respectively, both of such second and third widths being smaller than the first width.