CONTROL OF EXPOSURE ENERGY DELIVERY

摘要

Printing on photo-sensitive materials is effected by a linear array (10) of light emitting diodes (LEDs) energised by a power supply unit 20. The LEDs (101 ... 10x) are simultaneously enabled by a strobe pulse (S). Each LED has an individual input to control whether or not it is energised during a strobe pulse (S). The different exposures for each pixel are achieved by selectively controlling the LEDs (101 ... 10x) in each of a series of weighted strobe pulses whose base durations are in a power of 2 series. For this end the exposure for each pixel along the line is stored as an n-bit word and successive bits are taken from the words by a bit slicer (38) to be entered in an array control memory (40). Due to transients mostly associated with the power supply (20), the energisation of the LEDs does not exactly follow the rectangular strobe pulses. A dual compensation technique is disclosed. A pulse generator (42) generates the base series of pulses synchronised with the bit slicer (38) and firstly this base series is modified by a compensation pulse generator (44) to add (or subtract) an increment from each of the weighted series of pulses. A second compensation is made for the transient effect itself being a function of the number of LEDs (101 ... 10x) to be turned on for any strobe pulse (S). To this a counter (46) connected to the bit slicer (38) provides a count (p) of the number of 1's which is further used by the compensation pulse generator to modify the actual duration of the strobe pulses (S). All these operations may be done by means of software control. In particular the compensation pulse generator may use a look up table of strobe pulse durations covering both the weighted base pulse durations and the number (p) of LEDs turned on. The look up table may be calibrated to the individual LED array (10). The compensation technique may also be applied in other printing arrays, such as those using liquid crystal shutters. These have turn-ON transients although the number-on effect here is small.