| 摘要 |
Compensation for the effects of duty cycle on the output level of an imagi ng device, and for the effects of periodically varying distance between the output of the imaging device and its target, is accomplished electronically. An exemplary apparatus includes a source of radiation (generally a laser) having an output level th at varies both with an input power level and, undesirably, with the duty cycle. The apparat us also includes means for focusing the radiation onto a recording surface, and a power supply for driving the radiation source. In the case of a diode laser, the power supply delivers a variable current, the magnitude of which determines the power output of th e laser. The radiation source is operated to produce, on the recording surface, an imagewise pattern of spots, and an adjustment facility alters the output of the power supply to compensate for output-level variations resulting from duty cycle-that is, fr om the recent pattern of laser activity. In a first embodiment, the invention includes an up-down counter that receives, as input, the same binary image data in serial form that is used to determine the state of the radiation source as it scans over a recording medium (causing actuation of the source only when adjacent to a location on the recording medium that is to be written). The counter increments during clock cycles wh en the radiation source is active, and decrements during inactive cycles. As a result, the state of the counter generally reflects the recent pattern of radiation-source activity, and its contents are used to address a list of correction values. In a second embodiment, serial image data is applied to a shift register rather than a counter. This provid es a more precise record of the pattern of recent activity, and the instantaneous contents of the register are once again used to address a list of correction values. To compensate for the effects eccentric rotation on the output level of an imaging device, a function that inverts or at least counteracts the resulting power variations is applied to the power supply operating the radiation source.
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