Laser printer employing modulated laser providing a uniform gray scale across a media sheet

#This invention relates to laser printers which employ binary dot patterns to represent images and, more particularly, a means and apparatus for modulating the laser in a laser printer to assure a uniform gray level dot pattern across the width of a media sheet.

#FIG. 1a is a block diagram of a prior art laser printer which employs a binary dot pattern to represent images. A processor 10 feeds to a video controller 12, a raster image of binary values which represent an image to be imprinted on a page. Video controller 12, in response, feeds a series of binary data signals to a laser driver 14 which, in turn, modulates the output of a laser 16 in accordance with the binary data signals. Modulated beam 18 from laser 16 is directed at a rotating, faceted mirror 20 which scans beam 18 across an imaging lens 22. Lens 22 directs scanned beam 18 to a mirror 24 which redirects scanned beam 18 onto a moving organic photoconductor (OPC).

#FIG. 1b is a schematic side view of the optics of FIG. 1a, showing rotating faceted mirror 20, lens 22, mirror 24 and a drum 26 on which OPC 28 is positioned. In the known manner, laser beam 18 is scanned across OPC 28 to cause selective discharge thereof in accordance with the modulation of the beam.

#FIG. 1a is a block diagram of a prior art laser printer.

#FIG. 1b is a schematic side view of the optical components of the prior art system of FIG. 1a.

#FIG. 2 is a schematic of a media sheet showing gray scale variations across the surface thereof.

#Referring to FIG. 5, each of the modules therein which is common to that shown in FIG. 1a is commonly numbered. As described above, laser driver 14 switches laser 16 on and off, based on video signals received from video controller 12. Laser 16 is normally configured as a laser diode and responds to input pulses from laser driver 14 by generating constant intensity output optical pulses. As further indicated above, it is non-linearities present in the optical system comprising lens 22 and mirror 24 which create a non-linear gray level printing on a media sheet. More specifically, dots corresponding to a "one" binary level (for instance) are not of constant size across the entire width of the media sheet, thus giving rise to a gray level variation.

#Since the dot size non-linearity is constant from scan to scan, it has been determined that the non-linearity during each scan can be compensated by controlling laser driver 14 to output varying magnitude drive pulses which, in turn, induce a variation in intensity (i.e., power) of output optical beam 18. To accomplish such a correction, a function generator 30 is connected to laser driver 14 and modulates the output power of excitation pulses that are applied to laser 16.

#A supply voltage is applied to both function generator 30 and laser driver 14. The control output from function generator 30 preferably modulates the supply voltage within laser driver 14 to enable a concomitant variation of the excitation pulses that are output by laser driver 14 to laser 16. The timing of the control voltage output from function generator 30 is controlled by the beam detect signal, which also synchronizes the operation of video controller 12.