A laser printer provides a photoreceptor and a laser which produces an optical beam. A modulator is coupled to the laser and is responsive to binary data values to apply modulation signals to the laser to cause an image-wise modulation of the optical beam. Scan apparatus repetitively scans the optical beam across the photoreceptor to induce an image-wise charge state thereon. The scan apparatus causes, for one of the binary values, a variation in charge states corresponding to the one binary value across the photoreceptor. A function generator applies correction signals to the modulator to alter the optical beam in such a manner as to substantially prevent the variation of charge states. The function generator either causes a modulation of the voltage bias applied to the laser or modulates its pulse width output.