HP LaserJet 4 Service Manual - Page 132

Writing Stage At the writing station, the laser beam is used to discharge the drum's negative potential by focusing laser light onto areas of the photosensitive drum. This creates a latent electrostatic image, which later is developed into a visible image. Laser light is produced by a small modulated laser diode. The beam created by the diode shines onto a rotating polygonal mirror. As the mirror rotates, the beam reflects off the mirror and sweeps the page from left to right (see Figure 5-10). A set of lenses and mirrors focuses the horizontal sweeping beam through the laser beam access slot in the rear of the toner cartridge, and onto the photosensitive drum. Because the beam is sweeping the entire length of the drum and the drum is rotating, the entire circumference of the drum can be covered. The speed of the scanner motor (which turns the scanning mirror) and the speed of the main motor (which turns the drum) are synchronized, and each successive sweep of the beam is offset 1/600th of an inch. The beam can be turned on and off to place a dot of light every 1/600th of an inch in the horizontal direction. This is how the printer achieves its 600 x 600 DPI resolution. Figure 5-10 Image Writing 5-20 Functional Overview