HP LaserJet 5000 Service Manual - Page 115

Writing the Image During the writing process, a modulated laser diode projects the beam onto the rotating six-sided scanning mirror. As the mirror rotates, the beam reflects off the mirror, through a set of focusing lenses, through a slot in the rear of the Toner Cartridge, and onto the photosensitive drum. The beam sweeps the drum from left to right, discharging the negative potential wherever the beam strikes the surface. This creates a latent electrostatic image, which later is developed into a visible image. BDIC PCB Focusing Lens BD Lens Scanner Motor Scanning Mirror (Hexahedron) Laser Driver Cylindrical Lens Photosensitive Drum BD Mirror Figure 5-11 Writing the Image Because the beam is sweeping the entire length of the drum and the drum is rotating, the entire surface area 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 by 1/1200th of an inch. The beam can be turned on and off to place a dot of light every 1/1200th of an inch. This is how the printer achieves its 1200x1200 dpi resolution. After the writing process, the drum surface has an invisible (latent) electrostatic image. At the end of each sweep, the beam strikes the beam detect lens, generating the Beam Detect Signal (/BD). The BD signal is sent to the DC Controller Board, where it is converted to an electrical signal used to synchronize the output of data (/VDO) for one sweep (scan line) and to diagnose problems with the laser diode or scanner motor. 5-24 Functional Information EN