Writing stage At the laser/scanner assembly, two laser beams focus onto the photosensitive drum and discharge the drum's negative potential. This creates a latent electrostatic image, which later is developed into a visible image. Two small laser diode beams shine onto a rotating, six-sided mirror. As the mirror rotates, the beams reflect off the mirror and sweep the page from left to right. A set of lenses and mirrors focuses the horizontal sweeping beams through the beam access slots in the rear of the toner cartridge and onto the photosensitive drum. Because the beams sweep the entire length of the drum while the drum rotates, the entire circumference of the drum can be covered.
Each successive sweep of the beams is offset 1/1200th of an inch. This is how the printer achieves its 1200 dpi resolution. At the beginning of each sweep (before the beams reach the drum), the beam detect mirror reflects the beams. The momentary pulse of light goes to the engine controller PCB, which converts the light to an electrical signal used to synchronize other activities in the printer and to diagnose problems with the laser/scanner assembly. Portions of the drum not exposed to the laser still contain the uniform negative potential (placed there by the primary charging roller), but those portions exposed to light have been discharged to ground to form the latent electrostatic image.
Scanning mirror Scanner motor Engine controller Focusing lens /ACC Beam Detect mirror /DEC Cylindrical lens CNT0 CNT1 CNT2 Laser driver BDI formatter /VDATA1 VDATA1 /VDATA2 VDATA2 Mirror Figure 44. Laser/scanner system C7058-90936 Printer functionality and operation 83