Xerox Phaser 4500, Service Manual

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Theory of Operation 2-21
Laser Assembly
The Laser Assembly scans the surface of the drum with a laser beam. The Laser
Assembly consists of three components: the Laser Diode (LD) Assembly, the Scanner
Assembly, and the Start of Scan (SOS) board.
Laser Diode (LD) Assembly The LD Assembly produces a laser beam that is
turned on and off according to the print data signal. A single LD is used for 600 dpi,
and a dual LD is used for 1200 dpi, generating one beam for 600 dpi and two beams
for 1200 dpi.
Scanner Assembly
This assembly consists of a 12-sided polygonal mirror
mounted on the shaft of the scanner motor that reflects the beam to the drum surface
through lenses and mirrors, to scan the beam across the drum.
Start of Scan (SOS) Board When the laser beam strikes the sensor on the SOS
board, the signal generated indicates the initial position of the scan. The initial
position where a scan is started on each line is detected. Depending on the print
resolution selected, either one beam creates one line, or two beams create two lines at
a time.
When the laser beam scans from one end of the drum surface to the other while being
turned off and on, one line of a latent image is created. The resolution in the scanning
direction (from right to left) is determined by the rotational speed of the Scanner
Motor and by the speed at which the laser is modulated. The resolution in the process
direction (from top to bottom) is determined by the number of the scan beams and the
rotational speed of the print drum.