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24 February, 1999
LASER EXPOSURE
Auto Power Control (APC)
LD5V
LD
P D
VIDEO
Current
LEVEL
Control
IC
LD OFF
LD Drive Board
A259D501.WMF
Even if a constant electric current is applied to the laser diode, the intensity of the
output light changes with the temperature. The intensity of the output decreases as
the temperature increases.
In order to keep the output level constant, the output light intensity is monitored
through a photodiode (PD) enclosed in the laser diode. The photodiode passes an
electrical current that is proportional to the light intensity. The output is not affected
by temperature, so it faithfully reflects the changes in the LD output, without adding
anything itself.
Just after the main switch is turned on, the current control IC on the LD drive board
excites the laser diode at full power (power level 32) and stores the output of the
photodiode as a reference. The current control IC monitors the current passing
through the photodiode. Then it increases or decreases the current to the laser
diode as necessary, comparing it with the reference level. Such auto power control
is done during printing while the laser diode is active.
The laser power level is adjusted on the production line. Do NOT touch the variable
resistors on the LD unit in the field.
2-63
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