Service manual

5.3 GRADATION CONTROL (LASER POWER MODULATION)
The LD power for each gradations is defined by process control.
To form the latent image, the laser beam illuminates the image area of the
drum surface. The longer the laser is on and the stronger its intensity is, the
darker the developed pixel becomes. Modulating (changing) the width of the
pulse makes the on time of the laser longer or shorter. There are eight
possible pulse width levels in this model.
While the laser is on to make one dot, the intensity of the laser is controlled
by power modulation (PM). The laser’s intensity is controlled by the amount
of current sent to the laser diode. Modulating the power makes the laser
brighter or dimmer. There are 32 possible power levels, or laser intensity
levels.
The laser engine can use the 8 pulse width levels and 32 power levels to
create the 256 possible grayscale values for each pixel. However, in this
machine, out of the possible 256 gradations for each dot, only 16 are
actually used. Dithering creates 256 gradations for each color, when taken in
4 x 4 pixel units.
The power is modulated ONLY at the end of the active part of the on/off
cycle of the laser pulse. For example (see the diagram above), to make a
pixel with a grayscale value of 48, the laser pulse width level for that pixel will
be 2. The first period of the pulse will be at the full power (32), and the
second pulse will be at power 16 to make up the resultant value of 48 (32 +
16 = 48).
Rev. 11/96
32
24
16
8
PWM
(8 levels)
Data: 0
1 dot
Data: 16 Data: 32
Data: 48
Data: 255Data: 64 Data: 136
181 1
22 5 8
PM
(32 levels)
Data
Black
White
0
255
LASER EXPOSURE
A166/A187/A189 2-70 SM
CÓPIA NÃO CONTROLADA
CÓPIA NÃO CONTROLADA