Image Processing Reference
In-Depth Information
introduced between the corona wires and the dielectric surface. The grid is biased to
a voltage that approximates the potential required for charging the photoconductor
(typically
900 V) which provides better control over the amount of charge
on the photoconductor surface.
As charge builds up on the surface of the photoconductor, its voltage increases
and the current
600 to
flowing to the photoconductor decreases. When this current reaches
zero, charging stops. If V p is the photoconductor surface voltage, then the charging
rate in a scorotron charging system can be expressed as the plate current per unit
length, which is given by
I p ΒΌ S ( V g V p )
(
10
:
1
a)
where
I p is the plate current per unit length [A
=
m]
V p is the plate voltage
V g is the grid voltage
S is the slope of the current
-
voltage response curve
See Figure 10.3 for an ideal current
voltage response curve [3] and Figure 10.4 for
an actual response curve shown for a typical scorotron charging system when the
wire potential is held at 6 kV. Photoconductor current density (the surface current per
unit area) as a function of photoconductor position is shown in Figure 10.5.
Characterizing the PR by its capacitance per unit area C (C
-
V-m 2 ) given by
=
e r e 0
d
(
:
b)
10
1
I p
I p = S ( V g - V p )
S
1
V p
V g
Photoconductor voltage
FIGURE 10.3 Idealized current vs. voltage response curve for the scorotron charging system
constructed in the nominal operating regime.
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