Image Processing Reference
In-Depth Information
Fringe fields around
edges of solids
Negative
charge
V L
Image
(Discharged PR voltage)
To ground plane
Toner develops more along
edges of solid areas
Negative
charge
V L
Image
(Discharged PR voltage)
To ground plane
FIGURE 10.10
Development without electrode.
development nip. For a deeper review of these development systems, refer to Refs.
[3,4,9,13] and various proceedings of IS&T conferences on nonimpact printing.
To understand the basics of image development, we first consider the electric
field in the region of the latent image. If the toner is developed on the fully charged
area of the photoconductor and not on the discharge area, the sharp electrostatic
contrast between the charged and discharge areas of the photoconductor has fringe
fields, toner develops more along the
edges of the solid areas. Small areas, for example, lines, are well developed since
without a development electrode on top of the photoconductor only fringe
fields (Figure 10.10) [14,15]. Due to fringe
fields
associated with the edges of solids and lines exist in the development zone. These
edge effects produce halos giving rise to poor image quality.
When a grounded development electrode is placed near the photoconductor, the
filled-in with
more toner, giving improved solid area development. On the otherhand, the non-
image areas (background) appear toned, showing a light-gray appearance due to the
developed toner particles as shown schematically in Figure 10.11.
When the development electrode is biased with a control voltage, V bias , back-
ground development can be signi
field pattern in the solid area is improved. Hence, solid areas will be
cantly reduced. Figure 10.12 shows shift in
development
field pattern with a proper bias voltage, which is used to adjust the
gray scale in many copiers and printers. Figures 10.13 and 10.14 show the actual
development of an image without and with a development electrode biased with a
control voltage, respectively. Halos can be seen in Figure 10.13 due to fringe
fields
around edges of solids.
For CAD, toner is developed only on the charged areas of the photoconductor
since the toner is repelled due to a more positively biased discharged region. If V h is
the voltage on the charged area of the photoconductor and V L is the voltage on the
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