Printing System Models - Control of Color Imaging Systems: Analysis and Design - page 562

Photons

Electrostatic surface charge

CTL

e

CGL

Electron-hole pairs h

Aluminum
ground plane

UCL

Substrate (Mvlar)

FIGURE 10.6 Cross section of a two-layer PR.

charge. For example, during imaging no change in charge occurs in the surface
voltage whenever dark characters exist on the image. Wherever the white back-
ground existed, the photoconductor is discharged, thus producing an electrostatic
image of the document on the photoconductor. The photoconductor could be a thin

in thickness, or a drum that is used not only for electro-
photography in copying machines, printers, and facsimile machines, but also in
photoelectric transducers such as solar batteries and electrolytic luminescence elem-
ents, photo transducers, and in materials for optical disks. Detailed investigation of
the physics of charge-generation and charge-transport properties in photoconductors
can be found in Ref. [8]. Our main goal is to develop an exposure model for
photoconductors (with CGL and CTL) used in imaging systems such as copying
or printing systems that can be used in design of controller for the exposure process.
After the charging process, the charge on the photoconductor decays with
time due to light leakage between the charging and exposure stations. The amount
of decay depends on the amount of charge and the amount of thermally released
holes in the photoconductor. The dark decay period can be different from charging to
exposure stations and then from exposure to development stations. This decay rate
should be comprehended by the model if the period is long. The time evolution of the
surface charge and the photoinduced discharge (PID) is shown schematically
in Figure 10.7 [4]. During exposure, the charge generated during PID can spread
laterally within the CTL (shown schematically in Figure 10.6) due to electrostatic
repulsion resulting in the loss of image resolution. This repulsion is a function of the
photoconductor properties like mobility, thickness, etc. [7,9,10].
PID taking place in the CGL is a nonlinear function of exposure light intensity.
This nonlinear relationship is commonly called photoinduced discharge curve
(PIDC). Next we derive an expression for the PIDC based on the physics of the
photoconductor [11

film, usually about 25 m

- 13].

The field-dependent injection of charge from the CGL to CTL is usually

explained by the field-dependent collection ef ciency (CE) term, h , which gives

the probability that an absorbed photon will generate an electron hole pair that will

actually contribute to the discharge of the photoconductor [7]. In the Springett

-

-

Melnyk model [11], the field-dependent CE of a photogenerator is taken to be of

the form