Photosensitive Section - Epson FilmScan 200 Service Manual

FilmScan 200

2.1.1.1 Photosensitive Section

The photosensitive section converts the light energy into electrical signals and accumulates the resulting
signal charge over a short term. The description below explains the process of photo electrical conversion
to change the incoming light energy into accumulated signal charge. Refer to Figure 2-3.
Photodiode
MOS
Capacitor
Light
The above ,photo sensor sections are arranged in parallel rows, where the number of units in each
horizontal row is equivalent to the maximum number of pixels per line plus ƒ¿ (dummy). Figure 2-4
illustrates the arrangement.
2-2
Shift Electrode
Transmission
Section
Figure 2-3. Photo Sensitive section Operation
Figure 2-4. Photo-sensors Arrangement
The equivalent circuit at left shows the
operating principles of the photosensitive
section. The circuit consists of a photodiode,
a MOS capacitor, and a shift electrode.
(The transmission section, which follows the
photosensitive section, also is illustrated
simply in terms of capacitance.)
Light reflected from the original document
impacts the photodiode, which converts the
energy into a corresponding amount of
electric current. The resulting electric current
causes the MOS capacitor to accumulate
electrons in numbers proportional to the
strength of the originally received light.
When the shift electrode goes on, the
electrons accumulated in the MOS
capacitor are released.
The shift electrode then goes off, which
completes the transfer of the signal-charge
to the transmission section.
Transmission
Section
Photosensing
Section
Rev. A