Image Processing Reference
In-Depth Information
Field
Integrated
charge
quantity at a
sensor part
Integration period
(exposure time)
Discharge
pulse
Read pulse
Time
FIGURE 4.3
Operational principle of electronic shutter mode.
4.3 Electronic Shutter Mode
So far, the modes explained have been based on the premise that all signal charges gener-
ated by incident light are utilized as signal charges to form image information. But occa-
sionally shorter exposure times are desired by design, because longer time exposures
detract from the image sharpness when objects are in motion. Therefore, electronic shutter
modes that enable control of exposure times are commonly used. The operational princi-
ple is explained by Figure 4.3, which shows integration and discharge of generated signal
charges by incident light with time.
As sensor parts are kept illuminated by incident light, integrated photoelectric charges
continue to increase with time. But integrated charges can be completely removed by
applying a discharge pulse to sensor parts to flush them, as shown. In this way, refreshed
sensors restart new integration times until a read pulse is applied. The periods from dis-
charge to readout are real exposure times in which signal charges are integrated and used
as image signals. This can be controlled by setting discharge pulse timing. Electronic shut-
ter modes are indispensable for present-day cameras.
