Image Processing Reference
In-Depth Information
exposure are replaced by nonsaturated signals in short exposure, and they are synthesized
by signal processing. Thus, image information can be obtained under much higher illumi-
nation than in conventional CCDs. Examples of a captured image are shown in Figure 7.3.
While the darker scene cannot be captured as well under the same exposure conditions
as the highlighted scene using a conventional CCD camera, both areas are captured well
by a hyper-D CCD camera. Using this signal processing, the linear relationship between
light intensity and image signal as a whole image is not retained. This case is a successful
example of flexibility, capability, and high functionality owing to the combination of the
device with the electronic system, which was not possible in earlier film camera systems.
7.1.2.2 CMOS Image Sensor with Lateral Overflow Capacitor
The operational principle of this device
3,4
is explained in Figure 7.4. As shown in Figure 7.4a,
one field-effect transistor (FET) (M3) and one capacitor (CS) are added to a four-transistor
(4-Tr) pixel configuration composed of a PD, readout transistor (M1), reset transistor (M2),
amplify (or drive) transistor (M4), and raw select transistor (M5). The operation is shown in
Figure 7.4b. At time t1, by making M2 and M3 on-state, floating diffusion (FD) and column
(a)
(b)
FIGURE 7.3 (See color insert)
Examples of a captured dynamic range: (a) conventional CCD; (b) hyper-D CCD.
φ R
t1
t2
t3
φ S
φ T
M1
M3
M1
M3
M1
M3
M2
PD
PD
PD
CS
FD+CS
FD+CS
FD+CS
Saturated overflow
signal charges + N2
M3
N2
M1
M4
FD
t4
t5
t6
PD
M1
M3
M1
M3
M1
M3
M5
PD
PD
PD
φ X
CS
FD+CS
FD
FD
CS
N1
S1+N1
S2+N2
(b)
(a)
FIGURE 7.4
Wide dynamic range CMOS image sensor with lateral overflow capacitor: (a) pixel configuration; (b) sche-
matic diagram of operational principle. (Reprinted with permission from Akahane, N., Sugawa, S., Adachi, S.,
Mori, K., Ishiuchi, T., and Mizobuchi, K.,
IEEE Journal of Solid-State Circuits
, 41, 851-856, 2006.)
