Image Processing Reference
In-Depth Information
5
Types of I
mage Sensors
Image sensors must have the following functions: (1) a sensor part that converts incident light
to signal charges and stores them, (2) a scanning part that identifies the pixel address of each
signal charge packet, (3) a part that measures the amount of signal charge and converts it to an
electric signal, as discussed in Section 1.3. The choice of devices for these functional elements
determines the type of image sensor. From the viewpoint of signal procedure, the order of (2)
scanning part and (3) part measuring signal charge quantity may be swapped. Various types
of sensors have been proposed in the past.
1
By using technology available at the time, some
marketable sensors have been developed. Among them, charged-couple device (CCD), metal-
oxide semiconductor (MOS), and complementary metal-oxide semiconductor (CMOS) sen-
sors are representative in terms of greater acceptance in the marketplace. Concrete measures
of each functional element of the representative sensor types are shown in Table 5.1.
A
pn
-junction photodiode (PD) is employed in all types because of its performance and
productivity. Thus, a PD is the major sensing element, while a photogate is applied for
specific purposes.
In CCD sensors, a CCD is used as the scanning part. Originally, CCD was the name of a
device having a charge transfer function, not the name of the image sensor itself. In MOS
and CMOS sensors, a shift register or decoder for row/column selection and a metal-oxide
semiconductor field-effect transistor (MOSFET) switch are employed as scanning parts to
realize X-Y addressing.
A floating diffusion amplifier (FDA) is used as the charge quantity measuring part in
almost all CCD and CMOS sensors. The difference is that in CCD sensors the FDA is
equipped in a chip as a common amplifier, while in CMOS sensors an FDA is provided at
each pixel. Therefore, charge-voltage conversion is carried out at a common FDA in the
final stage in CCDs, while in CMOS sensors, this is done at each individual pixel ampli-
fier. In the case of MOS sensors, the voltage signal caused by a minute signal current is
detected at an off-chip amplifier.
5.1 CCD Sensors
First the principle of CCDs, then interline transfer CCDs (IT-CCDs), which are the main
type of CCD, are described.
5.1.1 Principle of CCDs
The CCD was proposed as a device to store and transfer electric charges by Bell Telephone
Laboratories in 1970.
2
The principle is shown in Figure 5.1. One electrode each of adja-
cently arrayed multiple capacitors are commonly connected to ground level, while
other electrodes are independent, as shown in Figure 5.1a. Then, commonly connected
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