Image Processing Reference
In-Depth Information
1.3 Functional Elements of Image Sensors
Section 1.2 showed that the output of an image sensor is the intensity information of the
incident light to each coordinate point. What functions must image sensors possess? First,
they must have the function to measure the photon number, that is, the light intensity that
comes to each coordinate point. Although, if it were possible,* an ideal method would be
to count the photon numbers directly one by one; this is not realistic using the current
technology. However, it can be achieved by using the following two steps. Initially, pho-
tons arriving at each coordinate point are absorbed to create signal charges (photoelectric
conversion) by image sensors and the generated signal charges are integrated and stored
(charge storage). In the second step, the stored charge quantity is measured and converted
to electric signals (charge quantity measurement).
Secondary sensors must have the function to identify the signals from which coordinate
point they emanate. Thus, image sensors must possess three functional elements: (1) a sen-
sor part that generates and stores the signal charges; (2) a scanning part that identifies or
addresses the coordinate point of the pixel of the signal; and (3) a signal charge quantity
measuring part that measures the signal charge quantity and converts it to electric signals
such as voltage, current, frequency, and pulse width.
Name of
elements
3. Charge quantity
measuring part
1. Sensor part
2. Scanning part
Measurement of signal
charge quantity and
conversion to electrical
signals
Addressing
(identification of
pixel address)
(space information)
Generation and storage
of signal charges
according to light intensity
(light intensity information)
Function
•
Charge-voltage conversion
•
Current-voltage conversion
•
Charge transfer
•
X-Y addressing by
shift register or
decorder
•
Photoelectric
conversion
Means
•
Charge-frequency conversion
•
Charge-pulse width conversion
Sensor part
Scanning part
Charge quantity measuring part
Signal
charge
(
x
,
y
,
c
,
t
)
(
i
+1,
j
)
(
I
,
j
)
Light
Pixel
(
x
-1,
y
) (
x
,
y
) (
x
+1,
y
)
Coordinate point
of pixel (
x
,
y
)
N
Light
Output voltage
Time
Image sensor
M
FIGURE 1.11
Functional elements of image sensors.
* It is possible to count the photon number directly only in the case of very low light intensity, when each
photon comes with a sufficient time interval.
