Image Processing Reference
In-Depth Information
4
Integratio
n Period and Scanning Mode
In this chapter, scanning modes are described before image sensors are discussed in the
next chapter. A description is given of how many separate pieces of information on an
image can be used to construct a complete image.
4.1 Progressive Mode
The plainest mode is the progressive mode. As shown in Figure 4.1a, complete image
information is constructed by taking image information from individual lines from the
first line to the final line by scanning the lines. Each scanning line corresponds to the
signal of that line or row of the sensor array. Signals of lines are output in series, as shown
in Figure 4.1b. A series of complete image signals is called a frame, and a series of divided
subimage signals is called a field. In progressive scanning mode, a frame is formed by one
field, therefore the frame information is equal to that of the field. One serial output starts
with the first scanning line, that is, the signal of the first line of the pixel array. Before the
second scanning line, a horizontal blanking interval (HBI) appears, which was necessary
for the electron beam from the end point of the first line to the top of the second line in
image pickup tubes and CRT displays. In the present image sensors, mainly vertical driv-
ing clocks are applied in HBI to avoid the superposition of synchronous noise caused by
their leakage. Hereafter, combinations of scanning line and following HBI appear serially
till the final scanning line. A vertical blanking interval (VBI) then follows to complete one
frame of scanning. A VBI was necessary for the electron beam to return from the end point
of the final line to the top of the first line as well as an HBI. HBIs and VBIs are also neces-
sary for present image sensors as periods to apply driving pulses, because such pulses are
overlapped in signal output as induction noise if applied during the signal output period.
4.2 Interlaced Mode
In the interlaced mode, complete image information is divided into subsets. An example of
one of the simplest interlaced modes is shown in Figure 4.2.
As shown in Figure 4.2a, complete image information is formed by two images: the first
image is made by odd lines and the second image by even lines. Thus, frame information
is constructed by the first field and second field, as shown in Figure 4.2b.
Therefore, in the serial output, the first field signal, made by odd line signals, is scanned;
after that, the second field signal, made by even line signals, is output. In the case of mov-
ing images, this sequence is repeated.
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