Image Processing Reference
In-Depth Information
6
Impacts of Digitization by Built-In Coordinate
Points on
Image Information Quality
Chapter 1 described the factors that compose image information (light intensity, space
[position], wavelength, time) and how all factors except light intensity are built-in coordi-
nate points in imaging systems. Image sensors measure the number of photons entering
the domain of each built-in coordinate point. This means that sensors integrate the signal
charge generated when an incident photon reaches three territories of each coordinate
point, that is, at each pixel area, through each color filter, and during the exposure period
of each frame. This operation is known as sampling of photon numbers at each coordinate
point. This chapter discusses the impacts of the sampling operation at each digitized built-
in coordinate point.
6.1 Sampling and Sampling Theorem
A specific example used here is space sampling. Sensors integrate signal charges
generated by incident light* that enters the sensor part formed in each pixel arranged in a
two-dimensional area. The output signal at each pixel during one exposure period is only
one, and the signal value is the incident light intensity information at the coordinate point.
Therefore, if the number of pixels is smaller, or the periodicity of sampling or the space
frequency are lower, then the image quality based on space information is low because of
coarse sampling, as shown in Figure 6.1.
Figure 6.2 explains how the spatial frequency in the obtained image information is
restricted. The solid line in the top frame indicates three kinds of frequency and four
input signals. A sine wave curve whose frequency is sampling frequency,
f
s
, is shown in
the bottom frame. The sampling pitch
p
is expressed as 1/
f
s
. Sampling operations are car-
ried out at the positions of the maximum point, as indicated by the up arrows. In the case
that the input signal frequency is sufficiently low compared with the sampling frequency,
f
s
, as shown in Figure 6.2d, a broken curve obtained by tracing the sampling point, which
is indicated by filled circles, accurately shows the same wave as the input signal. Both the
amplitude and the frequency are maintained.
Then how is it possible to reproduce high frequency? The case when the frequency is
half the sampling frequency, that is,
f
=
f
s
/2, is shown in Figure 6.2c. In this figure, the posi-
tions of the peaks and troughs (corresponding to white and black in the images) fit with
that of the sampling points shown by filled circles. The amplitude and frequency of the
reproduced curve shown by a broken line are retained, although the shape is a triangular
waveform. Because this is the condition in which peaks and troughs fit with the sampling
* Actually, this is light that passes through the color filter of a pixel.
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