Image Processing Reference
In-Depth Information
select (CS) are reset to the voltage source to start the exposure. After the reset operation,
the output of noise N2 existing in the combined capacitor [FD + CS] follows and is stored
in off-chip memory at time t2. During the exposure period, M3 is kept on-state so that
any existing oversaturated signal charges overflowing from the PD due to high illumi-
nance are stored in the FD and CS, as shown at time t3. Before applying the readout signal
charges from the PD, M3 is set to off-state. At this time, part of the oversaturated charges
and noise N2 existing in the FD are output as noise N1 at time t4. Readout of the nonsatu-
rated signal charge S1 from the PD to FD follows to output (S1 + N1) at time t5. The output
voltage corresponding to S1 is obtained as the difference between the output voltages of
N1 and (S1 + N1). Next, M3 is set to on-state to sum (oversaturated signal charges + N2) at
time t3 and signal charge S1 to get (S2 + N2), that is, the summation of all signal charges
and the initial noise N2. The summed charges amount is output by using (FD + CS) as the
charge quantity detective capacitor at time t6, and the output voltage corresponding to
the total signal charge S2 is obtained as the difference from the output voltage of noise N2
stored in off-chip memory. Because the noise charge quantity N2 at time t6 is the summa-
tion of N2 at time t2 and dark current generated at FD during the exposure period, they
are not the same as N2 at time t2. While frame memory is necessary to store each N2 of
each pixel at time t2, there is a proposal to substitute N2 by that of the next frame to avoid
memory installation. While reset noise cannot be canceled because there is no correlation
between the different reset operations in this case, since the signal level is higher in the
oversaturated situation, it can be thought to be highly tolerant for noise.
7.2 Space Information
The improvement in space (position) information is nothing less than progress in space
resolution. The most direct method to enhance the Nyquist frequency is to increase the
pixel number. The pixel interpolation array, described in Section 5.2.3.1, increased the hori-
zontal resolution by devising a pixel array without increasing the pixel number. This sen-
sor was produced for video cameras in the early 1980s. Because the scanning line number
is decided by the format of television systems, there was no need to increase the vertical
resolution. A sensor that also extends to vertical resolution in the same manner as for digi-
tal camera use is a pixel interleaved array CCD (PIA CCD).
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Square and interleaved pixel arrays in real space are shown in Figure 7.5a and b, with the
interleaved array indicating a rotation at an angle of 45° of the square array. The vertical,
horizontal, and diagonal pixel pitches in the square array are
p
,
p
, and
p
/√2, respectively.
Conversely, the vertical and horizontal pitches in the interleaved array are shortened to
p
/√2, while the diagonal pitch is
p
as shown in Figure 7.5b.
The Nyquist frequency obtained by Equation 6.1 is shown as frequency space in
Figure 7.5c. In an interleaved array, the Nyquist frequency is higher than that of a square
array in the vertical and horizontal directions, while that of the diagonal direction is
lower. Thus, some part of a higher resolution in the diagonal direction of a square array is
allocated to the vertical and horizontal directions in the interleaved array. Since the con-
figuration is only rotated at an angle of 45°, the sampling density, that is, the information
density, is the same, but the weight is changed in accordance with the directions.
So is there much point in it? The answer is “yes.” Watanabe et al.
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report that the human eye
has higher sensitivity in vertical and horizontal directions. Additionally, in a paper on PIA,
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