Image Processing Reference
In-Depth Information
only at pixels and at times when the input has changed is a big feature of this sensor, and
is quite different from frame-based sensors. Therefore, redundancy is significantly reduced
compared with the frame-based mode in which signals are read out at each pixel at each
frame regardless of whether input changes. The generated signal pulses in the quantization
block are also used for the reset operation in the second block, from where the next potential
monitoring starts. If the output of the first block, log
I
or
V
p
, changes as shown by the solid
line in Figure 7.12b, the output of the second block, A·d(log
I
), appears. When it reaches the
threshold of ON or OFF, which are the predefined amounts of change, a signal pulse is emit-
ted by the third block as shown in Figure 7.12c and the second block output is reset. From the
obtained time information when the predefined change occurred as shown in Figure 7.12c,
the signal shown by the dotted line in Figure 7.12b can be obtained as a reconstructed tem-
poral transition of log
I
or
V
p
. In this reconstruction, while it can be said that the light inten-
sity information is quantized by a predefined amount of change of the amplified differential
signal, the time information is obtained as high as the resolution determined by circuit char-
acteristics. Thus, the time information is not quantized in this sensor, since this is not a
frame-based sensor. In this sensor, the quantized factors, or built-in coordinate points, are
space and amount of change of time-derivative log-transformed voltage output of amplified
photocurrent
I
. Therefore, the signal output of the sensor is not the amount of integrated sig-
nal charge
S
(
r
,
t
), but time
T
when amount of change of time differential of log-transformed
voltage output of amplified photocurrent
I
reaches a predetermined quantity ±
A
·
∆[∂log
I
/∂
t
]
q
at pixel
r
k
, that is,
T
(±
A
·
∆[∂log
I
/∂
t
]
q
,
r
k
), where
A
is voltage gain of amplification.
The time resolution of this sensor is higher than 10 μs. The DR is 120 dB by logarith-
mic output and differential circuit. Because only input-varied pixels emit a signal pulse,
a moving object composes a cluster of those pixels, and it can be recognized as a moving
object in real time. This sensor has 26 transistors in a pixel.
Readers are directed to the website of the research institute of this sensor,
17
where vari-
ous interesting moving images are shown on the homepage.
7.4 Color and Wavelength Information
The physical quantity named “wavelength” exists in the natural world and “color” is a
perception generated by the human eye and brain. As mentioned in Section 6.4, since it is
quite difficult to obtain images with physically accurate wavelength information using the
current technology, a subjective color reproduction technique is commonly used for appli-
cations that are viewed by the human eye. In this field, a single-sensor color camera system
represented by the primary colors R, G, and B or the complementary colors magenta, yel-
low, cyan, and green and the three-sensor camera system with the primary colors are used.
7.4.1 Single-Chip Color Camera System
Color reproduction by three or four colors is an absolute approximation. Therefore, it is
possible to express more subtle shades of color by adding a new color with an appropriate
spectral response; however, this usually has side effects such as degradation of the SNR.
As system designs are determined by what characteristics should be featured as part of a
balanced overall performance, designs unsuitable for high sensitivity, which is the priority
for common imaging systems, are not commonly adopted. In a digital still camera system,
the Bayer configuration color filter remains dominant.
