Television – Camera – system and detail – Solid-state image sensor
Reexamination Certificate
1999-11-02
2004-08-31
Moe, Aung (Department: 2612)
Television
Camera, system and detail
Solid-state image sensor
C348S296000, C348S362000
Reexamination Certificate
active
06784935
ABSTRACT:
This application is based on Japanese patent application No. HEI 10-313334, filed on Nov. 4, 1998, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
a) Field of the Invention
The present invention relates to an image pickup apparatus and more particularly to an image pickup apparatus capable of picking up an image in a wide dynamic range and a method of controlling the image pickup apparatus.
b) Description of the Related Art
A solid state image pickup device has photodiodes disposed in a two-dimensional matrix shape and can take a two-dimensional image. Each photodiode corresponds to a pixel of an image.
FIG. 11
is a graph showing the photoelectric conversion characteristics of photodiodes of a solid state image pickup device. The abscissa represents the amount of light incident upon a photodiode, and the ordinate represents the voltage of a signal output from the photodiode. Characteristic curves A
1
, A
2
and A
3
show the photoelectric conversion characteristics of first, second and third photodiodes of the same solid state image pickup device.
Each of the characteristic curves A
1
to A
3
has a linear region R
1
with a small incidence light amount and a saturated region R
2
with a large incidence light amount. In the linear region R
1
, an output voltage is proportional to an incidence light amount. In the saturated region R
1
, the output voltage corresponding to incidence light is saturated.
The characteristics A
1
to A
3
of the photodiodes are the same in the linear region R
1
, whereas they are different in the saturated region R
2
. In the saturated region, the levels of output voltages of the photodiodes become irregular. In order to forcibly convert an output voltage of Vw or higher into a voltage Vw, a white clip process is performed.
If the white clip process is performed, the linear region R
1
is only a region which can be used for photoelectric conversion. The dynamic range capable of photoelectric conversion is therefore determined basically by the width of the linear region R
1
.
Solid state image pickup devices are used with digital still cameras and video cameras. The dynamic range of a solid image pickup device is very narrow as compared to that of human eyes and a photographic film. A narrow dynamic range may cause white or black crushed areas in an image.
In order to avoid this, techniques are known by which an image is picked up two times at different exposure times and the two images are synthesized. The details of the techniques will be described with reference to
FIGS. 12A
to
12
C.
Similar to
FIG. 11
, the abscissa of
FIGS. 12A
to
12
C represents an incidence light amount and the ordinate represents an output voltage.
FIG. 12A
is a graph showing the photoelectric conversion characteristics used by a first image pickup operation of long-time exposure. Since the exposure time is long, even if the incidence light amount per unit time is small, an output voltage is large. Therefore, the photoelectric conversion characteristics are subjected to the white clip process at a voltage Vw.
FIG. 12B
is a graph showing the photoelectric conversion characteristics used by a second image pickup operation of short-time exposure. Since the exposure time is short, an output voltage for an incidence light amount per unit time is lower than that obtained by the characteristics (
FIG. 12A
) for the long-time exposure. The photoelectric conversion characteristics shown in
FIG. 12B
are also subjected to the white clip process.
FIG. 12C
is a graph showing the photoelectric conversion characteristics obtained by synthesizing the first image pickup photoelectric conversion characteristics (
FIG. 12A
) and the second image pickup photoelectric conversion characteristics (FIG.
12
B). For example, the synthesizing method is a simple addition of the two characteristics.
By using the synthesized photoelectric conversion characteristics, the dynamic range of the solid state image pickup device can be broadened. Therefore, irrespective of whether the incidence light amount is large or small, all photodiodes of a solid state image pickup device can have the uniform photoelectric conversion characteristics.
In the synthesized photoelectric conversion characteristics, a slope in the large incidence light amount region is gentler than that in the small incidence light amount region. The characteristics with different slopes are approximately equal to the human visual sense characteristics. Therefore, any practical problem will not occur even if the synthesized characteristics are used with a solid image pickup apparatus.
The first image pickup operation of long-time exposure and the second image pickup operation of short-time exposure may be performed in a reverse order.
Next, the operation of the solid image pickup device performing the above process will be described with reference to
FIGS. 13
to
17
. In
FIGS. 13
to
17
, a hatched area is an area where electric charges are stored.
FIG. 13
is a plan view of an all-pixel read type solid state image pickup device. Signals of all pixels (photodiodes) can be read to an external at the same time as one frame image.
The solid image pickup device has: photodiodes
51
disposed in a two-dimensional matrix shape for photoelectric conversion; vertical charge transfer paths (VCCD)
52
for transferring electric charges in a vertical direction; a horizontal charge transfer path (HCCD)
53
for transferring electric charges in a horizontal direction; and an output amplifier
54
for outputting a voltage corresponding to electric charges to an external.
First, as shown in
FIG. 13
, an image pickup operation of long-time exposure is performed to store electric charges of a first image in the photodiodes
51
.
Next, as shown in
FIG. 14
, the electric charges of the first image stored in the photodiodes
51
are read and stored in the right side vertical charge transfer paths
52
. After this data read, the first image pickup operation of long-time exposure is terminated, and an image pickup operation of short-time exposure starts for a second image.
Next, as shown in
FIG. 15
, the electric charges of the first image on the vertical charge transfer paths
52
are transferred downward to the horizontal charge transfer path
53
. The horizontal charge transfer path
53
transfers the received electric charges from the right side to the left side to the output amplifier
54
. The output amplifier
54
outputs a voltage corresponding to the received electric charges. Namely, it outputs a signal of the first image.
During this period, as shown in
FIG. 16
, electric charges for the second image are being stored in the photodiodes
51
by the second image pickup operation of short-time exposure which started immediately after the data read operation shown in FIG.
14
.
Next, as shown in
FIG. 17
, the electric charges of the second image stored in the photodiodes
51
are read and stored in the right side vertical charge transfer paths
52
. After this data read, the second image pickup operation of short-time exposure is terminated.
Next, similar to
FIG. 15
, the electric charges of the second image in the vertical charge transfer paths
52
are transferred downward to the horizontal charge transfer path
53
. The horizontal charge transfer path
53
transfers the received electric charges from the right side to the left side to the output amplifier
54
. The output amplifier
54
outputs a voltage corresponding to the received electric charges. Namely, it outputs a signal of the second image.
Thereafter, the first and second images are synthesized as illustrated in
FIGS. 12A
to
12
C.
The operation of the solid state image pickup device described above does not pose any problem so long as the subject is stationary. However, if the subject is moving, the following problem occurs. Since there is a long time between the first image pickup operation of long-time exposure and the second image pickup operation of short-time exposure, the position of the subject during the first image
Uya Shinji
Yamada Tetsuo
Arent & Fox PLLC
Fuji Photo Film Co. , Ltd.
Moe Aung
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