Television – Camera – system and detail – Combined image signal generator and general image signal...
Reexamination Certificate
2000-04-18
2003-10-07
Garber, Wendy R. (Department: 2712)
Television
Camera, system and detail
Combined image signal generator and general image signal...
Reexamination Certificate
active
06630955
ABSTRACT:
This application is based on application No. H11-114397 filed in Japan on Apr. 22, 1999, the entire content of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image-sensing apparatus, and in particular to an image-sensing apparatus incorporating a solid-state image-sensing device having a characteristic that an output voltage thereof varies natural-logarithmically in accordance with the amount of received light.
2. Description of the Prior Art
FIG. 1
shows a solid-state image-sensing element (hereafter referred to as a “pixel”) employed in a conventional solid-state image-sensing device having a characteristic that an output voltage thereof varies natural-logarithmically in accordance with the amount of received light. The pixel shown in
FIG. 1
is composed of a photodiode PD that receives at its cathode a voltage Vdd
1
, an N-channel MOS transistor Tr
1
that has its drain and gate connected to the anode of the photodiode PD and that receives at its source a voltage Vss
1
, an N-channel MOS transistor Tr
2
that has its gate connected to the gate of the transistor Tr
1
and that receives at its drain a voltage Vdd
2
, and a capacitor C that has one end connected to the source of the transistor Tr
2
and that receives at the other end a voltage Vss
2
. Here, the voltage appearing at the node between the source of the transistor Tr
2
and the capacitor C is used as the output voltage Vout. The voltages mentioned above fulfill the relations Vdd
1
>Vss
1
and Vdd
2
>Vss
2
.
How this pixel works will be described briefly below. This pixel exploits the subthreshold characteristics that the transistor Tr
1
exhibits when its gate-source voltage is lower than the threshold voltage. When light is shone on the photodiode PD and, as a result, a current Ip flows through the transistor Tr
1
, the gate voltage Vg of the transistor Tr
1
varies natural-logarithmically in accordance with the current Ip. This gate voltage Vg causes a current to flow also through the transistor Tr
2
, and thus electric charge is accumulated in the capacitor C. As a result of this electric charge being accumulated in the capacitor C, the output voltage Vout appears, which is given as
Vout
=
Vss1
+
nkT
q
ln
(
q
nkTC
∫
Ip
ⅆ
t
)
(
1
)
where q represents the amount of electric charge carried by an electron, k represents the Boltzmann constant, n represents a constant determined according to the structure of the transistor, T represents the absolute temperature, and C represents the capacitance of the capacitor C.
A solid-state image-sensing device has a plurality of pixels, each having a characteristic as described above, arranged in an array, and is further provided with transfer means for transferring the output voltages from the individual pixels. In such a solid-state image-sensing device, the individual pixels have different sensitivities, and therefore, even if uniform light is shone on them, the voltages output from the individual pixels differ from one another. To overcome this problem, U.S. Pat. No. 5,289,286 proposes an image-sensing apparatus in which the differences in sensitivity among the individual pixels are compensated for.
The configuration of the image-sensing apparatus proposed in U.S. Pat. No. 5,289,286 mentioned above will be described briefly below with reference to
FIG. 6
, which shows a block diagram thereof. The image-sensing apparatus shown in
FIG. 6
is composed of an image-sensing device (hereafter referred to as the “sensor”)
1
that has a plurality of pixels each producing an output voltage varying natural-logarithmically in accordance with the amount of received light and that has transfer means for transferring the outputs from the individual pixels, an analog-to-digital converter (hereafter referred to as the “A/D converter”)
2
for performing analog-to-digital conversion on the voltage output from the sensor
1
, a memory
3
for storing the digital value obtained as a result of conversion performed by the A/D converter
2
, a correction calculation circuit (hereafter referred to as the “COR”)
4
for subtracting the shading data stored beforehand in the memory
3
from the image data fed from the A/D converter
2
during actual image shooting so as to correct the image data, and a device
5
, such as a storage device or printer, that is fed with the image data corrected by the COR
4
.
In this image-sensing apparatus configured as described above, first, uniform light is shone on the sensor
1
, and, for each pixel, the digital data representing the output voltage obtained therefrom is stored in the memory
3
. Then, during actual image shooting, the image data acquired is corrected on the basis of the above-mentioned digital data, called the shading data, that was stored in the memory
3
when uniform light was shone on the sensor
1
, and the thus corrected image data is output to the device
5
. By correcting the image data in this way, it is possible to correct the unevenness in sensitivity among the individual pixels of the sensor
1
.
Suppose that an amount L of light is shone on a pixel having a sensitivity &agr;. Then, the output voltage Vout
1
, which varies in accordance with the amount L of light and the temperature T of the sensor
1
, is given by Equation (2) below. On the other hand, when an amount L0 of uniform light is shone on the sensor
1
to store the shading data in the memory
3
, the pixel having a sensitivity a produces an output voltage Vout
2
as given by Equation (3) below (the temperature of the sensor
1
at this time is assumed to be T0). Subtracting Equation (3) from Equation (2) at each side thereof yields a difference Vd as given by Equation (4) below.
Vout1
=
Vss1
+
nkT
q
ln
(
q
nkTC
∫
L
·
α
ⅆ
t
)
(
2
)
Vout2
=
Vss1
+
nkT0
q
ln
(
q
nkT0
·
C
∫
L0
·
α
ⅆ
t
)
(
3
)
Vd
=
nkT
q
ln
(
q
nkT
·
C
∫
L
·
ⅆ
t
)
-
nkT0
q
ln
(
q
nkT0
·
C
∫
L0
·
ⅆ
t
)
+
nk
q
(
T
-
T0
)
ln
(
α
)
(
4
)
During image shooting, when the temperature of the sensor
1
equals to T0, the third term of the right side of Equation (4) equals 0, which means that the term that depends on the sensitivity &agr; of the pixel disappears. Therefore, in this case, by subjecting the image data fed from the individual pixels of the sensor
1
to correction as defined by Equation (4), it is possible to suppress the unevenness in sensitivity among the individual pixels. However, when the temperature of the sensor
1
differs from T0, the term that depends on the sensitivity &agr; of the pixel remains, and therefore, even if the image data fed from the individual pixels of the sensor
1
is subjected to correction as defined by Equation (4), it is not possible to suppress the unevenness in sensitivity among the individual pixels.
As described above, if the temperature at which the image data is acquired differs from the temperature at which the shading data is acquired, it is not possible to correct properly the unevenness in sensitivity among the pixels on the basis of that shading data. For this reason, the sensor
1
needs to be illuminated with uniform light to acquire appropriate shading data anew on every shooting occasion. Quite inconveniently, this requires either additional provision of a means for irradiating uniform light or manually performing shading with a diffusion cap fitted on the lens.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an image-sensing apparatus incorporating a solid-state image-sensing device that permits the unevenness in sensitivity among the individual pixels thereof to be corrected properly on the basis of previously stored shading data irrespective of temperature variation.
To achieve the above object, according to one aspect of the present invention, an image-sensing apparatus is provided with: a s
Garber Wendy R.
Minolta Co. , Ltd.
Sidley Austin Brown & Wood LLP
Tillery Rashawn N
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