Solid state imaging devices and driving methods that produce...

Details Solid state imaging devices and driving methods that produce... Solid state imaging devices and driving methods that produce...

1998-02-17

2002-08-06

Ho, Tuan (Department: 2612)

Television

Camera, system and detail

Solid-state image sensor

C348S322000

Reexamination Certificate

active

06429898

ABSTRACT:

FIELD OF THE INVENTION
The invention pertains to a solid-state imaging device and a method for operating the imaging device that provides wide dynamic range and multiple gray-scale image signals.
BACKGROUND OF THE INVENTION
Solid-state imaging devices using charge-coupled devices (“CCDs”) have been used as image sensors in electronic and video cameras. Solid-state imaging devices have been further developed to be sensitive to infrared radiation for applications such as thermal imaging or night vision.
With reference to
FIG. 15
, a prior-art solid-state imaging device for infrared imaging uses a two-dimensional array of PtSi Schottky diodes
61
. “Vertical” CCD arrays
62
are provided adjacent the columns of the PtSi diodes
61
and columnar transfer gates
63
are placed between respective vertical CCD arrays and columns of PtSi diodes
61
.
Electrodes &phgr;
1
-&phgr;
4
are connected to the vertical CCD arrays
62
. In
FIG. 15
, the electrodes &phgr;
1
-&phgr;
4
are shown connected to one of the vertical CCD arrays
62
corresponding to the first column of PtSi diodes
61
. For simplicity, the connections of the electrodes &phgr;
1
-&phgr;
4
to the other vertical CCD arrays
62
are not shown. One end of each of the vertical CCD arrays
62
is connected to a “horizontal” CCD array
64
. An output amplifier
66
is connected to an output buffer
64
a
of the horizontal CCD array
64
. The other ends of the vetical CCD arrays
62
are connected to a discharge line
67
.
The imaging device of
FIG. 15
provides interlaced output by providing odd and even fields. An operation in which an odd field is read out is described below. First, voltages are applied to the electrodes &phgr;
1
-&phgr;
3
, forming potential wells at the vertical CCD arrays
62
that receive signal charges accumulated by the PtSi diodes
61
. The accumulated charges are transferred to the potential wells with the transfer gates
63
. Signal charges on an n
th
row (where n is an odd integer) of PtSi diodes
61
and signal charges on an (n+1)
th
row are mixed in one potential well. Thus, signal charges for a single horizontal line are produced. Next, four different driving pulses are applied successively to the electrodes &phgr;
1
-&phgr;
4
, transferring the signal charges in the potential wells vertically to the horizontal CCD array
64
.
The horizontal CCD array
64
successively receives signal charges from rows of the diodes
61
; the signal charges are delivered to the horizontal CCD array
64
from the vertical CCD arrays
62
. These signal charges are transferred to the amplifier
66
during a horizontal scanning interval.
An even field is read by forming potential wells under the electrodes &phgr;
4
, &phgr;
1
, &phgr;
2
. Signal charges on an n
th
row (where n is an odd integer) of PtSi diodes
61
and signal charges on an (n−1)
th
row are mixed, whereby signal charges corresponding to a horizontal line are produced.
The readout operations are carried out so that signal charges for an even field or an odd field are read out sequentially during one field period, e.g. {fraction (1/60)} s for NTSC standard video. Readout of an odd field and an even field (one complete frame or image) requires {fraction (1/30)} s.
Imaging devices with electronic shutter capability (i.e. that have variable signal charge accumulation times) have been disclosed in Japanese Examined Patent Publication No. 1-18629. Electronic shutter operation can also be described with reference to FIG.
15
. First, unwanted charges accumulated on the PtSi diodes
61
are transferred to the vertical CCD arrays
62
via the transfer gates
63
. The vertical CCD arrays
62
are driven so that the unwanted charges are swept to the discharge line
67
.
After a specified accumulation time has elapsed after the discharge at the discharge line
67
, accumulated signal charges are transferred to the vertical CCD arrays
62
via the transfer gates
63
. The vertical CCD arrays
62
are driven so that the signal charges from the rows of PtSi diodes
61
are sequentially transferred to the horizontal CCD array
64
. The horizontal CCD array
64
then transfers the signal charges to the amplifier
66
. By varying the accumulation time, an electronic shutter operation is achieved.
With reference to
FIG. 16
, the output of the imaging device of
FIG. 15
with electronic shutter operation is shown as a function of blackbody temperature of an object being imaged. The blackbody temperatures were measured using a blackbody oven; such blackbody temperatures are analogous to luminance levels of visible light. The vertical axis indicates the number (in base-
10
exponential notation) of output electrons generated by the PtSi Schottky diodes
61
for objects as a function of blackbody temperature.
With reference to
FIG. 16
, output-characteristic curves
16
a
-
16
d
show outputs of the horizontal CCD array
64
for signal-charge accumulation times of {fraction (1/60)}, {fraction (1/500)}, {fraction (1/1000)}, and {fraction (1/1500)} s, respectively. For an accumulation time of {fraction (1/60)} s as shown by curve
16
a
, there is a large variation in charge output for objects with a range of blackbody temperatures from 20 C. to 80 C., and the images produced have a wide dynamic range. Objects associated with this range of blackbody temperatures are therefore imaged with multiple gray-scale levels. However, at blackbody temperature greater than about 100 C., signal charges fill and overflow the PtSi diodes
61
, and the output saturates. Thus, images of objects at blackbody temperatures greater than 100 C. have on a few gray-scale levels.
With reference to curve
16
d
, corresponding to an accumulation time of {fraction (1/1500)} s, the output charge changes rapidly with temperature for blackbody temperatures near 200 C. Objects at blackbody temperatures near 200 C. are therefore imaged with multiple gray-scale levels, and excellent images are formed of such objects. However, at blackbody temperature near 20 C., the output is low, and the output signal-to-noise ratio is low. Images of objects at these temperatures are noisy and have few gray-scale levels.
The curves
16
b
,
16
c
, corresponding to signal charge accumulation times of {fraction (1/500)} s and {fraction (1/1000)} s, respectively, have narrow ranges of blackbody temperatures at which images with multiple gray-scale levels are produced. As will be readily apparent from curves
16
a
-
16
d
, high-quality (i.e., wide dynamic range, multiple gray-scale) images are difficult to obtain of objects having large temperature variations.
The above description concerns exposure control with an electronic shutter, but a similar problem occurs with exposure control using lens apertures or filters. These exposure-control techniques also do not improve the dynamic range or the gray-scale output of the imaging device. While the description above concerns infrared imaging with infrared-sensitive imaging devices, it is similarly difficult for an imaging device sensitive to visible light to produce images with a wide dynamic range and multiple gray-scale levels.
It is apparent that imaging devices and methods are needed that produce images with a wide dynamic range and multiple gray-scale levels.
SUMMARY OF THE INVENTION
The invention provides solid-state imaging devices comprising a plurality of photosensors arranged in a matrix of rows and columns. The rows comprise odd-numbered rows and even-numbered rows. The photosensors accumulate signal charges corresponding to an incident light flux. “Vertical” transfer paths corresponding to each column of photosensors are provided to transfer signal charges (or unwanted charges) to a “horizontal” transfer path. The horizontal transfer path transfers the charges to an output amplifier that produces an electrical image signal corresponding to the distribution of the light flux intensity on the photosensors. Each of the vertical and horizontal transfer paths are preferably arrays of charged-coupled devices (CCDs).
Gates are provided to control the transfe

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