Solid state image sensing device

An improved solid state image sensing device includes a two-dimensional array type charge-coupled device. The read rate thereof is increases n times a standard read rate, where n is a positive integer larger than one, and the output signals of the charge-coupled device are added n times with respect to the same pixel or to the same combination of pixels. The sum of the addition stored in a memory is read at the standard read rate, so that the dynamic range of the solid state image sensing device is expanded.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a charge-transfer type image sensing 
device, and in particular to the enhancement of the performance thereof. 
2. Description of Background Art 
Many types of two-dimensional solid state image sensor are known, and 
charge transfer type devices such as CCDs which operate with lower noise 
are also known. 
Recently, the size of CCD image sensor continues to decrease with the 
number of pixels of a CCD image sensor increasing rapidly. Thus, area per 
pixel, that is cell size, is being reduced to a large extent. 
The reduction of cell size brings about problems with respect to sensor 
characteristics. The sensitivity and the amount of charges to be dealt 
with become lower rapidly because an ineffective area which does not 
contribute to the sensitivity exists and a two-dimensional effect arises, 
when compared with a case wherein the sensitivity and the amount of 
charges decrease in proportion to the cell size. 
Recently, a technique of forming a microlens arranged above a 
photosensitive part of a CCD sensor so as to expand the effective 
photoelectric transformation area has been used. Even if the cell size is 
reduced, the sensitivity can almost be kept the same by using the 
microlens, but the amount of charges to be dealt with decreases largely so 
as to lower the dynamic range of an image. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a solid state image 
sensing device having a large dynamic range for a small cell size. 
A solid state image sensing device according to the present invention 
comprises: (a) an image sensor for sensing an optical image, the image 
sensor including a two-dimensional array type charge-coupled device; (b) a 
adder for adding output signals of the charge-coupled device with respect 
to the same pixel or to the same combination of pixels n times, where n is 
a positive integer larger than one; (c) a memory for storing the sums of 
the addition by the adder; (d) a clock control for feeding first clock 
signals of n times a predetermined read rate of the charge-coupled device 
to the image sensor for the image sensing and the adder for the summation 
and second clock signals of the predetermined read rate to the memory for 
the read of the sums. 
It is an advantage of the present invention that the dynamic range of the 
total image sensing device can be expanded easily without by changing the 
structure of image sensing element itself. 
Further scope of applicability of the present invention will become 
apparent from the detailed description given hereinafter. However, it 
should be understood that the detailed description and specific examples, 
while indicating preferred embodiments of the invention, are given by way 
of illustration only, since various changes and modifications within the 
spirit and scope of the invention will become apparent to those skilled in 
the art from this detailed description.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings, wherein like reference characters designate 
like or corresponding parts throughout the several views, preferred 
embodiments of the present invention will be explained in detail. 
A two-dimensional array type charge-coupled device is classified into the 
interline transfer type and the frame transfer type. In an interline 
transfer type device (refer to FIG. 1) to be used in the embodiments, a 
plurality of pixels are arrayed as a matrix, and a storage portion shaded 
optically is provided between lines of photosensitive portion made of 
pixels. In each optical integration period, signals of a part or all of 
the pixels of a line are transferred together to storage portions, and 
then the signals are successively read from the storage portions in the 
remaining period which takes a large part of the optical integration time. 
Two storage modes, frame storage mode and field storage mode, are possible 
when an interline transfer type charge-coupled device is operated in a 2:1 
interlace mode. 
On the other hand, in a frame transfer type device, a plurality of pixel 
are also arrayed as a matrix, and a storage portion made of pixels and a 
storage portion shaded optically are arranged in series in the transfer 
direction. In each optical integration period, signals of photosensitive 
portions are transferred to storage portions rapidly, and the signals are 
successively read from the storage portions in the remaining period which 
takes a large part of the optical integration period. Only the field 
storage mode is possible for a frame transfer type device. 
The interline transfer type is presently used generally because smaller 
false signals called "smear" appear above and below an object of high 
light quantity and a smaller chip size is required. Though the present 
invention can also be applied to a frame transfer type of charge-coupled 
device, an interline type charge-coupled device is used in the embodiments 
for the image sensing element, and it is used in the field accumulation 
mode wherein an optical integration period of each pixel consists of a 
field. 
FIG. 1 shows schematically a charge-coupled device of the interline 
transfer type, wherein four Vertical lines of photosensitive portions each 
made of five pixels l1, l2, . . . , l5 are aligned in parallel to each 
other in the horizontal direction and storage portions S1, S2, S3 and S4 
are arranged between the lines. Arrows 111 designate the direction of 
read-out, while an arrow 112 designates the direction of horizontal scan. 
In order to transfer signals from the photosensitive portions to the 
storage portions arranged between two lines of photosensitive portions, 
signals of all pixels are transferred in a first field to the storage 
portions and the signals of two pixels in the vertical direction are 
added, while signals of all pixels are also transferred in a second field 
to the storage portions and the signals of two pixels in the vertical 
direction of a combination different from that in the first field are 
added. In other words, in a first field, two signals of adjacent pixels 
are summed to give S.sub.l1 +S.sub.l2, S.sub.l3 +S.sub.l4, and so on, 
wherein S.sub.p designates a signal of a pixel p, while in a second field, 
two signals of adjacent pixels of a different combination from that of the 
first period are summed to give S.sub.l2 +S.sub.l3, S.sub.l4 +S.sub.l5, 
and so on. Then, the signals are read successively from the storage 
portions in each field. 
FIG. 2 shows a single pixel portion of the charge-coupled device array 1, 
which is composed of a photodetecting portion 101 to perform photoelectric 
transformation, a portion 102 to transfer signal charges stored in the 
photodetecting portion 101 in a short time to a CCD portion 103, the CCD 
portion 103 to transfer the charges to a detector portion, and a channel 
stop portion 104 to isolate the pixel from adjacent pixels. 
When the size of a pixel is decreased, the area of the photodetecting 
portion 101 decreases, to thereby lower the sensitivity due to the 
decrease in photoelectric transformation area. Further, the decrease in 
the areas of the photodetecting portion (PD) 101 and the CCD portion 103 
lowers the maximum charges Q.sub.max (PD) and Q.sub.max (CCD) to be 
retained, respectively. If the signal charges exceed these values, they 
run over the pixel so as to deteriorate the quality of image. In general, 
a pixel is designed so that Q.sub.max (PD) is larger than Q.sub.max (CCD) 
because this condition is better for an image. In this case, Q.sub.max 
(CCD) is a basis of the amount of charges to be dealt with. 
FIG. 3 shows a block diagram of an embodiment of a solid state image 
sensing device according to the present invention. A charge-coupled device 
array 1 is an interline transfer type two-dimensional charge-coupled 
device, and it is operated in the field storage mode. 
A central processing unit 11 controls the driving of the charge-coupled 
device array 1. That is, the central processing unit 11 sends a signal to 
a timing generator 12 which generates clock signals. The clock signals are 
fed to a driver 13 which drives the charge coupled device array 1. As will 
be explained later in detail, the driving rate or read rate of the 
charge-coupled device array 1 is n times the standard read rate when the 
quantity of light is high, wherein n is an integer larger than one, while 
the driving rate is the standard read rate when the quantity of light is 
low. In this embodiment, n is four. It is preferable generally that the 
standard read rate agrees with the television broadcasting standard. 
An output signal of the charge-coupled device array 1 is amplified by an 
amplifier 2 to a suitable electric voltage, which is then converted by an 
analog-to-digital converter 3 into a digital data. A switch 4 is changed 
according to whether the driving mode is odd field mode or an even field 
mode. The digital data is once stored in a first memory (M1) 5 in the odd 
field mode or in a second memory (M2) 7 in the even field mode. Then, the 
digital data is fed to an adder 6 so as to add it to a sum of the previous 
data with respect to the same pixel, and the sum is stored in the first or 
second memory 5, 7. After n successive data are added, the sum is read 
from the first or second memory 5, 7 to be fed via a switch 8 to a 
digital-to-analog converter 9 which converts the sum into an analog signal 
at the standard read rate. The central processing unit 11 also controls 
the analog-to-digital conversion, the write and read of data of the 
memories 5, 7, the sum of data and the switches 4, 8 in correspondence 
with the driving of the charge-coupled device array 1. 
In a charge-coupled device type solid state image sensing element, when the 
readout rate is increased up to several times the standard read rate, the 
capacitances of the photodetecting portion 101 and of the CCD portion 103 
which govern the maximum charges Q.sub.max (PD) and Q.sub.max (CCD), 
respectively, do not lower so much. Especially, as the size of a pixel 
becomes smaller, the capacitance of the gate electrode becomes smaller, so 
that the above-mentioned high speed driving becomes more advantageous. 
Thus, when the read rate is increased n times the standard read rate, the 
density of signals per time is increased up to about n times at once. By 
using the memories 5, 7 and the adder 6, the signals are added n times 
with respect to the same pixels while the sums are read out from the 
memories 5, 7 at the standard read rate to reduce the density of signals 
per time to the usual one. The maximum charges to be dealt with in a pixel 
can be increased up to about n times. 
FIG. 4 shows a timing chart of the solid state image sensing element 
mentioned above, when the quantity of light is high. A frame consists of 
an odd field and an even field, and the odd field begins at t.sub.O while 
the even field begins at t.sub.E. The period of each field is chosen to be 
adapted, for example, with the standard read rate, namely 1/60 second at 
the NTSC standard. In the odd field, an image is sensed and the charge 
signals of the pixels of the charge-coupled device array 1 are transferred 
to the detecting part in four periods T.sub.O1, T.sub.O2, T.sub.O3 and 
T.sub.O4 each starting from t.sub.O1, t.sub.O2, t.sub.O3 and t.sub.O4 in a 
field. In each period T.sub.On (n= 1, 2, 3 and 4), the read pulse 
(.0..sub.T) is fed to the charge-coupled device array 1 so that signals S 
of two pixels of adjacent two arrays are summed to give S.sub.l1 
+S.sub.l2, S.sub.l3 +S.sub.4, . . . , and the signals are added by the 
adder 6 to be summed in the first memory 5. Then, the stored data are read 
out after a time T.sub.O /n passes, wherein T.sub.O designates the length 
of an odd field, and are added with signals to be read in the next period 
T.sub.O,n+1 and the sums are stored again in the first memory 5. While the 
data are written in the first memory 5, the sums which have been stored in 
the second memory 7 are read at the standard read out rate. 
Similarly, in an even field, an image is sensed and read in four periods 
T.sub.E1, T.sub.E2, T.sub.E3 and T.sub.E4 each starting from t.sub.E1, 
t.sub.E2, t.sub.E3 and t.sub.E4 in a field. In each period, T.sub.En (n=1, 
2, 3, 4), the read pulse (.0..sub.T) is fed to the charge-coupled device 
array 1 so that signals S of two pixels of adjacent two arrays are summed 
to give S.sub.l2 +S.sub.l3, S.sub.l4 +S.sub.l5, . . . , and the signals 
are added by the adder 6 to be stored in the second memory 7. Then, the 
stored data are read after a time T.sub.E /n passes and are added with 
signals to be read in the next period T.sub.E,n+1 and the sums are stored 
again in the second memory 7. While the data are written in the second 
memory 7, the sums which have been stored in the first memory 5 are read 
at the standard read rate. 
When the quantity of light is low, it is better that n is one. FIG. 5 shows 
a timing chart when n=1. In this case, the read pulse (.0..sub.T) is fed 
once in an odd or even field as usual. 
The memories 5 and 7 may be either analog or digital memories in principle. 
Because the driving of the charge-coupled device is faster by n times when 
compared with the usual driving, it is necessary that the successive 
processing of the analog-to-digital transformation, the readout from the 
memory 5, 7, the sum and the storage in the memory should be performed 
fast. Then, it is effective that the successive processing is performed by 
M processing units arranged in parallel (for example M=n) in order to 
reduce the processing rate by a factor (1/M). 
FIG. 6 shows a graph of the level of signal/noise plotted against the 
quantity of light in a log scale. By using FIG. 6, the improvement of the 
performance of the solid state image sensing device is explained below. 
Noises in the output signal of the solid state image sensing device 
consists of sensitivity non-uniformity noises (N.sub.p), photoelectric 
conversion noises (N.sub.s) and output noises (N.sub.A). The sensitivity 
non-uniformity noises come from the randomess of the sensitivity of the 
pixels. Because the sensitivity non-uniformity noises are proportional to 
the output signal, they become large at high quantities of light. The 
photoelectric conversion noises are shot noises which are generated at the 
photoelectric conversion in principle. Because they are proportional to 
the square root of the output signal, they become important at 
intermediate quantities of light. The output noises consist mainly of 
reset noises of the detecting part of the charge-coupled device array 1 
and noises of the amplifier 2; because the output noises are constant and 
independent of the output signal, they become important at low quantities 
of light. 
The maximum output signal to be dealt by the solid state image sensing 
device is designated as S.sub.1 in FIG. 6, and the photoelectric 
conversion, sensitivity non-uniformity noises and output noises 
accompanying the output signal S.sub.1 are designated as N.sub.S1, 
N.sub.P1 and N.sub.A1, respectively. Then, the total noises amount to 
.sqroot.N.sub.S1).sup.2 +(N.sub.p1).sup.2 +(N.sub.A1).sup.2. In this 
invention, the image sensing is performed n times faster than the standard 
read rate. Then, the maximum output signal is expanded up to n*S.sub.1. On 
the other hand, the photoelectric conversion noises increases up to 
.sqroot.n*N.sub.S1, the sensitivity non-uniformity noises up to 
.sqroot.n*n.sub.P1, and the output noises up to .sqroot.N*N.sub.A1. Then, 
the total noises amount to 
##EQU1## 
which is less than n.sqroot.(N.sub.S1).sup.2 +(N.sub.P1).sup.2 
+(N.sub.A1).sup.2. Thus, the dynamic range is expanded. 
On the other hand, when the quantity of light is low, it is preferable to 
drive the charge-coupled device array 1 at the standard read rate. When 
the Output signal is S.sub.2, which is much lower than S.sub.1, the total 
noises are .sqroot.(N.sub.S2).sup.2 +(N.sub.A2).sup.2 where N.sub.S2 and 
N.sub.A2 are the photoelectric conversion noises and the output noises, 
respectively. The sensitivity non-uniformity noises can be neglected 
because the quantity of light is low. If the quantity of light is kept the 
same and the output read rate is increased n times, the output signal does 
not change and is the same as S.sub.2 because the total integration period 
of the charge-coupled device array 1 does not change. That is, S.sub.2 
=n*S.sub.3 where S.sub.3 is the output signal of each integration period. 
On the other hand, the output noises increases due to the sum operations 
up to .sqroot.n*N.sub.A2. Thus, the total noises increases up to 
.sqroot.(n*N.sub.A2).sup.2 +(N.sub.S2).sup.2. Therefore, when the quantity 
of light is low, the sum processing is not performed. 
As shown in FIG. 7, only noises of the output are different among various 
kinds of noises between the addition driving mode and the conventional 
driving mode. Then, the dependence of the total noises on the quantity of 
light is expressed as 201 and 202 in the addition driving mode and in the 
conventional mode, respectively. Therefore, the total noises in the 
addition driving mode becomes larger than those in the conventional mode 
if the amount of light exceeds a critical quantity of light 203. Thus, the 
addition driving mode is adopted above the critical quantity of light, 
otherwise the conventional mode is used. Though the value of the critical 
quantity of light depends on the ratios of various kinds of noises, it is 
generally of the order of a tenth the saturation quantity of light in the 
conventional mode. 
FIG. 8 shows a flow of mode change performed by CPU 11. First, the 
conventional (non-addition) processing mode is set as the initialization 
(step S2). Then, the output value A of the digital-to-analog converter 9 
is averaged when the signals are read in the odd field of the n-th frame 
(step S4). Next, it is decided if the averaged value (A) is larger than 
the critical output A.sub.c (step S6). If the decision is decided to be 
YES, the addition processing is performed (step S8), otherwise the 
conventional (non-addition) processing is performed (step S10) when the 
data are written in the odd and even fields in the (n+1)-th frame. 
As explained above, the dynamic range of the total image sensing device of 
two-dimensional array type can be expanded easily without changing the 
structure of the image sensing element itself. Especially, image sensing 
devices are now digital, so that an image sensing device according to the 
present invention can be used easily in a system. 
The invention being thus described, it will be obvious that it may be 
varied in many ways. For example, the field storage mode is explained 
above. However, this invention can also be applied in a frame storage 
mode. Further, a frame transfer type device can also be used. Such 
variations are not to be regarded as a departure from the spirit and scope 
of the invention, and all such modifications as would be obvious to one 
skilled in the art are intended to be included within the scope of the 
following claims.