Image sensors having alternating arrays of high and low sensitivity picture elements

Image sensors comprising an alternating array of two or more sets of photoelectric devices provide enhanced detail and reproduction of intermediate illuminances. The sets of photoelectric devices are selected such that each set produces a different level of output in response to the same level of illuminance.

BACKGROUND OF THE INVENTION 
The present invention relates to a solid state image sensor for use in 
picture information processing devices such as facsimile 
transmitter/receivers, character readers and television. 
Various image sensing devices are presently known which are useful in 
picture information processing devices. These include charge transfer 
devices, such as CCD's and BBD's, and XY address devices, such as 
one-dimensional line sensors and two-dimensional area sensors. Each of 
these known devices, however, suffers from a common drawback with regard 
to accurately dealing with variations in illuminance caused by shading, 
colors, or differences in paper quality. 
A typical image sensor according to the prior art contains an array of 
photoelectric devices, each of which represents a picture element or pixel 
in the reproduced image. The illuminance from each region of the subject 
picture causes the corresponding photoelectric device to produce an output 
signal, a high illuminance leading to a high output signal. The output 
signal from each photoelectric device is then compared with some arbitrary 
threshold level, T to determine if the picture element should be black or 
white. 
FIG. 1 shows graphically the output currents from a conventional image 
sensor of the line type. T is the threshold output level for 
distinguishing picture elements between black and white. Any region of the 
subject picture that causes the photoelectric device to produce an output 
current greater than T for example I.sub.11, gives rise to a white picture 
element. Any region of the subject picture having an illuminance that 
causes the photoelectric device to produce an output less than T for 
example I.sub.12, gives rise to a black picture element. Similar 
principles are used in conjunction with filters to generate color images. 
From this description, it can be seen that the accuracy of reproduction for 
subject pictures having variable illuminance is troublesome. Picture 
regions which are gray are converted to either black or white, depending 
on where the threshold level is set, giving rise to loss of detail. This 
problem can also limit the utility of differing papers and inks, unless 
some provision is made to vary the threshold level to correct for 
differing reflectivities. Mechanisms are available to make this 
correction, but their use increases the cost and the time required for 
transmitting images. 
It is the object of the present invention to provide image sensing devices 
which correctly represent regions of intermediate illuminance, thus 
preserving detail in the transmitted image and minimizing the need for 
correction mechanisms. 
SUMMARY OF THE INVENTION 
Image sensors according to the invention, which correctly represent 
intermediate illuminances, comprise an array of two or more sets of 
photoelectric devices. The sets of photoelectric devices are selected such 
that they produce differing output levels in response to the same level of 
illumination. Such differing responses can be achieved, for example, by 
utilizing photoelectric devices having different effective photoelectric 
conversion areas, by using different biasing voltages, and by using 
different amplification factors. 
The sets of photoelectric devices are arranged in the image sensor in an 
alternating fashion, to provide a uniform distribution of each type of 
photoelectric device over the entire surface of the image sensor.

DETAILED DESCRIPTION OF THE INVENTION 
Image sensors according to the invention comprise an alternating array of 
two or more sets of photoelectric devices, said sets of photoelectric 
devices being selected such that each set produces a different level of 
output in response to the same level of illuminance. FIG. 2 illustrates 
one embodiment of the claimed invention. 
FIG. 2(a) shows a plan view of an image sensor containing two rows of 
alternating electrodes 2 and 3 having different sizes arranged on a 
substrate 1 (FIG. 2(b)). The electrodes do not touch one another. 
Extensions 4 from the electrodes 2 and 3 are provided for electrical 
connection with the remainder of the apparatus. An insulating film 5 is 
applied over the extensions 4. 
As shown in FIG. 2(b), a layer of amorphous silicon 6 is applied over the 
partially insulated electrodes. Finally, a transparent common counter 
electrode 7 is applied over the amorphous silicon 6. 
The principle by which an image sensor according to the invention operates 
is illustrated in FIGS. 3 and 4 for line type image sensors. FIG. 3 shows 
outputs for an image sensor having alternating photoelectric devices of 
two different photosensitivities. The high sensitivity devices which are 
positioned at odd numbered picture element positions produce output 
I.sub.1 when exposed to some high level of illuminance, while the low 
sensitivity devices which are positioned at the even-numbered picture 
element positions produce output I.sub.2 in response to the same level of 
illuminance, as represented by the response curve 31. Similarly, different 
outputs are produced by the high and low sensitivity devices, 
respectively, in response to some low level of illuminance, as illustrated 
by the response curve 32. T is the threshold output level used to 
discriminate between black and white picture elements. The terms 
photosensitivity and sensitivity of a photoelectric device as used 
hereinafter and in the claims shall refer to the ratio of the output level 
associated with a photoelectric device to the illumination to which the 
device is exposed. The output level may be varied, for example by changing 
the effective photoelectric conversion area, the bias applied to the 
device, or the amplification provided by any amplifier means associated 
with the device. 
At high levels of illuminance, both the high and the low sensitivity 
photoelectric devices produce outputs greater than T and yield white 
picture elements, as illustrated by response curve 31 in FIG. 3. At some 
lower illumination, the high sensitivity devices continue to produce 
outputs above T, but the outputs of the low sensitivity devices are less 
than or equal to T. This situation, depicted in FIG. 3 by response curve 
32, results in alternating white and black picture elements. At still 
lower illuminances, the output from the all the devices falls below T, as 
illustrated by the response curve 33, and all the picture elements become 
black, and remain so at still lower illuminances, as represented by the 
response curve 34. 
As a result of using an image sensor having the response characteristics 
depicted in FIG. 3, regions of intermediate illuminance are represented as 
a pattern of alternating dots or lines which appear visually as a gray. 
Thus, the object of achieving improved detail and reproduction of 
intermediate illuminances without the use of a correcting mechanism is 
obtained. 
Still greater improvement can be realized by utilizing photoelectric 
devices of three different photosensitivities, as shown in FIG. 4. Devices 
at picture element positions 1, 4, 7 and 10 make up one set having a first 
photosensitivity; devices at picture element positions 2, 5 and 8, and 
devices at picture element positions 3, 6 and 9 the other two sets having 
a second and a third photosensitivity, respectively. The first 
photosensitivity is higher than the second and the third. The second 
photosensitivity is greater than the third. 
At high illuminance, all of the outputs are above T, and produce white 
picture elements as depicted by the response curve 41. As the illuminance 
drops, the outputs of the devices having the third sensitivity fall below 
T, as illustrated by the response curve 42, and a pattern of 
"white-white-black" is produced. At still lower illuminance, as depicted 
by the response curve 43, two sets of devices having the second and third 
sensitivities produce outputs below T, yielding a pattern of 
"white-black-black". Finally, at some level of illuminance, all the 
outputs fall below T, as depicted by the response curve 44, and all black 
picture elements result. At still lower illuminations, as illustrated by 
the response curve 45, the elements remain all black. 
EXAMPLE 1 
A photoconductor such as that depicted in FIG. 2 was prepared. Square 
electrodes 2 and 3 consisting of chromiun (Cr) were alternately arranged 
on a glass substrate 1, as shown in FIG. 2(a), to form two arrays, each 
consisting of 3456 electrodes. The electrodes do not touch one another. 
The electrodes 2 and 3 had dimensions of 55 .mu.m.times.55 .mu.m and 46 
.mu.m .times.46 .mu.m, respectively. Extensions 4 from electrodes 2 and 3 
were covered with an insulating film 5 consisting of silicon dioxide. An 
amorphous silicon (a--Si) film 6 was formed to a thickness of 1 .mu.m by 
glow discharge decomposition of silane gas. The a--Si film 6 covered the 
entire surface of the image sensor. 
Finally, a common electrode 7 was formed on the a--Si using 
indium-tin-oxide (ITO). The ITO electrode had a width of 1 mm and a 
thickness of 700 .ANG., and was situated opposite the entire array of 
electrodes 2 and 3. 
A biasing voltage of +5 V was applied to electrodes 2 and 3 in this device 
during exposure to an illuminance of from 10 to 100 lux and the output 
currents were measured. As shown in FIG. 5, the output current from the 
device is proportional to illuminance. 
FIG. 6 shows the output levels from the device at two levels of 
illuminance, 50 and 100 lux. Response curve 61 shows the output currents 
observed at 100 lux, while response curve 62 shows the output currents at 
50 lux. In FIG. 6, the odd-numbered devices are those having the larger 
surface area. The ratio of the current values from the two sets of devices 
is approximately the same as the area ratio, irrespective of illumination. 
EXAMPLE 2 
For comparison purposes, image sensors were prepared and tested in a 
facsimile machine. Sensors according to the prior art were prepared by 
preparing a line of 3456 Cr electrodes measuring 50 .mu.m.times.50 .mu.m 
on a glass substrate. The sensor was then completed in accordance with 
Example 1. Sensors according to the invention were prepared using a single 
line array of 3456 Cr electrodes of two alternating sizes as described in 
Example 1. These sensors were used as intimate contact type image sensors 
in a facsimile machine. 
The test subject transmitted by facsimile was a photograph composed of a 
white portion having a reflectivity of 80%, a black portion having a 
reflectivity of 5%, and an intermediate colored portion having a 
reflectivity at 550 nm, the wavelength used, of 43%. 
Using sensors according to the prior art, all of the intermediate portion 
was reproduced as white upon facsimile transmission. Using sensors 
according to the invention, an image close to the original image was 
achieved. Microscopically, the intermediate colored portions were 
represented by stripes of black and white having the same width as the 
picture elements. These stripes can be replaced by dots, however, by using 
an array such as that shown in FIG. 2(a), wherein successive rows of 
electrodes are staggered. This produces a further improvement in image 
quality. 
The invention has been described above in terms of black and white images 
and exemplified using a photoconductor, but is in no way limited to these 
embodiments. One skilled in the art would understand that the principles 
of the invention are fully applicable to color as well as black and white 
image sensors. Furthermore, one would understand that any type of 
photoelectric device could be used, so long as the output from the device 
could be conveniently varied.