Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to a photoconductor
device for imaging a linear object and is particularly
applicable to line imaging in such applications as facsimile
read and print systems.
Conventionally, in a reading, or imaging
device, the object is imaged onto a detector array such as a
charge-coupled device (CCD) array. Such systems are
expensive.
The present invention provides a device
which is much cheaper to produce. The device comprises a
linear photo-conductive member and associated conductors and
connectors arranged as a matrix, in rows and columns. The
photoconductive member can be a continuous bar of photoconductive
material, or a plurality of closely spaced separate elements of
photoconductive elements. A detector circuit is associated with
the device.
The invention will be readily understood by
the following description of certain embodiments, by way of
example, in conjunction with the accompanying drawings, in
which:-
Figure 1 is a diagrammatic plan view of part of -
an array;
Figure 2 is a plan view of one form of photo-
conductive member and associated conductor patterns;
Figure 3 is a typical cross-section to a
greatly enlarged scale, on the line III-III of Figure 2-
Figure 1 illustrates, very diagrammatically, asection of a device or array. A plurality of photoconductive
elements lOa to lOn extend in a line. To one side are made
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connections via connectors 11, which are arranged in columns, in
the present example three connectors 11 forming a column. To
each column is connected a conductor 12a, 12b and 12c. On the
other side of the line of elements 10 are connectors 13. In
the example illustrated the connectors 13 are connected to
conductors 14a, 14b and 14c in rows, three elements 10 to each
row.
As stated, Figure 1 is a section of a device and
normally many more elements 10 would be provided. For example,
for reading of a line of print on a page there would be 1600
elements assuming a line 8" long and a resolution of 200 lines
per inch. These would be connected, via connectors 11 and 13
and conductors 12 and 14 as a 4Qx40 matrix, that is 40 rows and
40 columns. Each row would connect to 40 elements and each column
would connect to 40 elements.
While the elements 10 are shown as separate
from each other in Figure 1, the elements are preferably formed
as a continuous bar, as indicated by the dotted lines 15. The
continuous bar may be scribed, as by a laser, to reduce the
thickness of the bar between elements.
A device can operate as follows. Column 12a would
be taken to voltage V, typically 20V. columns 12b, 12c--------12n
would be grounded. A detector circuit would look at the currents
flowing through the elements lOa, lOb and lOc. Typical currents
would be 2.0ma (lOK) in the on state to 0.2ma (lOOK) in the off
state, the on and off states depending upon whether or not light
impinged on the elements. Thus by sequentially "looking" at the
elements it would be possible to produce signals, indicative of the
on or off state of an element, thus indicative of a light or dark
spot on the line being read or scanned and these signals can then
1~1973
be transmitted to a printer.
Some indication of the sensitivity and accuracy
of the system is as follows. Typical print reflectivities
exhibit contrast ratios of 10:1, although in the case of poor
print it may be as low as 5:1. The resistivity is approximately
inversely proportional to light power and will also be in the
ratio of 5:1 in the poor print example. Tnus on white prints the
resistance will be near 10,000 ohms and on dark prints the
resistivity will be near 50,000 ohms. As a worst case analysis,
with column 12a raised to 20V and the remaining columns grounded,
a detector on elements lOa, lOb and lOc must sense the difference
between 2.0 ma for a white area flowing to virtual ground and 0.4
ma for a black area flowing to vertical ground.
Possible parallel paths place as little as 250 ohms
(10,000/40) or as high as 1250 ohms in parallel with the detection
impedance which therefore should be below 25 ohms to reduce signal
shift to below 10%. The voltage signal generated across 25 ohms
by 2.0ma is 50mV and the signal generated by 0.4ma across 25 ohms
is 10 mV. This gives an effective on signal to off signal of 5:1.
The 50mV to lOmV swing is in the range where good signal to noise
ratio is possible.
Figure 2 illustrates one form of arrangement for
the linear photoconductive member and conductor patterns
associated therewith. The photoconductive member or bar is
~; indicated at 20. At one side of the bar 20 extends a pattern of
closely spaced electrical connectors 21, the connectors 21 being
parallel and extending from beneath the bar 20 and being connected
in columns or blocks to contact pads 22a to 22n. The connectors
21 correspond to connectors 11 of Figure 1 while the contact pads
22a-22n correspond to the conductors 12a-12c and so on of Figure 1.
-- 3 --
l9~3
Qn the other side of the bar 21 is a pattern
comprising a plurality of parallel transverse electrical conductors ~-
23a to 23n extending as rows across the substrate 24. From the
conductors 23 extend electrical connectors 25. A connector 25
connects between each conductor 23 to the bar 20, the connector
connecting to the bar in opposition to a connector 21. The
connectors from a particular conductor 23 are spaced apart so that
each connector 25 is connected to the bar opposite a connector 21 ~r
from a different contact pad 22. Thus, for example, assuming
that twenty connectors 21 extend from each contact pad 22, the
first conductor 23a is connected to the bar 20 by connectors 25
which are opposed to the first connector 21, then the twenty-first,
forty-first and so. The next conductor 23b is connected to the
second connector 21, the twenty-second, the forty-second and so on.
This continues for each conductor 23. By this arrangement, by
selection of a particular contact pad 22 and a particular conductor
23, an unique position on the bar 20 can be connected to a circuit.
- Contact pads 26 are provided at alternate ends of the conductors
23. In one method of forming the conductor patterns, the conductors
23 are formed on the substrate 24, a layer of electrically
insulating material formed over the conductors and then the
connectors 25 formed. Electrical connection is provided between
connectors 25 and conductors 23 through vias formed in the
insulating layer, the vias indicated at 27.
Figure 3 illustrates to a larger scale a typical
cross-section through the bar 20, connectors 21 and 25, conductors
23 and contact pads 22. Also shown in Figure 3 is insulating
layer 28, between connectors 25 and conductors 23.
In use as a detector or reading head, the device
is connected via leads from the contact pads 22 and 26 to drive
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1973
circuitry. The drive circuitry can be of conventional form and
effectively connects each pad 22 to a particular potential and
then sequentially connects each pad 26 to, for example, a unity
gain switch, the switches connected to a level detection circuit.
Light falling on the bar makes it conductive. Thus if a line of
print is imaged on the bar 20, each picture element will be either
photoconductive or non-conductive, depending upon whether the
related portion of the image is white or black. By stepping
connections to pads 22a-22n, and to pads 26, the whole bar can be
scanned to produce a pattern of output pulses from the detector
; circuit indicative of white (or light) picture elements.
A particular process for producing a device as
in Figures 2 and 3 is as follows. The electrical connectors 21
and 25 are formed by thin film techniques, either by first -~
forming a patterned mask on the substrate 24 and then forming the
conductors. Alternatively the substrate can be covered with the
~- connector material and then the material photolithographically
etched to produce the connectors. The layer of insulating
material 28 is applied and then the through connection vias
prepared. Conductors 23 are then formed by deposition through a
mask, or by a layer which is photolithographically etched to
produce the desired pattern. Conductors 23 can be thin film form.
Again, as an alternative, the conductors 23 can be deposited by
thick film techniques. Finally the photoconductive material
forming the bar 20 is deposited. This can again be thin film
techniques.
~ nstead of forming the insulating layer over the
entire pattern of connectors 23, the layer can be in sections,
the edge of a section inclined from the bar 20, to define the
particular position on each connector 25 at which the conductor
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make contact with the particular connector. The conductors 23
could be formed first on the substrate 24, then the areas of
insulating layer formed and then the connectors 25 formed.
One example of material for the bar 20 is
cadmium selenide, and a further example is cadmium sulphide.
The conductors and connectors can conveniently be of a three
part layer; a first layer of titanium for good adhesion to the
substrate, a second layer of palladium which gives good adhesion
to the titanium and provides good adhesion for the third layer
of gold. The three layers can be formed by evaporation.
The contact between the connectors and bar would
normally be ohmic contacts with the materials described, but
by appropriate choice of metal to semiconductor (bar) contacts,
rectifying junctions can be provided. Such junctions would
minimize any loop current problem in the detection circuitry.
An example of a rectifying junction contact is a sandwich
structure, with the photoconductor, for example, of cadmium
sulphide contained between an Indium (metal) contact and tin
oxide (semiconductor).
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