Apparatus for Point-by-Point Reading of Documents Using Photodetector Matrix

The present invention relates to an apparatus for the point-by-point reading of a document using a matrix of photodetector elements. It is used more particularly in analysis of documents for telescoping.

According to the prior art, represented in particular by ACTA ELECTRONICA, vol. 21, no. 1, 1978, pp. 55-79, the analysis means of telecopiers comprise a reading head constituted by a strip of photodetector elements. In general terms, for reading a standard A4 document, the strip has 1728 aligned photodetector elements making it possible to analyze one line of the document. In order to reduce the number of connections between the photodetector element and the electronic processing circuits, there is a multiplexed addressing of these photodetector elements. These addressing circuits are produced on the actual strip. FIG. 1 gives an example of such a known strip. In the special case of FIG. 1, the photodetector elements constituting the reading head 4 of the strip are photodiodes 2 aligned along axis XX, but could equally well be photoresistors, phototransistors and in general terms any element supplying a current proportional to the light radiation received. These photodetector elements are arranged in p groups of q elements. Two circuits E.sub.x and E.sub.y control switches making it possible to separately address each photodetector element. These switches are generally realized by means of MOS transistors, whose grid is connected to circuits E.sub.x and E.sub.y. Circuit E.sub.x has p connections, each making it possible to select all the photodetector elements of a particular group. Circuit E.sub.y has q connections, each making it possible to select all the photodetector elements occupying a particular rank in a group. When a photodetector element is simultaneously selected by circuit E.sub.x and by circuit E.sub.y, it is activated, i.e. it can supply a signal which is a function of the illumination received on connections S.sub.1, . . . S.sub.q of output circuit S.

The analysis of the state of each of the photodetector elements of the strip takes place in the following way. In the initial state, the p connections X.sub.1, X.sub.2, . . . X.sub.p of E.sub.x and the q connections Y.sub.1, Y.sub.2 . . . Y.sub.q of E.sub.y are in an electrical state such that all the switches are open. The first group of q photodetector elements is then selected by bringing about the appearance on connection X.sub.1 of circuit E.sub.x of an electrical state controlling the closing of switch I.sub.1. The first photodetector element of the group is then made active by bringing about the appearance on connection Y.sub.1 of circuit E.sub.y of an electrical state controlling the closing of switch I.sub.11. An electrical state then appears on connection S.sub.1 of output circuit S, which is a function of the activated photodetector element. This signal is collected and then switch I.sub.11 is opened. The same performed for each of the photodetector elements of the group. Switch I.sub.1 is then opened and then the same sequence of operations as for the first group is carried out for each of the other groups.

The main disadvantage of this MOS switch - photodiode system is the extraction of a low level signal on connections S.sub.1, S.sub.2 . . . S.sub.q of output circuit S. Thus, the closing of switches I.sub.11, I.sub.21 . . . I.sub.p1 controlled by connection Y.sub.1 of circuit E.sub.y induces on connection S.sub.1 of output circuit S a by no means negligible switching noise, which is prejudicial to the operation of said low level signal. In the same way, the closing of the switches respectively controlled by connections Y.sub.2, Y.sub.3, . . . Y.sub.q of circuit E.sub.Y induces a switching noise on the corresponding connections S.sub.2, S.sub.3 . . . S.sub.q of output circuit S.

The present invention mainly constitutes an original matrixing solution for the multiplexed photodetectors.

The object of the invention is to attenuate the switching noise, by substituting for the basic MOS switch - photodiode element, a basic element constituted by two diodes connected in opposite positions, one of these diodes being photosensitive.

More specifically, the present invention relates to an apparatus for the point-by-point reading of a document using a matrix of photodetector elements, comprising p groups of q basic elements aligned along an axis XX, each provided with an input terminal and an output terminal, a circuit having p connections, each being connected to the input terminal of each of the basic elements of a given group, an output circuit having q connections, each being connected to the output terminal of each of the basic elements occupying a particular rank in the different groups, wherein each basic element is constituted by a photodiode and a diode which is insensitive to light radiation, one of the electrodes of the photodiode being connected to the electrode of the diode of the same polarity, the other electrode of the photodiode being the output terminal of the basic element, the electrode of the diode not connected to the photodiode being the input terminal of the basic element and the addressing circuit with p connections is of the shift register type.

In a preferred manner, the diode of each basic element is a photodiode identical to the photodiode of the same basic element made insensitive to any light radiation. This preferred construction simplifies the realization of each strip.

The term identical photodiodes is understood to mean photodiodes constituted by the same semiconductor layers. The electrodes of these photodiodes can however, differ either by their nature, or by their thickness. The photodiodes can be of any random type, e.g. of the Schottky, PN, PIN (P intrinsic N), MIS (metal--insulant--semiconductor) or similar photodiodes. The photodiodes are produced by means of a semiconductor chosen from among hydrogenated amorphous silicon, polycrystalline silicon, gallium arsenide (GaAs) and cadmium selenide (CdSe).

According to a secondary feature, each basic element of axis YY perpendicular to axis XX is constituted by:

an insulating substrate,

two electrodes along axis YY, arranged symmetrically on either side of axis XX, a negatively doped semiconductor layer partly covering each electrode,

an intrinsic semiconductor layer covering the preceding layer and the interelectrode space,

a light radiation-transparent, Schottky electrode covering the preceding layer and constituting the input face of the photodetector,

a light radiation-opaque resin covering that part of the preceding electrode contained in a single half-plane defined by axis XX.

The photodiode is illuminated by the upper common electrode. It is also possible to illuminate the photodiode by the transparent substrate. The two embodiments can be envisaged with all diode and semiconductor types.

According to another secondary feature, each photodetector element of axis YY is constituted by:

a light radiation-transparent insulating substrate,

a light radiation-transparent Schottky electrode, positioned in axis YY in a half-plane defined by axis XX and constituting the input face of the photodetector,

a light radiation-opaque Schottky electrode arranged symmetrically with respect to the light radiation-transparent electrode in the other half-plane defined by axis XX,

an intrinsic semiconductor layer arranged along axis YY and partly covering each electrode,

a highly negatively doped semiconductor layer covering the preceding layers,

a light radiation-opaque electrode covering the preceding layer.

The invention also relates to an apparatus for the point-by-point reading of a colour document having on the same substrate several reading apparatuses according to the invention and which are aligned along a same axis XX and in which the basic elements of each reading apparatus are covered by a filter absorbing part of the optical spectrum.

For examplem it is possible to arrange on the same substrate three parallel reading apparatuses, which are displaced by a few hundred micrometers. In order to obtain a colour reading apparatus, it is possible e.g. to cover each reading apparatus with a coloured filter, whose colours are complementary and can be red, green and blue. Another way of obtaining a colour reading apparatus is, e.g. to choose photodiodes which are sensitive to part of the optical spectrum. The reading time of this colour reading apparatus is substantially identical to the reading time of the apparatus having a single reading head, because the three electrically independent reading heads operate simultaneously.

The invention is described in greater detail hereinafter relative to non-limitative embodiments and with reference to the attached drawings, wherein:

FIG. 1 schematically depicts a prior art strip of photodetector elements.

FIG. 2a schematically depicts a basic element of the apparatus.

FIG. 2b shows the current - voltage characteristic of the basic element of FIG. 2a.

FIG. 3 schematically depicts a strip of photodetector elements.

FIG. 4 is a perspective view of an embodiment of the basic element of the apparatus.

FIG. 5 is a perspective view of another embodiment of the basic element of the apparatus.

FIG. 2a shows a basic element 1 according to the invention forming a dipole. A photodiode 2 is connected in opposition with a diode 3. The common electrode can either be the cathode (as in FIG. 2) or the anode. The voltage V represents the potential difference at the terminals of the dipole from the electrode of diode 3 to the electrode of photodiode 2. The current I is counted positively from diode 3 to photodiode 2.

FIG. 2b shows the configuration of the current - voltage characteristic of the dipole. Photodiode 2 and diode 3 each have a saturation current or inverse current. This current is very low and is approximately 10.sup.-12 A. It is represented by the quantity I.sub.S.sbsb.2 for photodiode 2 and I.sub.S.sbsb.3 for diode 3. The quantity I.sub.ph represents the current produced by the photons illuminating photodiode 2.

The slashed curve represents the characteristic of basic element 1 when the photodiode 2 is not illuminated. No matter what the voltage at the terminals of the basic element 1, the current circulating in the system is between I.sub.S.sbsb.2 and I.sub.S.sbsb.3. The basic element 1 is non-conductive, and then behaves like an open switch.

The continuous curve represents the characteristic of basic element 1 when the photodiode 2 is illuminated. When the polarization voltage is below V.sub.o, the circulating current is between -I.sub.S.sbsb.3 and 0. The basic element 1 is then non-conductive. When the polarization voltage exceeds V.sub.0, it is conductive. The current is then proportional to the polarization voltage up to an asymptotic value equal to I.sub.S.sbsb.2 +I.sub.ph.

Basic element 1 is the essential part of the reading apparatus depicted in FIG. 3. The basic elements 1 are arranged in p groups of q elements aligned along the axis XX. The group of photodiodes 2 constitutes the reading head 1. The apparatus is completed by a shift register-type circuit E, each of the p connections of which is connected to all the non-photosensitive diodes of a group of basic elements 1, and by a circuit S, each of the q connections of which is connected to all the photodiodes occupying a particular rank in a group. The use of a shift register eliminates the switches conventionally used in the input circuit, so that the signals are less noisy.

The analysis of the basic elements 1 takes place in the following way. Circuit E supplies on connection E.sub.1 a voltage such that the elements of the first group are active, i.e. the characteristic of these elements is represented by the continuous line of FIG. 2b. In the case of the basic elements of FIG. 3, this corresponds to a voltage exceeding V.sub.0. On the other connections E.sub.2, E.sub.3, . . . E.sub.p, the circuit E supplies a voltage such that the elements of all the groups function in open switch form (dashed lines in FIG. 2b). In the case of the basic elements of FIG. 3, this corresponds to a voltage below V.sub.0 and these basic elements 1 are then non-conductive.

Output circuit S comprises a group of switches, constituted e.g. by CMOS transistors, and an operational amplifier for amplifying to a usable level the signal supplied by the basic elements. The state of the switches is such that the connections S.sub.1 . . . S.sub.q are permanently connected to ground. This considerably limits noise and stray currents and consequently improves the quality of the signal supplied by the operational amplifier.

In order to sequentially read each basic element 1 of the first group, it is merely necessary to control the switches in such a way that connections S.sub.1 . . . S.sub.q are successively connected to the operational amplifier.

A reading in parallel (i.e. simultaneously of all of the basic elements of one group is possible if circuit S contains an operational amplifier for each output connection S.sub.1 . . . S.sub.q, instead of a single operational amplifier.

When all the basic elements of the first group have been read, circuit E applies to connection E.sub.1 a voltage such that the basic elements of this group function as an open switch.

The basic elements of the other groups are successively read in the same way as those of the first group. The selection of the different groups takes place by sequentially applying to each connection E.sub.1 . . . E.sub.p a polarization voltage square wave pulse, which renders the basic elements connected to said connection conductive.

An advantage of the invention is the reduction of the parasitic noise on connections S.sub.1, S.sub.2 . . . S.sub.q of output circuit S. Another advantage relates to the simplification of basic element 1 of the strip. FIG. 4 shows an embodiment of a basic element 1 according to the invention. The basic elements 1 are arranged with a spacing of 100 .mu.m along axis XX. On an insulating substrate 11, each basic element 1 comprises two electrodes 12, 13, etched in a NiCr or In.sub.2 O.sub.3 coating of thickened 3000 .ANG.. These electrodes 12 and 13 are arranged along axis YY, axis XX passing between electrodes 12 and 13. This etching operation, as well as the subsequent coating deposition, is carried out by any known method. A hydrogenated amorphous silicon coating 14 of thickness 100 .ANG., which is highly doped with type N impurities, is deposited on part of each electrode 12 and 13. The function of coating 14 is to ensure a better ohmic contact. Coating 14 is covered by an intrinsic hydrogenated amorphous silicon coating 15 of thickness between 5000 and 10,000 .ANG.. On coating 15 is deposited a Schottky metal coating 16, which is transparent to light radiation and sufficiently conductive. This metal, for example can be platinum. Finally a light radiation-opaque resin layer 17 is deposited along axis YY in a single half-plane defined by axis XX. Thus, there are two diodes having a common electrode, i.e. the Schottky metal, and each having an electrode on the substrate, the diode which is not covered by resin layer 17 being photosensitive.

FIG. 5 shows another possible embodiment, in which the light rays reach the diode and photodiode across a transparent, insulating substrate 18. A coating 20 of Schottky metal is deposited, which is transparent to light radiation and sufficiently conductive and has a thickness of 100 .ANG., followed by a Schottky metal coating 19, which is opaque to light radiation and is of thickness 1000 .ANG.. For example, this Schottky metal can be platinum. Coatings 19 and 20 are partly covered by an intrinsic hydrogenated amorphous silicon coating 21 having a thickness between 5000 and 10,000 .ANG.. This layer 21 is covered by a hydrogenated amorphous silicon coating 22, which is highly doped with type N impurities and is of thickness 100 .ANG.. Coating 22 is covered with a NiCr electrode 23 of thickness 3000 .ANG..

Other possible embodiments permit the use of photodetectors with PIN, PN or MIS diodes. It is also possible to use any other semiconductor deposited in thin coating form, such as polycrystalline silicon GaAs or CdSe.