Image pick-up device

Abstract: A multichip linear image sensor (Fig. 1) for use in, for example, a facsimile machine or a photocopier comprises several chip image sensors, each including photosensors (2-1-1-2-1-m) and an output amplifier (9-1). To correct for the offset error of the output amplifier (9-1) caused by processing variations, each chip includes a circuit (11; 10,26) enabling the amplifier (9) to be isolated, so that its output can be sampled independantly of the image signal. To eliminate the offset error a correction circuit (Fig. 3) is provided as part of the video clamp circuit, for sampling the amplifier output and subtracting it from the subsequent image signals readout through the amplifier (9).

Description: FIELD OF THE INVENTION This invention relates to an image pick-up device for use for example in facsimile machines, optical document scanning machines or photocopying machines. BACKGROUND ART Image pick-up devices having photo conversion cells in a linear array are used in facsimile machine or optical reading machines. This kind of image sensor is called a linear image sensor, and is produced from for example a silicon wafer, so that the length of the sensor is restricted by the size of the wafer. Therefore it is quite difficult to produce an image sensor which has the same length as the width of a document which is read by the sensor. One solution of this problem was providing an optical system for reducing the size of the document so that the image sensor can read the full width of the document, but the optical system prevents the system from being small, and the resolution of the image cannot be maintained. Another solution was the adoption of a so called "multi-chip" type image sensor which has plural image sensor chips aligned in a straight line, as shown in our earlier European applications published as EP 0382540 and EP 0382568. This kind of multi-chip sensor can output a plurality of output signals from said plurality of image sensors through one single output line, but generally, the multi-chip sensor consists of a plurality of image sensor chips, so that there is a difference between output voltages from different image sensor chips due to processing variations from chip to chip. Therefore during the manufacturing process a plurality of sensor chips are classified depending on their characteristic and mounted and connected together to constitute one multi-chip sensor. The differences of output voltages of said sensor chips are caused by differences of offset voltage between output voltages of the image pick-up devices. Although the differences between sensor chips in one multi-chip sensor are reduced by this classifying, there can still be a difference of output voltage between multi-chip sensors despite the classifying, if the image pick-up device consists of a plurality of multi-chip sensors for reading the full width of the document. Further, although the chips of each multi-chip sensor are mutually matched, they differ between multichip sensors and hence the apparatus into which each one is fitted needs to be individually adjusted to account for the variations in offset voltage or other output level error parameter between multichip sensors, which is a further complex and expensive manufacturing operation. Since the offset voltage may drift or change depending on temperature, ageing or other factors, a single compensation set at the factory may in any case be insufficient for reliable long term operation. European Patent Specification No. EP-A-0392754 discloses a photoelectric conversion apparatus which includes means for eliminating noise components in the outputs of photo-conversion elements. A concern of the present invention is to provide an improved image pick-up device. Another concern is to provide an improved image pick-up device which can reduce the output voltage difference between sensor chips or multi-chip sensors. According to the present invention, there is provided a multi-chip line sensor device as set out in claim 1. This allows the offset of the output means to be separately sampled and stored, and used to correct the image signals. In a preferred embodiment, the invention further comprises means for subtracting said offset signal from the output signal which was produced by said photoconversion cells. Said predetermined signal may be separate from a power source. In a preferred embodiment, the subtracting means comprlses a storage capacitor storing the offset signal. It preferably also comprises a clamp circuit for clamping said output signal. Thus, the invention provides an image sensor chip which includes means for generating a signal allowing the offset voltage within the chip to be separated and compensated for by whatever circuit the chip is later installed within. If a corresponding compensating circuit is provided in the apparatus for which the chip is installed, an image sensor chip having any value of offset can be installed within the device thus avoiding the need for individual calibration of the device or laborious selection of particular chips for particular devices. Where the image sensors are provided as a linear sensor array for reading a document, for example, separate calibration of chips or multichip sensors within the middle of the array is possible. In order that the present invention may be more readily understood, an embodiment thereof will now be described by way of example and with reference to the accompanying drawings, in which: Fig. 1 illustrates a schematic circuit diagram of an image pick-up device of an embodiment of the present invention: Fig. 2 illustrates a timing chart corresponding to said circuit shown in Fig. 1; Fig. 3 illustrates a correction circuit connected to said image pick-up device; and Fig. 4 illustrates schematically the disposition of multichip sensor device incorporating an embodiment of the present invention in document scanning apparatus; and Fig. 5 illustrates schematically one particular apparatus including a sensor device according to this embodiment. DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT Fig. 1 illustrates a schematic circuit diagram of the embodiment of the present invention and Fig. 2 illustrates a timing chart of the embodiment. In the figure, sensor chips 1-1, 1-2,,1-n (surrounded by dashed lines) constitute a multi-chip sensor. In the sensor chips 1-1 to 1-n, photoconversion cells 2-1-1 to 2-n-m can convert an optical signal to an electrical signal. In this embodiment the photoconversion cells 2-1-1 to 2-n-m of the separate multichip sensors are aligned as if to constitute one line sensor, and consequently an image pick-up device of long length can be obtained so that it can read full size (e.g. A4 or A3) documents without any particular optical system. The multi-chip sensor starts operating in synchronism with a CLOCK signal supplied from a CLOCK terminal 22 when a START signal is supplied to a START terminal 21 which is connected to all the sensor chips. Receiving this START signal, memory address switching shift registers 13-1-1 to 13-n-k make pattern generating memories (ROM circuits or simple logic gate circuits) 12-1 to 12-n operate in synchronism with the clock signal so that, in a first stage, condensor reset MOS switches 5-1-1 to 5-n-m turn on together, and thus an initialisation of condensors 7-1-1 to 7-n-m is carried out. Next, by turning on MOS switches 4-1-1 to 4-n-m together in a second stage light signals from photoconversion cells 2-1-1 to 2-n-m are transferred to condensors 7-1-1 to 7-n-m. Then, in a third stage, reset MOS switches 3-1-1 to 3-n-m are turned on together for initialising the photoconversion cells 2-1-1 to 2-n-m. In this embodiment, during the above operation, a reset MOS switch 26-1 is turned on so that a resetting voltage (e.g. ground or O Volts) applied to a reset voltage terminal 24 is also applied to an input terminal of an output amplifier 9-1, through the action of a control circuit 11-1 (typically a logic gate circuit), operated in sychronism with the START signal as a predetermined voltage is applied to an input terminal 27-1 through a voltage terminal 25. At this time a MOS switch 10-1 is also turned on by the switch control circuit 11-1 so that the voltage V1 at an output terminal 23 consists of an offset voltage component SV1 of the output amplifier 9-1 and the reset voltage component VS applied from the terminal 24 (of, if the output amplifier 9-1 has other than unity again, by the reset voltage multiplied by the gain). After these operations, an output signal of the shift register 13-1-k is applied to the shift register 8-1-1 and the control circuit 11-1, so that a pulse ripples through the stages of the shift register 8-1-1 to 8-1-m and the switches 6-1-1 to 6-1-k are thus sequentially turned on. This causes the electrical signals stored in the condensors 7-1-1 to 7-1-m to be sequentially supplied to the output amplifier 9-1, where they are amplified and supplied from the output terminal 23 through the switch 10-1. The voltage V2 at the output terminal 23 thus comprises a signal component S1 from photoconversion cells 2-1-1 to 2-1-m in turn, together with the offset voltage component SV1 of the output amplifier 9-1. As mentioned later by subtracting the voltage V1 from the voltage V2 the offset voltage component can be eliminated. This subtracting operation can be carried out for example by a clamp circuit shown in Fig. 3. In this circuit, a multichip sensor device 31 corresponds to the device illustrated in Fig. 1. A clamp circuit is conventionally used in video or image signal processing to provide a constant DC level from a clamp voltage source. However, as discussed below, this circuit may also be employed with the image sensor of the above described embodiment to correct for the offset error of the output amplifier thereof. The output terminal 23 of the sensor is connected to the subtraction circuit 33, 35, 40. Before the reading out of the light signals, a signal V1 comprising the sum of the offset component SV1 of said output amplifier 9-1 and the reset voltage component VS is supplied from the image pick-up device 31 via the output line 23 to a first, input, terminal of a condensor 35; the signal may be buffered by an additional amplifier 33. A MOS switch 37 is kept on so that the second, output, terminal of the condensor 35 is maintained at reference clamping voltage VC denotes a clamp voltage supplied from a clamped voltage source 38. Next the switch 37 is turned off by a control signal source 36 clocked in synchronisation with, or controlled by the control circuit 11 so that the output terminal of the condensor 35 is floating. Therefore, while the signals output from the photoconversion elements are read out, an output signal V2 = S1 + SV1 from the amplifier 33 is applied to the input terminal of the condensor 35. Here S1 denotes the photoconversion signal component from the sensor chip 1-1. In this situation the voltage Vout at the output terminal of the condensor 35 is described by the following equation: Vout = VC + (S1 + SV1) - (SV1 + VS) = VC + S1 - VS Therefore as understood from the above equation, the offset voltage component SV1 is eliminated by this operation and the signal at the output terminal of said condensor 35 is supplied to a signal processing circuit 40 for subsequent use (e.g. in a photocopying operation, or for facsimile image encoding) through a further buffering amplifier 39. In this embodiment, since the chips are classified to have similar offset voltage components it is found that differences between offset voltage components between the amplifiers 9-1 to 9-m are negligable and therefore offset voltage components of output amplifiers 9-2 to 9-n can be practically eliminated by the subtracting operation of said offset voltage component SV1 for the first chip amplifier 9-1. Therefore in this embodiment, an output of the register 8-1-m is connected to the following register 8-2-1 and a control circuit 11-2, just after the reading out operation of the sensor chip 1-1, a following reading operation for a sensor chip 1-2 is carried out. However, it would be possible in the event that the chips were less well classified, or not classified, to provide separate offset voltage readout periods prior to the signal readout for each chip, to allow each chip offset to be separately subtracted. Likewise, if several multichip sensors are provided, there may be a separate offset readout period for each multichip sensor. As mentioned above output signals from sensor chips 1-2 to 1-n are read out sequentially without the offset voltage components, so that the difference of offset voltages between multi-chip sensors can be eliminated. The use of the clamping circuit of Fig. 3, which would in any case be present in many apparatus which include the linear sensor of Fig. 1, provides one convenient way of subtracting the offset information from the image output signal from the image sensor device, without requiring further subtraction components, merely by controlling the sampling times of the clamping circuit in cooperation with the image sensor so that the offset signal is sampled. The effect of the clamp circuit thereafter is then to apply to the output of the image sensor a signal responsive to the difference between the clamping voltage and the threshold level from the output amplifier of the image sensor. Referring to Fig. 4, in use, a sheet 100 (e.g. a document, for example A3 or A4 size) is transported in a direction Y by a transport system, illustrated as a pair of rotating transport rollers 110a, 110b. Disposed in a direction X transversely (e.g. normal) to the direction Y is a linear sensor unit 120 extending substantially the entire width of the document 100. An illumination source 130 illuminates the surface of the document 100, and the sensor 120 is exposed to the illuminated document. The output of the sensor 120 is then read out through a correction unit 140 which effects the subtraction of the output error in the output stage of the sensor 120 as discussed above, and may therefore comprise the components 33-39 as described with reference to Fig. 3. The corrected output signal is supplied to an image processing unit 150 which may apply, for example, density or gamma conversion. The image processing unit 150 may, for example, comprise an A/D converter and a digital processing circuit including a gamma correction look up table ROM. The processed signal from the image processing unit 150 is supplied to apparatus 160 which will utilise the image signal; for example, a facsimile transmitter for encoding and transmitting the image signal via telecommunications network, or a personal computer for utilising the image signal in a desktop publishing (DTP) application, or an image reproducing apparatus such as a photocopier. It will be clear from the foregoing that, in practice, the document may be retained static and the document transport system 110 may not be utilised; instead, scanning optics may be provided to effect the scanning in the Y direction. Equally, the correction circuit 140 may be a separate circuit to the clamp circuit, or the correction circuit 140 could comprise part of the image processing unit 150. Referring to Fig. 5, one particular application of the sensor according to this embodiment of the invention is in a portable photocopying unit. The unit comprises a chassis, a supply roller 1102 mounted within the chassis to supply a document along a document path through the chassis, a separator 1104 to separate document sheets, and a platen roller 1106 to move the document past the platen and out through an exit in the chassis. Disposed adjacent to the platen is a sensor unit 1100 comprising the linear sensor 120 and illumination source 130 of Fig. 4. An image recording head 1110 (for example an electrostatic recording head, an ink jet recording head or a laser recording head) is supplied with an image signal derived from the output of the sensor unit 1100 for recordal on a recording medium which is transported past the recording head 1110 by a recording platen roller 1112 receiving a continuous roll of recording medium (paper). A processing and control printed circuit board (PCB) 1130 receives the output of the sensor unit 1100, and includes the correction circuit 140 and the image processing unit 150, together with a control circuit for supplying the START and CLOCK signals to the lines 21, 22 and a control signal to the terminal 25, and control signals to the recording head 1110. An operations panel 1120 allows the user to control the operation of the PCB 1130; for example, by selecting the density correction to be applied by the image processing unit 150 thereon. A power supply unit (e.g. a battery) 1140 powers the PCB 1130 and the motors for the rollers 1102, 1106, 1112. The scope of the invention is in no way limited to the above mentioned embodiment. For example, the various buffer amplifiers 33, 39 may be varied or omitted depending upon the surrounding circuit components. Likewise, the capacitor circuit shown is only one example of a suitable circuit for sampling and storing the offset signal value for subsequent subtraction; other digital or analog stores could be employed. Further, the arrangement and reading out of image sensors is still not critical; any equivalent circuit arrangement (such as those described in EP 0382540 or EP 0382568 incorporated herein by reference) in which the circuit output means can be isolated from the image sensors to generate a signal including the offset level is equally applicable for the invention. Similarly, although application of the invention in a linear multichip sensor array has been described, two dimensional sensor arrays, for example, may equally benefit from the application of the invention. Each photoconversion cell could be of the self-amplifying type in which charges are accumulated in a control region of a transistor, as disclosed for example in our earlier European applications EP 0253678 or EP 0260956. Equally, however, any other type of photoconversion cell could be employed.

Claims: A multi-chip line sensor device comprising a plurality of line sensor chips (1-1, ... 1-n) arranged in a line, wherein each chip comprises a plurality of photo-conversion cells (2) and each chip also comprises an amplifier (9) for sequentially amplifying outputs of said plurality of photo conversion cells, control means (11) for sequentially operating one of said plurality of line sensor chips and, characterised in that each chip includes a resetting circuit (26) for resetting said amplifier before sequentially reading out said plurality of photo conversion cells by applying a fixed reference signal to said amplifier, and that said device comprises a common clamp circuit (32, 35, 36, 38) for clamping an output of said amplifier when said amplifier is being reset to hold an offset component (SV1) to be subtracted from said sequential output signal amplified by said amplifier so that an offset error among said plurality of chips is eliminated. A device according to claim 1, wherein said fixed reference signal comprises the reset signal for said amplifier. A device according to claim 1 or claim 2, wherein the clamp circuit includes means (35) for storing said offset component. A device according to claim 3, in which said means for storing comprise a capacitor (35). A device according to claim 4, wherein one of the terminals of said capacitor (35) is connected to an output of said amplifier (9) and another terminal of said capacitor (35) is connected to the clamp circuit (36-38). A device according to any preceding claim, wherein the or each group of photoconversion cells (2) and output means (9) are provided as a separate semiconductor chip device. A device according to any preceding claim, further comprising isolating means (5) for isolating the amplifier (9) from said photoconversion cells (2). A device according to any preceding claim, and further comprising at least one further group of photoconversion cells (1-2) and associated output means.

Inventors: NAKAMURA KENICHI, NAKAMURA, KENICHI
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1991
Main classification: H04N5
IPC codes: H04N3, H04N1, H04N5