JPH01137774A - Driving method for contact type image sensor - Google Patents

Abstract

PURPOSE:To stably fix the DC level of an output signal from a sensor, to reduce the number of shielded picture elements and to decrease the size by making a read period of an output signal from said picture elements longer than that of an output signal from an effective picture element. CONSTITUTION:Among the picture elements 18-22 provided on a substrate 1, only that 18 is the shielded picture element the incident light 3 to which is shielded by a shielding layer 2, and others are the effective picture elements. In the figure of waveforms, the periods 38-42 in a read period signal shows the read periods of output signals from the picture elements 18-22. The read period 38 of an output signal from the shielded picture element 18 is eight times as long as those 39-42 of output signal from the effective picture elements 19-22. Therefore, the period corresponding to the shielded picture element of a sensor output signal shown in the figure (b) can be made longer than the pulse width of a clamping pulse in figure (c) for fixing the DC level stably. As a result, even the only one shielded picture element can be equal in effect to a case there are eight pieces of shielded picture elements.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to a method of driving a contact type image sensor.

[Conventional technology]

In recent years, contact type image sensors have been increasingly used in facsimiles, OCR, and the like.゛In these contact-type image sensors, the DC level of the sensor output signal (hereinafter referred to as dark current) when there is no incident light changes depending on environmental conditions such as temperature, so an optical image of the document to be read is not formed. There is a method of fixing the DC level of the effective pixel when there is no incident light by providing light-shielded pixels in addition to the effective pixels, and fixing the DC level of the output signal of these shaded pixels to a predetermined level. Often used.

FIG. 5 is a structural diagram of a conventional contact type image sensor, and FIG. 6 is a time chart showing its operation. In FIG. 5, among pixels 91 to 102 provided on a substrate 9, pixels 91 to 98 are shielded from incident light 3 by a light shielding layer 8, and pixels 99 to 102 form an optical image of a document to be read. This is the effective pixel. FIG. 6(a) shows the successive period signal, and the period 11.1 to 122 in this readout period signal.
indicate output signal readout period signals of pixels 91 to 102, respectively. FIG. 6(b) shows a sensor output signal that is not being read, and FIG. 6(c) shows a clamp pulse for fixing the DC level of the sensor output signal.

[Problem that the invention seeks to solve]

In order to stably fix the DC level of the sensor output signal, the width of the clamp pulse 5 needs to be longer than a certain period of time. Therefore, in the conventional contact type image sensor described above, the output signal successive period of the shaded pixel is the same as the output signal successive period of the effective pixel (see Fig. 6 (a>)), so the scanning speed is fast and the output of each pixel is As the signal readout period becomes shorter, the number of necessary light-shielding pixels increases and the external size becomes larger.

[Means for solving problems]

A method for driving a contact type image sensor according to the present invention is to set the output signal readout period of one light-shielded pixel of a contact-type image sensor having a plurality of effective pixels on which light is incident and a light-shielded pixel that is shielded from light on a substrate to a number of effective pixels. A readout signal that is at least twice the output signal readout period of one output signal is applied to each pixel, and a signal is read out from each pixel during the output signal readout period of each pixel, and a clamp is applied during the output signal readout period of the shaded pixel. The configuration is such that the output level of the light-shielded pixel is fixed at a constant value by applying a pulse.

[Effect]

Among the means for solving the above problem, the DC level is stably fixed during the period corresponding to the shaded pixel of the sensor output signal by making the output signal readout period of the shaded pixel longer than the output signal readout period of the valid pixel. This is effectively equivalent to increasing the number of light-shielding pixels. As a result, even if the number of light-shielded pixels is reduced, the function is equivalent to that of a large number of light-shielded pixels.The number of light-shielded pixels can be reduced and the outer size of the contact image sensor can be made smaller.

[Example 1] Next, the present invention will be explained with reference to the drawings.

FIG. 1 is a structural diagram of a contact type image sensor used in a first embodiment of the present invention, and FIG. 2 is a time chart showing its operation. In FIG. 1, among the pixels 18 to 22 provided on the substrate 1, only the pixel 18 is a light-shielding pixel that is shielded from incident light 3 by the light-shielding layer 2, and the others are effective pixels. FIG. 2(a) shows a successive period signal. Periods 38 to 42 in this successive period signal 30 are output signal successive periods of the pixels 18 to 22, respectively, and an output signal readout period 38 of the shaded pixel 18 is an effective pixel period. Output signal read period 39 from 19 to 22
It is eight times longer than ~42. This lick Figure 2 (
A clamp pulse (not shown in b) for stably fixing the DC level is applied during the period corresponding to the shaded pixel of the sensor output signal.
The pulse width can be made longer than that shown in FIG. 2(C)).

As a result, in this embodiment, even one shaded pixel is effectively the same as the presence of eight shaded pixels.

[Embodiment 2] FIG. 3 is a structural diagram of a contact type image sensor used in a second embodiment of the present invention, and FIG. 4 is a time chart showing its operation. Pixel 5 provided on substrate 7 in FIG.
Among pixels 7 to 62, pixels 57 and 58 are light-shielded pixels that are shielded from incident light 3 by light-shielding layer 6, and the others are effective pixels. FIG. 4(a) shows a readout period signal, and periods 77 to 82 in this readout period signal are output signal readout periods for pixels 57 to 62, respectively, and output signal readout periods 77 and 78 for light-shielded pixels 57 and 58. is four times longer than the continuous output signal period 79 to 82 of the effective pixels 59 to 62, and is shown in FIG.
) is the period corresponding to the shaded pixel of the sensor output signal,
The pulse width can be made longer than the pulse width of the clamp pulse (FIG. 4(C)) for stably fixing the DC level.

〔Effect of the invention〕

As explained above, the present invention stably fixes the DC level of the sensor output signal and reduces the number of shaded pixels by making the output signal successive period of the shaded pixels longer than the output signal readout period of the effective pixels. This has the effect of reducing the external size.

[Brief explanation of the drawing]

1 and 3 are structural diagrams of the contact type image sensors used in the first and second embodiments of the present invention, FIGS. 2 and 4 are their time charts, and FIG. 5 is the conventional contact type image sensor. Figure 6 is a structural diagram of the shape image senna and its time chart. 18.57.58...shading pixel, 19,20°21.
22, 59, 60, 61, 62. ...Effective pixels, 38
, 77.78...output signal readout period of light-shielded pixel, 39
, 40, 41.42, 79, 80° 81.82...Output signal readout period of effective pixels.

Claims (1)

[Claims] A readout signal is applied to each pixel of a contact image sensor that has a plurality of effective pixels on which light enters and a light-shielded pixel that is shielded from light on a substrate, and a signal is read from each pixel during a readout period assigned to each pixel. In a method of driving a contact type image sensor, the output level of the light-shielded pixel is fixed to a constant value by applying a clamp pulse during the reading period of the light-shielded pixel output signal.
1. A method of driving a contact image sensor, characterized in that one output signal readout period is at least twice the output signal readout period of one effective pixel.