JPH0473901B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method of driving a contact type image sensor.

[Conventional technology]

In recent years, contact image sensors have been increasingly used in facsimile, OCR, and other applications. In these contact type image sensors,
Since 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, in addition to the effective pixels that form the optical image of the document to be read, there are A pixel is provided,
A method is often used in which the DC level of the effective pixel when there is no incident light is fixed by fixing the DC level of the output signal of these light-shielded pixels to a predetermined level.

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 the pixels 91 to 102 provided on the substrate 9, the pixels 91 to 98 are covered by the light shielding layer 8.
The incident light 3 is blocked by the pixel 99.
.about.102 are effective pixels on which an optical image of the document to be read is formed. FIG. 6a shows a readout period signal, and periods 111 to 122 in this readout period signal indicate output signal readout period signals of pixels 91 to 102, respectively. FIG. 6b shows the read sensor output signal, and FIG. 6c shows the 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 image sensor described above, the output signal readout period of the shaded pixels is the same as the output signal readout period of the valid pixels (see FIG. 6a), so the scanning speed is fast and the output signal of each pixel is As the 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 including a plurality of effective pixels on which light is incident and a light shielded pixel that is shielded on a substrate to the 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, by making the output signal readout period of the shaded pixel longer than the output signal readout period of the effective pixel, the DC level of the sensor output signal is stabilized during the period corresponding to the shaded pixel. This is effectively equivalent to increasing the number of light-shielded 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, so it is possible to reduce the number of light-shielded pixels and reduce the external size of the contact image sensor.

[Example 1] Next, the present invention will be described 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, pixel 1
Only 8 is a light-shielding pixel that is shielded from the incident light 3 by the light-shielding layer 2, and the others are effective pixels. FIG. 2a shows a readout period signal. Periods 38 to 42 in this readout period signal 30 are output signal readout periods of the pixels 18 to 22, respectively, and an output signal readout period 38 of the shaded pixel 18 is the effective pixel 19 to 42. This period is eight times longer than the output signal readout periods 39 to 42 of No. 22. Therefore, the period of the sensor output signal shown in FIG. 2b corresponding to the shaded pixels can be made longer than the pulse width of the clamp pulse (FIG. 2c) for stably fixing the DC level. 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. In FIG.
Among the pixels 57 to 62 provided above, pixel 57
and 58 are light-shielding pixels that are shielded from the incident light 3 by the light-shielding layer 6, and the others are effective pixels. FIG. 4a shows a readout period signal, periods 77 to 82 in this readout period signal are output signal readout periods of pixels 57 to 62, respectively, and output signal readout periods 77 and 78 of light-shielded pixels 57 and 58 are valid. Pixel 59 ~
A clamp pulse (FIG. 4 c) for stably fixing the DC level during a period that is four times longer than the output signal readout period 79 to 82 of No. 62 and corresponds to the shaded pixels of the sensor output signal shown in FIG. 4 b. can be longer than the pulse width of

〔Effect of the invention〕

As explained above, the present invention stably fixes the DC level of the sensor output signal by making the output signal readout period of the shaded pixels longer than the output signal readout period of the effective pixels, and the number of shaded pixels. There is an effect that the outer size can be made smaller by reducing the number of parts.

[Brief explanation of the drawing]

1 and 3 are structural diagrams of contact type image sensors used in the first and second embodiments of the present invention, FIGS. 2 and 4 are time charts, and FIG. 5 is a conventional contact type image sensor. Figure 6 is a structural diagram of the image sensor and its time chart. 18, 57, 58...shading pixel, 19, 20,
21, 22, 59, 60, 61, 62... Effective pixel, 38, 77, 78... Output signal read period of light-shielded pixel, 39, 40, 41, 42, 79, 80,
81, 82...Output signal readout period of effective pixels.

Claims (1)

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