JPH065882B2 - Image sensor output correction circuit - Google Patents

Description

The present invention relates to an image sensor output correction circuit used in a copying machine, a facsimile machine or the like.

[Prior Art] As is well known, the output voltage of an image sensor used as an image reading unit of a copying machine, a facsimile machine, or the like varies due to variations in the light amount of an illumination light source of an original, changes in ambient temperature, and the like. . Particularly, in a one-dimensional image sensor in which a plurality of sensors are arranged in the main scanning direction of an original, it is considered that the output voltage is uniform due to the variation in the light amount of the illumination light source in the main scanning direction and the difference in the sensor sensitivity for each pixel. Don't

Therefore, conventionally, in order to correct such fluctuations and non-uniformity of the output voltage of the image sensor, the image sensor output signal is AD-converted and then compressed and expanded in the density region according to the difference from the reference voltage. There is something like this.

[Problems to be Solved by the Invention] However, in the above-described conventional technique, the fluctuation width of the dark voltage is relatively large and the fluctuation width of the output voltage is large like the contact type amorphous silicon image sensor. When a sensor is used, there is a problem in that compression and expansion in the density region may be affected by the influence of dark voltage, and multi-tone output cannot be obtained accurately.

The present invention has been made to solve such a problem, and an object thereof is to provide an image sensor output signal capable of obtaining an accurate multi-gradation output even when the dark voltage of the image sensor is large. It is to provide a correction circuit.

[Means for Solving the Problems] In order to solve the above-mentioned problems, the present invention is an amplification means for amplifying an output signal of an image sensor, and an image sensor output signal amplified by changing the gain of the amplification means. To a predetermined level range, a memory means for storing the dark voltage value when the incident light of the image sensor is blocked, and a dark voltage value stored in the memory means according to the gain of the amplifying means. Correction means for correcting and outputting the corrected voltage and a calculating means for subtracting the dark voltage value corrected by the correcting means from the amplified output signal of the image sensor and outputting the subtracted value.

[Operation] The dark voltage value of the image sensor is measured and stored in the storage means before the start of image reading or at regular intervals.

On the other hand, the signal output from the image sensor with the start of image reading is amplified by automatic gain control so that the signal level thereof is always within a predetermined level range. This amplified signal is input to the calculating means, where the dark voltage component is subtracted. As a result, a multi-tone output signal that does not include a dark voltage component can be obtained.

[Examples] Hereinafter, the present invention will be described in detail based on illustrated examples.

FIG. 1 is a block diagram showing an embodiment of the present invention. In the figure, 1 is an AD converter (ADC) for converting an output signal Vs of an image sensor (not shown) into a digital signal X2 of a predetermined number of bits (for example, n bits), and 2 is an ADC.
If the converted value of 1 exceeds the maximum converted value and its overflow output becomes "H", the reference potential of the AD conversion operation of ADC1 is raised, and if it becomes "L", the reference potential is lowered. , An automatic gain control circuit (AGC circuit) 3 for controlling the conversion gain of the ADC1 so that the converted output value of the ADC1 is always in a constant value range, and 3 stores the dark voltage value X ₁ when the incident light on the image sensor is cut off. The first memory (RAM), the dark voltage value X ₁ is measured using an image sensor whose output signal is to be corrected immediately before the start of reading an original image or at regular intervals, and after conversion to a digital signal by the ADC 1. It is stored in the first memory 3. However, when the dark voltage value X ₁ is stored, the AGC circuit 2 does not perform differential operation, and the value X ₁ obtained by AD converting the current dark voltage of the image sensor itself is stored in the first memory 3.

Reference numeral 4 is a signal Y representing the conversion gain of the ADC ₁ output from the AGC circuit 2 for the dark voltage value X ₁ stored in the first memory 3.
A signal correction ROM for outputting corrected by _1, the lower address signals _{X 1,} the memory address is the superior address signals _{Y 1, Y 2 = X 1} · Y 1 ··· (1) The correction value Y ₂ of the signal X ₁ indicated by is stored.

5 is subtracted obtaining a difference signal _{X 3 = X 2 -Y 2 ···} (2) between the signal _{Y 2} outputted from the signal _{X 2} and the signal correcting ROM4 output from ADC1 with the start image reading Reference numeral 6 denotes a second memory (RAM) that stores a bright voltage value Y ₃ when reference light (white light or the like) is incident on the image sensor. The bright voltage value Y ₃ is the same as the dark voltage value X ₁ in the original image. Immediately before the start of reading or at a constant cycle, the output signal is measured by an image sensor as a correction target, converted into a digital signal by the ADC 1, and stored in the second memory 6. However, this bright voltage value Y ₃
The AGC circuit 2 does not operate during the storage of, and the value Y ₃ obtained by AD-converting the current bright voltage of the image sensor is stored.

7 is a signal correction ROM which outputs the correction on the basis of the bright voltage value Y ₃ that the output signal X ₃ stored in the second memory 6 the subtractor 5, the lower address signals X _3, the signals Y ₃ The correction value Z of the signal X ₃ represented by Z = (X ₃ / Y ₃ ) × 2 ⁿ (3) is stored in the memory address as the upper address. In the equation (3), n represents the number of output signal bits of the ADC1.

In the above configuration, the signal Vs output from the image sensor when the reading of the original image is started is converted into the digital signal X ₂ of n-bit configuration in the ADC 1. At this time, the conversion gain of the ADC 1 is controlled by the AGC circuit 2 so that the value of the signal X ₂ always falls within a fixed value range. The reason for controlling the conversion gain of the ADC1 is to improve the conversion accuracy.

On the other hand, when the reading of the original image is started, the signal correction R
The dark voltage value _{X 1} of the image sensor from OM4 ADC1
Signal Y ₂ corrected by the signal Y ₁ representing the conversion gain of
(= X ₁ · Y ₁ ) is output.

That is, the signal Y ₂ equivalent to passing the dark voltage value X ₁ through the converter having the gain Y ₁ is output from the signal correction ROM 4.

These two signals X ₂ and Y ₂ are input to the subtractor 5, and the signal Y ₂ is subtracted from the signal X ₂ . That is, the signal X ₂
The dark voltage component Y ₂ contained in is subtracted. As a result, the subtractor 5 outputs the signal X ₃ that includes only the image signal that does not include the dark voltage component.

Next, the signal X ₃ that does not include the dark voltage component is input to the signal correction ROM 7. At this time, the bright voltage value Y ₃ previously stored in the second memory 6 is also input to the ROM 7.

As a result, the ROM 7 outputs the signal Z represented by Z = (X ₃ / Y ₃ ) × 2 ⁿ .

That is, even if the density of the read image is constant, if the dark voltage X _{1 of the} image sensor increases, the signal X ₂ of the read image also increases as shown in the graph of FIG. However, at this time, the correction signal Y ₂ proportional to the increase of the dark voltage X ₁ is output from the signal correction ROM 4 and the signal X ₂ is output.
Subtracted from ₁ . As a result, as indicated by the broken line in the graph of FIG. 2, a signal X ₃ depending on only the density of the read image is taken out from the subtracter 5 regardless of the dark voltage X ₁ .

The absolute level of the signal X ₃ which depends only on the density of the read image becomes insufficient because the dark voltage component X ₁ is removed. Therefore, the absolute value of this signal X ₃ is amplified as shown in the graph of FIG. 3 by a proportional calculation based on the bright voltage value Y ₃ when the reference light is incident. As a result, the signal X ₃ that does not include the dark voltage component of the image sensor
Is converted into a gradation signal Z based on the brightness of the reference light and is output.

In this case, the dark voltage value X ₁ and the bright voltage value Y ₃ stored in the first memory 3 and the second memory 6 already include the fluctuation component of the optical system such as the fluctuation component of the light amount of the illumination light source. . Therefore, not only the fluctuation of the dark voltage but also the fluctuation of the image sensor output signal due to the fluctuation component of the optical system is corrected at the same time.

Although the output signal of the image sensor is corrected by digital signal processing in this embodiment, it may be configured by analog signal processing. In the case of analog signal processing, the AD converter 1 is composed of a variable gain type amplifier, the first memory 3 and the second memory 6 are composed of analog memories, and the signal correction ROMs 4 and 7 are analog operations. It may be composed of a circuit.

Further, in the above embodiment, it is assumed that the image sensor performs photoelectric conversion of a single pixel for simplification of description. However, like the one-dimensional image sensor, a plurality of image sensors are arranged in the main scanning direction of the document. When an array of sensors is used, the dark voltage value X ₁ and the bright voltage value Y ₃ are stored independently for each pixel sensor unit, and the entire circuit of FIG. 1 is time-divided for each pixel. Is used in.

[Effects of the Invention] As is clear from the above description, in the present invention, the dark voltage of the image sensor is measured in advance before the start of image reading, and the dark voltage is selected from the image sensor output signals after the start of reading. Since the components are subtracted before being output, it is possible to reliably take out a stable multi-tone output signal even when the dark voltage component varies greatly.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, FIG. 2 is a graph showing a relationship between dark voltage and an image sensor output signal, and FIG. 3 is a signal excluding the dark voltage component and a final output signal. It is a graph which shows the relationship with. 1 ... AD converter, 2 ... Automatic gain control circuit, 3 ... First memory, 4, 7 ... Signal correction ROM, 5 ... Subtractor, 6 ... Second
memory.

Claims (4)

[Claims] 1. An amplifying means for amplifying an output signal of an image sensor, an automatic gain control means for changing a gain of the amplifying means to control the amplified image sensor output signal within a predetermined level range, and an incidence of the image sensor. Memory means for storing dark voltage when light is cut off, correction means for correcting and outputting the dark voltage value stored in the memory means according to the gain of the amplifying means, and output signal of the amplified image sensor And an arithmetic means for subtracting and outputting the dark voltage value corrected by the correcting means. 2. The amplification means comprises an AD converter for converting an image sensor output signal into a digital signal of a predetermined number of bits, and the automatic gain control means has a conversion output value of the AD converter of a constant value. The image sensor output correction circuit according to claim (1), characterized in that the circuit is configured to control the reference potential of the AD converter so as to be within the range. 3. An amplifying means for amplifying an output signal of the image sensor, an automatic gain control means for changing a gain of the amplifying means and controlling the amplified image sensor output signal within a predetermined level range, and an incidence of the image sensor. First memory means for storing a dark voltage value when light is cut off, and first correcting means for correcting and outputting the dark voltage value stored in the first memory means according to the gain of the amplifying means. The first signal from the amplified output signal of the image sensor
Calculating means for subtracting and outputting the dark voltage value corrected by the correcting means, second memory means for storing a bright voltage value when the reference light enters the image sensor, and output of the calculating means for the second An image sensor output correction circuit comprising: second correction means for correcting and outputting based on the bright voltage value stored in the memory means. 4. The amplification means comprises an AD converter for converting an image sensor output signal into a digital signal of a predetermined number of bits, and the automatic gain control means has a conversion output value of the AD converter of a constant value. A circuit for controlling the reference potential of the AD converter so as to be within the range, and the second correction means is a division circuit for dividing the output of the calculation means by the bright voltage value and outputting the result. An image sensor output correction circuit according to claim (3).