JPH01114162A - Picture reader - Google Patents

Abstract

PURPOSE:To attain the correction of a dark voltage with high quality by amplifying a dark voltage to a prescribed level, digitizing the result and applying level conversion while the signal is converted into an analog signal by a D/A converter. CONSTITUTION:In shutting the light of a light source, a dark voltage is generated. Since an inverting input of an analog subtractor 2 connects to GND, the dark voltage output of a photoelectric converter is amplified without any correction by an amplifier 4 and the result is stored in a dark voltage storage circuit 6. In entering the operation to read the original, the original picture signal from the conversion section 1 is fed to a noninverting input of the subtractor 2. The digital dark voltage signal read from the circuit in this case is subjected to D/A conversion 7, then the attenuator 8 restores the amplified (4) dark voltage to the original level. The output of the subtractor 8 is fed to the inverting input of the subtractor 2 and the dark voltage is subtracted from the original picture signal by the subtractor 2. The picture signal whose dark voltage is corrected becomes a density proportion signal by a log converter 3, after A/D conversion 4 and the result is outputted via a selector 9 from a output terminal 13.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an image reading device, and particularly to an image reading device that reads halftone images.

[Conventional technology]

Conventionally, a first example of this type of image reading device is the one shown in FIG. 2 with the memory circuit 18 removed, and includes a photoelectric conversion section 14,
A/D converter 15, dark voltage storage circuit 16, and subtracter 1
7 and a control circuit 19.

Then, the document reflectance or signal from the photoelectric converter 14 is A
The /D converter 15 converts the signal into a digital image signal. Additionally, photoelectric conversion elements such as COD (charge-coupled device) image sensors and photomultiplier tubes are often used with DC coupling, and in this case, the DC level fluctuates due to temperature fluctuations, and this dark voltage fluctuation is reflected in the image. To prevent this, a shutter is placed in the middle of the optical path as a pre-operation before reading the document.
The dark voltage at that time is detected and written to the dark voltage storage circuit 16 according to the control signal from the control circuit 19. Next, the shutter is opened to read the actual document, and the dark voltage is written in the dark voltage storage circuit 16 according to the control signal from the control circuit 19 from the dark voltage storage circuit 16. The dark voltage is read out and subtracted using a digital subtracter 17.

However, in halftone image reading devices, not only the method proportional to the reflectance of the original as described above (the transmittance is also the same, so from now on we will explain the method proportional to the reflectance), but also the method proportional to the original density R = 10 -”. Figure 4 shows the conversion characteristics of the density proportional method and the reflectance (transmittance) proportional method. The graph of the density proportional method is as follows.
For convenience, the conversion output is set to 0 at a density of 2.4. When this conversion output is digitally decomposed, in the case of the reflectance proportional method, the amount of density information included in the rubel increases in the black image density region, and the black image density resolution decreases.

This problem can be solved by adopting a concentration proportional method.

For this reason, as shown in FIG.
There was an example of However, in this second example, if the concentration resolution of the output terminal 20 is to be made equal to the method in which the concentration proportional method signal is directly decomposed into bits, the A/D converter 15
The disadvantage is that the number of bits required for this is extremely large, making it impractical.

As a countermeasure against this, as shown in FIG.
An analog log converter 21 is connected between the A/D converter 15 and the A/D converter 15.
There was a third example of connecting .

[Problem that the invention seeks to solve]

However, the dark voltage correction in this third example is
This cannot be achieved with a simple subtraction circuit as shown in Figure 2, and when considering the digital signal after log conversion, subtraction of the original signal has the drawback that the circuit is extremely complex and correction is difficult. .

The purpose of the present invention is to solve the above-mentioned drawbacks, namely, when adopting the concentration proportional method, if log conversion is performed at the output end, the number of bits of the previous digital circuit becomes extremely large. An object of the present invention is to provide an image reading device that solves the problem that dark voltage cannot be easily corrected when log conversion is performed.

[Means for solving problems]

In order to solve the above-mentioned problems, the present invention includes a photoelectric conversion section that outputs an analog document reading image signal proportional to the document reflectance or document transmittance, this optical converter, and this A/D converter. A selector connected to the subsequent stage, a dark voltage storage circuit that is switched to an output terminal by this selector, a D/A converter that converts the digital output signal of this storage circuit into an analog signal, and a D/A converter that converts the digital output signal of this storage circuit into an analog signal. An attenuator for attenuating an output signal, a subtracter provided before a log (LOG) converter for subtracting the output signal of this attenuator from the original read image signal, and amplifying the output signal of this subtracter. An amplifier, two switch means interposed between the amplifier, the A/D converter, the attenuator, and the subtracter, and a control for controlling these switch means, a selector, and a dark voltage storage circuit. A configuration having a circuit is adopted.

[Effect]

Since the present invention is configured as described above, in the pre-operation,
The optical path entering the photoelectric conversion unit is blocked, the dark voltage is amplified by an amplifier, converted to a digital value by an A/D converter, and stored in a dark voltage storage circuit.Next, the control circuit controls the analog switch and selector. After switching, reading the stored data from the dark voltage storage circuit and converting it to an analog value with the D/A converter, the attenuator attenuates the gain amplified by the previous amplifier back to the original value, and the subtracter converts the gain to the original value. Subtract the dark voltage value from the original image signal.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings.

Referring to FIG. 1, which shows an embodiment of the present invention in a block configuration diagram, the image reading apparatus of the present invention includes a photoelectric conversion section 1, an analog subtracter 2 to which the output of the photoelectric conversion section 1 is applied to a positive input terminal, and an analog subtracter 2 whose output is applied to a positive input terminal. A log (1'og) converter 3 and an amplifier 4 connected to the subtracter 2, an analog switch 1O for switching and connecting these outputs, and an A/D converter 5 connected to the analog switch 10. This A/D converter 5
a selector 12 connected to the output side of
an output terminal 13 connected to one of the output terminals of the selector 12, a dark voltage storage circuit 6 connected to the other output terminal of the selector 12, and a D/A converter 7 that converts the output of the storage circuit 6 into D/A. , an attenuator 8 for attenuating this output, an analog switch 11 that switches the output of the attenuator 8 and the ground potential, and inputs the switched voltage to the negative input terminal of the analog subtracter 2; 10,11
It consists of a selector 12 and a control circuit 9 that controls the dark voltage storage circuit 6.

Next, the operation of this embodiment will be explained using FIG.

First, in FIG. 1, the analog switches 10 and 11 and the selector 12 are shown in a state in which they are operated before reading an original. In this pre-operation, for example, if a shutter or the like is placed in the middle of the optical path to block the light from the light source, a dark voltage is generated. Since the negative input terminal of the analog subtracter 2 is connected to the ground GND, the dark voltage output of the photoelectric converter 1 such as a COD image sensor is directly applied to the amplifier 4 without correction. The amplifier 4 is implemented to digitize the dark voltage signal to approximately the input full-scale allowable voltage of the A/D converter 5. The dark voltage that is accurately converted from analog to digital signal by the A/D converter 5 is stored in the dark voltage storage circuit 6. During the pre-operation, the control signal a of the control circuit 9 performs write control on the dark voltage storage circuit 6.

Next, the operation of reading the original begins. At this time, the control circuit 7
The control signal S operates the analog switches 10, 11 and the selector 12 to change the connection from the state shown in FIG. Further, the control signal a performs read control for the dark voltage storage circuit 6. The original image signal from the photoelectric converter 1 is applied to the positive input terminal of the analog subtracter 2 . At this time, the digital dark voltage signal read from the dark voltage storage circuit 6 is the D/A
After being converted into analog by converter 7, it is applied to attenuator 8. As described above, since the dark voltage has been amplified to a predetermined level by the amplifier 4, the attenuator 8 functions to return the amplified dark voltage to its original level. The output of attenuator 8 is analog spa.

It is applied to the negative input terminal of the '7/analog subtracter 2 via the switch 11. Therefore, the dark voltage is subtracted from the original image signal by the analog subtracter 2. This dark voltage corrected image signal is applied to the log converter 3 to become a density proportional signal, and then converted to a digital signal by the A/D converter 5 and outputted to the output terminal 13 via the selector 12.

Therefore, there is an advantage that the dark voltage can be easily corrected. Further, by performing the pre-operation for measuring the dark voltage for each original document or at regular intervals, it is possible to reliably correct fluctuations in the dark voltage due to temperature changes in the photoelectric conversion element.

〔Effect of the invention〕

As explained above, according to the present invention, an analog subtracter, a log converter, an amplifier, an A/D converter, a dark voltage storage circuit, a D/A converter, an attenuator, By having two analog switches and selectors, and a control circuit, the dark voltage signal read in the previous operation can be subtracted in an analog manner at the front stage of the log converter, making dark voltage correction easy. effective.

In addition, since the dark voltage is amplified to a predetermined level and then digitized, and the level is converted when it becomes an analog signal using a D/A converter, it is possible to accurately digitize the dark voltage, which is a very small voltage. This has the effect of realizing high-quality dark voltage correction.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing an embodiment of the present invention, Figs. 2 and 3 are block diagrams of conventional circuits, and Fig. 4 is a conversion characteristic diagram in the density proportional method and reflectance (transmittance) proportional method. It is. 1...Photoelectric conversion unit, 2...Analog subtracter, 3...LOG converter, 4...Amplifier, 5...A /D converter, 6...Dark voltage storage circuit, 7...D/A converter, 8...
...Attenuator, 9...Control circuit, 10°11.
...Analog switch, 12...Selector, 13...Output terminal, GND...Ground. Child 3T! ! 1 Figure 4

Claims (1)

[Claims] A photoelectric conversion section that outputs an analog document reading image signal proportional to document reflectance or document transmittance, a log converter that log-converts the output of this photoelectric conversion section, and an analog converter that converts this output into a digital image signal. Digital converter (
(hereinafter referred to as an A/D converter), a selector connected to a subsequent stage of the A/D converter;
A dark voltage storage circuit that is switched to an output terminal by this selector, a digital-to-analog converter (hereinafter referred to as a D/A converter) that converts the digital output signal of this storage circuit into an analog signal, and this D/A converter. an attenuator for attenuating the output signal of the attenuator, a subtracter provided before the log converter for subtracting the output signal of the attenuator from the document reading image signal, and a subtracter for amplifying the output signal of the subtracter. an amplifier, two switch means interposed respectively between the amplifier and the A/D converter, the attenuator and the subtracter; and controlling these switch means, the selector and the dark voltage storage circuit. , and a control circuit for correcting dark voltage fluctuations of the photoelectric conversion element.