JPH01147957A - Image reader - Google Patents

Abstract

PURPOSE:To improve the read accuracy and to reduce the cost of image reading by varying the comparison reference value based on a dark output characteristic corresponding to the temperature raising characteristic when the light source of a photoelectric conversion means is lighted. CONSTITUTION:The light projected from a light source 1 onto an original 2 is made incident in an image sensor 5 including the image information of an original, photoelectric-converted by the sensor 5 and outputted to an amplifier circuit 6 as an analog data. The output of the circuit 6 is given to a discrimination circuit 8, compared with a comparison reference value SL and outputted as a binary data. The comparison reference value SL is set as a value by dividing a prescribed voltage VC and the output voltage VB of a reference value setting circuit 9 by resistors R3, R4 and the voltage VB is set as a dark output VD corresponding to the temperature raise of the sensor. Thus, the output voltage VB of the circuit 9 is changed corresponding to the dark output characteristic and becomes the earth potential. Thus, the dark current included in the image element data is corrected by including the comparison reference value SL.

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 using a so-called contact type image sensor.

(Conventional technology) Recently, due to its small size and light weight, so-called close-contact type
Image reading devices using sensors are becoming popular. This close-contact image sensor has a photoelectric conversion element such as a CCD formed in the width of the document's reading size, and uses a light emitting element such as an LED as a light source, and directly photoelectrically converts the reflected light from the light source onto the document. It has been introduced into the device. Therefore,
A light source and a photoelectric conversion element are provided close to each other. Therefore, the characteristics of the photoelectric conversion element change upon receiving heat from the light source.

Therefore, in conventional image reading devices, for example, as described in Japanese Patent Application Laid-open No. 193665/1983,
The dark output of each pixel of the photoelectric conversion element (output when the light source is turned off) is stored in memory, and when reading a document, the output of the photoelectric conversion element is corrected for each pixel based on the dark output stored in the memory. ing.

(Problems to be Solved by the Invention) However, in such conventional image reading devices, the dark output of each pixel of the photoelectric conversion element is stored in a memory, and the dark output data stored in this memory is Since the output (light output) of the photoelectric conversion element when reading a document is corrected based on this, a memory is required to store the dark output data for each pixel, which not only increases cost but also requires the light source to be switched off during reading. It was necessary to turn off the light and update the dark output data. In other words, the temperature of the photoelectric conversion element increases due to the heat generated by the light source, but
With this temperature rise, the dark output of the photoelectric conversion element also changes,
If the optical output of the photoelectric conversion element is corrected based only on the dark output before reading starts, the correction will be insufficient. As a result, the accuracy of the image reading device deteriorates.

(Object of the Invention) Therefore, the present invention detects the dark output before the start of reading, determines a correction value for correcting the output of the photoelectric conversion element during reading based on this dark output, and also measures the temperature of the photoelectric conversion element. By changing the correction value based on the change characteristics of the dark output as it increases, the output of the photoelectric conversion element can be adjusted appropriately without installing memory or turning off the light source during reading to redetect the dark output. The purpose is to correct this to reduce costs and improve reading accuracy.

(Structure of the Invention) In order to achieve the above object, the present invention includes a light source that irradiates light onto a document, photoelectric conversion means that photoelectrically converts light reflected from the document, and a light source that is turned off when the document is not being read. A light source driving means turns on during reading, a discrimination means compares the output signal of the photoelectric conversion means with a comparison reference value to discriminate between black and white, and detects a dark output from the photoelectric conversion means when the light source is turned off before reading starts. Reference value setting means that outputs a comparison reference value to the discrimination means based on the dark output, and changes the comparison reference value based on the dark output characteristic corresponding to the temperature rise characteristic when the light source of the photoelectric conversion means is turned on; It is characterized by having the following.

Hereinafter, the present invention will be specifically explained based on examples.

1 to 4 are diagrams showing one embodiment of the present invention.

In FIG. 1, 1 is a light source, and the light source 1 includes, for example, a large number of LEDs (Light Emitting Di).
ode) are arranged in the width direction (main scanning direction) of the original 2.

The light source 1 is blinked by a drive current from a light source drive circuit (light source drive means) 3, and when turned on, projects light onto two surfaces of the document. The light reflected by the two surfaces of the original document 2 contains image information of the original document 2 and has different light intensities, and is introduced into the image sensor (photoelectric conversion element) 5 through the light guide system 4 . The image sensor 5 is a COD (Charge Couple)
ed Device), which is divided into a plurality of photoelectric conversion elements and arranged in the width direction (main scanning direction) of the original 2. Therefore, the image sensor 5
The light reflected from the sensor is divided into multiple pixels and photoelectrically converted.
It is output to the amplifier circuit 6 as serial analog data.

The amplifier circuit 6 includes an operational amplifier (OP) and resistors (R1), (R
2), which amplifies the output signal (analog data) of the image sensor 5 and outputs it to the switch 7. The switch 7 is actuated by a signal from a control circuit (not shown) to selectively connect the amplifier circuit 6 to the discrimination circuit (discrimination means) 8 and the reference value setting circuit (reference value setting means) 9. When the light source drive circuit 3 is turned on, the amplifier circuit 6 is connected to the discrimination circuit 8, and when the light source drive circuit 3 is turned off, the amplifier circuit 6 is connected to the reference value setting circuit 9. The discrimination circuit 8 has a comparator (COM) and resistors R1 and R4, and has a predetermined voltage Vc and an output voltage (Vg) of the reference value setting circuit 9.
) divided by the resistors R1, R, (comparison reference value'
) (SL) and the analog output (VA) of the image sensor 5 input from the amplifier circuit 6 are compared and output as a black and white binary digital signal.

On the other hand, the reference value setting circuit 9 includes a diode (D), a capacitor (C), a resistor (R3), and a buffer amplifier (BA
), and when connected to the amplifier circuit 6 by the switch 7, the image sensor 5 input via the amplifier circuit 6
The minimum value of the dark output VD (analog output of the image sensor 5 when the light source 1 is turned off) for one line is held in the capacitor C. Also, dark output ■ held in capacitor C. The minimum value of is discharged through resistor R6,
The time constant τ is set by the values of the capacitor C and the resistor R5. This discharge time constant τ is set in accordance with the dark output characteristics of the image sensor 5 when the temperature of the image sensor 5 increases due to heat generated by the light source 1.

That is, due to its structure and material characteristics, the image sensor 5 rises from the same temperature as the ambient temperature T0 as the light source 1 generates heat, as shown in FIG. It is saturated at a predetermined temperature T, at which the heat dissipation due to is in equilibrium. The time t to reach this predetermined temperature T is the light source 1,
It is determined by the structure, material, etc. of the contact type image sensor composed of the light guide system 4, the image sensor 5, etc., and exhibits a certain temperature rise characteristic.

Corresponding to this temperature rise characteristic of the image sensor 5, the dark output of the image sensor 5 exhibits a constant characteristic as shown in FIG. 3, and each photoelectric conversion element of the image sensor 5 has a similar dark output characteristic. show.

Therefore, the minimum value, maximum value, and average value of the dark output of each photoelectric conversion element of the image sensor 5 exhibit similar dark output characteristics as shown in FIG.

The discharge time constant τ of the capacitor C is set to a value corresponding to the dark output characteristic of the image sensor 5. Therefore, the reference value setting circuit 9 receives the dark output ■ of the image sensor 5. An output voltage (8) is output that decreases from the minimum value in accordance with the dark output characteristics of the image sensor 5.

Next, the effect will be explained.

-i, carrier density of each photoelectric conversion element of the image sensor 5 using COD increases due to excitation of light energy, and its conductivity changes to perform so-called photoelectric conversion. The conductivity of such an image sensor 5 changes depending on thermal energy, and exhibits temperature-dependent characteristics as shown in FIG. 3. That is, the image sensor 5 outputs an image signal according to the reflected light (light energy) from the original 2, but an unnecessary signal according to the temperature (thermal energy) is superimposed on this image signal. In particular, in so-called contact type or complete contact type image sensors, the light source 3 and the image sensor 5 are disposed extremely close to each other, so that the influence of thermal energy is large (see FIG. 2).

Now, as shown in FIG. 4, each photoelectric conversion element of the image sensor 5 changes the temperature of the image sensor 5 to the ambient temperature TO
(see Figure 2), the dark output ■ is a negative voltage value. , and the temperature of the image sensor 5 is the saturation temperature T1 (second
(see figure), it is assumed that the dark output VID is a positive voltage value.

Also, the ambient temperature T of the image sensor 5. Dark output VO at
The minimum value, maximum value, and average value of D are respectively N V o
o, X V oo, A V on, saturation temperature T1
Let the minimum value, maximum value, and average value of the dark output VID at the time be NVID, XvlD, and AVltl. moreover,
Now, the dark output VI at the saturation temperature T1 of the image sensor 5
The minimum value NVIEI of D is OV(
ground potential).

To use this image reading apparatus under such conditions, first, before reading the original 2, the switch 7 is switched to connect the amplifier circuit 6 to the reference value setting circuit 9. The dark output VD of the image sensor 5 at this time is amplified by the amplifier circuit 6 and held in the capacitor C of the reference value setting circuit 9. This dark output■. is the ambient temperature T. Dark output VOfl at
, and the dark output of all elements of the image sensor 5 is ■. . The minimum value N■. . It is. Also, the minimum value NV of this dark output v0ゎ. , increases during reading according to the characteristic curve of FIG.

Dark output ■. When the holding of 0 is completed, the switch 7 is switched to connect the amplifier circuit 6 to the discrimination circuit 8, and the light source drive circuit 3 is driven to turn on the light source 1.

Light projected onto the original 2 from the light source 1 is incident on the image sensor 5 including image information of the original 2, photoelectrically converted by the image sensor 5, and outputted to the amplifier circuit 6 as analog data for each pixel. The data is amplified by the amplifier circuit 6 and input to the discrimination circuit 8, where the serial analog data for each pixel from the image sensor 5 is compared with the comparison reference value SL and output as binary data. That is, analog data (pixel data) from the image sensor 5 is compared with a comparison reference value SL to determine whether it is black or white, and output as black and white binary data. As described above, the analog data from the image sensor 5 includes dark output due to thermal energy. However, the comparison reference value SL is set as a value obtained by dividing the predetermined voltage (2) and the output voltage (2)8 of the reference value setting circuit 9, and the output voltage V of the reference value setting circuit 9.

is set as the dark output V corresponding to the temperature rise of the image sensor 5. That is, the capacitor C of the reference value setting circuit 9 has the dark output ■ shown in FIG. , the minimum value N
V. . Or dark output ■ corresponding to the temperature of the image sensor 5 at that time. The minimum value NV, is held, and then, when the switching device 7 is switched and reading of the image information of the original 2 is started, the resistance is increased at a time corresponding to the minimum value NVD of the hold potential, that is, the dark output ■. Discharge via R3. This discharge time constant τ is set corresponding to the dark output characteristic shown in FIG. Therefore, the reference value setting circuit 9
The output voltage V! l changes in accordance with the dark output characteristics and becomes the ground potential. As a result, the output voltage VB and the predetermined voltage ■.

The comparison reference value SL obtained by dividing the difference between the two becomes higher in accordance with the dark output characteristic. Therefore, analog data (pixel data) output from the image sensor 5 changes depending on the temperature.
The dark output VD contained therein can be corrected by being included in the comparison reference value SL, and black and white of pixel data can be accurately determined without being affected by the dark output (2). As a result, the reading accuracy of the image reading device can be further improved.

In the above embodiment, the minimum value NvID of the dark output VID at the saturation temperature T of the image sensor 5 is set to the ground potential, but the invention is not limited to this. The minimum value NV of the dark output V1ゎ,
, and connect the final output potential of the reference value setting circuit 9 to its dark output ■.

It is sufficient to match the minimum value NV, .

Also, the ambient temperature T of the image sensor 5. Dark output ■.

Although ゎ is set to a negative potential, the dark output V of this image sensor 5 is
Since D changes depending on the material, the image sensor 5
dark output V. When the minimum value N V o n of Il is a positive potential or a negative potential close to ground potential, the reference value setting circuit outputs a dark signal. By holding the maximum value or average value of , an appropriate comparison reference value SL can be set.

For example, to hold the maximum value of dark output ■9,
As shown in FIG. 5, the reference value setting circuit 11 is composed of a diode D+, a capacitor CI, resistors R11, RI2, a buffer amplifier BA, and switches Sw, SW2. open, and close switch SW2 to output dark output v to capacitor C2.
Hold the maximum values XV and D of llo. When reading an image,
When switch SW2 is closed and switch SWl is opened, the charge on capacitor C2 is increased by time constant τI (τ1 is C2・R
11 and set according to the dark output characteristics shown in FIG. 3). Therefore, the dark output V at the ambient temperature T as the power supply 4 on the resistor RI2 side. , the maximum value of
. . Set the power supply V on the capacitor C2 side to a smaller value. When the maximum value XV+O of the dark output v1° at the saturation temperature T1 is adopted as, the output of the reference value setting circuit 11 increases from the hold value to the maximum value . Therefore, the maximum value X■. By correcting the influence of dark output from the pixel data of the image sensor 5 in which , is at a positive potential, it is possible to accurately discriminate between black and white.

Furthermore, when holding the average value of the dark output VD,
As shown in FIG. 6, the reference value setting circuit 12 may be a combination of the reference value setting circuit 9 and the reference value setting circuit 11. That is, the output V of the amplifier circuit 6. Dark output VD
The output of the reference value setting circuit 9 and the reference value setting circuit 11 is held by the reference value setting circuit 9 and the reference value setting circuit 11 depending on the value (positive value, negative value, etc.), and the output of the reference value setting circuit 9 and the reference value setting circuit 11 is connected to the resistor R8. Divide and output with R2□. Therefore, the dark output of the image sensor 5 is ■. , it is possible to hold and output the average value of
Dark output correction can be performed with higher precision.

(Effects) According to the present invention, it is possible to appropriately correct the output of the photoelectric conversion element in accordance with the dark output characteristics without providing a memory or turning off the light source during reading to redetect the dark output. This makes it possible to reduce costs and improve reading accuracy.

[Brief explanation of the drawing]

1 to 4 are diagrams showing an embodiment of the image reading device of the present invention, and FIG. 1 is a circuit diagram of the image reading device, and FIG.
Figure 3 shows the temperature characteristics of the image sensor of the image reading device, Figure 3 shows the dark output characteristics of the image sensor of the image reading device, and Figure 4 shows the temperature characteristics of the image sensor of the image reading device. FIG. 4 is a diagram showing dark output at ambient temperature and at saturation temperature. FIG. 5 is a reference value setting circuit diagram in another embodiment of the image reading apparatus of the present invention. FIG. 6 shows a reference value setting circuit in still another embodiment of the image reading apparatus of the present invention. 1...Light source, 2...Original f height, 3...Light source drive circuit (light source drive means), 4...
...Light guiding system, 5... Image sensor (photoelectric conversion element), 8.
...Discrimination circuit (discrimination means), 9.11.12...
...Reference value setting circuit (reference value setting means).

Claims (4)

[Claims] (1) A light source that irradiates light onto a document, a photoelectric conversion device that photoelectrically converts light reflected from the document, a light source driving device that turns off the light source when the document is not being read and turns it on when the document is being read, and a photoelectric conversion device that Discrimination means for determining black and white by comparing the output signal with a comparison reference value, detecting a dark output from the photoelectric conversion means when the light source is turned off before starting reading, and outputting a comparison reference value to the discrimination means based on the dark output. and a reference value setting means for changing the comparison reference value based on a dark output characteristic corresponding to a temperature rise characteristic when the light source is turned on of the photoelectric conversion means. (2) The image reading device according to claim 1, wherein the reference value setting means detects a minimum value of the dark output of the photoelectric conversion means. (3) The image reading device according to claim 1, wherein the reference value setting means detects a maximum value of the dark output of the photoelectric conversion means. (4) The image reading device according to claim 1, wherein the reference value setting means detects an average value of the dark output of the photoelectric conversion means.