JPH0671309B2 - Image sensor output correction method - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for correcting variations in outputs of light receiving elements of an image sensor having a large number of light receiving elements.

(Prior Art) An image reading apparatus such as a facsimile includes an image sensor that receives light from a document in order to read an image on a recording medium such as a document based on grayscale information of the image. In such an image sensor, when a large amount of light is applied to a large number of light receiving elements included in the image sensor, output signals (photoelectric conversion information) of similar levels can be obtained from the respective light receiving elements. desirable. However, in reality, the output of each light receiving element is not uniform.
Therefore, when such an image sensor is used to perform image processing such as reading even halftone densities, there arises a problem that density reproducibility cannot be obtained. Therefore, various image sensor output correction methods for the purpose of preventing this problem have been conventionally proposed.

An example of this type of conventional method is, for example, JP-A-62-1.
Some are disclosed in Japanese Patent No. 72866.

The method disclosed in this publication irradiates the image sensor with a uniform amount of light corresponding to halftone density, and uses photoelectric conversion output data obtained from each light receiving element of the image sensor as correction reference data, The feature is that the output of the light receiving element at the time of actual reading is corrected by the correction reference data.

Another conventional method for correcting the output of the image sensor is disclosed in Japanese Patent Publication No. 61-14702.

The method disclosed in this publication uses an image sensor,
Output Did of each pixel (synonymous with light receiving element) in the dark, and each pixel output Diw when a constant amount of light is emitted (light)
The pixel output Di obtained according to the shading of the image projected on the image sensor is corrected by the following formula, and the correction value Dix is used as the output signal. However, A is a constant.

Dix = A (Di-Did) / (Diw-Did) ... The method disclosed in Japanese Patent Publication No. 61-14702 will be further described with reference to FIGS.

FIG. 4 shows two picture elements (picture element 1) on the image sensor.
And picture element 2. 3] is a diagram showing the relationship between the light amount L irradiated to these picture elements and the voltage Di output from these picture elements. Light amount L is 0% in the dark and in the bright
It is 100%. The output voltage D ₁ of the picture element 1 changes as shown by 11 in the figure and the output voltage D ₂ of the picture element 2 changes as shown by 13 with respect to the change of the light amount L from dark to bright. Is expressed by an equation, the relationship is D ₁ = (D ₁ w−D ₁ d) L + D ₁ d D ₂ = (D ₂ w−D ₂ d) L + D ₂ d. In the method disclosed in Japanese Examined Patent Publication No. 61-14702, in order to obtain the output of A at the time of light in each picture element having such characteristics, D ₁
Or, for D _2, the calculation of A (D _1- D ₁ d) / (D ₁ wD ₁ d) A (D _2- D ₂ d) / (D ₂ wD ₂ d) is performed and the same light amount is calculated. The outputs of picture elements 1 and 2 are corrected to be equal. Then, the outputs Did and Diw at the time of darkness and at the time of bright time are obtained in advance for each picture element, and the above-mentioned calculation is performed, so that the corrected light quantity-output voltage characteristics as shown in FIG. Was getting

Further, the method disclosed in the above Japanese Patent Publication No. 61-14702,
It was carried out by an apparatus as described below. Sixth
The figure is a block diagram schematically showing the apparatus disclosed in this publication.

An output signal from the image sensor is input to the input terminal indicated by 21. When the output signal of the image sensor when the image sensor is dark, for example, when the output signal when the light source is off, is input, the analog switch indicated by 22 is turned on, and
The analog switch shown at 23 is turned off. Then, the output signal in the dark is converted into a digital signal by the A / D converter indicated by 24, and then stored in the RAM indicated by 25 as Did. Further, the signal called Did is converted into an analog signal by the D / A converter indicated by 27, if necessary, and then output to the subtractor indicated by 29. On the other hand, when the output signal of the image sensor when the image sensor is bright, for example, when the white original is illuminated with a predetermined amount of light, is input to the input terminal 21, the analog switch 22 is turned off. , And the analog switch indicated by 23 is turned on, and this signal is directly input to the subtractor 29. Then, the subtractor 29 subtracts the dark output signal stored in the RAM 25 from the output signal in the bright state (Diw-Did) and outputs it to the A / D converter 24. A / D
This converted signal is stored in RAM 26. Further, this signal is converted into an analog signal by a D / A converter indicated by 28, if necessary, and then output to a divider indicated by 30.

In the above-mentioned apparatus, both analog switches 22 and 23 are turned off when reading a document. Therefore, the output signal Di for each picture element from the document is subtracted from the subtractor 29 via the input terminal 21.
Input to the positive terminal of. Since Did corresponding to Di is input to the minus terminal of the subtractor 29, as a result, (Di-Did) is output to the output terminal of the subtractor 29. At this time, (Di-Did) is input to one terminal Y of the divider 30, and (Diw-Did) is input to the other terminal x of this divider 30, so If the divider 30 has a constant A, Dix = A (Di-Did) / (Diw-Did) ... is executed, and the correction output signal Dix is output from the output terminal indicated by 31. Will be done.

(Problems to be Solved by the Invention) However, the above-described conventional correction method does not perform highly accurate correction unless the output voltage with respect to the irradiation light amount of each light receiving element of the image sensor changes linearly. It is impossible.

Explaining with a specific example, for example, an image sensor of a contact type that reads a document at the same magnification, and an image sensor having a large number of light receiving elements like an image sensor for reading a wide document ( For the sake of explanation, in an image sensor having a structure in which a plurality of individual image sensors are arranged (hereinafter referred to as a multi-image sensor), the γ characteristics of the individual image sensors are different from each other, and they are attached to the individual image sensors. A light receiving element in which the output voltage with respect to the light amount changes non-linearly is likely to appear due to the influence of the characteristic variation among the control circuits. Therefore, it becomes difficult to perform highly accurate correction. This will be further described with reference to FIG.

FIG. 3 focuses on the average light receiving element (picture element) of each of the two individual image sensors having different r characteristics in the above-mentioned multi-image sensor, and outputs signals from these light receiving elements. This is a plot of the corrected output signal D with respect to the light amount L, which is corrected by the method disclosed in Japanese Patent No. 61-14702. In FIG. 3, the curve indicated by 33 is the LD characteristic of one light receiving element, and the curve indicated by 35 is the LD characteristic of the other light receiving element. As is clear from this figure, the corrected output signal and the light intensity at 0% light intensity (dark)
Regarding the correction output signal at 100% (bright), both individual image sensors make the desired correction regardless of the difference in γ characteristics, but the correction in the halftone region is not good, and in this region Variation in correction output signal (37 in the figure)
) Is determined by the difference in the γ characteristics of both individual image sensors.

The present invention has been made in view of such points,
Therefore, an object of the present invention is to consider that an image sensor having a large number of light receiving elements, particularly, a light receiving element unit or an individual image sensor unit, a photoelectric conversion signal corresponding to these light amounts changes non-linearly. An object of the present invention is to provide a method capable of accurately correcting the output signal of the image sensor.

(Means for Solving the Problem) In order to achieve this object, according to the present invention, when correcting variations in the output of each light receiving element of an image sensor having a large number of light receiving elements The output signal Dia when receiving a slightly darker first predetermined amount of light and the output signal Dib when receiving a second brighter amount of light that is slightly brighter than dark are obtained in advance, and each light receiving when reading the recording medium is performed. When the output signal from the element is Di, the correction output signal Dic of the signal Di is obtained by the following equation (1) (where K is a constant).

Dic = K (Di-Dib) / (Dia-Dib) (1) (Operation) According to the output correction method of the image sensor of the present invention, conventionally, the output correction is performed when the light amount is bright and dark. Instead of obtaining the reference signals, the corrected reference signals of Dia and Dib are obtained when the light amount is on the halftone side from bright and dark, respectively. Therefore, although the correction value of the output signal with respect to the light amount of the light receiving element near the light and dark is slightly different for each light receiving element, the variation of the correction output signal in the halftone region is smaller than that of the conventional one. Therefore, it becomes possible to effectively correct the output signal variation for each light receiving element in the halftone region, which is a particular problem.

(Embodiment) A CCD (Charge Couppled Devise) type contact image sensor, which is a multi-image having a plurality of image sensors (individual image sensors) having a large number of light receiving elements, since a read document is wide An embodiment of the correction method of the present invention will be described by taking as an example the case where the correction method of the present invention is applied to a multi-image sensor which is a sensor and has two individual image sensors having different γ characteristics from each other. To do.

First, in order to facilitate understanding, the method of the present invention is not applied to each individual image sensor as a whole, but an average light receiving element in each of two individual image sensors having different γ characteristics is used. Focusing attention, that is, focusing on the two light-receiving elements, one and the other, described with reference to FIG. 3, the correction method of the present invention will be described.

Output signals D ₁ a, D ₂ a when both light receiving elements receive a first predetermined amount of light that is slightly darker than bright, and an output signal when a second predetermined amount of light that is slightly brighter than dark is received D ₁ b and D ₂ b are obtained in advance. The term "light time" in this embodiment means the amount of light incident on the light receiving element from the paper when the light from the light source provided in the reading device is applied to the white paper at the light amount for reading the original. I am trying. Then, the first predetermined light quantity slightly darker than the light time means a light quantity of a predetermined ratio (for example, 90%) to the light quantity at the light time, and the second predetermined light quantity slightly brighter than the dark time means the first light quantity. The light intensity is considerably darker than the predetermined light intensity, and it is a predetermined ratio (for example, 10
%) Of light. It should be noted that the first predetermined light amount may be actually obtained by using a gray paper close to white instead of the above-mentioned white paper, or by using white paper but slightly weakening the light amount of the light source. The second predetermined light amount is
It may be obtained by using gray paper close to black instead of the above white paper, or by using white paper but considerably weakening the light amount of the light source.

Obtained D ₁ a, D ₂ a, and D ₁ b, using D ₂ b, of the output signals D ₁ and D ₂ with respect to the change in the amount of light emitted to each of the light receiving elements,
The corrected output signals D ₁ c and D ₂ c are obtained by the following equation (1). However, K is a constant, for example, a constant that can obtain a corrected output signal when the first predetermined light amount is K. Also, i is 1, 2 in this case.

Dic = K (Di-Dib) / (Dia-Dib) (1) FIG. 1 shows the corrected output signals D ₁ c and D obtained according to the equation (1).
2C is a characteristic curve diagram in which ₂ c is plotted with respect to the light amount L. FIG. In the figure, the curve indicated by 33a is the L of one light receiving element.
-D characteristic, and the curve indicated by 35a is the L of the other light receiving element.
-D characteristic. 33a and 35a are 33 and 3 shown in FIG.
Corresponds to curve 5. Further, in the figure, La represents the above-mentioned first predetermined light amount, and Lb represents the second predetermined light amount. As is clear from FIG. 1, according to the correction method of the present invention, the variation 37a of the correction output signal in the halftone region is smaller than that of the conventional one shown by 37 in FIG.

Next, in the case where the correction method of the present invention is applied to each light receiving element of two individual image sensors having different r characteristics in the multi-image sensor, a correction device suitable for implementing the present invention is used. An example will be described below.
FIG. 2 is a block diagram schematically showing the correction device.

Reference numeral 41 denotes a terminal to which an output signal from each light receiving element of the image sensor is input. In the case of this embodiment, the signal input to the input terminal 41 is obtained by multiplexing the outputs of the two individual image sensors. However, the signals of the two image sensors are time-divisionally processed, for example, so that they can be distinguished from each other.

Reference numeral 43 denotes an A / D converter for A / D converting the output signal of the image sensor. 45 is the output signal Di of each light receiving element of each image sensor when receiving a second predetermined amount of light which is slightly brighter than in dark
Indicates a ROM (Read Only Memory) in which b is stored. 47
Is the output signal of the A / D converter 43 and the Dib stored in the ROM45.
The arithmetic unit which performs addition and subtraction between and is shown. 49 is the output signal Dia when receiving a first predetermined amount of light that is slightly darker than when it is bright
And the ROM that stores the difference (Dia-Dib) from the above-mentioned Dib
Indicates.

Reference numeral 51 indicates a look-up table in which a table of the correction output signal Dic is stored in advance, which is composed of a ROM (may be abbreviated as a table ROM 51). Correction output signal D stored in this table ROM51
The ic is configured to be accessed by the signal output from the arithmetic unit 47 and (Dia-Dib) output from the ROM 49. Since the table ROM is used in this manner, the following consideration is given in this embodiment.

In the case of the present invention, since Dib is a value obtained from the light amount on the halftone side which is slightly brighter than in the dark,
When the operation (Di-Dib) is performed with Di, the operation result becomes a negative value. Depending on the design of the device, it may be a negative value, but since the table ROM 51 is accessed by (Di-Dib) in this embodiment, it is inconvenient if a negative value occurs, The arithmetic unit 47 is supposed to perform an operation of (Di-Dib + B). Here, B is a constant, and B in the case of this embodiment is the maximum value in Dib calculated for each light receiving element. When such a constant B is used, the corrected output signal Di is calculated by the following equation (2) which is a modification of the above equation (1).
You will be asked for c.

Dic = Kk (Di-Dib + B) / (Dia-Dib) (2) The correction output signal Dic obtained by accessing the table ROM51 with (Di-Dib + B) and (Dia-Dib) is the output indicated by 53. It is output from the terminal to the circuit in the subsequent stage.

Moreover, Dib and (Dia-Dib) are stored in advance in the ROM 45 and the ROM 49 by a method described below.

The second predetermined light amount indicated by Lb in FIG. 1 is set by setting the original for each individual image sensor to a gray original close to black or by considerably reducing the light amount of the light source. The output signal of each light receiving element of each individual image sensor at the second predetermined light amount is converted by the A / D converter 43, and the converted value is sequentially stored in the ROM 45 as Dib using the ROM writer. Next, the original for each individual image sensor is set to a gray original close to white, or the light quantity of the light source is slightly decreased, and the −predetermined light quantity indicated by La in FIG. 1 is set. The output signal of each light receiving element of each individual image sensor at the time of the first predetermined light amount is converted by the A / D converter 43 to obtain Dia, but in this embodiment, Dib in the ROM 45 is subtracted from this Dia ( Dia-Dib) is calculated and stored in ROM49. Then, from these ROMs 45 and 49, when the document is actually read, Dib and (Dia-Dib) corresponding to the output signal Di of each light receiving element of each individual image sensor
Are read out respectively.

When a document to be read is set on the reading device having the above-described configuration, the following processing is performed. When scanning a document, the output power of the light source is
%) Light intensity is obtained.

When the image sensor is scanned, the output signal Di from each light receiving element of the image sensor changes according to the density of the original and the A / D converter
Output for 3. The A / D converter 43 is for converting the A / D converted signal D
i is output to the plus terminal of the calculator 47. Also, arithmetic unit 4
Since Dib is input from the ROM 45 to the minus terminal of 7, the calculation unit 47 performs the calculation (Di-Dib + B) as described above.

Next, the computing unit 47 sends (Di-Dib +
B) is output. Also, since (Dia-Dib) is output from ROM49 to this table ROM, the table
The ROM 51 is accessed by (Di-Dib + B) and (Dia-Dib). As a result, the correction value Dic of the output signal Di from the light receiving element is output from the output terminal 53.

The present invention is not limited to the above embodiment.

In the above-described embodiment, an example in which the present invention is applied to a contact type multi-image sensor having a plurality of individual image sensors having different γ characteristics is described, but it is one image sensor and each light receiving element It is obvious that the correction method of the present invention is also effective for an image sensor having a large number of light-receiving elements that change non-linearly with respect to the amount of light each time, but change differently. Further, it is obvious that the present invention can be applied not only to the contact type but also to an image sensor used by forming an image with a condenser lens.

The configuration of the correction device described in the embodiment is merely an example, and it is obvious that various modifications can be made according to the design.

Further, in the above embodiment, the image sensor is described as an example of CCD, but it is clear that the present invention can be applied even if the image sensor is of another type such as MOS type or amorphous Si type. Is.

(Effects of the Invention) As is apparent from the above description, according to the output correction method of the image sensor of the present invention, the reference signal for the output correction at the light amount in the bright time and the dark time is conventionally obtained. However, these reference signals are obtained when the light amount is on the halftone side from the bright and dark states, respectively. Therefore, it is possible to reduce the variation of the correction output signal in the halftone region in which the grayscale is particularly problematic, as compared with the conventional case.

Therefore, even in the image sensor in which the photoelectric conversion signal with respect to the light amount changes non-linearly, the correction can be performed so as to reduce the output variation among the light receiving elements.

[Brief description of drawings]

FIG. 1 is a diagram for explaining the principle of an output correction method for an image sensor according to the present invention, and FIG. 2 is a diagram schematically showing a configuration example of an apparatus suitable for carrying out the method according to the present invention, and FIG. FIG. 4 is a diagram for explaining the conventional technique and the present invention, and FIGS. 4 to 6 are diagrams for explaining the conventional technique. 33a …… L-D characteristic after correction of one light-receiving element 35a …… L-D characteristic after correction of the other light-receiving element 37a …… Correction value variation 41 …… Input terminal, 43 …… A / D converter 45 …… ROM for Dib, 47 …… ROM for calculator 49 …… (Dia-Dib) 51 …… Table ROM for correction value Dic 53 …… Output terminal.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor, Naohiro Watanabe 1-7-12 Toranomon, Minato-ku, Tokyo Oki Electric Industry Co., Ltd. (56) Reference JP-A-62-200872 (JP, A)

Claims (1)

[Claims] 1. When correcting variations in the output of each light-receiving element of an image sensor having a large number of light-receiving elements, an output signal Dia and a signal when each light-receiving element receives a first predetermined amount of light that is slightly darker than when it is bright. When the output signal Dib when receiving a second predetermined amount of light slightly brighter than in the dark is obtained in advance, and when the output signal from each light receiving element when reading the recording medium is Di, the correction output signal of the signal Di An output correction method for an image sensor, wherein Dic is obtained by the following equation (1) (where K is a constant). Dic = K (Di-Dib) / (Dia-Dib) (1)