JPH0662246A - Correcting method for dark time signal for image reader - Google Patents

Abstract

PURPOSE:To accurately correct a dark time signal at an image reader. CONSTITUTION:The dark time signal from an array-shaped photosensor 11 is inputted to one terminal of a subtraction circuit 12, the other terminal of the subtraction circuit 12 is held at a fixed value, and a first correction value is stored corresponding to a signal outputted from the subtraction circuit 12 at that time. Next, the dark time signal from the photo-sensor 11 is inputted to one terminal of the subtraction circuit 12, the first correction value is inputted to the other terminal of the subtraction circuit 12, and error between a value corresponding to the signal outputted from the subtraction circuit 12 at that time and the first correction value is calculated. When the error is not settled within a previously decided range, a second correction value prepared based on the error is stored.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a dark signal correction method in an image reading apparatus.

[0002]

2. Description of the Related Art When a document surface is read by an optical sensor having a plurality of cells such as CCDs arranged in an array, an offset (dark signal) is produced from the output of the optical sensor even in the dark. Varies from cell to cell. Therefore, the offset value is stored in advance for each cell, and the dark correction is performed by subtracting the offset value when reading the document surface.

[0003]

However, in the conventional correction method, since the correction data is created only once, there are problems that the correction error becomes large and the offset error of the electric system cannot be removed. there were.

It is an object of the present invention to provide a method capable of accurately correcting a dark signal in an image reading device.

[0005]

According to the method of correcting a dark signal of an image reading apparatus in the present invention, a dark signal from an array of photosensors is input to one terminal of a subtraction circuit and the other of the subtraction circuits is input. Of the first correction value corresponding to the signal output from the subtraction circuit at that time, and the dark signal from the photosensor to one terminal of the subtraction circuit. A step of inputting the first correction value to the other terminal of the subtraction circuit and at the same time obtaining an error between the value corresponding to the signal output from the subtraction circuit and the first correction value; Storing a second correction value created based on the error when the error is not within a predetermined range.

[0006]

1 is a block diagram showing an embodiment, and FIG. 2 is a flow chart showing the operation of FIG.

The optical sensor 11 comprises a plurality of cells arranged in an array and is composed of a linear sensor such as a CCD. The subtraction circuit 12 inputs the photoelectric conversion signal to its plus input, and inputs the correction data output from a D / A conversion circuit 17 described later to its minus input to the gain amplifier 18
And input through the switch 19. A /
The D conversion circuit 13 is for A / D converting the output from the subtraction circuit 12. The line buffer memory 14 is A /
The output from the D conversion circuit 13 is temporarily stored. The CPU 15 performs a comparison operation between the data from the line buffer memory 14 and the data from the RAM 16. The RAM 16 stores the correction data from the CPU 15 and the like. The D / A conversion circuit 17 D / A converts the correction data and sends it to the gain amplifier 18. The switch 19 inputs either the output of the gain amplifier 18 or the ground terminal to the subtraction circuit.

Next, the operation of FIG. 1 will be described with reference to the flow chart of FIG.
It will be described with reference to FIG.

With the switch 19 set to the ground side (a), the photosensor 11 is set to the dark state and the dark signal is input to the subtraction circuit 12. The signal from the subtraction circuit 12 is input to the line buffer memory 14 through the A / D conversion circuit 13 (b). The data of the line buffer memory 14 is stored in the RAM 16 as an initial correction value (first correction value) (c). That is, the data corresponding to the dark signal for each cell of the optical sensor 11 is stored in the RM 16.

Next, the switch 19 is set to the gain amplifier 18
Side (i.e., the RAM 16 side) (d), the dark signal from the optical sensor 11 is input to the subtraction circuit 12 again (e). The signal from the subtraction circuit 12 is the A / D conversion circuit 13
Input to the line buffer memory 14 through the CPU
The value 15 is compared with the correction value in the RAM 16, that is, the initial correction value, and the error between them is obtained (f). When the error is smaller than the predetermined constant value (g), the initial correction value is set as the final correction value, and the correction value creation operation is ended.

If the error is larger than a certain value (g), a new correction value (second correction value) is created instead of the initial correction value (first correction value) (h). Usually, half of the error is added to or subtracted from the initial correction value to create a new correction value. Then, the new correction value is stored in the RAM 16 instead of the initial correction value (i).

When the new correction value is stored in the RAM 16 in this way, the operations d to i are performed again. That is, the error between the data based on the dark signal from the optical sensor 11 and the new correction value (second correction value) is obtained again. Then, when the error is smaller than the above-mentioned fixed value, the second correction value is set as the final correction value, and the correction value creating operation is ended. When the error is larger than the constant value, the new correction value (third correction value)
The correction value) is stored in the RAM 16.

In the same manner, the above operations d to i are performed until the error becomes smaller than a constant value.

[0014]

According to the present invention, since the correction operation is performed until the error becomes smaller than a fixed value, it is possible to accurately correct the dark signal.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.

FIG. 2 is a flow chart showing the operation of FIG.

[Explanation of symbols]

 11 ... Optical sensor 12 ... Subtraction circuit 15 ... CPU 16 ... RAM

Claims (1)

[Claims] 1. A dark signal from an array of photosensors is input to one terminal of a subtraction circuit and the other terminal of the subtraction circuit is held at a constant value, and at that time is output from the subtraction circuit. The process of storing the first correction value corresponding to the signal, and the dark signal from the photosensor is input to one terminal of the subtraction circuit and the first correction value is input to the other terminal of the subtraction circuit. , A step of obtaining an error between the value corresponding to the signal output from the subtraction circuit and the first correction value at that time, and when the error is not within a predetermined range, it is created based on the error. A method for correcting a dark signal of an image reading apparatus, the method including storing a second correction value.