JP2888627B2 - Image reading device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an image reading device.

[Prior Art] Generally, in an image reading apparatus such as an image scanner, a so-called 1: 1/2 optical system in which an original placed on an original platen glass is scanned by a light source and a mirror and exposed on an image sensor or a photosensitive member. Are often adopted.
As shown in FIG. 7, this 1: 1/2 optical system irradiates a document P on a fixed document table glass 100 with a light source 101 and forms image light from the document P reflected by mirrors 102, 103, and 104. An image is formed on the image sensor 106 and the like by the lens 105. Here, by scanning the mirror unit 108 equipped with the mirrors 103 and 104 at half the scanning speed of the lamp unit 107 equipped with the light source 101 and the mirror 102, the optical path length from the document surface to the imaging lens is reduced. Is always constant. The unit scans in the direction (sub-scanning direction) perpendicular to the longitudinal direction of the image sensor, but the scanning amount for one line at the same size reading is the reading in the longitudinal direction (main scanning direction) of one pixel of the image sensor. It is set to be equal to the width.

[Problem to be Solved by the Invention] The image scanner configured as described above is often used for computer input. by the way,
The read image data is processed into any of binary, halftone (pseudo halftone), and gray scale (multi-value output) and output to the host computer.

In the above-described embodiment, the accuracy of determination is required because the number of black and white divisions of the document increases as the latter increases. On the other hand, in the image scanners having the various processing methods described above, the reading speed of the original is the same regardless of the output mode selected, and even in the case where the accuracy is not so required, the processing time is different from that of other modes. The efficiency (operability) is poor because it is the same as in the case.

Therefore, an object of the present invention is to provide an image reading apparatus which has solved the above problems.

Means for Solving the Problems To achieve the above object, the present invention provides a photoelectric conversion unit for reading an image of a document, and a scanning unit for moving a position of the image read by the photoelectric conversion unit in a sub-scanning direction. A signal processing means for outputting image data read by the photoelectric conversion means at a desired number of gradations, and a read cycle of the photoelectric conversion means in proportion to the number of gradations of the image data output from the signal processing means. And control means for controlling the moving speed of the scanning means.

According to the present invention, the read cycle of the photoelectric conversion unit and the moving speed of the operation unit are controlled in proportion to the number of gradations of the image data output from the signal processing unit, so that the number of gradations of the image data is controlled. The read time is optimized in proportion to
Processing time is reduced.

[Example] Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

First Embodiment FIGS. 2A and 2B schematically show the configuration of an image reading apparatus according to an embodiment of the present invention. In FIG. 1, reference numeral 1 denotes an image sensor such as a CCD as a reading unit, which is provided in the apparatus main body 2. An original placing glass 3 is provided on the upper surface of the apparatus main body. The original P placed on the original placing glass 3 is exposed and scanned by an image scanning means 4 to expose an image on the image sensor 1. The image scanning means 4 includes a lamp unit 5 and a mirror unit 6 that move and scan in parallel with the original placing glass 3, and a lens 7 fixedly placed in the apparatus main body 2. The lamp unit 5 also includes a light source L for illuminating the document P, and a mirror unit 6 that reflects image reflected light from the surface of the document P illuminated by the light source L.
The first mirror M1 reflecting to the side allows the mirror unit 6
Is composed of second and third mirrors M2 and M3 which turn image light 9 reflected by the first mirror M1 toward the image sensor 1. The lamp unit 5 moves and scans the guide rails 8a and 8b at both ends at twice the speed of the mirror unit 6 in order to keep the optical path length constant in the entire image reading area.

Reference numeral 9 denotes a pulse motor which is a driving source for scanning the lamp unit 5 and the mirror unit 6, and its rotation is transmitted to a driving pulley 10 via a gear train. Generally, in this type of image reading apparatus, a pulse motor is often used as a drive source because position control and speed control are easy. The reduction ratio of the gear train and the outer diameter of the driving pulley 10 are supplied to the pulse motor 9 by n pulses (n: integer, n1).
, The image scanning means is set to move by one line at the time of reading at the same size. Thus, the sub-scanning density can be easily controlled by controlling the number of pulses. For example, when performing double-magnification reading, a 2 / n pulse is applied to the motor during the reading cycle of one main scanning line. Conversely, when reducing to 1/2, 2n pulses may be given.

The drive pulley 10 is wound around two wires 11a and 11b joined at one point on the way. One end of each wire is fixed to the lamp unit 5 on the way, and the mirror unit 6
The pulleys 12a and 12b at both ends are fixed to the main body so as to make a half circumference. The other end of the wire is connected to the pulleys 12a and 12b by a tension spring 13 so as to make a half circumference. As a result, the mirror unit 6 scans at half the speed of the lamp unit 5 based on the principle of a moving pulley, so that the optical path length is always kept constant over the entire scanning area.

Usually, the lamp unit 5 and the mirror unit 6 are set at home positions near a document reading start position by a home position sensor (not shown). When a reading start command is received from a host computer (external device), a motor driving pulse synchronized with a reading synchronization signal of the image sensor is given to the motor 9, the motor 9 rotates, and the exposure scanning system starts scanning to read the read image. The data is transmitted to the host computer via the image processing circuit.

By the way, the required image processing method differs depending on the content of the original to be read (characters, line drawings, pictures, photographs, etc.) and the purpose of the data (OCR, image file, etc.). Select the image processing method and set the reading mode of the image reading device.

Image processing of the image reading apparatus includes binary, halftone (pseudo halftone), and gray scale (multi-value output). Binary is a method of dividing a document into black and white with a certain density, and is suitable for reading characters, line drawings, and the like. The halftone is a method (dither method) that expresses an intermediate density in a pseudo manner by allocating black and white in a group of a plurality of pixels (for example, 4 × 4, 8 × 8).

While the above two methods replace the image information of each pixel with either white or black and represent it with one bit, the gray scale expresses the image information of one pixel with a plurality of bits. For example, in the case of a gray scale having 8-bit accuracy, halftones from white to black can be represented by 256 gradations.

By the way, the reading speed of the image reading apparatus depends on the output of the image sensor, that is, the brightness of the light source illuminating the original, the brightness of the lens, the sensitivity and the accumulation time of the image sensor, the dark output of the image sensor and the noise of the signal processing circuit. Depends on the level. Among them, the output of the image sensor and the output during dark are proportional to the length of the accumulation time, and the noise level of the signal processing circuit is peculiar to the circuit. To output the required number of tones, the difference ΔV between the output levels of two consecutive tones must be greater than the sum of the dark output and the noise (see FIG. 3). The accumulation time is t, the output of a white document of the image sensor at the accumulation time t is good _Vout (t), the maximum value of the dark output is _Vdrk (t),
_Assuming that the maximum value of the noise level of the signal processing circuit is V _noise and the number of output gradations is n, if the proportional constant of the dark output of the image sensor is sufficiently smaller than the sensitivity, V _out (t) = n × ΔV> n × (V _drk (t) + V _noise ) (1) It is necessary to determine the accumulation time so that the above equation is satisfied, and this determines the reading speed.

As can be seen from the above equation, the more the number of output gradations, the more
It is necessary to obtain a large image sensor output, and for this purpose, the accumulation time must be lengthened. In the conventional image reading apparatus, the reading speed is determined by the accumulation time when the number of gradations is the maximum in the specification.

From the above equation, it can be seen that the accumulation time may be shorter if the number of read gradations is smaller. Therefore, by setting an accumulation time according to the number of read gray scales and switching the storage time, when the number of gray scales is small, the read time can be significantly reduced as compared with the related art.

FIG. 1 is a processing block diagram of read image data of an image reading apparatus according to the present invention. The analog image signal from the image sensor 1 is amplified by an amplifier 21, then converted into an n-bit digital image signal by an A / D converter 22 and input to an image processing circuit 23. In accordance with an instruction from the CPU 24, the image processing circuit 23 performs processing such as binarizing the digital signal or converting the density gradation.

The CPU 24 receives commands such as start and stop of reading from the host computer 31 through the interface circuit 26, and controls the image reading apparatus according to a predetermined procedure. Also, the image processing circuit 23
It supplies threshold values for shaping and data for shading correction. Further, as a feature of the present invention, the CPU 24 has a role of switching the drive frequency of the image sensor 1 and the pulse motor 9 via the timing generator 29 in accordance with a read mode command from the host computer 31. 27 and 28 are circuits for driving the pulse motor 9 and the image sensor 1 in accordance with control signals from the CPU 24, respectively.

The output from the image processing circuit 23 is
And sent to the host computer 31 via the interface circuit 26 in accordance with a transfer command from the host computer 31.

Normally, when a read command is sent from the host computer 31 to the image reading apparatus, one of binary, halftone and gray scale modes is set, and the information is transmitted to the CPU 24 via the interface circuit 26. Is done. The CPU 24 starts the reading operation after switching the driving frequency of the image sensor 1 and the motor 9 to a value corresponding to the set reading mode.

In an image reading apparatus which needs to recognize halftones (halftones such as gray scale or dither) of a maximum of n bits, that is, ²ⁿ gradations, the accumulation time satisfying the above-mentioned expression (1) is represented by t _n , When the white document reading output of the image sensor at this time is V _out (t _n ), the output level difference between two consecutive gradations is ΔV = V _out (t _n ) / 2 ⁿ (2).

On the other hand, when trying to obtain an output of 2 ^m (m <n) gradation with this image reading device, the required white document reading output is V _out (t _m ) = ΔV × 2 ^m (3) ( From equations (2) and (3), V _out (t _m ) = 2 ^mn × V _out (t _n ) (4)

Since the output of the image sensor is proportional to the accumulation time, 2 ^m
In the case of gradation output, the accumulation time is shorter than in the case of 2 ⁿ gradation output.
The reading speed can be shortened by 2 ^mn , that is, the reading speed can be increased by 2 ^nm . For example, in an image reading apparatus capable of outputting 8 bits, that is, 256 gradations, 256/2 for binary.
= 128 times, 256/16 = 8 for 4 × 4 matrix dither
Reading can be performed at twice the speed.

FIG. 4 is a diagram for explaining processing of the obtained image sensor output. FIG. 6A shows the correspondence between the image sensor output and the number of gradations when a document is read at 2 ⁿ gradations. As shown in this figure, for each value ΔV obtained by dividing the white document output by 2 ⁿ , 0 (black)
To 2 ⁿ -1 (white).

FIG. 3B shows a state when the original is read at 2 ^m gradation. As described above, at this time, the accumulation time is 2 ^mn times that of the ²ⁿ gradation, and therefore, the white document output is also 2 ^mn times that of the ²ⁿ gradation. At this time, ΔV is the amount of 2 ^nm increments in 2 ⁿ gradations of the white document output. Therefore, the CPU 24 instructs the image processing circuit 23 to assign one gradation number for every 2 ^nm increments.

According to the above method, when reading is performed with a reduced number of gradations, the reading speed can be greatly improved.

<Embodiment 2> Figs. 5 and 6 are diagrams illustrating a second embodiment of the present invention.

A variable-precision A / D converter 22A is connected downstream of the amplifier 21 connected to the image sensor 1. This A /
The D converter 22A can switch the precision to any one of n bits, m bits (m <n),... 1 bit based on a signal from the CPU 24. The CPU 24 switches the driving frequency of the image sensor 1 and the motor 9 by changing the setting of the timing generator 29 together with the accuracy of the A / D converter 22A according to the setting of the host computer 31. Thus, the original is read at a speed suitable for the number of reading gradations.

In the above-described embodiment, when reading is performed at 2 ^m gray scale, ΔV is obtained by 2 ^nm of the step size at 2 ⁿ gray scale. However, according to the configuration of this embodiment, ΔV is obtained. / D converter 2
At 2A, A / D conversion is performed in increments of ΔV. Therefore, for grayscale and binary, A / D converter 2
The output of 2A is directly output to the host computer 31 via the image processing circuit 23 and the interface circuit 26. In the case of halftone such as dither, the output of the A / D converter 22A is binarized by the image processing circuit 23 and then output to the host computer 31 via the interface circuit 26.

Third Embodiment The above two embodiments are very effective means for improving the performance of an image reading apparatus. Although the reading speed can be set according to the number of gradations as described above in view of the performance of the image sensor, the performance of the motor as the scanning drive source often cannot catch up with this. For example, in an image reading device that can read 256 gradations, it is necessary to operate the motor at 128 times the speed for binary reading, but use a motor that can cover such a wide speed range Is not practical.

Therefore, it is an object of the present embodiment to increase the speed within a possible range at the time of reading when the number of gradations is reduced.

This embodiment can be adopted in any of the circuit forms of the above embodiments.

When the frequency of the motor at 2 ⁿ gradations is f _2n and the upper limit of the rotatable frequency is f _max , when reading 2 ^m gradations such that f _max / f _2n <2 ⁿ / 2 ^{m Can} not read at a speed of 2 ^nm . Therefore, in such a case, the frequency of the motor is set to a value as high as f _max or less, and the drive frequency of the image sensor is also switched to a value corresponding to this.

At this time, the white document reading output of the image sensor is set to V
_out (t _m ′) Assuming that the output level difference per gradation is ΔV ′, since t _m ′> t _m , from the equation (3), ΔV ′ = V _out (t _n ′) / 2 ^m > ΔV (5) That is, it can be seen that an output sufficient for reproducing the gradation is obtained.

According to the present embodiment, when the gradation is lowered,
The speed can be increased as much as the performance of the motor permits.

In the description of the present embodiment, an image scanner having a 1: 1/2 scanning optical system has been described.However, the present invention is not limited to this, and the present invention can be implemented in an integrated scanning optical system or a document transport type image scanner. .

[Effects of the Invention] As described above, according to the present invention, the reading time can be optimized in proportion to the number of gradations of the image data, and the processing time can be reduced.

[Brief description of the drawings]

FIG. 1 is a block diagram of an embodiment of the present invention, FIG. 2 (a) is a plan view of the embodiment, FIG. 2 (b) is a side view thereof, and FIG. FIG. 4 is an explanatory diagram of a process when the number of gradations is switched, FIG. 5 is a block diagram of another embodiment of the present invention, and FIG. FIG. 7 is an explanatory view of a process in the case of switching, FIG. 7 is a side view of a conventional example. 1 ... Image sensor, 9 ... Pulse motor, 22 ... A / D converter, 23 ... Image processing circuit, 24 ... CPU, 27 ... Motor drive circuit, 28 ... Image sensor drive circuit, 29 ... Timing generator.

Claims (5)

(57) [Claims] 1. A photoelectric conversion unit for reading an image of a document, a scanning unit for moving a position of the image read by the photoelectric conversion unit in a sub-scanning direction, and an image data read by the photoelectric conversion unit A signal processing unit that outputs the number of gradations, a control unit that controls a reading cycle of the photoelectric conversion unit and a moving speed of the scanning unit in proportion to the number of gradations of image data output from the signal processing unit, An image reading apparatus comprising: 2. The reading cycle in proportion to the number of gradations is determined by a time required for the photoelectric conversion means to obtain a minimum output required for recognizing a change in density on the document. 2. The image reading device according to claim 1, wherein the value is a value obtained by multiplying the value by: 3. An A / D converter for converting the image data into digital data, wherein the desired number of gradations is smaller than the maximum number n of gradations readable by the photoelectric conversion means. 2. The image reading apparatus according to claim 1, wherein when the number is m, consecutive n / m gradations of the output of the A / D conversion means are assigned to one gradation. 4. An apparatus according to claim 1, further comprising A / D conversion means for converting said image data into digital data, wherein the conversion accuracy of said A / D conversion means is changed in proportion to said gradation number. Item 2. The image reading device according to Item 1. 5. A reading cycle of said photoelectric conversion means, which is proportional to said number of gradations, is necessary for said photoelectric conversion means to recognize a minimum output necessary for recognizing a change in density on said document. A value obtained by multiplying a time required to obtain a minimum output by the number of gradations, or a minimum cycle in which a driving source of the scanning means can operate. Item 2. The image reading device according to Item 1.