JP2004064768A - Image reading apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image reader capable of reading a printed matter as a good quality of image without creating a moire even when the printed matter is to be read. <P>SOLUTION: In the image reader, a reading object 2 of print dots regularly arranged is imaged by a sensor 5 as an assembly of imaging elements regularly arranged through an optical system including a lens 4 to convert the image into an electrical signal. A optical low-pass filter 9 whose trap point is half or less of a sampling period of the sensor 5 is installed in the midway of an optical path that leads to the sensor 5 from the reading object 2. <P>COPYRIGHT: (C)2004,JPO

Description

[Claims]
1. An image to be read having image information to be read, such as a photograph or printed matter, is connected to a sensor formed as an assembly of image pickup elements which are photoelectric conversion elements through an optical system such as a lens. In an image reading apparatus for obtaining an image signal which is an electric signal by performing image processing and performing predetermined electrical processing,
An optical low-pass filter is installed in the optical path from the reading target to the sensor, and the trap point of the optical low-pass filter is configured to be at least half of the spatial sampling frequency of the sensor.
Further, the optical low-pass filter is composed of a plurality of types having different optical characteristics, such as a trap point, and can be selectively inserted into the optical path from the reading target to the sensor, and the position thereof is the light of the lens. An image reading apparatus configured to move back and forth along an axis.
2. An image to be read having image information to be read, such as photographs and printed matter, is connected to a sensor formed as an assembly of image pickup elements which are photoelectric conversion elements through an optical system such as a lens. In an image reading apparatus for obtaining an image signal which is an electric signal by performing image processing and performing predetermined electrical processing,
An optical low-pass filter is installed in the optical path from the object to be read to the sensor, and the trap point of this optical low-pass filter is one half or more of the spatial sampling frequency of the sensor, and the period of the printed dots of the printed object to be read Configured to match
Further, the optical low-pass filter is composed of a plurality of types having different optical characteristics, such as a trap point, and can be selectively inserted into the optical path from the reading target to the sensor, and the position thereof is the light of the lens. An image reading apparatus configured to move back and forth along an axis.
3. The image reading apparatus according to claim 1, wherein the optical low-pass filter is a crystal filter.
4. The image reading apparatus according to claim 1, wherein the optical low-pass filter is formed of a diffraction grating.
DETAILED DESCRIPTION OF THE INVENTION
[0001]
[Industrial application fields]
The present invention relates to an image reading apparatus, and is particularly useful when a printed material (hereinafter simply referred to as a printed material) in which an image is formed by expressing the density by a large number of regularly arranged printing dots is used as a reading object. Is.
[0002]
[Prior art]
2. Description of the Related Art Image reading apparatuses are known as apparatuses that read image information to be read, such as photographs and printed materials, and convert them into electrical signals.
[0003]
FIG. 9 is a structural diagram showing the main part of this type of image reading apparatus. As shown in the figure, this image reading apparatus illuminates a reading object 2 with an illuminating means 1 such as a fluorescent lamp, and an image of the reading object 2 obtained by illuminating with the illuminating means 1 includes a mirror 3 and a lens 4. The image is formed on the sensor 5 through the optical system. The sensor 5 is an assembly of image pickup elements composed of a CCD as a photoelectric conversion element, and is usually a one-dimensional line sensor. Therefore, the illumination unit 1, the mirror 3, and the lens 4 are integrated with the sensor 5. The image reading unit 6 to be moved moves in a direction perpendicular to the longitudinal direction of the sensor 5 (hereinafter referred to as a sub-scanning direction), and scans and reads the entire image of the reading object 2 having a planar shape. is there. When the sensor is an area sensor having an image sensor arranged in a two-dimensional plane, there is a case where it is not necessary to scan.
[0004]
The image formed on the sensor 5 is stored in the memory as electrical image information, or is supplied to a television monitor or a printer after a predetermined correction.
[0005]
The reading object 2 is placed on a reading window 7 which is a flat surface formed of a transparent member such as glass so that the reading surface 2 a faces the mirror 3 and can form an image on the sensor 5. Further, in order to ensure a constant exposure level at the time of image reading, a reference is made so that the mirror 3 can be opposed to the starting point position in the scanning direction so as to determine a reference level (usually a white level) that becomes a reference prior to image reading. A concentration plate 8 is provided. The reference density plate 8 is formed with a uniform density as a whole, and the whole formed with a white color is called a reference white board, which is usually used.
[0006]
Thus, when the image is read by the image reading apparatus, the reading object 2 is placed on the reading window 7 with the reading surface 2a facing down, and the image reading unit 6 is caused to travel in the sub-scanning direction. Are sequentially taken into the sensor 5 and predetermined electrical processing is performed. The electrical processing at this time is executed while performing exposure adjustment so that the white level becomes constant with reference to the level (for example, white level) of the reference density plate 8 taken prior to reading of the reading object 2. .
[0007]
[Problems to be solved by the invention]
By the way, when the printed matter is read by the image reading device, the reading is read by the sensor 5 because the arrangement of the print dots of the printed matter is regular and the arrangement of the image pickup elements of the sensor 5 is regular. Moire may occur in the image.
[0008]
The principle and mode of generation of moire at this time will be described with reference to FIG.
[0009]
FIG. 10A is an explanatory diagram showing the relationship between the image pickup device 5a of the sensor (CCD) 5 and the print dots D of the print on the surface of the image pickup device 5a when a printed product having a uniform density on one surface is imaged. 10B shows the sampling period of the sensor 5, FIG. 10C shows the printing dot period, FIG. 10D shows the output signal of the sensor 5, and FIG. 10E shows the image signal output.
[0010]
FIG. 10A to FIG. 10E are examples in the case where the period of the print dots D is the limit resolution. Here, the limit resolution is an index indicating that the printing dot D can be resolved so far when the sampling period of the sensor 5 and the period of the printing dot D coincide with each other.
[0011]
In the case of this example, the cycle of the printing dots D is twice the sampling cycle of the sensor 5, that is, half of the sampling frequency, and if the printing dots D can be resolved correctly, the density becomes uniform and moire does not occur.
[0012]
On the other hand, FIG. 11A is an explanatory diagram showing the same relationship as FIG. 10A when the period of the printing dots D is shorter than the period at the limit resolution (in the case shown in FIG. 10A). In this case, moire occurs. That is, the output signal of the sensor 5 becomes irregular as is apparent from FIGS. 11 (b) to 11 (d) showing the print dot cycle, the output signal of the sensor 5 and the image signal output in this case. In addition, the same state is repeated every nine pixels. This becomes moire. Therefore, even when a printed matter having a uniform density on one side is imaged, the density is not uniform.
An object of the present invention is to provide an image reading apparatus that can read a printed image as a high-quality image without causing moire even when a printed material is to be read.
[0013]
[Means for Solving the Problems]
The configuration of the present invention that achieves the above object is characterized by the following points.
[0014]
1) An image to be read having image information to be read, such as photographs and printed matter, is imaged on a sensor formed as an assembly of image pickup elements which are photoelectric conversion elements through an optical system such as a lens, In an image reading apparatus that obtains an image signal that is an electrical signal by performing predetermined electrical processing,
An optical low-pass filter is installed in the optical path from the reading target to the sensor, and the trap point of the optical low-pass filter is configured to be at least half of the spatial sampling frequency of the sensor.
Further, the optical low-pass filter is composed of a plurality of types having different optical characteristics, such as a trap point, and can be selectively inserted into the optical path from the reading target to the sensor, and the position thereof is the light of the lens. Being configured so that it can move back and forth along the axis.
2) An image to be read having image information to be read, such as photographs and printed matter, is imaged on a sensor formed as an assembly of image pickup elements which are photoelectric conversion elements through an optical system such as a lens, In an image reading apparatus that obtains an image signal that is an electrical signal by performing predetermined electrical processing,
An optical low-pass filter is installed in the optical path from the object to be read to the sensor, and the trap point of this optical low-pass filter is one half or more of the spatial sampling frequency of the sensor, and the period of the printed dots of the printed object to be read Configured to match
Further, the optical low-pass filter is composed of a plurality of types having different optical characteristics, such as a trap point, and can be selectively inserted into the optical path from the reading target to the sensor, and the position thereof is the light of the lens. Being configured so that it can move back and forth along the axis.
3) In 1) or 2) above, the optical low-pass filter is a crystal filter.
4) In 1) or 2) above, the optical low-pass filter is a diffraction grating.
[0015]
[Action]
According to the present invention having the above-described configuration, the optical image of each print dot constituting the image of the printed matter is separated by passing through the optical low-pass filter, and is averaged as a whole. Prevents the occurrence of moiré.
[0016]
For example, as shown in FIG. 1, when the separation width of the normal image (see FIG. 1 (a)) by the optical low-pass filter is matched with the sampling period of the sensor (see FIG. 1 (d)), the separation is performed. The image is as shown in FIG. 1 (b), and after combining the normal image and the separated image, they are averaged as shown in FIG. 1 (c). In other words, this case corresponds to a case where the trap point of the optical low-pass filter is half of the spatial sampling frequency corresponding to twice the sampling period, and the black and white repetitive image is synthesized while its separated image is inverted. The black and white contrast of the image is zero.
[0017]
Therefore, when the separation width is made to coincide with the sampling period of the sensor as described above, the image of the print dot D shown in FIG. 11 is passed through the optical low-pass filter 10 and shown in FIG. The separated image is separated from the normal image, and the output signal at this time is averaged as shown in FIG. As a result, the generation of moire is prevented.
[0018]
【Example】
Embodiments of the present invention will be described below in detail with reference to the drawings.
[0019]
FIG. 3 is a structural diagram showing the main part of the image reading apparatus according to the embodiment of the present invention together with its control system. The image reading apparatus of this embodiment is different from the conventional image reading apparatus in that an optical low-pass filter 10 is added, but the basic structure as the image reading apparatus is the same as that of the image reading apparatus shown in FIG. Therefore, the same parts as those in FIG.
[0020]
As shown in FIG. 3, the image signal converted into an electrical signal by the sensor 5 is controlled by the gain controller 10 and then converted into a digital signal by the A / D converter 11, and the digitized image signal S0. Is output as
[0021]
The control unit 12 performs predetermined control on the dimming circuit 13 and the timing circuit 14 together with the gain controller 10 and the A / D converter 11 as in the prior art, and the image signal when the image on the reference density plate 8 is read. Based on S0, exposure adjustment is performed via the gain controller 10, the dimming circuit 13, the timing circuit 14, and the like so that this becomes a predetermined level set in advance.
[0022]
At this time, the light control circuit 13 controls the light quantity of the illumination means 1. The timing circuit 14 controls the readout timing of charges accumulated in the sensor 5, that is, the exposure time.
[0023]
In the present embodiment, the optical low-pass filter 9 is disposed between the lens 4 and the sensor 5 in the optical path from the reading target to the sensor 5. The optical low-pass filter 10 can be composed of a crystal filter (birefringent plate) or a diffraction grating. The optical low-pass filter 9 has a trap point that is at least one half of the spatial sampling frequency of the sensor 5 or at least one-half of the spatial sampling frequency of the sensor 5, and the reading target 2. It is configured to coincide with the printing dot cycle of a certain printed matter.
[0024]
Therefore, according to the present embodiment, it is possible to reduce the occurrence of moire caused by reading an image constituted by regularly arranged printing dots by the sensor 5 constituted by regularly arranged imaging elements.
[0025]
Next, the principle and aspect of the moire reduction will be described.
[0026]
4 is an explanatory diagram conceptually showing the relationship between various printing dots and moire, FIG. 5 is a characteristic diagram of an optical low-pass filter, and FIG. 6 is a characteristic diagram of a double-structure optical low-pass filter. In these drawings, LP / mm on the horizontal axis indicates how many periods of black and white pairs of printed dots are in 1 mm on the surface of the sensor 5, and the vertical axis indicates the response. Further, the “use video band” means the frequency band of the image output signal shown in FIGS. 10 and 11, the “limit resolution point” means that the print dot D has the relationship shown in FIG. 10, and the sensor sampling spatial frequency. The spatial sampling frequencies of the sensor 5 shown in FIG. 2 (FIGS. 10 and 11) are respectively shown.
[0027]
Accordingly, there are S black and white line pairs at the sensor sampling spatial frequency. The frequency of the limit resolution point is half of the sensor sampling spatial frequency, and the number of line pairs is S / 2.
[0028]
As shown in FIG. 4, moire occurs when the spatial frequency of the printing dots D exceeds the limit resolution. In addition, a to g in the figure are spatial frequencies when various printing dots D are imaged on the surface of the sensor 5. Therefore, although it changes depending on the reduction or enlargement magnification of the optical system, since the spatial frequency of all the printing dots D is S / 2 or more, that is, in the moire generation range, moire occurs at positions a ′ to g ′. It is shown that.
[0029]
Characteristic 1 in FIG. 5 is a crystal filter characteristic, and characteristic 2 is a diffraction grating filter characteristic. As shown in the figure, the trap points of both coincide with each other and are located in the vicinity of b and c at a frequency of S / 2 or higher, so that it is most effective when the print dot D has a spatial frequency of b and c. Moire is reduced.
[0030]
As shown in FIG. 5, the characteristic 1 and the characteristic 2 have different characteristics in the higher band than the trap point, so the characteristic 2 is more suitable when it is desired to reduce the band beyond the trap point.
[0031]
FIG. 6 shows a case where the optical low-pass filter 9 for reducing the moire by suppressing the printing dots D having various spatial frequencies in FIG. 4 is configured. The trap point of the optical LPF-A is the trap point A, which is set between S / 2 and S. At this time, the intermediate portion between the trap point A and 3S / 2 is lifted only by the optical LPF-A. Therefore, the trap point B of the optical LPF-B is set at the most lifted position of the intermediate portion. As a result, the total characteristic is a product of the optical LPF-A and the optical LPF-B, and becomes a characteristic indicated by a dotted line in FIG. Thereby, the band beyond the trap point A can be sufficiently suppressed.
[0032]
On the other hand, if the optical low-pass filter having the overall characteristics shown in FIG. 6 is used, it is effective for preventing moire when reading the printed dots D of various frequencies. In this case, the amplitude is about 35% at the limit resolution point. It becomes a characteristic (response), and the resolution deteriorates accordingly. Therefore, when the spatial frequency of the print dot D to be read 2 is specified, for example, if the print dot D has only the spatial frequency at the position f in FIG. 4, only the optical LPF-B is used. good. In this case, the amplitude characteristic at the limit resolution point is about 75%, and the degradation of the resolution is alleviated.
[0033]
As described above, the optical low-pass filter 9 is selected to have an appropriate characteristic in accordance with the spatial frequency of the printing dot D, and in some cases, a plurality of types are appropriately combined to reduce moire and simultaneously resolve. The deterioration of feeling can be reduced as much as possible.
[0034]
Regarding the trap point setting, since it is meaningless to set a frequency lower than the limit resolution point (that is, moire does not occur at this frequency), the trap point is set to be higher than the limit resolution point.
[0035]
The above description is focused on the main scanning direction (perpendicular to the sub-scanning direction), but the situation is the same for the sub-scanning direction. The same is true if the direction is the same). Accordingly, in order to prevent the occurrence of moire, an optical low-pass filter is required in both the main scanning direction and the sub-scanning direction.
[0036]
Therefore, the actual optical low-pass filter 9 is configured as shown in FIGS. Each of the optical filters 9 shown in FIGS. 7A and 7B is a composite of two types of filters. In particular, the optical filter 9 shown in FIG. 7A includes a first trap 9a in the main scanning direction, a second trap 9b in the same direction, phase plates 9c, 9d and 9e, and a first trap in the sub-scanning direction. 9f and the second trap 9g in the sub-scanning direction are sequentially overlapped. The optical filter 9 shown in FIG. 7B has a first trap 9h in one main scanning direction (vertical direction) and sub-scanning direction (horizontal direction) formed on one surface, and the other surface on the opposite side. Similarly, a second trap 9i in both directions is formed.
[0037]
In the above embodiment, the optical low-pass filter 9 is fixedly installed, but the present invention is not limited to this configuration. For example, one point of a support member of the optical low-pass filter 9 is rotatably supported, and the optical low-pass filter 9 is rotated from the reading object 2 to the sensor 5 by rotating around this point about the optical axis of the lens 4. In addition, a plurality of optical low-pass filters 9 may be prepared, and one or a plurality of optical low-pass filters 9 may be selectively inserted into the optical path. You may comprise so that it may insert. In the former case, in the case of ordinary originals and photographs other than printed matter that forms an image with the printing dots D, an image that does not deteriorate the resolution is obtained without using the optical low-pass filter 9. Can do. That is, moire removal can be appropriately performed according to the type of the reading object 2. In the latter case, in addition to this effect, a filter having optimum filter characteristics can be selected according to the spatial frequency of the printing dot D or the like.
[0038]
Further, as shown in FIG. 8, the optical low-pass filter 9 is disposed between the lens 4 and the sensor 5, and the position of the optical low-pass filter 9 is changed via a screw shaft 16 driven by a motor 15. You may comprise so that it may move back and forth along. In this case, when the optical low-pass filter 9 approaches the lens 4, the trap frequency is lowered and the resolution is deteriorated. Therefore, the position is appropriately adjusted according to the type of the reading object 2.
[0039]
Further, moire can be reduced without using the optical low-pass filter 9. For example, by placing the reading object 2 on the reading window 7 through a spacer, the image formed on the sensor 5 is blurred, or the sensor 5 is slightly moved from the in-focus position to blur the image. Can be reduced.
[0040]
【The invention's effect】
As specifically described above with reference to the embodiments, according to the present invention, when an image of a printed matter composed of regularly arranged dots is converted into an electrical signal by a sensor that is an assembly of image pickup elements having a regularly arranged arrangement. Generation of moire can be reduced.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram for explaining the operation of an optical low-pass filter according to the present invention.
FIG. 2 is an explanatory diagram illustrating the principle of moire removal in the present invention.
FIG. 3 is a structural diagram showing an image reading apparatus according to an embodiment of the present invention.
FIG. 4 is an explanatory diagram conceptually showing the relationship between various print dots and moire.
FIG. 5 is a characteristic diagram illustrating an example of characteristics of an optical low-pass filter.
FIG. 6 is a characteristic diagram showing another example of the characteristics of the optical low-pass filter.
FIG. 7 is a structural diagram illustrating a specific configuration example of an optical low-pass filter.
FIG. 8 is an explanatory diagram showing an extracted position adjustment mechanism of an optical low-pass filter in another embodiment of the present invention.
FIG. 9 is a structural diagram showing a conventional technique.
FIG. 10 is an explanatory diagram illustrating an example of a positional relationship between printed dots and an image sensor of a sensor.
FIG. 11 is an explanatory diagram conceptually showing the principle of moiré generation.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Illumination means 2 Reading object 4 Lens 5 Sensor 5a Image pick-up element 7 Reading window 9 Optical low-pass filter 12 Control part

Claims (7)

An image to be read having image information to be read, such as a photograph or printed matter, is imaged on a sensor formed as an assembly of imaging elements that are photoelectric conversion elements via an optical system such as a lens, In an image reading apparatus that obtains an image signal that is an electrical signal by performing electrical processing,
An image reading device characterized in that an optical low-pass filter is installed in the optical path from the reading object to the sensor, and the trap point of the optical low-pass filter is configured to be at least half of the spatial sampling frequency of the sensor. apparatus. An image to be read having image information to be read, such as a photograph or printed matter, is imaged on a sensor formed as an assembly of imaging elements that are photoelectric conversion elements via an optical system such as a lens, In an image reading apparatus that obtains an image signal that is an electrical signal by performing electrical processing,
An optical low-pass filter is installed in the optical path from the object to be read to the sensor, and the trap point of this optical low-pass filter is more than half of the spatial sampling frequency of the sensor, and the period of the printed dots of the printed object to be read An image reading apparatus configured to match the above. The image reading apparatus according to claim 1 or 2, wherein the optical low-pass filter is a crystal filter. The image reading device according to claim 1 or 2, wherein the optical low-pass filter is formed of a diffraction grating. The image reading apparatus according to any one of claims 1 to 4, wherein an optical low-pass filter can be selectively inserted in an optical path from a reading target to a sensor. . The optical low-pass filter is composed of a plurality of types having different optical characteristics such as its trap point, and each optical low-pass filter is configured to be selectively inserted into the optical path from the reading target to the sensor. Any one of the image reading devices according to any one of claims 1 to 4. The optical low-pass filter is disposed between the lens and the sensor of the optical system, and the position thereof can be moved back and forth along the optical axis of the lens. An image reading apparatus according to claim 4.

Images