JP3139102B2 - Image reading device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image reading apparatus which discretely obtains an image (color image) using an image pickup device such as a CCD, and more particularly, has at least two types of low-pass characteristics in the arrangement direction of the image pickup device. The present invention relates to an image reading apparatus suitable for, for example, a color scanner, a color facsimile, and the like, which removes a moiré phenomenon by using an optical means and reads the image with high accuracy.

[0002]

2. Description of the Related Art Generally, in a color image reading apparatus using an image pickup device such as a CCD having a discrete pixel structure, image information is spatially sampled optically to obtain an output image.

In this case, if a high spatial frequency component higher than the sampling frequency is included in the color image, a false signal such as a structure or a color tone not included in the color image is generated.

That is, a frequency component that cannot be collected by an output device (reading device) (a frequency component exceeding the Nyquist frequency) cannot be reproduced as image information, and a phenomenon called so-called waveform distortion (aliasing) occurs. This causes moiré fringes and false colors to be formed in the output image.

For this reason, conventionally, a low-pass filter is arranged in a part of an optical system of an image reading apparatus to limit a high-frequency component of a color image to remove an influence of aliasing.

As a conventional low-pass filter, a filter utilizing birefringence such as a quartz plate or the like, for example, US Pat.
As proposed in Japanese Patent No. 01, there has been often used one formed by periodically changing at least one surface in a trapezoidal shape.

FIG. 11 is a schematic view of a main part of a conventional image reading apparatus when this kind of low-pass filter is used for a part of an optical system.

In FIG. 1, reference numeral 111 denotes an illuminating means which comprises a light source such as a halogen lamp or a fluorescent lamp.
Reference numeral 112 denotes a platen glass (document carrier), and 113 denotes a color image (color document), which is mounted on the platen glass 112. Reference numeral 114 denotes an image forming unit which forms a color image 113 at substantially the same magnification on a light receiving unit 115 described later.

Reference numeral 116 denotes a low-pass filter, which is formed by using birefringence of quartz or the like or by periodically changing the surface shape of an optical member as proposed in US Pat. No. 4,998,801. Moiré fringes are removed.

Reference numeral 115 denotes a light receiving means, which comprises, for example, a one-dimensional line sensor (CCD) or the like, and converts image information from the color image 113 into an electric signal. This line sensor 115 is, for example, as shown in FIG.
A plurality of image sensors set to have spectral sensitivities corresponding to the three primary colors (red), G (green), and B (blue) are repeatedly arranged on one line. In the figure, R, G, and B color filters are deposited on each image sensor surface, and a color image is read by three color lights of R, G, and B.

In the image reading apparatus shown in FIG. 11, a color image 113 placed on a platen glass 112 is illuminated by an illuminating means 111 and the color image 113 is illuminated by an image forming means 1.
The light passes through a low-pass filter 116 to form an image on the surface of the light receiving means 115 at the same magnification, thereby sequentially reading the color images 113.

[0012]

However, the conventional image reading apparatus has the following problems.

For example, as shown in FIG. 12, halftone reproduction of a color image (color original) is performed by printing a halftone dot having a periodic pattern as shown by enlarging a part A of the color image in FIG. Is made larger or smaller.

Therefore, for example, when the above method is applied to an image reading apparatus having an optical system having a high resolution,
A beat with the pixel pitch of the element of the two-dimensional line sensor is generated, which may cause a so-called moire phenomenon.

As a result, when the color image shown in FIG. 12 is output through a conventional reading device, there is a problem that unevenness occurs in a low-frequency area on the color image as shown in FIG. There was a point.

In particular, when the imaging magnification of the imaging means approaches unity, a very high return response component appears even near the spatial frequency of the one-dimensional line sensor as shown in FIG. Tended to be remarkable.

For this reason, even if a low-pass filter for preventing the normal moire phenomenon shown in the image reading apparatus of FIG. 11 is used, a very high return response component in a frequency region higher than the spatial frequency where the MTF value is set to 0 is used. There was a problem that appeared.

As a result, the aliasing moire generated by sampling the aliasing response component causes the image quality of the output image to be significantly impaired.

On the other hand, if the spatial frequency at which the MTF value of the low-pass characteristic of the low-pass filter is set to 0 is set to a value higher than the spatial frequency of the image pickup device, there is a problem that a normal moire phenomenon occurs.

In particular, when a one-dimensional line sensor (CCD) for reading a color image in which a plurality of image sensors are arranged on one line shown in FIG. 16 is used, the halftone dots of the output image are R, G, and B. There is a problem that erroneous color image processing is performed depending on which element in one pixel divided into three colors forms an image.

For this reason, the color image is processed in a color different from the color of the actual color image, which causes a phenomenon in which a so-called color moiré or false color is formed in the output image.

According to the present invention, by providing at least two types of optical means having low-pass characteristics in the optical path between the image forming means and the light receiving means, both normal moire and folded moire can be effectively removed. It is an object of the present invention to provide an image reading apparatus capable of reading an image (color image) with high accuracy and obtaining a high-quality image.

[0023]

According to a first aspect of the present invention, there is provided an image reading apparatus, wherein an image placed on a document surface is illuminated by an illuminating unit, and the image is formed by an image forming unit. In an image reading apparatus which forms an image on a surface and reads the image, the same shape is formed in an optical path between the image forming means and the light receiving means.
And at least two optical members comprising
A position on the optical path where the optical member is separated from the light receiving means by a predetermined distance
And optical means having at least two types of low-pass characteristics in the arrangement direction of the imaging elements.

According to a second aspect of the present invention, in the first aspect of the present invention, at least two kinds of low-pass characteristics of the optical means are such that an MTF value is set to 0 at two different spatial frequencies. Is about 1: 1.2-
It is characterized by the fact that the relation of 3 is satisfied. According to a third aspect of the present invention, in the first aspect of the present invention, the imaging magnification of the imaging means is substantially equal. According to a fourth aspect of the present invention, in the first aspect of the present invention, the light receiving unit is configured to divide a range corresponding to a pixel to be read of the image in a direction in which the imaging elements are arranged and read the image. According to a fifth aspect of the present invention, in the first aspect of the present invention, at least one of the at least two types of low-pass characteristics of the optical means has a low-pass characteristic in a direction perpendicular to a direction in which the imaging devices are arranged. And

[0025]

FIG. 1 is a schematic view of a main part of an image reading apparatus according to a first embodiment of the present invention.

In FIG. 1, reference numeral 1 denotes an illuminating means, which comprises a light source such as a halogen lamp or a fluorescent lamp. Reference numeral 2 denotes a platen glass (document carrier), and 3 denotes a color image (reflection type color document), which is placed on the platen glass 2. Reference numeral 4 denotes an image forming unit which forms a color image 3 at the same magnification on a light receiving unit 5 described later.

Numeral 20 denotes an optical means, as will be described later, a low-pass filter 1 of two transparent optical members having the same shape.
1 and 12 having different low-pass characteristics with respect to the arrangement direction of the imaging elements. The two low-pass filters 11 and 12 are provided at predetermined distances from the light receiving unit 5 in the optical path between the imaging unit 4 and the light receiving unit 5.

As shown in FIG. 2, one of the low-pass filters 11 has a low-pass characteristic in which the MTF value is 0 in a low-frequency region, thereby eliminating a normal moire phenomenon.

The other low-pass filter 12 is shown in FIG.
As shown in FIG. 2, the low pass characteristic of the low pass characteristic shown in FIG. 2 has a low pass characteristic in which the MTF value becomes 0 at a value near the spatial frequency at which the return response component becomes the maximum, thereby eliminating the aliasing moire phenomenon.

In this embodiment, the low-pass characteristics as shown in FIG. 4 are obtained by using the two low-pass filters 11 and 12 in combination. That is, as shown in FIG. 4, the MTF value is low at the cutoff frequency of the image sensor, and is higher than the cutoff frequency.
A low-pass characteristic with a low value is obtained. As a result, in the present embodiment, both the normal moire phenomenon and the moire phenomenon caused by folding are effectively removed.

Numeral 5 denotes a light receiving means, which comprises, for example, a one-dimensional line sensor (CCD) or the like.
As shown in FIG. 6, a plurality of three image sensors set to have spectral sensitivities corresponding to the three primary colors of R (red), G (green) and B (blue) are repeatedly arranged on one line. Make up.

Further, R, G, and B color filters are deposited on each image pickup element surface so that an image can be read by three color lights of R, G, and B. That is, color image 3
The color image is read by dividing the range corresponding to the read pixel of the image sensor in the direction in which the image sensors are arranged.

In this embodiment, the reading means 30 comprises the illumination means 1, the imaging means 4, the optical means 20, and the light receiving means 5.

In this embodiment, the color image 3 placed on the platen glass 2 is illuminated by the illuminating means 1, and the color image 3 is illuminated by the imaging means 4 with the low-pass filter 11.
12 to form an image at the same magnification on the surface of the light receiving means 5.

The color image is digitally read by the light receiving means 5 based on the predetermined color light. Then, the platen glass 2 is line-scanned in a direction parallel to the paper surface or the reading means 30 is line-scanned in a direction parallel to the paper surface, whereby the color images 3 on the surface of the platen glass 2 are sequentially read.

Next, each of the low-pass filters 11 and 12
The relationship between the MTF value and the spatial frequency at which the MTF value is set to 0 will be described.

For example, when the spatial frequency at which the MTF value first becomes 0 when the low-pass filter 11 is used is f1 and the spatial frequency at which the MTF value first becomes 0 when the low-pass filter 12 is used is f2, the spatial frequency is f1. The frequency f2 is set so as to be substantially the same as the value of the spatial frequency at which the aliasing response component becomes maximum in a higher frequency region than the spatial frequency f1.

However, in the high-frequency region, M
The TF value also decreases. For this reason, in the present embodiment, the ratio of each of the low-pass filters 11 and 12 to the spatial frequencies f1 and f2 is set to be approximately 1: 1.2 to 3 in accordance with the imaging operation of the imaging unit 4. ing. This relation of the ratio is such that the moire phenomenon can be effectively removed and sufficient low-pass characteristics can be obtained.

As shown in FIGS. 5A and 5B, the shape of each of the low-pass filters 11 and 12 in the present embodiment has a periodic structure having a trapezoidal pattern (trapezoidal shape) in cross section. That is, the transparent plate (low-pass filter) is formed of a trapezoidal pattern having a minute height on the emission side on the emission side surface 11b (12b). As a result, the light beam that has passed through the imaging means 4 causes diffraction interference when passing through the low-pass filter, and the MTF value is set to 0 at a certain arbitrary spatial frequency.

In the present embodiment, two low-pass filters 11 and 12 having the same shape are installed on the optical path at positions separated from the light receiving means 5 by a predetermined distance, and different low-pass characteristics are obtained.

FIGS. 6 (A) and 6 (B) are explanatory diagrams of the spatial frequency for setting the MTF value to 0 by installing low-pass filters 11 and 12 having the same shape according to the present embodiment at arbitrary positions on the optical path. It is.

For example, the deflection angles of the low-pass filters 11 and 12 are θ ₁ and θ ₂ , respectively.
The optical path lengths from 1, 12 to the light receiving means 5 are L1, L2, respectively.
In this embodiment, the deflection angles θ ₁ and θ ₂ are set so that θ ₁ = θ ₂ and the optical path lengths L 1 and L 2
Each optical element is set such that 2 satisfies L1> L2. At this time, the low-pass filter 11 sets the MTF value to 0 in a low-frequency region by the low-pass filter 12,
As a result, different low-pass characteristics are obtained as shown in FIGS.

As described above, in this embodiment, two different low-pass characteristics can be obtained only by arbitrarily setting the positional relationship between the two low-pass filters 11 and 12 having the same shape as described above. Both of the normal moiré and the moiré caused by the folding are effectively removed, and a high-quality output image is obtained.

Also, since the two low-pass filters in this embodiment have the same shape, they have the advantage that they can be easily manufactured and can be manufactured at low cost.

In this embodiment, the cross-sectional shape of at least one of the two low-pass filters 11 and 12 is formed so that low-pass characteristics can be obtained in the direction perpendicular to the arrangement direction of the pixels of the one-line sensor. In addition, the moire phenomenon which occurs in the mechanical original (image) scanning direction can be removed at the same time.

In such an image reading apparatus, the spatial frequency of the image sensor of the one-line sensor becomes lower and the MTF value of the imaging means becomes higher as the imaging magnification of the imaging means becomes closer to unity. The present invention can be said to be a very effective means since the moire phenomenon appears remarkably.

The present invention is also effective for removing a color moiré phenomenon occurring in a color image reading one-line sensor (CCD) as a light receiving means.

FIGS. 7 (A) and 7 (B) are schematic views of a main part for explaining two types of low-pass characteristics according to the second embodiment of the present invention. In this figure, the same elements as those shown in FIG. 6 are denoted by the same reference numerals.

The present embodiment is different from the first embodiment in that the optical means is constituted by a low-pass filter 71 made of one transparent optical member. A trapezoidal pattern having different shapes is formed on the incident side surface 71a and the exit side surface 71b of the low-pass filter 71, thereby obtaining two low-pass characteristics.

That is, as shown in FIG. 8, the inclination angles of the inclined surfaces 72, 73 of the trapezoidal patterns A, B are changed to the incident side surface 71.
a and the emission-side surface 71b are formed so as to be different from each other, and the optical path lengths L1 and L2 from the incidence-side and emission-side surfaces of the low-pass filter 71 to the surface of the light receiving means 5 are L1L2.
, And each incident side surface 71a
And the respective deflection angles θ ₁ and θ ₂ from the emission-side surface 71b.
Are set such that θ ₁ > θ ₂ .

As a result, the two types of low-pass characteristics shown in FIGS. 2 and 3 are obtained, and finally, the low-pass characteristics shown in FIG. 4 are obtained. Have gained.

Further, the low-pass filter 71 in this embodiment occupies a small space and requires a small number of holding members for holding the low-pass filter 71, thereby simplifying the entire apparatus and reducing the cost. I have.

FIG. 9 is a schematic diagram of a main part for describing two types of low-pass characteristics according to the third embodiment of the present invention. In this figure, the same elements as those shown in FIG. 6 are denoted by the same reference numerals.

This embodiment is different from the second embodiment in that, of the low-pass filter 91 made of a transparent optical member, the light-exit surface 91b of the light-incident and light-exit surfaces 91 and 91b is different as shown in FIG. The trapezoidal patterns A and B are alternately and repeatedly formed, and the incident side surface 91a is formed from a flat surface.
That is, we obtained two low-pass characteristics.

That is, as shown in FIG. 10, the two trapezoidal patterns A and B are formed so that the inclination angles of the inclined surfaces 92 and 94 are different from each other, and the pitch P is smaller than the beam diameter of the light beam. ing. As a result, the low-pass characteristics as shown in FIG. 4 are obtained, and the same effects as in the first and second embodiments are obtained.

If a pattern having a low-pass characteristic is formed on the incident surface 91a in a direction perpendicular to the arrangement direction of the image pickup devices, the direction perpendicular to the arrangement direction of the image pickup devices can be obtained by using only one low-pass filter. Can be removed at the same time.

As described above, in each embodiment, when a color image is read by a light receiving means using an image pickup device such as a CCD in a unity magnification imaging system, a moire phenomenon is removed to obtain a high quality image.

In each of the above embodiments, an example using a reflection type original (image) has been described. However, the present invention can be applied in the same manner as the above-described embodiment even when a transmission type original is used. it can.

[0059]

According to the present invention, as described above, by providing optical means having at least two types of low-pass characteristics in the arrangement direction of the image pickup devices in the optical path between the image forming means and the light receiving means, as described above. It is possible to achieve an image reading apparatus that can always reduce the influence of the moiré phenomenon regardless of the imaging magnification of the image unit and can obtain a high-quality image.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a main part of an image reading apparatus according to a first embodiment of the present invention.

FIG. 2 shows a low-pass filter 11 according to the first embodiment of the present invention.
Explanatory diagram of MTF value of

FIG. 3 shows a low-pass filter 12 according to the first embodiment of the present invention.
Explanatory diagram of MTF value of

FIG. 4 is an explanatory diagram of an MTF value of the optical unit according to the first embodiment of the present invention.

FIG. 5 is an explanatory view of a main part of the low-pass filter according to the first embodiment of the present invention.

FIG. 6 is a schematic diagram of a main part for describing low-pass characteristics according to the first embodiment of the present invention.

FIG. 7 is a schematic diagram of a main part for explaining low-pass characteristics according to a second embodiment of the present invention.

FIG. 8 is an explanatory view of a main part of a low-pass filter according to a second embodiment of the present invention.

FIG. 9 is a schematic diagram of a main part for describing low-pass characteristics according to a third embodiment of the present invention.

FIG. 10 is an explanatory view of a main part of a low-pass filter according to a third embodiment of the present invention.

FIG. 11 is a schematic view of a main part of a conventional image reading apparatus.

FIG. 12 is an explanatory diagram of a color image.

FIG. 13 is an enlarged explanatory view of a part of FIG. 12;

FIG. 14 is an explanatory diagram when the color image shown in FIG. 12 is output by a conventional image reading apparatus.

FIG. 15 is an explanatory diagram of an MTF value of a conventional low-pass filter.

FIG. 16 is an explanatory diagram of a light receiving unit.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 illumination means 2 original glass table 3 original 4 image forming means 5 light receiving means 20 optical means 11, 12, 71, 91 low-pass filter 30 reading means

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G02B 27/46

Claims (5)

(57) [Claims] 1. An image reading apparatus for illuminating an image placed on a document surface by illuminating means, forming the image on a light receiving means surface having a plurality of image sensors by an image forming means, and reading the image. The same shape in the optical path between the imaging means and the light receiving means
At least two optical members having a shape
Position the optical member on the optical path at a predetermined distance from the light receiving means.
An image reading apparatus, comprising: an optical unit having at least two types of low-pass characteristics in the arrangement direction of the imaging devices by being installed in the respective devices. 2. The at least two kinds of low-pass characteristics of the optical means are to set the MTF value to 0 at two different spatial frequencies, and the ratio of the two spatial frequencies at this time is approximately 1: 1.2 to 3. The image reading device according to claim 1, wherein the image reading device is formed by the following relationship: 3. The image reading apparatus according to claim 1, wherein an image forming magnification of said image forming means is substantially equal. 4. The image reading apparatus according to claim 1, wherein said light receiving means divides a range corresponding to a read pixel of said image in a direction in which said image sensors are arranged and reads the image. 5. The image according to claim 1, wherein at least one of the at least two types of low-pass characteristics of the optical means has a low-pass characteristic in a direction perpendicular to a direction in which the imaging devices are arranged. Reader.