JPH0870371A - Color image reader - Google Patents

Abstract

PURPOSE: To obtain a color image reader which is capable of simultaneously reading the color image information of the same line and exactly focusing all the plural light beams in which the spectroscopic characteristic is uniform in all the areas in a main scanning direction and color separations are performed on plural line sensors with the same magnification. CONSTITUTION: A color image reader illuminates an original by a light source 1 and makes the reflected light from an original 3 form images on the different one-dimensional picture element columns 61, 62 and 63 of a light receiving element 6 for every wavelength component by an image formation lens 4 and a color separation means 5. The image formation lens 4 has the telecentric system optical characteristic set so that the main light beam of emitted light may be parallel to an optical axis. The color separation means 5 has plural color separation elements 5a and 5b in which dichroic films 513 and 523 reflecting only specified wavelength light and total reflection mirrors 511 and 521 are laminated via transparent layers 512 and 522.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color image reading device,
More specifically, the present invention relates to a reading device that reads a color image by separating it into three primary colors.

[0002]

2. Description of the Related Art Conventionally, there has been known a color original reading device in which a color image is color-separated into three primary colors and the color-separated light in each wavelength region is photoelectrically converted by a light receiving element to be read.

For example, the apparatus shown in FIG. 8 uses an image forming lens 11 for forming an image of reflected light from an original 3 placed on a platen glass 2 by using a white light containing a desired wavelength region as a light source 1. And blue (blue) light, green (green) light on one chip,
A light receiving element 6 in which three one-dimensional pixel rows 61, 62, 63 set to have spectral sensitivity characteristics of three colors of red (red) light are arranged in the sub-scanning direction is provided, and color is obtained while scanning in the sub-scanning direction. It reads an image.

Further, as shown in FIG. 9, white light including the entire desired wavelength range is used for the light source 1, and in the optical path between the image forming lens 11 and the light receiving element 6 for forming an image of the reflected light from the original 3, Dichroic films 81, 8 that reflect only the desired wavelength range
2. A color separation means composed of a beam splitter 8 in which a total reflection mirror 83 is laminated via a transparent layer 84 is arranged, and the reflected light is color separated by the beam splitter 8 to form a plurality of one-dimensional pixel rows 61, 62, 63. A device for making an imaging sensor integrally provided with ... receive light is also known from Japanese Patent Laid-Open No. 3-201861.

Further, as shown in FIG. 10, white light including the entire desired wavelength range is used for the light source 1, and it is formed in the optical path between the image forming lens 11 and the light receiving element 6 for forming an image of the reflected light from the original 3. ,
A beam splitter 9 in which dichroic films 91 and 92 and a total reflection mirror 93 are laminated via a transparent layer 94, a dichroic film 101 and 102, and a total reflection mirror 103 are a transparent layer 104 as color separation means for reflecting only a desired wavelength range. A plurality of line sensors 61 and 6 are color-separated into parallel rays by using a beam splitter 10 laminated via
A structure in which an image sensor including 2, 63, ... Is received is also known from the disclosures of JP-A-62-234106, JP-A-1-237619, and JP-A-2-180465. .

[0006]

However, the above-mentioned conventional color image reading apparatus has the following problems.

First, in the color image reading apparatus shown in FIG. 8, since the three line sensors 61, 62, 63 sequentially read, a delay memory is required to perform color processing of the color image in real time, and therefore the image is read. There is a problem that the processing becomes complicated and the cost becomes high. Further, it is difficult to obtain a clear image due to color shift due to mechanical scanning blur during sub scanning.

Further, in the color image reading apparatus shown in FIG. 9, since the light path length of the light flux of each color is different, it is necessary to use an image forming lens having a sufficiently deep depth of focus in order to obtain a clear image. There are difficulties. In addition, if an attempt is made to adjust the optical path length by inclining the light-receiving surface of the sensor, it is necessary to increase the angle, and the light-receiving sensitivity decreases, and due to the characteristics of an ordinary imaging lens, the light rays of the three primary colors are Since the optical path lengths are different, the image forming magnifications for the respective color components are different, and as a result, there is a problem in that an error occurs in the image forming magnification in the main scanning direction.

Further, in the color image reading apparatus shown in FIG. 10, the light path lengths of the light fluxes of the respective colors are equal, but the structure of each color separation element becomes complicated and each component light must pass through a plurality of layers. However, there is a disadvantage that the loss of the light amount of each component light is large.

Further, in the color image reading apparatus shown in FIGS. 8, 9 and 10, it is preferable that the light beams of the three primary colors are accurately focused on the plurality of line sensors 61, 62, 63. Due to the influence of the chromatic aberration of the image lens 11, the rays of each color do not form an image on the same plane. As a countermeasure against this, it is conceivable to use a lens having a small chromatic aberration, but in order to eliminate the chromatic aberration of the imaging lens 11 in a desired wavelength region at all, a considerably expensive lens such as an apochromat must be used, which is not realistic. .

Further, in the color image reading apparatus shown in FIGS. 9 and 10, since the dichroic film utilizing the multilayer interference film is used as the color separation means, the color separation characteristics have angle-of-view dependency and the main scanning. There is a problem that it is difficult to obtain uniform spectral characteristics over the entire area of the direction.

The present invention has been made in view of the above problems, and an object of the present invention is to simultaneously read color image information on the same line, and
It is an object of the present invention to provide a color image reading device having uniform spectral characteristics in the entire area in the main scanning direction and capable of accurately focusing all of a plurality of color-separated light rays on a plurality of line sensors at the same magnification.

[0013]

A color image reading apparatus of the present invention comprises a light source, a light receiving element formed by arranging a plurality of one-dimensional pixel rows in parallel on one chip, and an original document illuminated by the light source. A color image having an image forming lens for condensing reflected light and color separation means for color separating the light passing through the image forming lens and forming an image on a different one-dimensional pixel array for each wavelength component. In the reading device, the imaging lens has a telecentric system optical characteristic in which the principal ray of the emitted light is set to be parallel to the optical axis, and the color separation means only emits light of a specific wavelength. It is characterized by comprising a plurality of color separation elements in which a dichroic film that reflects light and a total reflection mirror are laminated with a transparent layer in between.

Further, the imaging lens in the color image reading apparatus of the present invention is characterized in that due to the axial chromatic aberration, the focal length of blue light is short and the focal length of red light is long with respect to the focal length of green light. .

Further, the image forming lens in the color image reading apparatus of the present invention is characterized in that the focal lengths for blue light, green light and red light due to the axial chromatic aberration are equal to each other.

The color separation means in the color image reading apparatus of the present invention comprises a first color separation element for separating white light into blue light and yellow light, and white light into blue green light and red light. And a second color separation element that performs

Further, the thickness of the transparent layer between the dichroic film of the color separation means and the total reflection mirror in the color image reading apparatus of the present invention compensates for the difference in focal length between the color lights in the imaging lens. It is characterized by being set as follows.

Further, the light receiving element in the color image reading apparatus of the present invention is a three-line sensor having three one-dimensional pixel rows for blue light, green light and red light.

[0019]

With the above arrangement, when reading a color image, the image information of a single line with no positional deviation can be read. Further, the image information of this single line is separated by the optical means at an interval corresponding to the array pitch of the line sensors arranged for each color component, and at the same time, each color light ray has a different optical path length for each color component. After that, it is guided to the line sensor. At this time, the thickness of the transparent layer laminated between the dichroic films, the beam splitter, and the line so as to offset the difference in the optical path length for each color component in the color separation means to offset the shift amount of the focal position due to the longitudinal chromatic aberration of the imaging lens. By setting the installation angle of the sensor, it becomes possible to accurately focus all of the plurality of color-separated light rays on the plurality of line sensors without using an expensive lens such as an apochromat. In addition, since a telecentric system is used for the imaging lens, the spectral characteristics of color separation are uniform over the entire area in the main scanning direction, and the effect of lateral chromatic aberration, which is a problem when different optical path lengths are used for each color component, It can be lost.

[0020]

1 and 2 are a plan view and a side view of an optical system showing an embodiment of a color image reading apparatus of the present invention.

In the figure, reference numeral 1 is a white slit light source whose emitted light flux includes almost the entire visible wavelength range, and 2 is a platen glass on which an original 3 is placed. The white light emitted from the light source 1 is a platen. The light passes through the glass 2 and is reflected by the original 3, and the reflected light is reflected on the light receiving element 6 by the imaging lens 4 having the telecentric system optical characteristics set so that the principal ray of the emitted light is parallel to the optical axis. It is imaged. Reference numeral 5 is a color separation means for separating the reflected light from the original 3 into information of the three primary colors, and is composed of two color separation elements 5a and 5b. The color separation elements 5a and 5b are dichroic mirrors in which a dichroic film and a total reflection film are laminated with a transparent layer in between. The reflected light color-separated by the color separation unit 5 is separated into the three primary colors and is incident on the light receiving element 6 which is an image sensor. As described above with reference to FIG. 2, the light receiving element 6 has three one-dimensional pixel rows 61, 62, 63, and the color separation means 5
The image information decomposed into the three primary colors is output as three electric signals.

The telecentric system means an optical system in which either the entrance pupil or the exit pupil exists at infinity.
In this telecentric system, an aperture stop is arranged in an image space, an object space focal plane, or a position conjugate with them, or a collimator lens for converting the light emitted from the convex lens system into parallel light is provided on the output side of the convex lens system. This can be realized by using the arranged combination lens.

The image forming lens 4 in the embodiment has a telecentric optical characteristic in which the principal ray of the emitted light is set to be parallel to the optical axis, and the light ray emitted from the image forming lens 4 is a full image. In the long range, the color separation means 5 is guided under the same angle condition. Therefore, even when the spectroscopic characteristics pass through the dichroic film, which theoretically has angle dependence, the spectroscopic characteristics in the entire area in the main scanning direction are constant and uniform. Further, since the chief ray from the imaging lens 4 to the light receiving element 6 is set to be parallel to the optical axis,
Even when an image is formed on the light receiving element 6 by the color separation means 5 with a different optical path length for each color component, the image magnification does not change and the image magnification of each color component is the same, and the main scanning between the colors is performed. There is no magnification error in imaging in the direction.

Further, the imaging lens 4 in this embodiment is
Blue light, green light, which is color-separated in the color separation means 5,
Considering that red light passes through different optical paths, the difference in the optical path length of each component light is canceled out, and all the color-separated color lights are focused in advance on the same plane of the light receiving element 6 in advance. It is designed by adjusting the amount of chromatic aberration. The adjustment amount of the longitudinal chromatic aberration will be described later.

The color separation means 5 in the embodiment comprises two sets of dichroic mirrors 5a and 5b. The dichroic mirror 5a has a total reflection film 51 on the glass substrate 510.
1 is coated, a thin glass plate 512 having a predetermined thickness, which is a transparent layer, is adhered and laminated thereon, and a dichroic film 513 for reflecting a blue light component is coated on the surface part of the thin glass plate 512. The dichroic film 51 is shown in FIG.
3 shows the spectral characteristics of No. 3. Also, the dichroic mirror 5b
Is a glass substrate 520 coated with a total reflection film 521, and a thin glass plate 52 having a predetermined thickness, which is a transparent layer, is formed thereon.
2 is adhered and laminated, and a dichroic film 523 for reflecting a blue-green (cyan) color light component is coated on the surface of a thin plate glass 522. The dichroic film 52 is shown in FIG.
3 shows the spectral characteristics of No. 3. Here, the thickness of the thin plate glass 512 and the thin plate glass 522 determines the separation width and the optical path length of each color component light to be color-separated, and the setting of the thickness will be described later.

Here, the dichroic mirrors 5a and 5b.
The thickness of the thin glass plates 512 and 522, which are transparent layers, is the tilt angle of the surface of the dichroic mirrors 51 and 5b with respect to the optical axis and the three-dimensional one-dimensional pixel rows 61, 62 and 63 on the light receiving element 6 shown in FIG. It is determined by the pitches P1 and P2 between them. The procedure for obtaining the thickness of the thin glass plates 512 and 522, which are the transparent layers of the dichroic mirrors 5a and 5b, will be described below with reference to FIGS.

Transparent layer of dichroic mirror (refractive index n
₅ , the thickness t ₃ ), the incident angle θ ₃ of the light flux, the refraction angle θ ₄ , and the beam separation width d ₂ are shown in FIG.

Now, consider the blue component light reflected by the uppermost surface 513 of the dichroic mirror 5a and the yellow component light reflected by the total reflection film surface 511.

A point O is formed on the surface of the dichroic mirror at an angle of θ _3.
Of the white light incident on, the blue component light is reflected at an angle θ ₃ 'which is symmetric with respect to the perpendicular of the surface, while the yellow component light other than the blue component light is refracted at the refraction angle θ ₄ to the next surface. Go to At this time, the relationship between the angles θ ₃ and θ ₄ is defined by the refraction formula n ₁ sin
It is determined by θ ₃ = n ₅ sin θ ₄ .

The yellow component light refracted at the point O is totally reflected at an angle θ ₄ '(= -θ ₄ ) at the point P, and the refraction angle θ from the point Q.
₃ 'emitted by (= - [theta] _3), travels in parallel across a blue component light and distance d ₂ which is reflected at the first surface.

Here, d ₂ is represented by OQ · cos θ ₃ . However, OQ is the distance between the point O and the point Q. The same applies to OP, PQ, and OR described later. OQ is 2t
_{Since 3} · tan θ ₄ , d ₂ = 2 · t ₃ · tan θ
₄ × cos θ ₃ .

The effective optical path length difference between the optical path length of the blue component light from the point O to the point R and the optical path length of the yellow component light from the point O to the point P to the point Q, that is, The optical path length difference ΔL converted into a vacuum is represented by the following equation.

ΔL = {(OP + PQ) / n ₅ }-(OR / n ₄ ) = {(2t ₃ ) / (n ₅ cos θ ₄ )}-{(2t ₃ · tan θ ₄ × sin θ ₃ ) / n ₄ } (n _4: refractive index of air, where a 1) Therefore, the refractive index n ₄ = 1 in air, the refractive index n ₅ = 1.517, the light incident angle theta ₃ = 22.5 degrees transparent layer glass In this case, if the thickness t ₃ of the transparent layer glass is 166 μm, then d ₂ = 80 μm. At this time, the effective optical path length difference between the blue component light and the yellow component light is ΔL-193
μm.

Next, the blue component light reflected by the uppermost surface 523 of the dichroic mirror 5b after being reflected by the dichroic mirror 5a, and the green component light of the yellow component light,
Consider the red component light reflected by the total reflection film surface 521.

A point S is formed on the surface of the dichroic mirror at an angle θ _3.
The blue component light that is incident on and the green component light of the yellow component light that is incident on the point O at an angle θ _{3 are} at an angle θ that is symmetric with respect to the perpendicular of the surface.
While reflected at ₃ ', the red component light other than the green component light of the yellow component light is refracted at a refraction angle θ ₄ and goes to the next surface. At this time, the relationship between the angles θ ₃ and θ _{4 is} as follows:
₄ sin θ ₃ = n ₅ sin θ ₄ is determined.

The red component light refracted at the point O is totally reflected at the angle P ₄ '(=-θ ₄ ) at the point P, and the refraction angle θ from the point Q.
_{It is} emitted at ₃ ′ (= −θ ₃ ), and travels in parallel with the green component light reflected on the first surface with an interval d ₂ (see FIG. 6).

Here again, the refractive index in air n ₄ =
1, the transparent layer glass has a refractive index n ₅ = 1.517 and a light incident angle θ ₃ = 22.5 degrees, the transparent layer glass has a thickness t _2.
= 166 μm, d ₂ = 80 μm can be obtained.

Therefore, the white light after being reflected by the two dichroic mirrors 5a and 5b under the above-mentioned conditions is separated by 80 μm from each other, and the blue component light, the green component light,
It is separated into red component light. At this time, the effective optical path length of the green component light with respect to the blue component light is about 193 μm long, and the effective optical path length of the red component light with respect to the blue component light is about 38 μm.
6 μm longer.

Therefore, the amount of longitudinal chromatic aberration of the imaging lens 4 is set so as to meet these conditions. FIG. 7 shows the relationship between the image forming position and the MTF for each color component light of the image forming lens used in this embodiment. As can be seen from the figure, the image forming position showing the peak of MTF of the green component light is M of the blue component light.
It is located approximately in the middle of the peak position of TF and the peak position of MTF of red component light, and the intervals between the respective peaks are separated by about 0.2 mm. By combining such an imaging lens 4 and the dichroic mirrors 5a and 5b, the shift of the imaging position due to the difference in the optical path length of each color component light by the color separation means 5 is offset by the amount of longitudinal chromatic aberration of the imaging lens 4. It is possible to form the separated color component light of the three primary colors of green, blue, and red on the one-dimensional pixel array without defocusing.

In the color image reading apparatus of the embodiment, the one-dimensional pixel rows 61, 62, 63 on the light receiving element 6 are arranged at equal intervals, but two dichroic mirrors of the color separation means 5 are used. The thicknesses 512 and 522 of the transparent glass layers 5a and 5b can be independently set, and the imaging lens 4
Since the interval between the image forming positions of the respective color components can be adjusted to some extent depending on the amount of longitudinal chromatic aberration of the light receiving element 6
The upper one-dimensional pixel rows do not have to be arranged at equal intervals.

In the color image reading apparatus of the embodiment, the image forming positions of the respective color components due to the longitudinal chromatic aberration of the image forming lens 4 are in the order of blue component light, green component light and red component light. By doing so, there is an advantage that the loss of the amount of blue component light in the color separation means 5 can be minimized and a light source with a small amount of blue component light can be effectively used. However, in some cases, the order of the green component light, the blue component light, and the red component light, or the order of the green component light, the red component light, and the blue component light can be set.

[0042]

According to the color image reading apparatus of the present invention,
Even when an imaging lens with chromatic aberration is used, the optical path length of the color-separated light flux of each color can be canceled by the color separation means in accordance with the longitudinal chromatic aberration amount of the imaging lens, and all of the plurality of light rays can be canceled out. It is possible to accurately focus on each of the plurality of one-dimensional pixel rows on the light receiving element. As a result, the reading performance of the color image reading device can be improved, and reading with less color misregistration can be performed. Further, the design and manufacture of the imaging lens can be facilitated, and the cost of the entire device can be reduced.

Further, uniform spectral characteristics can be obtained in the entire area in the main scanning direction, and the reading performance of the color image reading device can be improved.

Further, it is possible to accurately focus all of the plurality of color-separated light rays on the plurality of line sensors at the same magnification.

[Brief description of drawings]

FIG. 1 is a plan view of a color image reading apparatus showing an embodiment of the present invention.

FIG. 2 is a side view of a color image reading apparatus showing an embodiment of the present invention.

FIG. 3 is a diagram showing spectral characteristics of a blue reflection dichroic mirror in an example.

FIG. 4 is a diagram showing spectral characteristics of a blue-green reflecting dichroic mirror in an example.

FIG. 5 is a diagram illustrating optical path lengths of blue component light and yellow component light in the dichroic mirror in the example.

FIG. 6 is a diagram illustrating optical path lengths of blue component light, yellow component light, and red component light in the dichroic mirror in the example.

FIG. 7 is a correlation diagram showing a relationship between image forming positions of green, blue and red component light components and MTF values in the image forming lens in the embodiment.

FIG. 8 is a configuration diagram of a conventional color image reading device.

FIG. 9 is a configuration diagram of another conventional color image reading device.

FIG. 10 is a configuration diagram of still another conventional color image reading apparatus.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Light source, 2 ... Platen glass, 3 ... Original, 4 ... Imaging lens, 5 ... Color separation means, 6 ... Light receiving element, 7 ... Optical axis, 51
3, 523 ... Dichroic film, 512, 522 ... Transparent layer, 511, 521 ... Total reflection film, 61, 62, 63 ... One-dimensional pixel array

Continuation of front page (51) Int.Cl. ⁶ Identification number Office reference number FI technical display location // G02B 13/22

Claims (6)

[Claims] 1. A light source, a light-receiving element formed by arranging a plurality of one-dimensional pixel rows in parallel on one chip, an imaging lens for condensing reflected light from a document illuminated by the light source, and a combination thereof. In a color image reading device including color separation means for color-separating light passing through an image lens and forming an image on a different one-dimensional pixel array for each wavelength component, the imaging lens is The chief ray has optical characteristics of a telecentric system set so as to be parallel to the optical axis, the color separation means, a dichroic film that reflects only light of a specific wavelength and a total reflection mirror transparent layer. A color image reading apparatus comprising a plurality of color separation elements laminated through the above. 2. The image forming lens has a short focal length of blue light and a long focal length of red light with respect to a focal length of green light due to an axial chromatic aberration.
The described color image reading device. 3. The color image reading apparatus according to claim 1, wherein the image forming lenses have equal focal distances for blue light, green light, and red light due to axial chromatic aberration. . 4. The color separation means includes a first color separation element that separates white light into blue light and yellow light, and a second color separation element that separates white light into bluish green light and red light. The color image reading apparatus according to claim 1, further comprising: 5. The thickness of the transparent layer between the dichroic film and the total reflection mirror in the color separation means is set so as to cancel out the difference in focal length between the color lights in the imaging lens. The color image reading device according to claim 1, wherein 6. The light receiving element is for blue light, green light,
The color image reading apparatus according to claim 1, wherein the color image reading apparatus is a three-line sensor having three one-dimensional pixel rows for red light.