JPS6378663A - Image reader - Google Patents

Abstract

PURPOSE:To always form images of originals on a line sensor regardless of the wavelength of light by installing on an optical path from a color image to the line sensor a transparent member which removes color aberration. CONSTITUTION:A transparent glass 5a with a thickness t1 is installed between a 'Selfoc' lens 3 and the line sensor S, and on a plane perpendicular to the optical aixs of the lens 3. In order to obtain G-colored image information, a light source 2a is replaced with a G-colored single emission light source 2b, while the transparent glass 5a, is replaced with the transparent glass 5b of a thickness t2 (> t1). Thus, if a device uses light beams R, G and B with the same distance between the 'Selfoc(R)' lens 3 and the line sensor 3 and different wavelengths, the device can always obtain the best focus on the line sensor by replacing different glasses 5a-5c with thickness (t) in accordance with the light beams R, G and B.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention is an image reading device, more specifically, an image reading device that has a pixel array that illuminates a color image with an illumination light source, and reads this color image with a converging light guide lens. The present invention relates to an image reading device that forms an image on a line sensor, reads image information, and photoelectrically converts the image information to obtain a digital signal.

(Prior technology) There are various image reading methods that optically separate color image information, form an image on a reading line sensor, photoelectrically convert it, and convert it into a digital signal.The representative one is one of various image reading methods. As a method, a document containing image information is illuminated with white light, and the reflected light is optically reduced and projected onto a line sensor such as a COD through a normal lens, and multiple color separation filters are placed in the middle of the optical path. There is a method in which the color separation reading is performed while sequentially replacing the filters, and a method in which the illumination light source is separated into colors and the irradiation of each of a plurality of colors is read in reverse. These are conventional methods in which a single line sensor repeatedly reads three colors in sequence.

In addition, for a reading method that emphasizes speed, the optical axis of a single optical system is changed by using a gicroic mirror to change the optical path of each, and multiple line sensors are arranged, and at the same time, each line sensor is connected to a different color filter. There is a method of extracting the color-separated signals through the process.

In a color image reading method using a normal spherical lens, in order to improve chromatic aberration in the optical axis direction (shift of the best image forming point) and chromatic aberration in the angle of view direction (color shift), a spherical lens consisting of multiple lenses is used. Chromatic aberration is removed at the lens design stage by selecting the glass material to minimize the difference in refractive index that occurs for each color. This countermeasure is easy to take, and the variation in imaging accuracy on the sensor of the color separation by the color separation filter is improved, and the problem of chromatic aberration does not occur.

However, if a 1-magnification imaging system is constructed using a compound lens that combines three or more spherical lenses, the distance between the object plane and the image plane will be four times the focal length f of the lens, making the reading optical system larger. There was a drawback.

In recent years, with the practical use of a converging light guide lens (Nippon Sheet Glass Co., Ltd., product name SELFOC Lens), a compact image reading device that integrates a reading optical system and a 1x contact type reading line sensor has been put into practical use. has been made into

Figure 5 is an explanatory diagram of a conventional image reading device using SELFOC. . This focusing lens element 3' has a parabolic refractive index distribution, and the light rays incident on this lens element 3' have the same meandering period (P=2πrT) regardless of the incident angle.
), the SELFOC lens 3 is sandwiched between two frame plates 3a, and a black silicone resin or the like is filled between the frame plates 3a to remove flare light around the SELFOC lens 3. has been done. In addition, the side frame 3b is made of a glass cloth-based epoxy resin black laminate that has almost the same characteristics as the thermal expansion of the lens.
The aim is to reduce thermal strain and increase strength.

In the figure, the conjugate length indicated by TC is TC'=Z +2i.

The TC can be made to have approximately 15 to 70 layers, thereby making the image reading device more compact.

The above device irradiates light emitted from a light source 2 onto an original document having color image information placed on a holder 1, and uses a line sensor (CO
D, 1-magnification contact type sensor) Image information is formed on S by moving the line sensor S and SELFOC lens 3 relative to the document, and the line sensor S photoelectrically converts the image information. The electric signal generated is A/D converted to obtain a digital signal of image information. Note that Wl) is the effective length of the SELFOC lens.

(Problems to be Solved by the Invention) Next, using FIG. 6, chromatic aberration that occurs when reading a color image will be described. If the light source 2 is, for example, an LED light source, the LED light source emits a monochromatic light beam necessary for color separation reading, irradiates the document, and the reflected light is introduced from the end face of the lens element 3' (for example, at the position of the arrow). Chromatic aberration of a lens is caused by the refractive index nQ on the optical axis and the refractive index distribution constant T, which gives a slightly different refractive distribution depending on the wavelength, and the light incident on the lens has a meandering period P that differs depending on the wavelength, and the periods of R, G, and B. are different as shown in the figure.

The magnitude of chromatic aberration ΔP/P is expressed by the following formula.

In Fig. 7, a indicates the R light source, b indicates the G light source, and C indicates the B light source. Image points 4a and 4b on the line sensor S
, 4c, and as described above, the TC differs for each wavelength due to chromatic aberration, and for this reason, if image transmission is performed under a white light source, the resolution will decrease. When reading color images, in order to reduce this chromatic aberration as much as possible, we try to improve it by selecting glass materials so that it can withstand practical use.However, as with countermeasures against chromatic aberration in spherical lenses, chromatic aberration can be reduced by selecting and combining the materials of multiple lenses. Unlike the case of improvement, there is a limit to the improvement with only a single glass, and there is a problem that chromatic aberration becomes an obstacle to obtaining high density and high image quality.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and it is an object of the present invention to provide an image reading device in which an image of a document is always formed on a line sensor regardless of the wavelength of light.

(Means for Solving the Problems) In order to achieve the above object, the present invention provides a transparent member for removing chromatic aberration in the optical path from the color image to the line sensor.

(Function) Because of the above configuration, an image is formed on the line sensor regardless of the wavelength of the light.

(Example) The present invention will be explained below based on illustrated embodiments.

A, b, and c in FIG. 1 show the first embodiment of the present invention, and are between the SELFOC lens 3 and the line sensor S, and are orthogonal to the optical axis of the lens 3. A transparent glass 5a having a thickness tl is placed on the surface. 2a
is a single light emitting light source with R color, and when this light source 2a illuminates the color image (original) placed on 1, an R color image of the original is formed on the line sensor S. Thereby, image information of the R color of the document can be obtained from the line sensor S.

To obtain G color image information, the light source 2a is replaced with a G color single-emission light source 2b, and the transparent glass 5a has a thickness of t2 (>tt
).In order to obtain image information of B color in the same way, replace the light source 2b with the single light emitting light source 2 of B color.
C, and the transparent glass 5b is replaced with a transparent glass 5C having a thickness of 13 mm (>t2).

By the way, the value refracted by the transparent glasses 5a to 5C is determined by the thickness t of the glass, and the position where the line sensor should be installed is TC+Δt. The distance to move away is Δt1
．． The refractive index of the glass plate is considered. The refractive index n of the glass plate is generally 1.52.

Under the condition that the distance relationship between the SELFOC lens 3 and the line sensor S is constant, the respective lights iiG and H determined by the chromatic aberration of the light rays G and B having different wavelength ranges shown in FIG. If the distances from the best focus point to the top of the line sensor are Δt2 and Δt3 (not shown), the respective glass thicknesses t2. t3 can be found. Therefore, even if the distance between the SELFOC lens 3 and the line sensor S is constant and the light rays R, G, and B are different in wavelength, the glasses 5a to 5C having different thicknesses L can be By replacing the lenses accordingly, it is possible to always obtain the best focus on the line sensor S, completely eliminate monochromatic aberration, and obtain high-density, high-quality color separation signals.

A, b, and c in FIG. 2 show the second embodiment, and 2'
is a white light source, and 6(R), 6(G), and 6(B) are color separation filters made of glass with different thicknesses.
These color separation filters 8 (R), 6 (G),
6 If (B) is replaced and set in the illustrated position according to the color separation, R color with completely removed chromatic aberration,
Color separation signals of G and B colors can be obtained.

Figure 3 and Figure 4 show a third embodiment, which is a reading system of a serial scanning reading device, and in which a SELFOC lens 3 and a line sensor S have different thicknesses. Color separation filter 8 for R, G, and B colors
R, 8G, 8B are installed, and this color separation filter 8R
,8G.

8B, white light source 2', SELFOC lens 3, and line sensor S integrally scan back and forth along the guide rail 7 to convert the color image information on the document M into R, G, and B colors.
The three color separated signals are obtained simultaneously. Color separation filters 8R and 8G of this embodiment.

8B can be installed in a low position without replacing it.

Second. The thickness of the color separation filter in the third embodiment is set in the same manner as in the first embodiment.

Note that the color separation filter and the transparent glass in the above embodiments can also be made of transparent plastic. In addition, the thickness of the transparent glass is changed to eliminate chromatic aberration,
It may be made of a transparent material with a constant thickness and different refractive indexes.Alternatively, as a modification of the embodiment shown in FIG. 6, an integrated reading sensor in which line sensors are arranged in double rows in parallel on the same substrate The chromatic aberration members may be configured correspondingly.

(Effects) In summary, this invention can form an image on the line sensor regardless of the wavelength of the light resulting from color separation, so chromatic aberration can be completely removed and color separation signals with high density and high image quality can be produced. can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a schematic configuration diagram of a first embodiment of an image reading device according to the present invention, FIG. 2 is a schematic configuration diagram of a second embodiment of the present invention, and FIGS. Schematic configuration diagram of the embodiment, fifth
The figure is an explanatory diagram of a conventional image reading device, and FIGS. 6 and 7 are explanatory diagrams of chromatic aberration. Explanation of symbols 2a, 2b, 2c...R, G, B color single emission light source 3...Selfoc lens S...Line sensor 5a, 5b, 5c...Transparent glass Fig. 1 Fig. 2 Figure Cb Q Figure 6 Figure 7 Cb a

Claims (5)

[Claims] (1) A color image is irradiated by an illumination light source, this color image is read by a focusing light guide lens, and imaged on a line sensor having a pixel array to read image information, and this image information is photoelectrically converted. What is claimed is: 1. An image reading device that obtains a digital signal by using a transparent member that removes chromatic aberration in an optical path from the color image to the line sensor. (2) The image reading device according to claim 1, wherein the transparent member has a color separation filter function. (3) The image reading device according to claim 1, wherein the transparent member has a different thickness depending on the color separation. (4) The image reading device according to claim 1, wherein the transparent member is made of a material having a different refractive index depending on the color separation. (5) The transparent member is composed of a plurality of members that remove chromatic aberration corresponding to color separation, and the members are replaced according to the color separation. The image reading device described.