JPH0468756A - Picture reader - Google Patents

Abstract

PURPOSE:To reduce uneven luminous quantity distribution on a face of a light receiving element while preventing the deterioration in the MTF of an image forming optical system in its sagital direction by providing an optical member whose transmittivity differs from an incident angle in the vicinity of an aperture position in the image forming optical system. CONSTITUTION:An optical member 4 is made of a member such as a visual field selection glass having an incident angle dependency in which its transmittivity differs from its incident angle and arranged in the vicinity of an aperture 3. The transmission luminous quantity of an optical flux from each of object points a, b, c is controlled by the optical member 4. That is, the transmittivity of luminous flux is limited so as to be increased in the order of that from the object points a, b, c and the luminous flux passes through a radiation pupil Ik and is made incident each of points a', b', c' on a face 5 of a light receiving element through an image forming optical system 2. Thus, the luminous quantity distribution on the light receiving element face is corrected in an excellent way without use of a shading plate.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to an image reading device, and particularly to an end portion of a light receiving element such as a line sensor (CCD) for image reading in the direction in which the elements are lined up (main scanning direction). The present invention relates to an image reading device suitable for, for example, a copying machine, a facsimile machine, an image scanner, etc., which effectively reduces light fall-off in the document surface and reads image information on the surface of a document with high precision with a simple configuration.

(Prior Art) Conventionally, in an image reading device that uses a light receiving element such as a line sensor to read image information on a document surface illuminated by an illumination system, the document surface is illuminated by a light source such as a fluorescent lamp. was. The reflected light beam from the document surface is then passed through an imaging optical system to form an image on the document surface onto the light receiving element surface, and the output electric signal from the light receiving element is processed and read by an image processing section. . Two-dimensional image information on the document surface is read by moving the document surface or the image reading means in the sub-scanning direction.

In such an image reading device, the light intensity distribution on the surface of a light receiving element such as a line sensor (corresponding to an output signal from the light receiving element) is designed to minimize the drop in light intensity at both ends thereof. That is, the illuminance distribution on the light receiving element surface is made as uniform as possible to improve image reading accuracy.

Generally, when a document surface is illuminated by a light source consisting of a line-shaped light emitting surface, the amount of emitted light differs between the center and the edges of the light emitting surface, and especially when a fluorescent lamp is used as the light source, the amount of light at the edges is lower than that at the center. considerably less compared to

FIG. 4(A) is an explanatory diagram showing the light distribution characteristics of such a fluorescent lamp when the fluorescent lamp is used.

As shown in the figure, the light intensity at both ends of the fluorescent lamp in the horizontal direction is approximately 40% lower than that at the center, and this may be one reason for the illuminance distribution between the center and both ends on the document surface. It was uneven.

In addition, when image information on the document surface is imaged through an imaging optical system, due to vignetting of the light flux from the periphery of the screen passing through the imaging optical system and the cosine fourth power law, the image information on the light receiving element surface becomes more central. For example, in the case of an imaging optical system with a half angle of view of 20°, the end portion on the light receiving element surface is 22 compared to the center
The light intensity will decrease by %.

Therefore, when the reflected light flux from the document surface illuminated by such a fluorescent lamp passes through the imaging optical system and forms an image on the light receiving element surface, the output signal from the light receiving element is as shown in Figure 4 (A). (B3 is multiplied by the sum of 4 [A (C).
It becomes V. That is, the output at both ends is 47% higher than the output value at the center.

Therefore, in order to solve the above-mentioned problems, in the conventional image reading device, a fifth (as shown in 2I) is used to adjust the amount of light beams passing from the center and periphery of the original 59 in front of the imaging optical system 51. A shading plate 52 having an opening is provided to adjust the amount of light incident on the light receiving element 53.

In the figure, the surface of a document 59 illuminated by a light source (for example, a fluorescent lamp) 58 is moved in the sub-scanning direction by a transport roller (not shown), and the reflected light beam from the surface of the document 59 is sequentially reflected by mirrors 55, 56, 56, 56, 55, 56, 56, 56, 56, 56, 56, 56, 56, 56, 56, 56, 56, 56, 56, 56, 56, 56, 56, 56, 56, 56, 56, 56, 56, 56. 54, the optical path is bent, and the light enters the imaging optical system 51 via the shading plate 52. Then, the original 59 is placed on the surface of the light receiving element 53 by the imaging optical system 51.
The image information on the 59 sides of the original is sequentially read by forming an image.

FIG. 6 is an enlarged explanatory diagram of light amount adjustment by the shading plate 52 of a part of the image reading device shown in FIG. In this figure, the same elements as those shown in FIG. 5 are given the same reference numerals.

Reference numeral 52a indicates the diameter of the light beam from the document surface that can pass through the imaging optical system 51.

In the figure, a shading plate 52 having an opening that is larger at the periphery than at the center reduces the light flux near the axis by, for example, about 33% compared to the light flux farthest off-axis. Further, the opening of the shading plate 52 is determined so that the light beam corresponding to the most off-axis position, for example, an image height of 12.5 mm, is not eclipsed.

That is, the shape of the opening of the shading plate 52 is formed so that the amount of light passing through it increases sequentially from the center toward the ends. As a result, as shown in No. 4 [1 (DJ), the light intensity drop at the edge of the light-receiving element surface relative to the center is about 30%, which is the allowable value.

FIG. 4(D) is an explanatory diagram showing an output signal from the light receiving element 53 when the light intensity drop at the end on the surface of the light receiving element 53 is corrected by the shading plate 52.

(Problems to be solved by the invention) However, conventional image reading devices have light-receiving elements such as line sensors (CCDs) arranged in front of the imaging optical system in the main scanning direction (hereinafter referred to as "mewa"). The image forming optical MTF in the direction of the system (ModuIation)
There was a drawback that it lowered the function.

Figures 7 (A) and (B) show the MT of the sharpness and sharpness at each image height when a shading plate with different apertures in the sharp and sharp directions is not placed in front of the imaging optical system and when it is placed in front of the imaging optical system, respectively.
FIG. 3 is an explanatory diagram showing F characteristics.

When the shading plate in Figure (A) is not placed, the MTF values of the sharpness and sharpness are almost the same on the axis (image height Omm), or at the image height of 6.5mm, 9.5mm, and 12.5mm. In this case, Saji's MTF@ is higher than Meri's MTF (1).

On the other hand, when the shading plate shown in Figure (B) is placed in front of the imaging optical system, the aperture size of the sash is smaller than that of the sash, so the F number of the sash is darker than the F number of the sash. Another drawback is that the MTF of the sash decreases at a narrow image height (center portion) of the aperture due to the influence of diffraction.

Also, as mentioned above, due to the influence of the cosine fourth law of the imaging optical system, the amount of light falls off at the edges of the light receiving and receiving element surface, which has a wide field of view, compared to the center, which increases. Furthermore, the opening in the sagittal direction at the center of the shading plate had to be narrowed.

Therefore, the narrow central region of the aperture causes a partial deterioration of the MTF characteristics, and for this reason, the maximum half-field angle of the image reading device using the above-mentioned shading plate is limited to about 25 degrees.

This hinders the widening of the field of view of the imaging optical system and becomes a major obstacle to shortening the object-to-image distance.

The present invention effectively prevents a decrease in the MTF of the surge of the imaging optical system by providing an optical member having appropriate optical characteristics in a part of the imaging optical system that constitutes a part of the image reading device. Another object of the present invention is to provide an image reading device that can read an image on a document surface with high precision by reducing the amount of light falling off at the end portion of the surface of a light-receiving element.

(Means for Solving the Problems) The image reading device of the present invention forms an image of a document surface illuminated with a light beam from a light source onto a light-receiving element surface via an imaging optical system, thereby forming an image on the document surface. In an image reading device that reads information,
It is characterized in that a part of the imaging optical system is provided with an optical member whose transmittance varies depending on the angle of incidence for adjusting the illuminance distribution on the surface of the light receiving element.

In particular, the present invention is characterized in that the optical member is constituted by a parallel plane plate having a characteristic that the amount of transmitted light increases as the incident angle increases, and is disposed near the aperture of the imaging optical system.

(Example) FIG. 1 shows f! of the present invention. 141 is a cross-sectional view of a main part showing the image forming state of the imaging optical system 2 used in the image reading apparatus of the No. 141 embodiment. FIG.

The imaging optical system of this embodiment is arranged at approximately the same position as the imaging optical system 51 in the image reading apparatus shown in FIG. 145, for example.

This figure shows how light beams from various positions on the object surface (original surface) l are incident on a light-receiving element surface (combination surface) 5 of, for example, a line sensor (CCD).

That is, 1 is an object surface, which is a document surface, and 2 is an imaging optical system, which forms an image on the object surface 1 on a light-receiving element surface 5 at a predetermined magnification. 3 is a diaphragm, and 4 is an optical member according to the present invention, which is formed of a member such as a viewing field selection glass whose transmittance varies depending on the incident angle, and is arranged near the position of the diaphragm 3. ing.

The optical member 4 according to this embodiment has an optical property that the transmittance increases as the incident angle increases. Reference numeral 5 denotes a light receiving element surface composed of, for example, a one-dimensional line sensor (CCD).

In this embodiment, the light beams based on the image information emitted from the object points ab and c on the object plane l become substantially parallel light beams after passing through the entrance pupil I0 of the imaging optical system 2, and are arranged near the aperture 3 position. The light is incident on the optical member 4 at angles θ, θゎ, and 0 degrees, respectively.

The optical member 4 controls the amount of transmitted light from each of the object points a, b, and c. In other words, the exit pupil is restricted so that the transmittance of the light beams from object points a, b, and c increases in that order!
, it exits the imaging optical system 2 and enters each point a', b', c' on the light receiving element surface 5. Then, an image of the document l is formed on the light receiving element surface 5, thereby sequentially reading image information on the document surface.

Next, the optical features of the optical member 4 according to the present invention will be explained.

As shown in FIG. 2(A), the optical member 4 has transmittance characteristics that vary depending on the angle of incidence of the incident light beam. It is formed so that all the light is transmitted (100% transmittance) when the light is incident at a certain angle.

Also, when the incident angle is 0 degrees from the point C on the axis to the optical member 4, it is about 67% (transmittance 67%) compared to when the incident angle is θ1.
) is allowed to pass through. That is, the angle of incidence θ,
If the transmittance is 1 when the incident angle is 0, the transmittance is 0.67 when the incident angle is 0.

In this manner, in this embodiment, the dependence of the optical member 4 on the incident angle is configured such that the transmittance decreases sequentially as the incident angle decreases. And the light receiving element surface (imaging surface)
This effectively reduces the fall in the amount of light at the edges of the center of the screen. That is, the illuminance distribution on the light receiving element surface 5 is made as uniform as possible, thereby reading image information on the document surface with high precision.

FIG. 2(B) shows the transmittance characteristics of the optical member 4 according to the present invention shown in FIG. 2(A) with respect to the incident angle, and the transmittance characteristics shown in FIG.
) The angle of view at image height a (12.5 mm) shown by the solid line is 2.
The multiplication is made to match the cosine fourth power law of the imaging optical system which is 0°. FIG.

As shown in the figure, the output signal from the ends of the light receiving element (corresponding to the amount of light incident on the light receiving element) is higher than that from the center. When this is multiplied by the light distribution characteristics of the fluorescent lamp shown in FIG. 4(A), the result is as shown in FIG. 2(C). In other words, it is possible to obtain good light distribution characteristics with less drop in the amount of light at the end portions on the surface of the light-receiving element.

As shown in FIG. 4(C), in the conventional image reading device, the light distribution characteristic (solid line in FIG. 4(B)) based on the cosine fourth law of the imaging optical system 4 and the light distribution of the fluorescent lamp are combined. Characteristics (Figure 4 (
When A)) is combined, the amount of light falls off by 53% at the edges compared to the center on the light receiving element surface.In this example, as shown in Figure 2 (C), The drop in light intensity at the edge of the light-receiving element surface is reduced to about 30%. The light intensity drop at this time is approximately the same as the light distribution characteristic when a conventional shading plate is used, as shown in No. 411(D).

In this way, in this embodiment, by arranging an optical member having incidence angle dependence near the aperture position of the imaging optical system, the light intensity distribution on the light receiving element surface can be adjusted without using a shading plate. This allows for good correction. This also reduces the number of parts and makes it easier to make the entire device more compact.

Moreover, in this embodiment, since there is no change in the F number in the meridian direction and the sagittal direction of the imaging optical system, the MT of the imaging optical system
It has the advantage of not causing a decrease in F.

FIG. 3 is a schematic diagram of the main parts showing the imaging state of the imaging optical system used in the image reading apparatus according to the second embodiment of the present invention.

In this example, we used an optical member with incident angle dependence and an MT with a larger aperture diameter at the center compared to conventional shading plates.
By using a shading plate configured so as not to reduce the F performance too much, the decrease in the amount of light at the periphery of the screen is further reduced.

In the figure, 32 is a shading plate having an aperture for controlling the amount of light beam passing from an unillustrated original, and 31 is an imaging optical system, which is provided within the imaging optical system 31. An optical member having the above-mentioned incidence angle dependence is provided near the aperture position.

33 is a light receiving element, for example, a one-dimensional line sensor (C
CD), etc. 32a indicates the diameter of the light beam that can pass through the imaging optical system 31.

The area where the opening of the shading plate 32 and the beam diameter 32a overlap determines the amount of light that passes through the imaging optical system 31.

In this embodiment, a shading plate 32 having a wider opening in the center than the conventional shading plate shown in FIG. 6 is effectively combined with the above-mentioned optical member having angular dependence. This corrects the light amount distribution on the light receiving element surface.

Generally, in order to make the entire device more compact, it is necessary to shorten the distance between the object and image, and as a result, the imaging optical system has to have a wider angle of view. It becomes very difficult.

For example, as shown by the dotted line in FIG. 4(B), the image height is 12.
．． Assuming that the angle of view at 5 mm is 27'', the amount of incident light at the edge of the light receiving element surface (corresponding to the output signal from the light receiving element) is reduced by 36% compared to the center.

This means that the amount of light falling off at the edges is 14% higher than when the angle of view is 20 degrees when the image height is 12.5 mm, as shown by the solid line in the figure. Therefore, in this embodiment, in order to reduce the amount of light falling off at the end on the light receiving element surface due to the wide angle of view of the imaging optical system, we used an optical system that has an incident angle dependence on the aperture position of the imaging optical system. The members are efficiently combined with the shading plate 32 shown in FIG. 6, which has a wider opening in the center than the conventional shading plate and does not significantly reduce the MTF.

This reduces the drop in the amount of light at the edge of the light receiving element surface due to the wide angle of view of the imaging optical system, thereby obtaining an appropriate output signal from the light receiving element 33.

With this configuration, in this embodiment, the entire apparatus can be made compact, and a decrease in the MTF of the imaging optical system when a shading plate is used is effectively prevented.

(Effects of the Invention) According to the present invention, an optical member having a different transmittance depending on the incident angle is provided near the aperture position in the imaging optical system that forms an image of image information on the document surface onto the light receiving element surface. , while preventing the MTF of the imaging optical system from decreasing in the sharp direction, it is possible to reduce the unevenness of the light amount distribution on the light receiving element surface, and moreover, it is possible to read images with high precision while making the entire #It compact. An image reading device can be achieved.

[Brief explanation of drawings]

Figure 1 is the first example of the present invention! FIG. 2(A) is a sectional view of a main part showing the imaging state of the imaging optical system of the image reading device of the example, and FIG.
B) is the cosine fourth law using the imaging optical system and Figure 2 (A)
An explanatory diagram of the light distribution characteristics when the transmittance characteristics of the optical members shown in FIG. 3 is an explanatory diagram showing the output signal from the light-receiving element when combined, FIG. Figure 4 (A) is an explanatory diagram showing the light distribution characteristics of a fluorescent lamp, Figure 4 (B) is an explanatory diagram showing the light distribution characteristics according to the cosine fourth law of the imaging optical system, and Figure 4 (C) is an explanatory diagram showing the light distribution characteristics of a fluorescent lamp.
is an explanatory diagram showing the output signal from the light receiving element when the light distribution characteristics of Fig. 4 (A) and (B) are multiplied together;
D) is an explanatory diagram showing the output signal from the light-receiving element due to the light intensity correction of the shading plate, FIG. 5 is a schematic diagram of the main part of the optical system of a conventional image reading device, and FIG. 6 is the image reading device of FIG. 5. 71J(A), CB) is an explanatory diagram showing the MTF characteristics of the imaging optical system in the sharpness and sharpness directions with respect to the presence or absence of a shading plate. In the figure, l is the object plane, 2.31 is the imaging optical system, 3 is the aperture,
Reference numeral 4 designates an optical member 45 as a light-receiving element surface, 32 as a shading plate, and 33 as a light-receiving element.

Claims (3)

[Claims] (1) In an image reading device that images a document surface illuminated with a light beam from a light source onto a light receiving element surface via an imaging optical system and reads image information on the document surface, one of the imaging optical systems An image reading device comprising an optical member having a different transmittance depending on an incident angle for adjusting the illuminance distribution on the surface of the light receiving element. (2) The image reading device according to claim 1, wherein the optical member is provided near a diaphragm position of the imaging optical system. (3) The light receiving element is composed of a one-dimensional line sensor,
2. The optical member has a characteristic that the amount of transmitted light increases as the incident angle increases.
The image reading device described.