JP3067167B2 - Image reading device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image reading apparatus, and more particularly, to an end portion of a light receiving element such as a line sensor (CCD) for image reading in the arrangement direction (main scanning direction) of elements. The present invention relates to an image reading apparatus suitable for, for example, a copying machine, a facsimile, an image scanner, or the like, which effectively reduces a drop in the amount of light and reads image information on a document surface with a simple configuration.

2. Description of the Related Art Conventionally, in an image reading apparatus that reads image information on a document surface illuminated by an illumination system using a light receiving element such as a line sensor, the document surface is illuminated by a light source such as a fluorescent lamp. I was Then, an image on the document surface is imaged on the light receiving element surface via the image forming optical system with the reflected light beam from the document surface, and the output electric signal from the light receiving element is processed and read by the image processing unit. . Then, 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 apparatus, a light amount distribution on a light receiving element surface of a line sensor or the like (corresponding to an output signal from the light receiving element)
As for, the drop in the amount of light at both ends is made as small as possible. That is, the illuminance distribution on the light receiving element surface is made as uniform as possible to improve the image reading accuracy.

Generally, when the document surface is illuminated with a light source having a linear light emitting surface, the light emission amount differs between the central portion and the end portion of the light emitting surface, especially when a fluorescent lamp is used as the light source, the light amount at the end portion is in the central portion. Considerably less than in comparison.

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

As shown in the figure, the both ends in the longitudinal direction of the fluorescent lamp have a light intensity drop of about 40% compared to the center part. One of the causes is that the illuminance distribution at the center part and both ends on the document surface is reduced. Was uneven.

When the image information on the document surface is imaged through the image forming optical system, the light beam from the peripheral portion of the screen passing through the image forming optical system is further centered on the light receiving element surface due to the vignetting and the cosine fourth power rule. And the difference in light quantity between the two ends. For example, in the case of an imaging optical system having a half angle of view of 20 °, the edge on the light receiving element surface is 22% smaller than the center as shown by the solid line in FIG. 4 (B).
The amount of light falls.

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 FIG. (B)
As shown in FIG. 4 (C), for example, the center output value is 600 mV, and the end output value is 282 mV.
That is, the output at both ends is only 47% of the output value at the center
It becomes.

Therefore, in the conventional image reading apparatus, in order to solve the above-mentioned problem, as shown in FIG. 5, an opening for adjusting the amount of light flux passing from the central portion and the peripheral portion of the original 59 is provided in front of the imaging optical system 51. A shading plate 52 having a portion is provided to adjust the amount of light incident on the light receiving element 53.

In the figure, a document 59 illuminated by a light source (eg, a fluorescent lamp) 58 is moved in the sub-scanning direction by a transport roller (not shown) to sequentially reflect light beams reflected from the document 59 onto a reflection mirror 5.
The light path is bent through 5, 56, 54 and the shading plate 52
Is incident on the imaging optical system 51 via. Then, an image of the original 59 is formed on the light receiving element 53 by the imaging optical system 51, and the image information on the original 59 is sequentially read.

FIG. 6 is an enlarged explanatory view of light amount adjustment by the shading plate 52 of a part of the image reading apparatus shown in FIG. In this figure, the same elements as those shown in FIG. 5 are denoted by the same reference numerals. Reference numeral 52a denotes a light beam diameter of a light beam from the document surface that can pass through the imaging optical system 51.

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

That is, the shape of the opening of the shading plate 52 is formed so that the amount of passing light flux increases sequentially from the center to the end. As a result, as shown in FIG. 4 (D), the drop in the amount of light at the end with respect to the center on the light receiving element surface is an allowable value.
%.

FIG. 4 (D) shows a light receiving element using a shading plate 52.
Light receiving element 53 when the light intensity drop at the end on the 53 surface is corrected
FIG. 4 is an explanatory diagram showing output signals from the control unit.

(Problems to be Solved by the Invention) However, in the conventional image reading apparatus, the arrangement direction of light receiving elements such as a line sensor (CCD) in front of the imaging optical system, that is, the main scanning direction (hereinafter, “merit” or “merit direction”) ) In the mirror scanning direction, that is, in the sub-scanning direction (hereinafter, referred to as “sag” or “sag direction”). There is a disadvantage that the MTF (Modulation Transfer Function) in the direction is reduced.

FIGS. 7 (A) and 7 (B) show the MT of merging and sagittal due to each image height when shading plates having different apertures in the merri direction and the sagittal direction are not arranged in front of the imaging optical system.
FIG. 4 is an explanatory diagram showing F characteristics.

When the shading plate shown in FIG. 3A is not arranged, the MTF values of the merging and the sagittal are substantially the same on the axis (image height 0 mm), but the sagittal MTF values are 6.5 mm, 9.5 mm and 12.5 mm. The MTF value of is higher than the MTF value of Merri.

On the other hand, when the shading plate shown in FIG. 3B is arranged in front of the image forming optical system, the sagittal F-number becomes darker than Merri's F-number because the size of the opening is smaller in Saji than in Merri. Under the influence of diffraction, the MTF of the saji has a disadvantage that it decreases at an image height (center portion) where the aperture is narrow.

In addition, as described above, as the angle of view becomes wider due to the influence of the cosine fourth law of the imaging optical system, the amount of light drop at the end portion increases as compared with the center portion on the light receiving element surface. Furthermore, the opening in the center of the shading plate in the sagittal direction must be narrowed.

Therefore, the narrow central region of the opening causes further deterioration of the MTF characteristic. Therefore, the maximum half angle of view of the image reading apparatus using the above-mentioned shading plate is limited to about 25 degrees.

This has hindered the widening of the angle of view of the imaging optical system, and has been a major obstacle in reducing the distance between object images.

The present invention effectively prevents a decrease in the MTF of the image forming optical system by providing an optical member having appropriate optical characteristics in a part of the image forming optical system constituting a part of the image reading apparatus. It is another object of the present invention to provide an image reading apparatus capable of reducing a light amount drop at an end portion on a light receiving element surface and reading an image on a document surface with high accuracy.

(Means for Solving the Problems) An image reading apparatus according to the present invention provides a light receiving element surface formed of a one-dimensional line sensor via an image forming optical system on a document surface illuminated with a light beam from a linear light emitting surface light source. In an image reading apparatus that forms an image on the original surface and reads image information on the original surface, the image is transmitted to the stop position of the imaging optical system or in the vicinity thereof in accordance with an incident angle for adjusting the illuminance distribution on the light receiving element surface. An optical member made of a field-selection glass having a different ratio is provided.

In particular, the present invention is characterized in that the field-of-view selection glass has a characteristic that the amount of transmitted light increases as the incident angle increases.

(Embodiment) FIG. 1 is an end view of an essential part showing an imaging state of an imaging optical system 2 used in an image reading apparatus according to a first embodiment of the present invention.

The image forming optical system according to the present embodiment is disposed at substantially the same position as the image forming optical system 51 in the image reading apparatus shown in FIG. FIG. 1 shows how light beams from various positions on an object surface (original surface) 1 are incident on a light receiving element surface (imaging surface) 5 such as a line sensor (CCD).

That is, 1 is a document surface which is an object surface, and 2 is an image forming optical system which forms an image on the object surface 1 on the light receiving element surface 5 at a predetermined magnification. Reference numeral 3 denotes an aperture, and 4 denotes an optical member according to the present invention, which is formed of a member such as a field-of-view selection glass having an incident angle dependency whose transmittance varies depending on the incident angle. Hereinafter, it is referred to as “near the position of the diaphragm 3”.)

The optical member 4 according to the present 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 beam based on the image information emitted from each of the object points a, b, and c on the object plane 1 becomes substantially parallel light beam after passing through the entrance pupil _IO of the imaging optical system 2 and is in the vicinity of the stop 3 position. At an angle θ _a , θ _b , and 0 degree, respectively.

The optical member 4 controls the amount of light transmitted from each of the object points a, b, and c. That is, the exit pupil is restricted so that the transmittance increases in the order of the light beams from the object points a, b, and c.
After passing through I _K , the light exits the imaging optical system 2 and enters each point a ′, b ′, c ′ on the light receiving element surface 5. Then, the image of the document 1 is formed on the light receiving element surface 5, and the image information on the document surface is sequentially read.

Next, the optical characteristics of the optical member 4 according to the present invention will be described.

The optical member 4 has different transmittance characteristics depending on the incident angle of the incident light beam as shown in FIG. 2 (A). For example, it formed such that all passes (100% transmission) when incident at an angle theta _a to the optical member 4 from a viewpoint of the outermost abaxial.

When incident at an angle 0 degrees c to the optical member 4 points on Matajiku approximately 67% as compared with the case of the incident angle theta _a is (67% transmission)
Only to be transmitted. That is, assuming that the transmittance at the incident angle θ _a is 1, the transmittance at the incident angle 0 is
It has the characteristic to be 0.67.

As described above, in the present embodiment, the incident angle dependence of the optical member 4 is configured so that the transmittance decreases sequentially as the incident angle decreases. The drop in the amount of light at the end from the center on the light receiving element surface (imaging surface) 5 is effectively reduced. 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 accuracy.

FIG. 2 (B) shows the transmittance characteristic with respect to the incident angle of the optical member 4 according to the present invention shown in FIG. 2 (A) and the image height a '(12.5 mm) shown by the solid line in FIG. 4 (B). 20) angle of view
It is explanatory drawing which shows the light distribution characteristic at the time of multiplying the cosine fourth power rule of the imaging optical system which is (Sigma).

As shown in the figure, the output signal from the end of the light receiving element (corresponding to the amount of light incident on the light receiving element) is higher than the central part. When this is multiplied by the light distribution characteristic of the fluorescent lamp shown in FIG. 4 (A), the result becomes as shown in FIG. 2 (C). That is, it is possible to obtain a good light distribution characteristic in which the light amount drop at the end on the light receiving element surface is reduced.

This is because, as shown in FIG. 4C, in the conventional image reading apparatus, the light distribution characteristic of the imaging optical system 4 according to the cosine fourth power rule (solid line in FIG. 4B) and the light distribution of the fluorescent lamp. Characteristics (4th
FIG. 2C shows that in the present embodiment, the light intensity drop at the end portion with respect to the center portion on the light receiving element surface when crossing FIG. The drop in the amount of light at the end on the light receiving element surface is reduced to about 30%. The drop in the amount of light at this time is substantially equal to the light distribution characteristic when a conventional shading plate is used, as shown in FIG. 4 (D).

As described above, in this embodiment, by disposing the optical member having the incident angle dependence near the stop position of the imaging optical system, the light amount distribution on the light receiving element surface can be improved without using a shading plate. Can be corrected.
This also facilitates the downsizing of the entire apparatus with a reduced number of parts.

Moreover, in this embodiment, there is no change in the F-number between the merging direction and the sagittal direction of the imaging optical system.
It has the feature that it does not cause a decrease in F.

FIG. 3 is a schematic view of a main part showing an image forming state of an image forming optical system used in an image reading apparatus according to a second embodiment of the present invention.

In this embodiment, the optical member having the incident angle dependency and the MT having a larger opening diameter at the center compared to the conventional shading plate are used.
By using a shading plate configured so as not to significantly reduce the F performance, a decrease in the amount of light at the peripheral portion of the screen is further reduced.

In the figure, reference numeral 32 denotes a shading plate having an opening for controlling the amount of light flux from a document (not shown), and 31 denotes an image forming optical system, and a stop provided in the image forming optical system 31. An optical member having the aforementioned incident angle dependence is provided near the position.

33 is a light receiving element, for example, a one-dimensional line sensor (CC
D) etc. Reference numeral 32a denotes a light beam diameter that can pass through the imaging optical system 31.

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

In this embodiment, light is received by effectively combining the shading plate 32 having a wider central opening than the conventional shading plate shown in FIG. 6 and the above-described optical member having an angle dependency. The light amount distribution on the element surface is corrected.

In general, in order to reduce the size of the entire apparatus, it is necessary to reduce the distance between the object and the image. As a result, the imaging optical system has a wider angle of view, and as a result, it is necessary to uniform the illuminance distribution on the light receiving element surface. It becomes very difficult.

For example, as shown by the dotted line in FIG.
Assuming that the angle of view at 2.5 mm is 27 °, the amount of incident light (corresponding to an output signal from the light receiving element) at the end on the light receiving element surface is reduced by 36% with respect to the center.

This indicates that the drop in the amount of light at the end is increased by 14% as compared with the case where the angle of view is 20 ° when the image height is 12.5 mm as indicated by the solid line in the figure. Therefore, in this embodiment, an optical member having an angle-of-incidence dependence on the stop position of the imaging optical system in order to reduce a drop in the amount of light at the end on the light receiving element surface due to the wide angle of view of the imaging optical system. And a shading plate 32 that has a wider opening at the center than the conventional shading plate shown in FIG. 6 and does not significantly reduce the MTF.

As a result, a drop in the amount of light at the end on the light receiving element surface due to the widening angle of view of the imaging optical system is reduced, and an appropriate output signal is obtained from the light receiving element 33.

With this configuration, in the present embodiment, the overall size of the apparatus is reduced, and the reduction of the MTF of the imaging optical system when a shading plate is used is effectively prevented.

(Effect of the Invention) According to the present invention, an optical system made of a field-selection glass having a transmittance different according to an incident angle near an aperture position in an imaging optical system for forming image information on a document surface on a light receiving element surface. By providing members, it is possible to reduce the unevenness of the light amount distribution on the light receiving element surface while preventing the MTF in the sagittal direction of the imaging optical system from decreasing, and to achieve a high-precision, compact design of the entire device. An image reading device capable of reading an image can be achieved.

[Brief description of the drawings]

FIG. 1 is a sectional view of an essential part showing an image forming state of an image forming optical system of an image reading apparatus according to a first embodiment of the present invention, and FIG. 2 (A) shows transmittance characteristics of an optical member according to the present invention. FIG. 2B is an explanatory diagram of light distribution characteristics when the cosine fourth law of the imaging optical system is multiplied by the transmittance characteristic of the optical member shown in FIG. 2A. FIG. 2C is an explanatory diagram showing an output signal from the light receiving element when the light distribution characteristic of FIG. 2B is multiplied by the light distribution characteristic of the fluorescent lamp, and FIG. 3 is a second embodiment of the present invention. FIG. 4A is an explanatory diagram showing a light distribution characteristic of a fluorescent lamp, and FIG. 4B is an image forming optical system of an example image reading apparatus. FIG. 4C is an explanatory view showing light distribution characteristics according to the cosine fourth law of the system, and FIG. 4C is a light receiving element obtained by multiplying the light distribution characteristics shown in FIGS. 4A and 4B. FIG. 4D is an explanatory view showing an output signal, FIG. 4D is an explanatory view showing an output signal from a light receiving element by light amount correction of a shading plate, FIG. 5 is a schematic view of a main part of an optical system of a conventional image reading apparatus, and FIG. FIG. 6 is an enlarged explanatory view of a part of the image reading apparatus of FIG. 5, FIG.
(B) is an explanatory view showing the MTF characteristics of the imaging optical system and the sagittal direction with respect to the presence or absence of the shading plate. In the figure, 1 is an object plane, 2 and 31 are imaging optical systems, 3 is an aperture, 4
Is an optical member, 5 is a light receiving element surface, 32 is a shading plate,
33 is a light receiving element.

Claims (2)

(57) [Claims] An image of a document surface illuminated with a light beam from a linear light emitting surface light source is formed on a light receiving element surface of a one-dimensional line sensor via an image forming optical system, and image information on the document surface is converted. In the image reading apparatus to be read, an optical member made of a field-selection glass having a different transmittance depending on an incident angle for adjusting an illuminance distribution on the light receiving element surface is provided at or near a stop position of the imaging optical system. An image reading apparatus comprising: 2. The image reading apparatus according to claim 1, wherein the field-of-view selection glass has a characteristic that the amount of transmitted light increases as the incident angle increases.