JP3453875B2 - Document reading device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a document reading apparatus such as a copying machine or a bill / check reading apparatus, and more particularly to a configuration of a portion for setting a document (subject). 2. Description of the Related Art Conventionally, in this type of apparatus, for example,
Documents, such as a copying machine with an automatic document feeder disclosed in US Pat. No. 43712 and a color image reading apparatus disclosed in JP-A-618809, are set flat, As in the cylindrical scanning type image data input device disclosed in US Pat.
A document as a subject is set linearly in the scanning direction. In general, the focal point of oblique rays toward the periphery of a lens is closer to the lens than the on-axis focal point, so that the image plane of the lens tends to be a curved dish instead of a correct plane. This is called field curvature. For example, it is known that a box camera has a warped film surface, but since the attached lens is a simple lens, astigmatism cannot be corrected and the image plane of the lens is curved. That is, the influence of the curvature of field is corrected by warping the film surface to match the curved image surface. [0004] On the other hand, distortion is an aberration that occurs when the principal ray passing through the center of the stop has an excessive or insufficient refraction due to the same cause as spherical aberration. An image near the axis has a small inclination angle of the chief ray, and an image is formed according to the paraxial magnification.However, since the lens passing point of the chief ray moves away from the optical axis toward the periphery of the screen, a magnification larger than the paraxial magnification,
Alternatively, the image tends to be formed at a small magnification. If an image can be formed at the same magnification everywhere on the screen, the figure and the image of the object will be completely similar. However, if the magnification becomes larger toward the periphery, the whole becomes a pincushion type. , Resulting in barrel distortion. FIG. 4 is a graph qualitatively showing the relationship between Seidel's five aberrations and the half angle of view ω.
The half angle of view ω is defined in FIG. Since the field curvature tends to increase in proportion to the square of ω and the distortion tends to increase in proportion to the cube of ω, when designing an optical system to be used up to a large range of ω, correction of the field curvature and distortion is performed. Must be taken into account. In a conventional document reading apparatus, in order to reduce the angle of view 2ω, a mirror is used to increase the distance by bending the optical path and increase the conjugate length (distance between object and image), or a complicated combination of optical systems such as lenses. A method of optically correcting aberration is used. However, as described above, the optical path is bent by the mirror to increase the distance,
Increasing the conjugate length increases the overall size of the device and the number of components. Further, the method of optically correcting aberration by combining optical systems such as lenses complicates the lens configuration.
Therefore, it is desired to reduce the size of the device and to simplify the optical system configuration. FIG. 5 shows the relationship between the conjugate length and the angle of view (2ω). In FIG.
A is read into the sensor 14A via A. The distance from the original 10A to the sensor 14A is called a conjugate length, and the angle at which the original 10A is seen from the photographing lens 12A is called an angle of view 2ω.
FIG. 5B shows a case where the conjugate length is shorter than that of FIG. 5A. A document 10B having the same size is read by the sensor 14B having the same size. In this case, the angle of view 2ω increases as the conjugate length decreases. If the conjugate length is reduced by omitting the optical path bending by the mirror in the conventional apparatus, the angle of view 2ω increases,
As shown in FIG. 4, the influence of curvature of field and distortion increases rapidly. In the case of a lens for a copying machine, if the curvature of field is large even in a part, the part is slightly out of focus, and the image quality deteriorates. Therefore, it is important to reduce the influence of curvature of field and the like. When trying to avoid field curvature and distortion by using a lens configuration or the like, there is a disadvantage that the configuration becomes complicated.
If the lens configuration remains simple, the conjugate length must be increased to reduce the angle of view, and it has been difficult to reduce the size of the device. The present invention has been made in view of such circumstances,
An object of the present invention is to provide a compact document reading apparatus with a short conjugate length without complicating the lens configuration. In order to achieve the above object, the present invention provides a method for writing an optical image of a document through a photographing lens.
An image is formed on the light receiving surface of the
In the original reading apparatus for reading an original image Te, the original
The photographing lens and the line sensor
It is arranged linearly along the optical axis of the lens,
The original is moved in the longitudinal direction of the line sensor so as to cancel the aberration of the photographing lens in the longitudinal direction of the sensor.
And holding means in a curved state . According to the present invention, a document set curved in a direction to cancel the aberration of the photographing lens causes image field curvature and distortion due to the aberration when the image is formed by the photographing lens. Since the document side is curved in advance so as to correct the distortion by detecting the aberration on the image forming surface, an image is formed on a surface that matches the light receiving surface of the imaging unit as a result. Therefore, the imaging unit can read the original image without distortion. A preferred embodiment of a document reading apparatus according to the present invention will be described below in detail with reference to the accompanying drawings. FIG. 1 is a schematic explanatory view of a document reading apparatus according to the present invention. The document 20 is configured to be read by the line sensor 24 via the photographing lens 22. The original 20 is in its longitudinal direction (X
The line sensor 24 is arranged so that the longitudinal direction of the sensor is parallel to the longitudinal direction of the document 20. For convenience of description, the longitudinal direction of the document 20 is defined as the X axis, the minor axis direction is defined as the Y axis, and the optical axis is defined as the Z axis. FIG. 2 is a sectional view taken along a plane (XZ plane) parallel to the longitudinal direction of the line sensor 24 including the Z axis in FIG. The document 20 is held between the curved glass 26 and the holding member 28, and the document 20 is forcibly curved. FIG. 3 qualitatively shows the aberration characteristics of the field curvature and distortion of the taking lens 22. FIG. 3A shows the characteristic of the curvature of field, in which the vertical axis represents the angle (half angle of view ω) of the parallel light beam with respect to the optical axis, and the horizontal axis represents the amount of aberration. FIG.
(B) shows the characteristic of distortion, and the vertical axis shows the incident angle (half angle of view ω).
The horizontal axis shows the difference between the distance between the correct focal position and the actual focal position from the optical axis as a percentage, and minus represents a barrel type and plus represents a pincushion type. Note that the aberration characteristics differ according to the selected optical system such as a lens.
The present invention is not limited to the aberration characteristics shown in FIGS. 3A and 3B, but can be used in various optical systems by forming a document curve in accordance with the aberration characteristics generated by the selected optical system. The curved surfaces of the curved glass 26 and the holding member 28 in FIG. 2 are formed so as to forcibly bend the document surface in a direction to cancel the aberration characteristics in FIGS. 3 (a) and 3 (b). When the field curvature and the distortion (distortion) are ideally corrected, the aberration characteristics are respectively shown in FIG.
And FIG. 3 (b '). Generally, since the aberration is three-dimensional, the curved surface of the curved glass 26 or the like must be spherical. However, in the embodiment of FIG. The aberration may be corrected only in the direction (X-axis direction). Therefore, it is not necessary to form a curved surface for aberration correction in the Y-axis direction. In the document reading apparatus constructed as described above, the light receiving surface of the line sensor 24 is linear in the longitudinal direction, and the image forming surface of the document 20 via the photographing lens 22 is the light receiving surface of the line sensor 24. In the straight line direction.
The image of the document 20 set in a curved state is taken by the photographing lens 22.
Due to this aberration, the field curvature and distortion occur, but since the original side is curved in advance so as to detect the aberration on the image forming surface and correct the distortion, it eventually matches the light receiving surface of the reading sensor 24. Image on the surface. Therefore, the line sensor 24 can read the original image in which the distortion has been corrected. In FIGS. 1 and 2, the original 20 is read by the line sensor 24 by being sent in the Y-axis direction while being forcibly bent by being sandwiched between the curved glass 26 and the holding member 28. The curved glass and the holding member may cover the entire document, or may have a structure that covers only the portion of the document that the line sensor 24 is about to read. As for the feeding of the read original in the Y-axis direction, the original may be moved while the image pickup side (the imaging lens 22 and the line sensor 24) is fixed as described above, or the original may be fixed. Alternatively, the image may be read by moving the imaging side. In the above embodiment, the case where the line sensor is used as the reading sensor has been described. However, the present invention is not limited to this. For example, an area image sensor can be used as the sensor. In this case, since the aberration caused by the photographing lens occurs in both the horizontal direction and the vertical direction, the image plane is affected by the aberration in both directions. Therefore, in order to obtain an image in which the influence of the aberration is completely corrected, the original is curved spherically. There is a need.
However, in practice, the influence of the aberration is particularly significant in the longitudinal direction of the image sensor. Can be done. As described above, according to the document reading apparatus of the present invention, the document is set in a direction in which the aberration of the photographing lens is canceled so as to cancel out the aberration on the light receiving surface of the imaging means. Thus, an image with corrected aberration can be obtained, and a correct original image can be read. As a result, the conjugate length can be shortened while the photographic lens configuration is kept simple, and the apparatus can be reduced in size at low cost.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view showing an outline of a document reading apparatus according to the present invention. FIG. 2 is a plan view of the document reading apparatus shown in FIG. 1. FIG. FIG. 9A is a qualitative graph of aberration characteristics in an optical system, where (a) is the field curvature before correction, (a ′) is the field curvature after correction, (b) is distortion before correction, and (b ′) is after correction. FIG. 4 is a graph qualitatively showing the relationship between Seidel's five aberrations and the half angle of view ω. FIG. 5 is an explanatory diagram showing the relationship between the conjugate length and the angle of view 2ω. EXPLANATION OF SYMBOLS 20 .document 22 .photographic lens 24 .line sensor 26 .curved glass 28 .suppressing member

Claims (1)

(57) [Claims] (1) An optical image of a document is written through a photographing lens.
An image is formed on the light receiving surface of the
In the document reading apparatus for reading a document image by scanning , the document, the photographing lens and the line sensor
The original is arranged on a straight line along the optical axis of the photographing lens, and the original is moved by the line sensor so that aberration of the photographing lens is canceled in the longitudinal direction of the line sensor.
An original reading apparatus, comprising: means for holding in a state of being curved in a longitudinal direction .