Image reading apparatus

Abstract: This image reading apparatus includes a light source (1) for illuminating an object, a lens array (5) for focusing reflected light from the object into a predetermined position, and an image sensor (3) for reading an image formed by the focused light. The lens array (5) includes a transparent base (5a), and a diffraction element (8) provided on at least an incident plane and an emission plane of the base (5a). The lens array constitute an array including a plurality of lens areas. The employment of the diffraction element renders a shorter imaging distance of light from the object and thus a reduction in the size of the apparatus.

Description: The present invention relates generally to image reading apparatuses and, more particularly to reducing the size of and simplifying the structure of a lens array in an image reading apparatus for illuminating an image with light from an illuminating light source, imaging light from the image onto a line sensor through the lens array and electrically reading image information. Description of the Background Art Office machines or communication apparatuses such as copiers or facsimiles comprise image reading apparatuses which optically read image information represented on the original of paper or drawings and convert the read information into an electric signal to be taken out. While there are various types of the image reading apparatuses, the image reading apparatus with a representative structure comprises a light source, an optical lens and an image sensor. As to an operation of the image reading apparatus, in general, the apparatus illuminates the original having image information with white light from the light source, projects reflected light thereof being optically reduced in size onto a line sensor such as a CCD (Charge Coupled Device) image sensor through the optical lens, converts this projected light into an electrical signal and reads image information. However, in an optical system employing a spherical lens as the optical lens, a spacing between the original and a light acceptance plane of image information is increased, resulting in an increase in the size of the apparatus. In order to avoid this increase in the size of the apparatus, a reflecting mirror for bending an optical path of rays of light from an image is usually employed in the optical system to reduce the size of the apparatus. However, the provision of a device such as the reflecting mirror results in a complicated structure of the apparatus with a disadvantage that the apparatus is not effectively made compact. Meanwhile, in recent years, due to practical application of a focusing lens for conducting light, an image reading apparatus, the size of which is reduced by integrally forming a reading optical system and an equal-scale magnification adhesion type reading line sensor together, has been in practical use. This image reading apparatus will now be described with reference to Figs. 15 to 17. Referring to Fig. 15, this image reading apparatus comprises a light emitting diode 1 (not shown) to serve as a light source, a refractive index profile type lens array 2 constituting the optical system, and a line sensor 3 including a plurality of CCD image sensors for converting image information into an electric signal. Next, referring to Fig. 16, the refractive index profile type lens array 2 comprises a large number of focusing lens elements 2a of a cylindrical shape with a diameter of approximately 1 mm. These focusing lens elements 2a are interposed between frame plates 2b on both sides thereof. The frame plate 2b is formed of a glass fabric based epoxy resin black lamination plate (FRP) having almost the same thermal expansion property as the focusing lens element 2a so as to reduce thermal distortion and increase strength thereof. Black silicon rubber resin 2c is filled in spaces between the focusing lens elements 2a and the frame plate 2b. This silicon rubber resin 2c serves to remove flare light around the focusing lens elements 2a. An operation of the image reading apparatus will now be described with reference to Fig. 17. Illumination light is first directed from the light emitting diode 1 to the surface of the original 4. Light from the image, reflecting on the surface of the original 4, is introduced into the refractive index profile type lens array 2 to be focused. This light is then transmitted through the refractive index profile type lens array 2 and is then imaged on the surface of the line sensor 3. The line sensor 3 converts this light from the image into an electrical signal to read image information. A conjugate length TC of the optical system employing this refractive index profile type lens array 2 is expressed as TC = Z + 2ℓ₀ and is approximately 15-70 mm specifically. This enables a distance between a plane of the original and a plane on which an image is read to be shorter compared to the conventional spherical lens, resulting in a smaller-scale image reading apparatus. However, while the image reading apparatus employing the refractive index profile type lens array 2 can be reduced in size, it provides a disadvantage that a manufacturing cost of the apparatus is increased due to a difficulty in manufacturing the refractive index profile type lens array 2. Some causes of the above disadvantage are shown below: a. The refractive index profile type lens array 2 comprises a complex of three materials, the focusing lens elements 2a, the frame plate 2b and the silicon rubber resin 2c and thus is complicated to be manufactured. b. All the focusing lens elements 2a are required to have uniform properties so as to have an equal refractive index profile constant and also an increased accuracy in the manufacture. Further, the refractive index profile type lens array 2 also has another disadvantage that the removal of the flare light by the black silicon rubber resin 2c as a filler decreases the efficiency in utilizing incident light. An image forming apparatus similar to the one described above is known from the document US-A-4,560,866. In this prior art apparatus, the light is guided in the lens array parallel instead of rectangularly to the illuminated object. A mirror at the input side and a mirror at the output side of the lens array serve to deflect the light from the vertical direction to the horizontal direction and back to the vertical direction. A lens array which can be more easily manufactured than the lens arrays of the above described apparatuses is known from Patent Abstracts of Japan, Vol. 12, No. 259 (P-733) [3106], 21 July 1988 & JP-A-63-46403. This lens array comprises a plurality of micro-Fresnel lenses on both the front and rear faces of a rectangular base plate. Patent Abstracts of Japan, Vol. 7, No. 280 (E-216) [1425], 14 December 1983 & JP-A-58-159059 disclose a lens system in which a sensor is formed on the lower face of a lens substrate. Patent Abstracts of Japan, Vol. 7, No. 258 (E-211) [1403], 17 November 1983 & JP-A-58-143665 disclose an image sensor arrangement in which a plurality of light emitting elements and photoelectric converting devices are arranged within a housing. It is the object of the present invention to provide a small-sized image reading apparatus. The image reading apparatus of the present invention is defined by the teaching of appended claim 1. According to this teaching, the length of the lens arrangement or optical focusing means is reduced in that the light within the base of said means is reflected at least twice between the entrance position and the exit position. Thereby, the width of the base is somewhat broadened against the width of a base in which the light is irradiated directly from the entrance position to the exit position, however, the length is reduced considerably. Therefore, over all the size of an image reading apparatus can be reduced against the size of a prior art apparatus. It is especially advantageous to provide diffraction elements at all reflection positions, which measure broadens the freedom in adjusting optical properties of the optical focusing means. E. g. the distances between the object and the optical focusing means and/or the distance between said means and the sensor (in case that the sensor is not arranged on the base) are adjustable in a wider range. Further, it is possible to prevent chromatic aberration by using such plural diffraction elements. The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view of a schematic structure of an image reading apparatus according to a prestage of the present invention; Fig. 2 is a plan view of a diffraction element array employed in the image reading apparatus of Fig. 1; Fig. 3 is a view for illustrating imaging of light from an image in the image reading apparatus shown in Fig. 1; Fig. 4 is a perspective view of a schematic structure of an image reading apparatus according to a first embodiment of the present invention; Fig. 5 is a view of a light from an image, which is imaged by a diffraction element array 5 employed in the image reading apparatus of the first embodiment; Fig. 6 is a view of light from an image, which is imaged by the diffraction element array 5 employed in an image reading apparatus of a second embodiment which is a variation of the first embodiment; Fig. 7 is a perspective view of a schematic structure of an image reading apparatus of a third embodiment of the present invention; Fig. 8 is a plan view of a diffraction element array employed in the third embodiment; Fig. 9 is a view for illustrating imaging of light from an image in the image reading apparatus of the third embodiment; Fig. 10 is a perspective view of a schematic structure of an image reading apparatus according to a fourth embodiment of the present invention; Fig. 11 is a view for illustrating imaging of light from an image by a diffraction element array employed in the fourth embodiment; Fig. 12 is a view for illustrating imaging of light from an image by a diffraction element array of an image reading apparatus according to a fifth embodiment of the present invention; Fig. 13 is a view for illustrating imaging of light from an image by a diffraction element array of an image reading apparatus according to a sixth embodiment of the present invention; Fig. 14 is a view for illustrating imaging by a diffraction element array of an image reading apparatus according to an seventh embodiment of the present invention; Fig. 15 is a perspective view of a schematic structure of a conventional image reading apparatus; Fig. 16 is a front view of a refractive index profile type lens array employed in the conventional image reading apparatus; and Fig. 17 is a sectional view of a schematic structure for the operation of the conventional image reading apparatus. The embodiments of the present invention will now be described with reference to the figures. The prestage of the present invention will be described with reference to Figs. 1-3. Referring to Fig. 1, the image reading apparatus comprises a light emitting diode 1 (not shown) and a line sensor 3 including a diffraction element array 5 and a CCD image sensor. The line sensor 3 is provided at a position opposing to the original 4 through the diffraction element array 5. Referring to Fig. 2, the diffraction element array 5 comprises a rectangular base 5a formed of a transparent material such as glass or plastic. One plane of the base 5a in an elongated direction comprises a plane 6 of incidence, while the other plane opposite this incident plane 6 comprises a plane 7 of emission. A plurality of diffraction elements 8 such as Fresnel lens are arranged planar on the surfaces of the incident plane 6 and the emission plane 7. The diffraction elements 8 cover the overall surfaces of the incident plane 6 and the plane 7 of emission. The action of this diffraction element array 5 will now be described with reference to Fig. 3. Image information P on the surface of the original 4 which accepts illuminating light from the light emitting diode is directed toward the incident plane 6 of the diffraction element array 5, transmissively through a diffraction element 8a provided on this incident plane 6 and enters the diffraction element array 5. After forming an inverted real image Q₁ of the image information P within the diffraction element array 5, the image information P is transmitted through a diffraction element 8b formed on the plane 7 of the diffraction element array 5 to form an erecting real image at the position of Q. The image information P is then photoelectrically converted by the line sensor 3 (not shown) placed in the position of Q to be read. In this figure, a distance between the image information P on the surface of the original 4 and the erecting real image formed on the line sensor 3, i.e., the conjugate length TC is represented by a total sum of a differential distance ℓ₁ on the side of an object, a differential distance ℓ₂ on the side of an image and a thickness Z of the base of the diffraction element array 5. The differential distances ℓ₁ and ℓ₂ on the respective sides of the object and the image are determined by the form of the grating of the diffraction elements 8a and 8b provided respectively on the incident plane 6 and the emission plane 7 of the diffraction element array 5. Each diffraction element such as Fresnel lens has the form of uniform micro-spaced and curved grating. A method of manufacturing such an diffraction element comprises the steps of producing a mold with the form of grating formed by an electron beam direct writing method, reduction type exposure and the like, and then producing a replica by employing this mold. This method enables mass production of this diffraction element and thus a considerable reduction in cost compared to the conventional refractive index profile type lens array or the like. Further, a diffraction efficiency of the diffraction element can be twice or more that of a step type diffraction element by employing a blazed diffraction grating, and it becomes possible to inhibit cross-talk due to a diffraction of an unrequired diffraction degree, resulting in no particular problems in practical use. As shown in Fig. 2, the diffraction elements 8 are formed without any spacing over the entire surface of the incident plane 6 or the emission plane 7 of the base 5a. This formation enables utilization efficiency of light to be enhanced compared to the conventional refractive index profile type lens array 2. The image reading apparatus of the first embodiment in accordance with the present invention will now be described with reference to Figs. 4 and 5. This embodiment is featured by employing a prism 9 for the base 5a of the diffraction element array 5 compared to the first embodiment. The overall structure of the apparatus is identical to that of the prestage, as shown in Fig. 4. Referring to Fig. 5, the prism 9 of this embodiment is a polygonal prism comprising a plane 6 of incidence, a plane 7 of emission, and four planes of reflection, 10a, 10b, 10c and 10d. Diffraction elements 8a and 8b are provided respectively on the incident plane 6 and the emission plane 7 of this prism 9. In operation, an optical path of light from an image, which is transmitted through the diffraction element 8a formed on the incident plane 6 and then enters the prism 9, is first bent at the reflection plane 10a of the prism 9 and then at the reflection plane 10b to once form an inverted real image Q₁. Further, this optical path is then bent at the reflection planes 10c and 10d and is transmitted through the diffraction element 8b formed on the emission plane 7. The light transmitted is then introduced outside to form an erecting image at a predetermined position of Q. In the diffraction element array 5 employing this prism 9, a shorter straight distance of the optical path is achieved on due to the bending of the optical path of the light within the prism 9. Consequently, a thickness Z of the prism 9 can be reduced compared to that of the diffraction element array 5 shown in Fig. 3. This results in a reduction in the conjugate length TC of the diffraction element array and thus a reduction in the size of the apparatus. The second embodiment, which is a variation of the first embodiment, is shown in Fig. 6. In this embodiment, other diffraction elements 8c-8f are further provided respectively on the reflection planes 10a-10d of the prism 9 shown in Fig. 5 so as to decrease an accuracy required for the angle of reflection in the prism. That is, in the reflection caused by utilizing the reflection planes of the prism 9 shown in Fig. 5, a position of the imaging point Q is deviated due to an angular error of the reflection planes. However, in a flection of a luminous flux utilizing a diffraction by hologram employing the diffraction element, in general, a direction of the diffraction of the luminous flux is determined by spacings in a grating, so that this direction is least affected by an angular error of a plane on which the diffraction element is formed. Such an effect in the diffraction element enables a decrease in the accuracy of the reflection angle of the prism and a reduction in manufacturing cost thereof. Further, it is possible to form gratings on the diffraction elements 8c-8f provided on the reflection planes of this prism 9 so that these diffraction elements 8c-8f may serve as lenses in the same manner as the diffraction elements 8a and 8b provided respectively on the incident plane 6 and the emission plane 7. Such a construction results in enhanced mass-productivity in the form of the diffraction elements 8a-8f provided on the incident plane 6, the emission plane 7 and the reflection planes of the prism 9, respectively. This further causes the differential distances ℓ₁ and ℓ₂ on the respective sides of the object and the image to be adjustable in a wider range, so that the differential distances can arbitrarily be determined in accordance with the structure of the system of the image reading apparatus. Moreover, the transmissive structure of the plurality of diffraction elements 8a-8f shown in Fig. 6 prevents an occurrence of chromatic aberration. That is, the diffraction elements, in general, can eliminate in principle the chromatic aberration for light with a single wavelength. However, a general image reading apparatus employs a light emitting diode which emits light with multiwavelength to an illuminating light source for the original. Therefore, the chromatic aberration occurs with only a diffraction element on one plane. Thus, grating patterns are formed on the diffraction elements so as to mutually correct the chromatic aberration by diffraction at the plurality of diffraction elements on two or more planes, so that a clearly visible image without any chromatic aberration can be read. This action will be described in more detail in case of employing two diffraction elements, for example. When the first diffraction element involves a change in the angle of diffraction of light in accordance with a change in wavelength, grating patterns of the first and second diffraction elements can be designed by numerical calculation of a computer so that the second diffraction element corrects this change in the angle of diffraction at the first diffraction element to direct the light toward the same focal point. This effect of correction of the chromatic aberration is not limitative to this combination of two diffraction elements, but can be attained with a combination of a plurality of diffraction elements. Next, the third embodiment of the present invention will be described with reference to Figs. 7-9. Fig. 7 is a perspective view of a schematic structure of the image reading apparatus of this embodiment. As shown in this figure, the basic structure of the apparatus of this embodiment is the same as those shown in Figs. 1 and 4. The feature of this embodiment resides in the form of arrangement of the diffraction elements 8 in the diffraction element array 5 shown in Fig. 8. Referring to Fig. 8, the diffraction element array 5 comprises a rectangular base 5a. One plane of the base 5a in an elongated direction comprises a plane 6 of incidence of light from an image, while the other plane opposing to this incident plane 6 comprises a plane 7 of emission (not shown). On the incident plane 6 of the base 5a are formed a diffraction element array 11 for incidence and a diffraction element array 12 for reflection, in which arrays a plurality of diffraction elements 8 are closely arranged in series. Similarly, on the emission plane 7 of the base 5a are formed the diffraction element array 12 for reflection and a diffraction element array 13 for emission (not shown), in which arrays the plurality of diffraction elements 8 are arranged in series. Reference is now made to Fig.9 in describing a formation of an image by employing the diffraction element array 5 in this embodiment. Image information P on the surface of the original 4 is illuminated with illumination light from the light emitting diode 1, and light from the image is then focused by and transmissively through a diffraction element 8a constituting the diffraction element array 11 for incidence, formed on the incident plane 6 of the array 5, thereby being introduced into the element 8a. The light introduced in the diffraction element 8a is diffracted by a diffraction element 8c formed on the emission plane 7 of the base 5a and then reaches a diffraction element 8d formed on the incident plane 6 of the base 5a. This light is then diffracted by this diffraction element 8d and reaches a diffraction element 8b formed on the emission plane 7 of the base 5a. After once forming an inverted real image Q₁ within the base 5a of the diffraction element array 5, the image information P is emitted outside by the diffraction element 8b to form an erecting real image at a predetermined position Q. In this embodiment, an optical path of the light from the image is bent within the diffraction element array 5 by forming the diffraction elements 8c, 8d for reflection respectively on the incident plane 6 and the emission plane 7 of the array 5, thereby reducing a straight distance of the optical path. This results in a reduction in a thickness Z of the diffraction element array 5, and consequently enables a reduction in the conjugate length TC of the diffraction element array 5, implementing a smaller-scale apparatus. The fourth embodiment of the image reading apparatus according to the present invention will now be described with reference to Figs. 10 and 11. This embodiment is characterized in that the light emitting diode 1 serving as a light source is integrally formed with the diffraction element array 5. That is, the light emitting diode 1 is adhesively fixed onto the emission plane 7 of the diffraction element array 5, as shown in Figs. 10 and 11. The light emitting diode 1 to be attached comprises a semiconductor chip la of the light emitting diode and a package portion lb thereof. Diffraction elements 8e and 8f for focusing light emitted from the light emitting diode 1 are provided respectively on the emission plane 7 of the diffraction element array 5, positioned in an emission direction of the light emitting diode 1, and on the incident plane 6 of the array 5. This provision of the diffraction elements 8e and 8f requires no converging lens to be provided at an emitting position of the package of the light emitting diode 1 and thus results in a simplified structure of the light emitting diode 1. The size of the apparatus is reduced by integrally forming the light emitting diode 1 with the diffraction element array 5. Next, the fifth embodiment of the present invention is a variation of the fourth embodiment described above. In this embodiment, as shown in Fig. 12, the diffraction element 8f provided on the surface of the incident plane 6 of the diffraction element array 5 and the diffraction element 8a provided at an incident position of light from an image are integrally formed together to be employed as a converging lens of the light emitting diode 1. In an operation of these diffraction elements 8f and 8a, illumination light directed from the light emitting diode 1 is focused by the diffraction element 8e and therewith diffracted toward the diffraction element 8a provided on the incident plane 6. Since a luminous flux from the light emitting diode 1 is in the same optical path direction as minus first order diffraction light of the diffraction element 8a, it is diffracted toward an image P and is appropriately converged to illuminate the image. The light reflected from the image P due to this illumination light is again transmitted through the diffraction element 8a to be introduced into the diffraction element array 5. As described above, the diffraction element 8a serves both as a lens for focusing and transmitting the light from the image and as the converging lens for the light emitting diode. The image reading apparatus according to the sixth embodiment of the present invention, which is a variation of the fifth embodiment described above, is characterized by a simplified structure of the light emitting diode 1. That is, as shown in Fig. 13, the semiconductor chip la of the light emitting diode 1 is directly and adhesively fixed onto the surface of the diffraction element array 5 at a predetermined position of the emission plane 7 of the array 5. This structure requires no package of the light emitting diode 1 and thus attains the simpler and smaller-size apparatus. The image reading apparatus according to the seventh embodiment of the present invention is characterized by integrally forming the line sensor 3 to be attached onto the emission plane 7 of the diffraction element array 5 compared to the sixth embodiment described above. That is, as shown in Fig. 14,the line sensor 3 is fixedly attached to a position on the emission plane 7, from which light from an image is emitted outside. The diffraction element 8b which has been provided at this position is omitted. Increasing effects, as lenses, of the respective diffraction elements 8a-8c and 8d, provided on the diffraction element array 5 results in such a structure that the light from the image is imaged on the surface of the line sensor 3 attached onto the emission plane 7 of the diffraction element array 5. This structure requires no differential distance ℓ₂ on the side of the object, which is required in the above described embodiment, and thus the conjugate length TC of the diffraction element array 5 is substantially reduced to achieve the reduction in the size of the apparatus. The diffraction grating such as Fresnel lens or hologram is employed for the diffraction element in the above embodiment. As described heretofore, according to the present invention, since a lens array of the image reading apparatus is configured by forming the diffraction elements on the incident plane and the emission plane of the base, an image reading apparatus with low cost and available for mass production by employing replica manufacturing technology or the like can be implemented. Furthermore, this embodiment is structured such that the light from the image is bent within the lens array so as to be able to reduce the length of the optical path thereof, the reductions in the thickness of the lens array and in the optical path of the optical system can achieve the reduction in the size of the apparatus. In the other embodiments of the present invention, the integral formation of the lens array with the light source or the image sensor in the image reading apparatus results in the apparatus with the simplified structure and the reduced size.

Claims: An image reading apparatus comprising: a) a light source (1) illuminating a surface of an object with illumination light; b) an optical focusing means (5) for receiving light reflected from said object within an entrance position, for transmitting said light towards an exit position, and for forming an image of said object at a predetermined image position, said optical focusing means comprising: b1) a transparent base (5a) having an incident plane, comprising said entrance position, and an emission plane comprising said exit position; b2) a first diffraction element (8a) positioned in said entrance position; and b3) a second diffraction element; and c) an image sensor array (3) provided at said image position and converting said image into an electrical signal to read the image of said object; characterized in that said optical focusing means (5) is arranged to reflect said light within said base (5a) at least twice between said entrance position and said exit position. The image reading apparatus in accordance with claim 1, wherein said base (5) is a prism having V-form with an entrance side leg and an exit side leg, each of which have an inside surface positioned towards the inside of the "V" and an outside surface positioned towards the outside of the "V", whereby the light from the entrance position is first reflected from a first reflection position at the inside surface of the entrance leg to a second reflection position at the outside surface of said leg, from there to a third reflection position at the outside surface of the exit leg, from there to a fourth reflection position at the inside surface of the exit leg, and finally, from there, to the exit position at which said second diffraction element (8b) is provided. The image reading apparatus in accordance with claim 2, wherein a diffraction element (8c, 8d, 8e, 8f) is provided at each of said first to fourth reflection positions. The image reading apparatus in accordance with claim 1, wherein said base (5) is a rectangular base having an entrance surface comprising said entrance position, and an exit surface comprising said exit position, in which base the light when entering at the entrance position is deflected by said first diffraction element (8a) towards a position in said second diffraction element (8c) is positioned on said exit surface, is there reflected towards a position in which a third diffraction element (8d) is positioned and is then reflected to the exit position. The image reading apparatus in accordance with claim 4, wherein a fourth diffraction element (8b) is arranged at said exit position. The image reading apparatus in accordance with claim 4, wherein said image position coincides with said exit position, and therefore said image sensor is arranged directly on said base (5) in said exit position. The image reading apparatus in accordance with one of the preceding claims, wherein said diffraction element (8a, 8b, 8c, 8d, 8e, 8f) are arranged to compensate for chromatic abberation. The image reading apparatus in accordance with one of the preceding claims, characterized in that said diffraction elements (8a, 8b, 8c, 8d, 8e, 8f) are Fresnel lenses.

Inventors: KURATA YUKIO, TSUJI SHIGEKI, YOKOTA TAIZO, KURATA, YUKIO, TSUJI, SHIGEKI, YOKOTA, TAIZO
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1990
Main classification: H04N1
IPC codes: H01L31, G02B5, H04N1, G02B3