ELECTROSTATIC COPYING APPARATUS

Abstract: The electrostatic copying apparatus according to the invention is adapted for copying at variable ratios and has an optical device for performing slit exposure scanning of an original document and for pro­ jecting its image onto a photosensitive member. The distribution of the illuminance of a document illumi­ nating lamp (70) in the widthwise direction is pre­ scribed such that when a lens (78) is held at the equal scale position, the decaying characteristics of the lens in the widthwise direction are offset to render the illuminance distribution in the widthwise direction on the photosensitive member (10) sub­ stantially uniform, and when the lens is held at the predetermined ratio position, an exposure adjusting plate (150) is positioned partly in a light path leading from the document to the photosensitive member (10), said exposure adjusting plate (150) being adapted for compensating variations in illuminance on the photosensitive member (10) which are ascribable to the displacement of the optical axis in the width­ wise direction caused by the movement of the lens (78) from the equal scale position to the pre­ determined ratio position and to variations in the degree of focusing on the photosensitive member (10) caused by the change of the projecting ratio and also to variations in the speed of slit exposure scanning.

Description: This invention relates to an electrostatic copying apparatus of the kind referred to in the precharacterizing portion of patent claim 1. Such a copying apparatus is known from US-A-4 279 497. Various types of electrostatic copying processes and apparatuses have been proposed, and come into commercial acceptance, which can copy an original document selectively at two or more ratios, for example at a ratio of 1:1 and on a reduced or enlarged scale at a predetermined magnification ratio. These conventional processes and apparatuses are adapted for the selection of variable magnification ratios, however, have not proven to be entirely satisfactory, and are not free from various inconveniences and defects as will be understood from the detailed description of the invention which follows with reference to the accompanying drawings. US-A-4 279 497 discloses an electrostatic copying apparatus adapted for copying at different magnifications, comprising an optical device for performing slit exposure scanning of an original document and projeting its image onto a photosensitive member, a lens for projecting the image of the document onto the photosensitive member, said lens being adapted to be selectively held either at a first position for projecting the image of the document at a magnification 1:1 onto the photosensitive member or at a second position spaced a predetermined distance from said first position in a direction oblique to the optical axis for projecting the image of the document at a different magnification onto the photosensitive member, the slit exposure scanning being performed at a speed selectable according to the magnification, and means arranged to move an exposure adjusting plate (150) partly into the light path leading from the document to the photosensitive member, whereby the illumination on the photoconductive meter in the widthwise direction is rendered substantially uniform when the lens is held at said first position, and when the lens is displaced to said second position, the variations of the illumination on the photosensitive member due to the displacement of the lens and to the different selected speed of the slit exposure scanning are compensated. It is the object of the present invention to overcome or eliminate the various inconveniences and defects of conventional electrostatic copying apparatuses capable of giving copies at variable ratios, and to improve them in various respects. This object is accomplished with a copying apparatus as claimed in claim 1. Dependent claims are directed on features of preferred embodiments of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a simplified sectional view showing one embodiment of a copying apparatus constructed in accordance with this invention; Figures 2, 3 and 4 are diagrammatic views for illustrating the widthwise positioning of a projected image on a photosensitive member in the case of reduced (or enlarged) scale copying; Figure 5 is a partial sectional view showing the principal parts of an optical device used in the copying apparatus shown in Figure 1; Figure 6 is a partial perspective view showing one supporting frame of the optical device used in the copying apparatus shown in Figure 1 and its related elements; Figure 7 is a sectional view showing a part of the copying apparatus shown in Figure 1; Figure 8 is an exploded perspective view showing a lens assembly in the optical device used in the copying apparatus shown in Figure 1 and members used to mount the lens assembly; Figure 9 is a partial sectional view showing the mode of mounting a lens assembly in the optical device used in the copying apparatus shown in Figure 1; Figure 10 is a partial perspective view showing the other supporting frame of the optical device used in the copying apparatus shown in Figure 1 and its related elements; Figures 11-A and 11-B are partial sectional views showing the other supporting frame of the optical device used in the copying apparatus shown in Figure 1 and its related elements at the 1:1 magnification position and the reduced scale position, respectively; Figures 12-A, 12-B and 13 are diagrammatic views for illustrating variations in illumination and their adjustment in the case of reduced (or enlarged) scale copying; Figure 14 is a top plan view of an exposure adjusting plate in the optical device used in the copying apparatus shown in Figure 1. With reference to the accompanying drawings, the present invention will be described in greater detail. The general construction of the copying apparatus of the invention capable of giving copies at different magnifications will be described at same length with reference to Figure 1 showing its one embodiment. The illustrated copying apparatus has a substantially parallelpipedal housing shown generally at 2. On the upper surface of the housing 2 is disposed a transparent plate 4 on which to place an original document to be copied. The transparent plate 4 is supported by a supporting frame (not shown) mounted on the upper surface of the housing 2 for free movement in the left and right directions in Figure 1. As will be described in detail, in the performance of a copying process, the transparent plate 4 is caused to make a preparatory movement toward the right in Figure 1 from its stop position shown by solid lines in Figure 1 to its start-of-scan position shown by a two-dot chain line 4A in Figure 1; then to make a scanning movement toward the left in Figure 1 from the start-of-scan position to its end-of-scan position shown by a two-dot chain line 4B; and thereafter to make a returning movement from the end-of-scan position to its stop position. An openable and closable document holding member (not shown) for convering the transparent plate 4 and the document thereon is also mounted on the supporting frame (not shown) on which the transparent plate 4 is supported. Within the housing 2, a horizontal base plate 6 is disposed to divide the inside of the housing 2 into an upper space and a lower space. Substantially centrally in the lower space is rotatably mounted a cylindrical rotating drum 8 constituting a supporting base for a photosensitive member, and a photosensitive member 10 is disposed on at least a part of the peripheral surface of the rotating drum 8. Instead of the rotating drum 8, there may be used an endless belt-like element known to those skilled in the art, and the photosensitive member 10 may be disposed on at least a part of the surface of the endless belt-like element. Around the rotating drum rotated in the direction of an arrow 12 are disposed successively in its rotating direction a charging corona discharging device 14, a charge-eliminating lamp 16 to be operated during reduced scale copying, a developing device 18, a transfer corona discharging device 20 and a cleaning device 22. The charging corona discharging device 14 charges the photosensitive member 10 to a specified polarity substantially uniformly. An exposure zone 24 exists between the charging corona discharging device 14 and the charge-eliminating lamp 16. In the exposure zone 24, the image of the original document on the transparent plate 4 is projected by an optical device to be described hereinbelow, thereby forming a latent electrostatic cimage on the photosensitive member 10. As will be described hereinbelow, the charge-eliminating lamp 16 is operated when reduced scale copying is performed. The lamp 16 illuminates one side portion of the photosensitive member 10 which has been charged by the corona discharger 14 but on which the image of the original document has not been projected in the exposure zone 24. Thus, the electric charge on this one side portion is removed. The developing device 18 which may be of any known form applies toner particles to the latent electrostatic image on the photosensitive member 10 to develop it into a toner image. The transfer corona discharging device 20 applies a corona discharge to the back of a copying paper to be contacted with the surface of the photosensitive member 10 in a transfer zone 26, thereby transferring the toner image on the photosensitive member 10 to the copying paper. The illustrated cleaning device 22 is selectively held at its operating position shown by a solid line in Figure 1 or its non-operating position shown by a two-dot chain line. When the cleaning device 22 is held at the operating position, a blade 28 made of an elastic material is pressed against the surface of the photosensitive member 10, and by the action of the blade 28, the residual toner particles on the photosensitive member 10 after transfer are removed from it. In the lower portion of the housing 2, there are provided a copying paper feed mechanism shown generally at 30 and a copying paper conveying mechanism shown generally at 32 for conveying a copying paper from the paper feed mechanism 30 through the transfer zone 26. The illustrated paper feed mechanism 30 is known per se and comprises a cassette-receiving section 34, a paper cassette 36 to be mounted detachably on the cassette-receiving section 34 and a feed roller 38. The feed roller 38 is rotated selectively in the direction shown by an arrow 40, and feeds a plurality of sheet-like copying papers placed in the stacked state in the cassette 36 one by one to the paper conveying mechanism 32. The illustrated paper conveying mechanism 32 comprises a delivery roller unit 42 for receiving, and conveying, copying paper P fed from the paper feed mechanism 30, a guide plate unit 44, a conveying roller unit 46, a guide plate unit 48 for guiding the copying paper P from the conveying roller unit 46 into the transfer zone 26, a roller 50 for peeling off the copying paper P from the photosensitive member 10 in the transfer zone 26 and carrying it away from the transfer zone 26, a guide plate 52, a fixing roller unit 54, a guide plate 56, a discharge roller unit 58 and a receiving tray 60 for receiving the copying paper P discharged out of the housing 2 from the discahrging roller unit 58. One set of rollers in the fixing roller unit 54, i.e. those rollers which are located at its upper part, include a heating element (not shown) therein. Thus, by these rollers the surface of the copying paper P having a toner image transferred from the photosensitive member 10 is pressed and heated to fix the toner image on the copying paper P. To the fixing roller unit 54 is attached a peeling-guide member 62 for peeling the copying paper P from the roller surface and guiding it downstream. A charge-eliminating lamp 64 is disposed above the guide plate 52. The charge-eliminating lamp 64 serves to irradiate light onto the paper P conveyed to the guide plate 52 and thereby erasing the charge remaining on the paper P, and also to irradiate light onto the photosensitive member 10 in a zone between the corona discharging device 20 and the cleaning device 22 thereby erasing the charge remaining on the photosensitive member 10 after transfer. In the upper space of the housing 2 above the horizontal base plate 6, there is provided an optical device shown generally at 66 which projects the image of an original document placed on the transparent plate 4 onto the photosensitive member 10 to effect slit exposure when the transparent plate 4 makes a scanning movement toward the left in Figure 1 from its start-of-scan position shown by the two-dot chain line 4A to its end-of-scan position shown by the two-dot chain line 4B. The illustrated optical device 66 has a document illuminating lamp 70 for illuminating the document on the transparent plate 4 through a document illuminating opening 68 formed on the upper surface of the housing 2, and for projecting the light reflected from the document onto the photosensitive member 10, a first reflecting mirror 72, a second reflecting mirror 74, a third reflecting mirror 76, a lens assembly 78 and a fourth reflecting mirror 80. The reflecting light from the document illuminated by the lamp 70 is successively reflected by the first reflecting mirror 72, the second reflecting mirror 74, and the third reflecting mirror 76, and then reaches the fourth reflecting mirror 80 through the lens within the lens assembly 78. It is reflected by the fourth reflecting mirror 80, and finally reaches the photosensitive member 10 in the exposure zone 24 through an opening 82 formed in the horizontal base plate 6. Between the opening 82 and the photosensitive member 10 is provided a colored glass 83 known per se which compensates the color characteristics of the photosensitive member 10. A slit exposure width-regulating member 84 for regulating the width, in the moving direction of the photosensitive member 10 (the moving direction of the transparent plate 4), of a light path leading to the photosensitive member, i.e. the slit exposure width, is also disposed between the opening 82 and the photosensitive member 10. In the illustrated copying apparatus, there are further provided a blower 86 composed of a Silocco-type fan and a blower 88 composed of an ordinary impeller-type fan at the left side end portion of the housing 2 in Figure 1. The blower 86 sucks air from outside the housing 2 through a suction hole 90 formed on the upper surface of the housing 2, and discharges air through a discharge hole 92 formed on the left side surface of the housing 2, thereby cooling the transparent plate 4 heated by the illuminating lamp 70. The blower 88, on the other hand, sucks air from the lower space of the housing 2 below the horizontal base plate 6 and discharges it through the discharge hole 92 formed on the left side surface of the housing 2, thereby preventing the heat of the fixing roller unit 54 from being transmitted to the photosensitive member 10 and thereby from deteriorating the photosensitive member 10. The illustrated copying apparatus is constructed such that the copying process can be performed selectively in at least two copying magnification ratios, for example either 1:1 magnification copying or reduced scale copying at a ratio of about 0.7 in length and about 0.5 in area is selectively carried out. This feature will be described in detail later on, and for the time being the basic principle of copying at different magnifications in the illustrated copying apparatus is briefly described below. In the illustrated copying apparatus, the rotating drum 8 is rotated always at a predetermined speed irrespective of the ratio of copying. The paper conveying mechanism 32 also conveys the copying paper P through the transfer zone 26 always at a predetermined speed irrespective of the ratio of copying, namely at substantially the same speed as the moving speed of the photosensitive member 10 disposed, on the peripheral surface of the rotating drum 8. In contrast, the transparent plate 4 is caused to make a scanning movement at a speed varying according to the ratio of copying, and the optical device 66 projects the image of an original document placed on the transparent plate 4 onto the photosensitive member 10 at a prescribed ratio of copying. Specifically, when the copying process is performed at a ratio of substantially 1, the transparent plate 4 is caused to make a scanning movement substantially at the same speed as the moving speed of the photosensitive member 10 (and the moving speed of the copying paper through the transfer zone 26), and the optical device 66 projects the image of the original document at a 1:1 magnification. However, when the copying process is carried out at a predetermined ratio of copying, for example at a length ratio of M (e.g., M=about 0.7), the transparent plate 4 is caused to make a scanning movement at a speed corresponding to VM where V is the speed employed in the case of performing equal scale copying, and consequently, the size, in the moving direction of the photosensitive member 10 (scanning direction), of a latent electrostatic image formed on the photosensitive member 10 is reduced (or enlarged) to M times. At the same time, the optical device 66 projects the image of the original document placed on the transparent plate 4 onto the photosensitive member 10 at a ratio of M as a result of the lens assembly 78, second reflecting mirror 74 and third reflecting mirror 76 being moved respectively to prescribed positions as will be described in detail hereinbelow. As a result, the widthwise size of the latent electrostatic image formed on the photosensitive member 10 is reduced (or enlarged) to M times. In this way, a latent electrostatic image reduced (or enlarged) to M times in length is formed on the photosensitive member 10, and the reduced (or enlarged) latent electrostatic image is developed to a toner image and transferred to a copying paper. Thus, a reduced (or enlarged) copied image is obtained. Widthwise positioning of a projected image on the photosensitive member It is well known to those skilled in the art that in a so-called transfer-type electrostatic copying apparatus adapted to form a latent electrostatic image or a toner image on the photosensitive member 10 disposed on the peripheral surface of the rotating drum 8, and contact a copying paper P with the surface of the photosensitive member 10 in the transfer zone 26 thereby transferring the latent electrostatic image or the toner image on the photosensitive member 10 to the copying paper P, the copying paper P adheres fairly strongly to the surface of the photosensitive member 10 in the transfer zone 26 by the action of electrostatic charge, and it is not always easy to peel off the copying paper P from the photosensitive member 10 after transfer. In order to cope with this situation, a paper separating channel 94 is formed at one side portion of the rotating drum 8, and the photosensitive member 10 is disposed inwardly of the channel 94 as in clearly shown in Figure 2. The copying paper P is contacted with the photosensitive member 10 in such a manner that its one side edge portion extends outwardly beyond one side edge 10a of the photosensitive member 10 by a predetermined width w₁ and is positioned in an area where the channel 94 is formed, i.e. a nonimage area for paper separation. In peeling off the paper P from the photosensitive member 10, the action of a peeling nail-like member 96 (Figure 1) projecting from the channel permits accurate separation of the copying paper P from the photosensitive member 10. When substantially 1:1 magnification copying is carried out in a copying apparatus of the aforesaid construction, the image of the original document O is projected onto the rotating drum 8 in register with the widthwise position of the copying paper P with respect to the rotating drum as shown by solid lines in Figure 2. In other words, the image of the original document O is projected substantially at a ratio of 1 onto the rotating drum such that one side edge portion of the image of the document O extends beyond the side edge 10a of the photosensitive member 10 by the predetermined width w₁ and is thus located at a nonimage area for paper separation where the channel 94 is formed. As will be readily seen from Figure 2, therefore, the portion having the width w₁ of one side edge portion, of the original document O is located correspondingly to the predetermined width w₁ of one side edge portion of the copying paper P and forms a nonimage area in which a copied image is not formed on the copying paper P. However, since the predetermined width w₁ of one side edge of the original document is usually a white background having no image to be copied, no particular inconvenience is caoused if that portion becomes a non-copying portion. When the copying process is perfomed at a copying ratio of M (M=W₂/W₁) using an original document having a total width of W₁ and a copying paper P' having a total width of W₂ in the conventional variable ratio electrostatic copying, the same method as in the case of performing the copying process at a copying ratio of substantially 1 is employed. Specifically, the image of the original document to be projected onto the rotating drum 8 on a reduced (or enlarged) scale at a length ratio of M is registered with the widthwise position of the copying paper P' with respect to the rotating drum 8. In other words, the image of the original document O is positioned widthwise such that a portion having the predetermined width w₁ of one side edge portion of the projected image on the rotating drum 8 extends beyond the side edge 10a of the photosensitive member 10 and is positioned in a nonimage area for paper separation in which the channel 94 is formed. When the copying process is carried out at a copying ratio in length of M in such a conventional copying with different magnifications, the width of one side edge portion of the copying paper P' in which no copied image is formed is w₁ as in the case of equal scale copying. But the non-copying width of one side edge portion of the original document O projected onto the paper separating nonimage area of the rotating drum 8 is increased (or reduced) to w₂ (w₂=w₁/M) , and the portion having the width w₂ will not be converted to a copied image. This is unnatural in that while the non-copying width of one side edge portion of the original document O is w₁ in the case of substantially equal scale copying, it is w₂ (w₂=w₁/M) when the copying process is performed at a length ratio of M. Particularly in the case of reduced scale copying (i.e., M<1), the non-copying width at one side edge portion of the original document O which is not converted to a copied image is increased from w₁ to w₂ (w₂=w₁/M) . This causes the inconvenience that not only the white background area at one side edge portion of the original document O, but also that part of the original document O at which an image to be copied is present will not be converted to a copied image. In an attempt to solve or eliminate the aforesaid problem or defect of the conventional electrostatic copying with different magnifications, the specification of Japanese Laid-Open Patent Publication No. 28068/1980 discloses that in the case of reduced (or enlarged) scale copying, the ratio M' of the total width W₃ of an image projected on the rotating drum 8 to the total width W₁ of the original document O (M'=W₂/W₁) is made lower (or higher) than the ratio M of the total width W₂ of a copying paper to the total width W₁ of the original document O (M=W₂/W₁) to provide W₃=W₂-w₁, and the image of the original document O projected onto the rotating drum 8 at a length ratio of M' is positioned widthwise while being registered with the image-forming portion (that part of one side edge portion which is other than the portion having a width w₁) of the copying paper P'. By this contrivance, the entire width W₁ of the original document O is imaged as a copied image in the image-forming portion (W₂-w₁) of the copying paper P. The method disclosed in Japanese Laid-Open Patent Publication No. 28086/1980, however, has one or more disadvantages described below. (a) When the copying process is carried out substantially at a ratio of 1, the non-copying width w₁ at one side edge portion of the original document O (this portion is usually a white background) is not imaged as a copied image on the copying paper P. In contrast, reduced (or enlarged) scale copying is unnatural in that a non-copying width does not exist and the entire width W₁ of the original document O is imaged as a copied image on the image-forming portion (W₂-w₁) of the copying paper P' (hence, when one side edge portion of the original document is a white background, a white background having a fairly larger width than w₁ occurs in one side edge portion of the copying paperP'). (b) There is an unnatural feeling because a considerable difference exists between the ratio M of the width W₂ of the copying paper P' to the width W₁ of the original document O (M=W₂/W₁) and the ratio M' of the width W₃ of the copied image on the copying paper P' to the width W₁ of the image of the original document O (M'=W₃/W₁) . The above disadvantages can be overcome by performing the copying process such that irrespective of the ratio of copying, only that portion having a predetermined width w₁ at one side edge portion of the original document O is always projected as a non-copying portion onto a paper separating nonimage area (an area where the channel 94 is formed) constituting the supporting base. With reference to Figure 3, this feature will be described. When the copying process is performed substantially at a ratio of 1, the image of the original document O is projected on the rotating drum 8 while it is registered with the widthwise position of the copying paper P with respect to the rotating drum as in the conventional practice, as shown by a solid line in Figure 3. Hence, as in the conventional practice, that portion having a predetermined width W₁ at one side edge portion of the original document O is projected onto the paper separating nonimage area (the area in which the channel 94 is formed) on the rotating drum 8 while it is located correspondingly to the predetermined width w₁ of one side edge portion of the copying paper P; and thus it becomes a non-copying portion which is not imaged as a copied image on the copying paper P. On the other hand, when the copying process is performed in a reduced (or enlarged) mode at a length ratio of M (M=W₂/W₁), the projected image of the original document O on the rotating drum 8 is positioned widthwise so that the inside edge Q of the non-copying portion having the predetermined width w₁ in one side edge portion of the original document O in the case of performing substantially equal scale copying corresponds with the inside edge of the paper separating non-image area on the rotating drum 8, i.e. the one side edge 10a of the photosensitive member 10, as shown by a two-dot chain line in Figure 3. Thus, only that portion having the predetermined width w₁ in one side edge portion of the original is always projected onto the paper-separating nonimage area of the rotating drum 8 irrespective of the ratio of copying. It will thus be appreciated easily by reference to Figure 3 that in performing the copying process in a reduced (or enlarged) mode, only that portion having the predetermined width w₁ in one side edge portion of the original document O is located within a portion of the predetermined width w₁ in one side edge portion of the copying paper P or P' as in the case of performing substantially equal scale copying, and the non-copying width at one side edge portion of the original document O is always maintained at the predetermined vlue w₁ irrespective of the ratio of copying. Accordingly, unnaturalness does not occur even in the case of reduced (or enlarged) scale copying. On the other hand, if the image of the original document O projected on the rotating drum 8 at a length ratio of M (M=W₂/W1) as shown by a two-dot chain line in Figure 3, one side edge R₁ of the projected image on the rotating drum 8 is located inwardly (outwardly in an enlarging copying mode) of one side edge P'₁ of the copying paper P' always positioned in place with respect to the rotating drum 8 by a slight width x. Hence, when in a reduced (or enlarged) copying mode a copying paper P' having the same total width W₂ as the total width W₂ of the projected image on the rotating drum 8 is used or in other words the ratio M of the width W₂ of the copying paper used to the total width W₁ of the original document O (M=W₂/W₁) is made substantially the same as the ratio M of the total width W₂ of the projected image on the rotating drum 8 to the total width W₁ of the original document O (M=W₂/W₁) , the other side edge R₂ of the projected image on the rotating drum 8 is located outwardly (inwardly in an enlarged copying mode) of the other side edge P₂' of the copying paper P' by a slight width x, as illustrated in Figure 3, For this reason, when substantially 1:1 magnification copying is carried out, the other side odge O₂ of the original document O is substantially registered with the other side edge P₂ of the copying paper P. But in a reduced copying mode, that portion having a slight width w₃ at the other side edge portion of the original document O extends beyond the other side edge P₂' of the copying paper P' and is not imaged as a copied image (this, houwever, will usually not give rise to any particular problem since that portion having the width w₃ in the other side edge portion of the original document O is usually a white background). In an enlarging copying mode, the other side edge Q₂ of the original document O is located slightly inwardly of the other side edge of the copying paper. This minor inconvenience may be removed by adjusting the total width of the image of the original document O projected on the rotating drum 8 to W₄ which is slightly smaller than the total width W₂ of the copying paper P', or in other words, by making the ratio M" of the total width W₄ of the projected image on the rotating drum 8 to the total width W₁ of the original document O slightly lower than the ratio M of the total width W₂ of the copying paper P' to the total width W₁ of the original document O (M=W₂/W₁) . As will be seen from Figure 4, the other side edge R₂ of the projected image on the rotating drum 8 is registered with the other side edge P₂' of the copying paper P' and therefore, the other side edge O₂ of the original document O is registered with the other side edge P₂' of the copying paper P', thereby forming a reduced copied image. In an enlarged scale copying mode, the total width of the image of the original document O projected onto the rotating drum 8 is made slightly larger than the total width of the copying paper, or in other words, the ratio M"' of the total width of the projected image on the rotating drum 8 to the total width W₁ of the original document is made slightly higher than the ratio of the total width of the copying paper to the total width W₁ of the original document O. As a result, the other side edge of the projected image on the rotating drum 8 can be registered with the other side edge of the copying paper, and therefore, the other side edge O₂ of the original document O can be registered with the other side edge of the copying paper, thereby forming an enlarged copied image. If the above method described with reference to Figure 4 is employed, there will of course be some difference between the ratio M of the width W₂ of the copying paper P' to the width W₁ of the original document O and the ratio M"(M"') of the copied image formed on the copying paper P' to the image of the original document O. Since, however, such a difference corresponds to the slight width x mentioned above and is extremely small, it does not render the copied image unnatural. In contrast, since the corresponding difference in the method disclosed in the above-cited Japanese Laid-Open Patent Publication No. 28086/1980 corresponds to a predetermined width w₁ (w₁ x), it is considerably large and renders the copied image unnatural. Mounting and moving mechanisms for the optical device The copying apparatus of the invention illustrated in Figure 1 is constructed such that it can perform a copying process at two or more selectively prescribed ratios, more specifically in a substantially equal scale mode or in a reduced scale mode at a predetermined ratio (for example, about 0.7 in length and about 0.5 in area). As already stated hereinabove, when the copying process is performed in a substantially equal scale mode, the optical device 66 projects the image of an original document placed on the transparent plate 4 onto the photosensitive member 10 disposed on the peripheral surface of the rotating drum 8 substantially at a ratio of 1. In a reduced scale mode at a predetermined ratio of copying, the optical device 66 projects the image of the original document placed on the transparent plate 4 at the above-mentioned predetermined ratio onto the photosensitive member 10 disposed on the peripheral surface of the rotating drum 8. When the image of the original document placed on the transparent plate 4 is to be projected onto the photosensitive member 10 substantially at a ratio of 1, the constituent elements of the optical device 66 are positioned as shown in Figure 1. In contrast, when the image of the original document placed on the transparent plate 4 is to be projected on a reduced scale at a predetermined ratio onto the photosensitive member 10, some of the constituent elements of the optical device 66 (in the illustrated embodiment, the lens assembly 78, the second reflecting mirror 74 and the third reflecting mirror 76) are moved as prescribed. The lens assembly 78 is moved in a direction inclined at a predetermined angle to the optical axis of the optical device 66, and is thus caused to approach the photosensitive member 10, in order to position the reduced projected image, for example, as described hereinabove with reference to Figure 3 or 4 with respect to the photosensitive member 10. The second reflecting mirror 74 and the third reflecting mirror 76 are moved away slightly from the lens assembly 78 so that even when the lens assembly 78 is caused to approach the photosensitive member 10, the focal distance f of the lens placed in the lens assembly 78, the distance a between the lens and the original document placed on the transparent plate 4, and the distance b between the lens and the photosensitive member 10 are maintained in the relation 1/f=1/a + 1/b. One example each of mounting and moving mechanisms for achieving the change of the positions of some of the constituent elements of the optical device 66 (in the illustrated embodiment, the lens assembly 78, the second reflecting mirror 74 and the third reflecting mirror 76) according to the ratio of copying will be described below with reference to Figures 5, 6 and 7 taken in conjunction with Figure 1. To the upper surface of the horizontal base plate 6 (Figures 1, 6 and 7) disposed within the housing 2 (Figure 1) is fixed by means of a pair of mounting blocks 100 an inclined guide rod 98 extending inclinedly at an angle ϑ (Figure 5) with respect to the optical axis of the optical device 66 which extends in the left and right directions in Figures 1, 5 and 7. A pair of upstanding pieces 104 formed at one side portion of the supporting frame 102 for the lens assembly 78 are slidably mounted on the inclined guide rod 98. As can be seen from Figure 7, the under surface of a main portion 106 of the supporting frame 102 is separated some distance from the upper surface of the horizontal base plate 6, and at the under surface of the main portion 106 is formed a supporting block 108 which is in contact with the upper surface of the horizontal base plate 6 and when the supporting frame 102 is moved along the inclined guide rod 98, is caused to slide over the upper surface of the horizontal base plate 6. Hence, the supporting frame 102 is accurately supported in the desired condition when its pair of upstanding pieces 104 are mounted on the inclined guide rod 98 and the supporting block 108 comes into contact with the upper surface of the horizontal base plate 6. In relation to the supporting frame 102, a position-setting member 110 is also fixed to the horizontal base plate 6, and upstanding stop pieces 112a and 112b are formed at opposite ends of the position-setting member 110. When the supporting frame 102 is held at the equal scale position shown by a solid line in Figures 5 to 7 (as will be described below, when the supporting frame 102 is held at this equal scale position, the optical device 66 projects the image of an original document substantially at a ratio of 1 onto the photosensitive member 10), the edge of a projecting piece 114 formed at one side portion of the supporting frame 102 abuts against the stop piece 112b as shown in Figures 5 and 6. On the other hand, when the supporting frame 102 is held at the reduced scale position shown by a two-dot chain line in Figures 5 and 7 (as will be stated below, when the supporting frame 102 is held at this reduced scale position, the optical device 66 projects the image of the original document on a reduced scale at a predetermined ratio onto the photosensitive member 10), a part 116 (Figure 6) of the front edge of the main portion 106 of the supporting frame 102 abuts against the stop piece 112a. A projecting piece 118 is formed at the other side edge portion of the supporting frame 102, and a permanent magnet 120 is fixed to the under surface of the projecting piece 118. In relation to the permanent magnet 120, detecting switches S1 and S2 for detecting the permanent magnet 120 are provided on the horizontal base plate 6. Furthermore, as will be described in greater detail hereinafter, the detecting switch S1 detects the permanent magnet 120 when the supporting frame 102 is held at the aforesaid equal scale position or its vicinity, and the detecting switch S2 detects the permanent magnet 120 when the supporting frame 102 is held at the aforesaid reduced scale position or its vicinity. The lens assembly 78 of the optical device 66 is mounted on the supporting frame 102 as prescribed. The mechanism of mounting the lens assembly 78 on the supporting frame 102 will be described with reference to Figures 8 and 9 taken in conjunction with Figure 6. The lens assembly 78 is comprised of a substantially hollow cylindrical lens housing 122, and one or more (usually a plurality of) lenses 124 placed in the lens housing 122. In order to mount the lens assembly 78 on the supporting frame 102 as prescribed, a linking member 126 and a supporting member 128 are used in the illustrated embodiment. The linking member 126 has a hollow cylindrical portion 130 having an inside diameter corresponding to the outside diameter of the lens housing 122 of the lens assembly 78 and a flange portion 132 projecting from the cylindrical portion 130 radially toward both sides. A radially extending screw hole 134 is formed in the cylindrical portion 130, and a pair of axially extending screw holes 136 are formed in the flange portion 132. The linking member 126 is fixed to the lens assembly 78 by fitting it over a given position of the central part of the lens housing 122, threadably inserting a setscrew (not shown) through the screw hole 134, and causing the end of the setscrew to abut against the surface of the lens housing 122, or threadably fitting it with a corresponding screw hole (not shown) formed in the lens housing 122. On the other hand, the supporting member 128 has a base portion 138 and a projecting supporting piece 140 upstanding from the base portion 138. In the projecting supporting piece 140 is formed a relatively large notch 142 extending from its upper end edge to its lower end edge. The notch 142 has an introductory portion 142a extending downwardly from the upper end edge of the projecting supporting piece 140 with a slightly larger width than the outside diameter of the cylindrical portion 130 of the linking member 126 and a tapering portion 142b extending downwardly from the introductory portion 142a in a tapering manner. A pair of through-holes 144 located on the opposite sides of the notch 142 are formed in the projecting supporting piece 140. The supporting member 128 is fixed to the supporting frame 102 by fixing its base portion 138 to the upper surface of the main portion 106 of the supporting frame 102 by a suitable method such as welding or screwing. In mounting the lens assembly 78 having the linking member 126 fixed thereto on the supporting frame 102 having the supporting member 128 fixed thereto, the lens assembly 78 is inserted through the introductory portion 142a of the notch 142 and set on the tapering portion 142b, and as shown in Figure 9, the peripheral surface of the cylindrical portion 130 of the linking member 126 is placed on the side edges of the tapering portion 142b. Then, the flat one surface of the flange portion 132 of the linking member 126 is contacted with the adjoining flat one surface of the projecting supporting piece 140. Set screws 146 are screwed into the pair of screw holes 136 formed in the flange portion 132 of the linking member 126 through the pair of through-holes 144 formed in the projecting supporting piece 140. Thus, the lens assembly 78 is fixed to the projecting supporting piece 140. According to the above-described mounting mechanism using the linking member 126 and the supporting member 128, the peripheral surface of the cylindrical portion 130 of the linking member 126 is brought substantially into point-to-point or line-to-line contact with both side edges of the tapering portion 142b of the notch 142 thereby accurately defining the vertical and lateral positions of the lens assembly 78 with respect to the supporting frame 102. Furthermore, the flat one surface of the flange portion 132 of the linking member 126 is contacted with the adjoining one flat surface of the projecting supporting piece 140, thereby accurately defining the axial position of the lens assembly 78 with respect to the supporting frame 102 and also accurately positioning the axis of the lens assembly 78 with respect to the supporting frame 102 as prescribed (more specifically, so that it extends perpendicularly to the projecting supporting piece 140). Accordingly, without expertise, the lens assebly 78 can be mounted as prescribed onto the supporting frame 102 with relative simplicity and ease. If desired, it is possible to form the linking member 126 as a unit with the lens housing 122 of the lens assembly 78 and to form the projecting supporting piece 140 as a unit with the supporting frame 102. It is also possible to omit the cylindrical portion 130 of the linking member 126 and to place the peripheral surface of the lens housing 122 directly onto the tapering portion 142b of the notch 142 in the projecting supporting member 140. As shown in Figures 5, 6 and 7, a projecting piece 148 is formed at one end portion (the right end portion in Figures 5 and 7) of the supporting frame 102, and an exposure adjusting plate 150 is mounted on the projecting piece 148. A pair of laterally spaced slots 152 (only one of them is shown in Figure 6) are formed in the exposure adjusting plate 150. By screwing setscrews 154 into the projecting piece 148 through these slots 152, the exposure adjusting plate 150 is mounted on the projecting piece 148 such that its position can be freely adjusted (namely, the amount of the plate 150 projecting from the projecting piece 148 can be adjusted freely). The configuration, operation, effect, etc. of the exposure adjusting plate 150 itself will be described hereinafter in greater detail. With reference to Figures 5 and 10, a supporting frame 156 is also mounted on the horizontal base plate 6 (Figures 1, 6 and 7) in addition to the supporting frame 102. The supporting frame 156 has a pair of laterally spaced side plates 158a and 158b and a member 160 connected between the pair of side plates 158a and 158b. Furthermore, the second reflecting mirror 74 and the third reflecting mirror 76 (Figures 1 and 7) of the optical device 66 are mounted as prescribed between the pair of side plates 158a and 158b. A pair of linking brackets 162 are secured to the outside surface of the side plate 158a. On the other hand, a guide rod 166 extending substantially parallel to the optical axis of the optical device 66 is fixed to the horizontal base plate 6 (Figures 1, 5 and 7) by means of a pair of fixing blocks 164. The above pair of linking brackets 162 are slidably linked to the guide rod 166. A short shaft 168 is fixed firmly in the inside surface of the side plate 158b, and a roller 170 above the horizontal base plate 6 is rotatably mounted on the short shaft 168 (see Figures 11-A and 11-B also). The supporting frame 156 can be moved along the guide rod 166 when the pair of linking brackets 162 slide with respect to the guide rod 166 and the roller 170 rotates over the horizontal base plate 6. As will be described in greater detail, the supporting frame 156 is selectively held at the equal scale position shown by a solid line in Figure 5 and also in Figure 10 (as will be stated hereinafter, when the supporting frame 156 is held at this equal scale position, the optical device 66 projects the image of an original ducument onto the photosensitive member 10 substantially at a ratio of 1), and the reduced scale position shown by a two-dot chain line in Figure 5 (as will be stated hereinafter, when the supporting frame 156 is held at this reduced scale position, the optical device 66 projects the image of the original dicument onto the photosensitive member 10 on a reduced scale at a predetermined ratio). The optical device 66 also has a moving mechanism shown generally at 172 for selectively holding the supporting frame 102 and the supporting frame 156 at the aforesaid equal scale position and the reduced scale position. As shown in Figures 5 and 10, provided on the horizontal base plate 6 (Figures 1, 6 and 7) is a mounting member 174 having a base portion 174a fixed to the horizontal base plate 6 and a mounting portion 174b upstanding from the base portion 174a, and a drive source constructed of a reversible electric motor 176. In the illustrated embodiment, the reversible motor 176 has an output shaft 178 projecting forwardly in Figure 10 through the mounting portion 174b of the mounting member 174, and the output shaft 178 constitutes an input shaft of the moving mechanism 172. Needless to say, it is possible, if desired, to mount a separate input shaft for the moving mechanism 172 rotatably, and drivingly connect the input shaft to the reversible motor 176. The moving mechanism 172 further includes a first moving arrangement 180 for moving the supporting frame 102 according to the rotation of the shaft 178 and a second moving arrangement 182 for moving the supporting frame 156 according to the rotation of the shaft 178. With reference to Figures 5 and 10 taken in conjunction with Figure 6, the first moving arrangement 180 includes a pulley 184 fixed directly to the shaft 178, and a rope 186, conveniently a wire rope, is wrapped about the pulley 184 through nearly one turn. As will be described hereinafter, the pulley 184 is rotated between the angular position shown in Figure 11-A and the angular position shown in Figure 11-B by the reversible electric motor 176. Conveniently, in order to prevent generation of slippage between the pulley 184 and the rope 186 during this rotation of the pulley 184, that part of the rope 186 which does not separate from the pulley 184 is accurately fixed to the pulley 184 by means of a setscrew 188 (Figures 11-A and 11-B). One side of the rope 186 wrapped about the pulley 184 extends along a rotatably mounted guide pulley 190 and is connected by means of a tension spring 194 to a linking piece 192 fixed to the projecting piece 114 formed at one side portion of the supporting frame 102. The other side of the rope 186 wrapped about the pulley 184 extends along rotatably mounted guide pulleys 196 and 198 and is connected by means of a tension spring 200 to the linking piece 192 fixed to the supporting frame 102. The guide pulleys 190 and 198 guide the rope 186 so that it extends substantially parallel to the inclined guide rod 98 between the guide pulley 190 and the linking piece 192 and between the linking piece 192 and the guide pulley 198. The second moving arrangement 182 will be described with reference to Figures 11-A and 11-B taken in conjunction with Figures 5 and 10. The second moving arrangement 182 includes a wheel 202, conveniently a sprocket wheel, directly fixed to the shaft 178. A short shaft 206 is fixed to one of a pair of side plates 204 disposed in laterally spaced apart relationship within the housing 2 (Figure 1) (the horizontal base plate 6 is disposed between this pair of side plates), and a wheel 208, conveniently a sprocket wheel, is rotatably mounted on the short shaft 206. A wrapping power transmission member 210, conveniently a chain, is wrapped about the wheels 202 and 208, and a cam 212 to be rotated as a unit with the wheel 208 is also mounted on the short shaft 206. The cam 212 is comprised of a cam plate having on its peripheral surface two arcuate acting surfaces having different radii, i.e., a small-radius acting surface 214a and a large-radius acting surface 214b, and a transit surface 214c located between the two acting surfaces on its peripheral surface. A fan-like member 216 is mounted on the outside surface of the side plate 158a of the supporting frame 156, and a short shaft 218 is fixed into the fan-like member 216, and a roller 220 constituting a cam follower is rotatably mounted on the end portion of the short shaft 218. The lower end portion of the fan-like member 216 is pivotably linked to the side plate 158a by a linking pin 222 and a setscrew 226 is screwed into the side plate 158a through an arcuate slit 224 having its center at the linking pin 222. As a result, the fan-like member 216 is mounted on the side plate 158a so that its angular position of pivoting about the linking pin 222 as a center can be freely adjusted. It will be appreciated that when the pivoting angular position of the fan-like member 216 with respect to the side plate 158a is changed, the position of the roller 220 in the longitudinal direction of the guide rod 166 with respect to the supporting frame 156 will be changed. In relation to the supporting frame 156, the guide rod 166 has also mounted thereon a compression spring 228 one end of which acts on one of the pair of fixing blocks 164 and the other of which acts on one of the pair of linking brackets 162. The compression spring 228 elastically urges the supporting frame 156 toward the right in Figures 11-A and 11-B, and elastically presses the roller 220 constituting the cam follower against the peripheral surface of the cam 212. The operation of the moving mechanism 172 described hereinabove is summarized below. For example, in moving the supporting frames 102 and 156 from the equal scale position shown by a solid line in Figure 5 to the reduced scale position shown by a two-dot chain line in Figure 5, the reversible electric motor 176 is rotated normally to rotate the shaft 178 in the directoin of an arrow 230 (Figures 10 and 11-A). As a result, the pulley 184 of the first moving arrangement 180 is rotated in the direction of arrow 230. When the pulley 184 is rotated in the direction of arrow 230, the rope 186 is moved in the direction of arrow 230, and thus the supporting frame 102 is moved in the direction of arrow 230. When the supporting frame 102 is moved to the reduced scale position shown by the two-dot chain line in Figure 5, the part 116 of the front edge of the main portion 106 of the supporting frame 102 is caused to abut against the stop piece 112a. On the other hand, when the supporting frame 102 is moved to the reduced scale position or its vicinity, the detecting switch S2 detects the permanent magnet 120 fixed to the supporting frame 102. As will be described in detail hereinafter, even when the detecting switch S2 has detected the permanent magnet 120, the reversible motor 176 is not deenergized; but it is deenergized after the lapse of a predetermined delay time from the time when the detecting switch S2 detected the permanent magnet 120. Accordingly, after the supporting frame 102 has abutted against the stop piece 112a, the reversible motor 176 continues to be in the energized state for a certain period of time. As a result, the supporting frame 102 cannot further move in the direction of arrow 230, whereas a force tending in the direction of arrow 230 acts on the rope 186 to stretch the tension spring 194 elastically. Thus, the supporting frame 102 is pressed elastically against the stop piece 112a by the action of the tension spring 194 and thereby accurately held at the required reduced scale position. When the reversible electric motor 176 is normally rotated to rotate the shaft 178 in the direction of arrow 230 (Figures 10 and 11-A), the wheel 202 of the second moving arrangement 182 is also rotated in the direction of arrow 230, and the wheel 208 is rotated in the direction of arrow 230 through the wrapping power transmission member 210. Incident to the rotation of the wheel 208, the cam 212 is rotated in the direction of arrow 230 from the position shown in Figure 11-A, and when the reversible motor 176 is deenergized, the cam 212 is held at its angular position at which the large-radius acting surface 214b acts on the roller 220 forming the cam follower, as shown in Figure 11-B. When the cam 212 is rotated from the angular position shown in Figure 11-A to the angular position shown in Figure 11-B, the action of the cam 212 causes the supporting frame 156 to move from the equal scale position shown in Figure 11-A to the reduced scale position shown in Figure 11-B against the elastic biasing action of the compression spring 228, and is thus held accurately at the reduced scale position shown in Figure 11-B. When the reversible motor 176 is deenergized, the cam 212 needs not to be precisely held at its predetermined angular position, and so long as the large-radius acting surface 214b of the cam 212 is positioned in an angular range in which it acts on the roller 220, the supporting frame 156 is accurately held in the required reducing position. In moving the supporting frames 102 and 156 from the reduced scale position shown by the two-dot chain line in Figure 5 to the equal scale position shown by the solid line in Figure 5, the reversible electric motor 176 is reversely rotated to rotate the shaft 178 in the direction shown by an arrow 232 (Figures 10 and 11-B). As a result, the pulley 184 of the first moving arrangement 180 is rotated in the direction of arrow 232. When the pulley 184 is rotated in the direction of arrow 232, the rope 186 is moved in the direction of arrow 232, and as a result, the supporting frame 102 is moved in the direction of arrow 232. When the supporting frame 102 is moved to the equal scale position shown by the solid line in Figure 5, the edge of the projecting piece 114 formed in one side portion of the supporting frame 102 is caused to abut against the stop piece 112b. On the other hand, when the supporting frame 102 is moved to the equal scale position or its vicinity, the detecting switch S1 detects the permanent magnet 120 fixed to the supporting frame 102. As described in more detail hereinafter, however, even when the detecting switch S1 has detected the permanent magnet 120, the reversible motor 176 is not deenergized; but it is deenergized after the lapse of a certain period of delay time from the time when the detecting switch S1 detected the permanent magnet 120. Accordingly, even after the supporting frame 102 has abutted against the stop piece 112b, the reversible motor 176 continues to be in the energized state for a certain period of time. As a result, the supporting frame 102 cannot be moved further in the direction of arrow 232, whereas a force tending in the direction of arrow 232 acts on the rope 186 to stretch the tension spring 200 elastically. By the action of the tesion spring 200, the supporting frame 102 is pressed elastically against the stop piece 112b and thereby held accurately at the required equal scale position. On the other hand, when the reversible electric motor 176 is rotated reversely to rotate the shaft 178 in the direction of arrow 232 (Figures 10 and 11-B), the wheel 202 of the second moving arrangement 182 is rotated in the direction of arrow 232, and the wheel 208 is rotated in the direction of arrow 232 through the wrapping power transmission member 210. Incident to the rotation of the wheel 208, the cam 212 is rotated in the direction of arrow 232 from the position shown in Figure 11-B, and when the reversible motor 176 is deenergized, the cam 212 is held at an angular position at which its small-radius acting surface 214a acts on the roller 220 constituting the cam follower. When the cam 212 is rotated from its angular position shown in Figure 11-B to its angular position shown in Figure 11-A, the supporting frame 156 is moved from the reduced scale position shown in Figure 11-B to the equal scale position shown in Figure 11-A by the elastic biasing action of the compression spring 228, and is thus accurately held at the equal scale position shown in Figure 11-A. In the case of holding the supporting frame 156 at the equal scale position, too, the cam 212 needs not to be precisely held at its predetermined position at the time when the reversible motor 176 has been deenergized. So long as the small-radius acting surface 214a of the cam 212 is held in an angular range in which it acts on the roller 220, the supporting frame 156 is accurately held at the required equal scale position. The moving mechanism 172 provided in the optical device 66 has excellent advantages, among which are: (a) Since the rope 186 is utilized to move the supporting frame 102 whose moving distance is relatively large and the cam 212 is utilized to move the supporting frame 156 whose moving distance is relatively small, the supporting frames 102 and 156 which have to be moved in different directions can be moved in the required relationship by a relatively simple and inexpensive mechanism having a single drive source (i.e., the reversible electric motor 176); (b) It is extremely difficult, if not impossible, to precisely prescribe the time of deenergization of the drive source (i.e., the reversible motor 176). According to the above moving mechanism, the supporting frames 102 and 156 can be accurately held at the required positions (i.e., the equal scale position and the reduced scale position) even if a considerable error exists in the time of deenergization of the drive source. Exposure adjusting plate The illustrated copying apparatus of this invention is constructed such that the copying process can be performed at selectively prescribed two or more ratios of copying, more specifically in a substantially equal scale mode and a reduced scale mode at a predetermined ratio (e.g., about 0.7 in length and about 0.5 in area). In this type of copying apparatus, when substantially equal scale copying is changed to reduced scale (or enlarged scale) copying at a predetermined ratio, the amount of exposure on the photosensitive member 10 changes. In order, therefore, to obtain a good copied image in the case of the reduced (or enlarged) scale copying, it is important to adjust the amount of exposure on the photosensitive member 10 properly in changing the substantially equal scale copying to the reduced (or enlarged) scale copying at a predetermined ratio. Figure 12-A diagrammatically shows the projection of an original document O onto the photosensitive member 10 as a projected image I on a substantially equal scale by means of a lens L. It is well known to those skilled in that art that in the projected state shown in Figure 12-A, light from a point p on the original document O which falls at an incidence angle of α to the lens L is decayed to cos⁴α times at point p' on the projected image I owing to the widthwise light decaying property of the lens L. In order, therefore, to make the distribution of illuminance in the widthwise direction at the projected image I substantially uniform by adjusting the light decaying property of the lens L, the specific illuminance Z p at point p of the original document O should be adjusted to a value given by the following equation. wherein f is the focal distance of the lens L, B is the total width of the original document O, and x is the distance from one side edge of the document O to the point p. In order to satisfy this requirement, the document illuminating lamp 70 (Figures 1 and 7) of the optical device 66 in the illustrated copying apparatus is constructed such that its brightness is gradually increased from its center in the widthwise direction toward its side end as is well known, and it illuminates the document O placed on the transparent plate 4 (Figures 1 and 7) at the illuminance defined by equation (1) above thereby to offset the widthwise decaying property of the lens L and to make the distribution of illuminance of the projected image I in the widthwise direction substantially uniform. Thus, in the case of performing substantially equal scale copying, the width, in the moving direction of the photosensitive member 10 (the moving direction of the transparent plate 4), namely the slit exposure width, of a light path leading from the original document O to the photosensitive member 10 may be substantially the same along the entire width of the photosensitive member 10. In the illustrated embodiment, the slit exposure width regulating member 84 (Figures 1 and 7) defining the slit exposure width between the lens L and the photosensitive member 10 defines the slit exposure width which is substantially the same along the entire width of the photosensitive member 10. When the copying process is carried out in a reduced (or enlarged) scale mode at a predetermined ratio M, the lens assembly 78 of the optical device 66 in the illustrated copying apparatus is moved in a direction inclined at a predetermined angle with respect to the optical axis of the optical device 66. Hence, the state of projecting the original document O onto the photosensitive member 10 as a projected image I on a reduced (or enlarged) scale at a predetermined ratio M by the lens L is as shown diagrammatically in Figure 12-B. In order to simplify the description, Figure 12-B shows the case in which as described hereinabove with reference to Figure 2, the reduced (or enlarged) projected image I is positioned widthwise such that its one side edge corresponds with one side edge of the projected image I on an equal scale (accordingly, some correction is required as described below when the projected image I is positioned widthwise as described above with reference to Figures 3 and 4). Variations in the illuminance of the projected image I in the state shown in Figure 12-B will now be considered. Firstly, when variations in illuminace owing to the widthwise displacement of the optical axis of the lens L are considered, the specific illuminance at point p' of the projected image I corresponding to the point p of the original document O changes to the value defined by the following equation (2) owing to the widthwise displacement of the optical axis of the lens L in regard to the specific illuminance Z p(x) in the equal scale projection of the image of the document O. In the above equation (2), D is the distance between the lens L and the projected image I and is exressed by D = f(1 + M) . F is the distance from one side edge of the projected image I to the optical axis of the lens L and expressed by the following formula. G is the distance from one side edge of the projected image I to point p', and expressed by G = M(B - x) . Secondly, since the projected image I is M times the size of the original document O, the point p' of the projected image I collects light in an amount 4/(1 + M)² times that in the case of the substantially equal scale projection. Hence, owing to the projection at a ratio of M, the illuminance of the point p' of the projected image I changes to the value shown by the following eqation (3) with regard to the specific illuminance Z p(x) which is obtained in the case of the substantially equal scale projection. When the copying is carried out at a predetermined ratio M, the speed of slit exposure is changed to 1/M times the speed employed in the case of the substantially equal scale copying. Specifically, in the illustrated embodiment, the moving speed of the transparent plate 4 (the moving speed of at least a part of the optical device in a copying apparatus of the type in which slit exposure is carried out by moving at least a part of the optical device instead of moving the transparent plate) is changed to 1/M times that employed in the case of the substantial equal scale copying. Accordingly, the exposure time changes to M times that employed in the case of the substantially equal scale copying. However, as shown in Figures 1 and 7, when the exposure width is regulated between the lens L and the photosensitive member 10 by the slit exposure width regulating member 84, the optical slit exposure width based on the original document O is changed to 1/M times that in the case of the substantially equal scale copying according to the predetermined ratio M. This change in the optical slit exposure width offsets the change in the exposure time. On the other hand, when the slit exposure width is regulated between the original document O and the lens L, the optical slit exposure width based on the original document does not change even when the ratio M changes. Hence, owing to the change of the exposure time to M times that in the case of the substantially equal scale copying, the specific illuminance at p' of the projected image I changes to the value shown by the following equation (4) as compared with the case of the substantially equal scale copying. Z 3p'(x) = Z p(x) .M (4) Hence, when the slit exposure width is regulated between the lens L and the photosensitive member 10, the specific illuminance Z p'(x) of point p' of the projected image I projected at a predetermined ratio M changes to the value expressed by the following equation (5) as compared with the case of the substantially equal scale copying because of the changes represented by the above equations (2) and (3). When the slit exposure width is regulated between the original document O and the lens L, the illuminance changes to the value given by the following equation (6) as compared with the case of the substantially equal scale copying because of the changes expressed by the above equations (2), (3) and (4). When the copying process is carried out in a reduced (or enlarged) scale mode at a predetermined ratio M by adjusting the changes in illuminance expressed by equation (5) or (6), the illuminance of the projected image I in the widthwise direction is made substantially uniform in the following manner. Specifically, when the copying process is carried out in a reduced (or enlarged) scale mode at the predetermined ratio M, an exposure adjusting plate 150 (Figures 5, 6 and 7) is positioned in the light path between the lens L and the projected image I on the photosensitive member 10 or between the original document O and the lens L so as to change the slit exposure width; consequently, the amount of exposure of the point p' on the projected image I is made substantially the same as that in the case of the substantially equal scale copying. In other words, by changing the slit exposure width,the amount of exposure of the point p' on the projected image I is adjusted to The amount of decrease (or increase) of the slit exposure width for providing the aforesaid amount of exposure can be obtained by approximate calculation by a computer made for example according to the following theory. With reference to Figure 13, it can be assumed that in practice the light leaving the lens L arrives at the projected image I while forming innumerable oblique cones. Suppose that the projected image I is equally divided into n portions (for the simplicity of description, it is divided into two equal portions in Figure 13) in the direction of the slit exposure width (the up and down direction in Figure 13), and the light leaving the lens L and forming (n+1) oblique cones arrives at the projected image I. If the slit exposure width is narrowed by v at a position apart from the lens L by distance y , a change in the total amount of light of the projected image I is determined by the ratio of the sum of the cross sectional areas of the oblique cones shut off by the exposure adjusting plate 150 to the sum total of the cross sectional areas of all oblique cones at the position at distance y . If n is taken as 2 for the simplicity of explanation, the radius r of each oblique cone at the position at distance y from the lens L is given by the following equation. r = (1 - y D ) f/2N (7) Wherein N is the so-called F number of the lens L is expressed by N = f / D. Thus, the total sum S' of the cross sectional areas shut off at the position at distance y from the lens L (the cross sectional areas of the hatched portions) is given by S' = S₁ + S₂ + S₃, and As illustrated in Figure 13, H represents the length from one end (the upper end in Figure 13) of the slit exposure width at the position at distance y from the lens L to the center of each oblique cone. The total sum S of the entire cross sectional areas of the three oblique cones at distance y is given by S = 3πr². Hence, by decreasing the slit exposure width by v , the ratio of total amount of light of the projected image I becomes On the basis of this theory, the v value can be calculated by a computer so that by making n sufficiently large, the value of S-S'/S approximates the aforesaid value (When the slit exposure width is changed between the lens L and the projected image I). As already stated hereinabove, in the illustrated copying apparatus, the exposure adjusting plate 150 is mounted on the supporting frame 102 on which the lens assembly 78 of the optical device 66 is also mounted. As can be easily seen from Figure 7, when the supporting frame 102 is moved to the reduced scale position shown by the two-dot chain line in Figure 7 in order to hold the lens assembly 78 at the reduced scale position, the exposure adjusting plate 150 is caused to advance into the light path between the lens assembly 78 and the photosensitive member 10, more specifically between the fourth reflecting mirror 80 and the opening 82 formed in the horizontal base plate 6, and is located partly in the light path. When the exposure adjusting plate 150 is held at the position shown by two-dot chain line in Figure 7, the slit exposure width V regulated by the slit exposure width regulating member 84 (Figures 1 and 7) is partly narrowed by the partial shielding action of the exposure adjusting plate 150 as shown in Figure 14 (the amount of narrowing, v , is prescribed as described above), and thus, the change in the amount of exposure expressed by equation (5) can be fully compensated for. On the other hand, when the slit exposure width V employed in the substantially equal scale copying must be enlarged at least partly in order to compensate for the change in the amount of exposure expressed by equation (5) or (6) as in the case of enlarged scale copying, the restraining of at least one end of the slit exposure width by the regulating member 84 (Figures 1 and 7) is released, and the aforesaid at least one end of the slit exposure width is regulated by the exposure adjusting plate 150 to be partly positioned in the light path. It is noteworthy that in the illustrated copying apparatus the exposure adjusting plate 150 is mounted on the supporting frame 102 on which the lens assembly 78 is also mounted, and when the supporting frame 102 is moved to the position shown by the two-dot chain line in Figure 7 in order to hold the lens assembly 78 in the reduced scale position, the exposure adjusting plate 150 is necessarily positioned in the light path, and therefore, no particular moving and positioning mechanism for the exposure adjusting plate 150 is required. It should also be noted that in the illustrated copying apparatus constructed in accordance with this invention, the exposure adjusting plate 150 is caused to advance into the light path by being moved not substantially perpendicularly to the optical axis but in a direction inclined thereto by a predetermined angle γ, as can be easily understood from Figure 7. When the exposure adjusting plate 150 is moved into the light path inclinedly at a predetermined angle γ to the optical axis, the amount of change in the slit exposure width relative to the amount of movement of the exposure adjusting plate 150 is relatively small, and therefore, the slit exposure width can be varied with sufficient accuracy even if tolerable errors (for example, tolerable errors in the configuration of the exposure adjusting plate 150 or the incoming position of the exposure adjusting plate 150) in regard to the amount of advancing of the exposure adjusting plate 150 into the light path in the case of reduced (or enlarged) scale copying are relatively large. Control circuit for movement of the optical device In the illustrated copying apparatus it is essential that the supporting frame 102 (and the lens assembly 78 and the exposure adjusting plate 150 mounted on it) and the supporting frame 156 (and the second reflecting mirror 74 and the third reflecting mirror 76 mounted on it) should be moved selectively from the equal scale position shown by the solid line in Figure 5 to the reduced scale position shown by the two-dot chain line in Figure 5 or from the aforesaid reduced scale position to the aforesaid equal scale position according to the desired ratio of copying selected, more specifically according to whether the copying is carried out in a substantially equal scale 1:1 magnification mode or in a reduced scale mode at a predetermined ratio. As stated hereinabove, this movement of the supporting frames 102 and 156 is achieved by the operation of the drive source, i.e. the reversible electric motor 176 (Figure 5), of the moving mechanism 172.

Claims: An electrostatic copying apparatus, adapted for copying at different magnifications, comprising an optical device for performing slit exposure scanning of an original document and projecting its image onto a photosensitive member (10), a lens (78) for projecting the image of the document onto the photosensitive member, said lens being adapted to be selectively held either at a first position for projecting the image of the document at a magnification 1:1 onto the photosensitive member or at a second position spaced a predetermined distance from said first position in a direction oblique to the optical axis for projecting the image of the document at a different magnification onto the photosensitive member, the slit exposure scanning being performed at a speed selectable according to the magnification, and means arranged to move an exposure adjusting plate (150) partly into the light path leading from the document to the photosensitive member, whereby the illumination on the photoconductive member in the widthwise direction is rendered substantially uniform when the lens is held at said first position, and when the lens is displaced to said second position, the variations of the illumination on the photosensitive member due to the displacement of the lens and to the different selected speed of the slit exposure scanning are compensated, characterized by a supporting frame (102) supporting both the lens (78) and the exposure adjusting plate (150) and being movable between at least two positions, whereby in the first position of the supporting frame the lens is held at said first position and the exposure adjusting plate is held outside said light path, and in the second position of the supporting frame the lens is held at said second position and the exposure adjusting plate is moved partly into said light path. An electrostatic copying apparatus according to claim 1, characterized in that said exposure adjusting plate (150) is positioned partly in the light path leading from the lens (70) to the photosensitive member (10). An electrostatic copying apparatus according to claim 1, characterized in that said exposure adjusting plate (150) is positioned partly in the light path leading from the document to the lens (70). An electrostatic copying apparatus according to one of the claims 1 to 3, characterized in that when the lens (78) is held at the second position, the lens projects the image of the document on a reduced scale onto the photosensitive member. An electrostatic copying apparatus according to claim 1, characterized in that the exposure adjusting plate (150) is advanced into the light path by being moved obliquely to the optical axis.

Inventors: HIROBE, JUNICHI, IRIE, YOICHIRO, IWAO, NORIYUKI, MURAKAMI, MASAHIRO, NAKANISHI, TSUGIO, TSUTSUI, EIJI YAMADA HEIGHTS 202, WAKIKAIDO, TAKAHIRO, YOSHIOKA, MASAHIRO
Applicants: MITA INDUSTRIAL CO. LTD.
Application year: 1982
Main classification: G03G15
IPC codes: G03B27, G03G21, G03G15