IMPROVEMENT IN IMAGE-FORMING APPARATUS

Abstract: An image-forming apparatus equipped with an optical means (6), an electrostatic image-forming means (4) and a control means (8). The optical means (6) projects a scanned light beam to be controlled according to an image to be formed. The image-forming means (4) receives the scanned light beam projected from the optical means (6) and forms an image corresponding to the scanned light beam. The control means (8) controls the optical means (6) and the image-form­ ing means (4).

Description: This invention relates to an image-forming apparatus comprising an optical means for projecting a light beam to be scanned, which optical means is to be controlled according to an image to be formed, an electrostatic image-forming means for forming an image corresponding to the light beam to be scanned including an electrostatographic material to be moved through an exposure zone to be irradiated with the light beam to be scanned by the optical means and a control means for controlling the optical means and the image-forming means, said control means being adapted to control the optical means by feeding to the optical means image signals for modulating the light beam to be scanned according to the image to be formed and to control the image-forming means by controlling the speed of movement of the electrostatographic material in the image forming means characterized in that the optical means includes means for changing the diameter of the light because to be scanned and in that the control means changes the feeding of the image signal to the optical means according to the change of the diameter of the light became to be scanned. In recent years, image-forming apparatuses of the above-described type have been proposed and gained commercial acceptance as output devices or printing devices in computer systems, word processing systems, etc. An example of prior art device is disclosed in US-A-4 140 903. It is known form DE-A-3 212 194 to increase the diameter of the scanning light beam when operating the image-forming apparatus at a higher speed. The prior image-forming apparatuses, however, have problems or defects to be overcome. Among these are: (1) In the optical means of the aforesaid image-forming apparatus, a rotating polygon mirror assembly rotataing at high speeds is generally used as means for scanning a light beam. This rotating polygon mirror assembly, however, produces considerable noises which are displeasing to the operator and other persons. (2) In the optical means of the aforesaid image-forming apparatus, a light beam generating means such as a laser tube is used. Such light beam generating means decreases in function after use for a long period of time, and the light beam output is reduced. In order, therefore, to obtain the desired image, it is necessary to detect the light beam from the light beam generating means and when it is found that the output of the light beam is reduced, exchange the light beam generating means. In the conventional image-forming apparatus, a light beam detector is disposed at the end portion of an exposure area of the image-forming means, i.e. an area to which the scanned light beam is projected from the optical means, which is not utilized for image formation, and the scanned light beam is detected by this light beam detector. Such a detecting method in the conventional image-forming apparatus, however, has some defects. For example, according to this method, the light beam from the light beam generating means is not directly detected. What is detected is the scanned ligh beam scanned by the scanning means. Hence, a deterioration in the light beam caused by malfunctioning of devices which affect the light beam, such as the scanning means and a modulating means existing between the scanning means and the light beam generating means, is detected as a deterioration in the light beam generating means. Furthermore, the scanned light beam is projected onto the light beam detector repeatedly at time intervals. The duration of projection, however, is only very short, and the judgement of the sanned light beam so detected is relatively difficult. (3) With the aforesaid image-forming apparatus, it is desired both to obtain an image of high quality, and to increase the speed of image formation at the sacrifice of some reduction in image quality. Therefore, the speed of image formation is desired to be changed as required. The conventional image-forming apparatus cannot meet this requirement. A first object of this invention is to provide an image-forming apparatus in which noises generated by a rotating polygon mirror assembly in an optical means are reduced. A second object of this invention is to provide an image-forming apparatus in which a light beam from a light beam generating means of an optical means can be directly detected without causing inconveniences and the function of the light beam generating means can be checked accurately and easily. A third object of this invention is to provide an image-forming apparatus in which the speed of image formation can be changed as required, and therefore it is possible to select whether to obtain an image of high quality or to obtain an image at a fairly high speed at the sacrifice of some reduction in image quality. In another aspect of this invention, the optical means has a rotating polygon mirror assembly which is substantially closed by a casing and an upstream side lens assembly and a downstream side lens assembly mounted on the casing. In still another aspect, between a modulating means and a scanning means of the optical means, there is provided a light beam detector which shields a non-modulated light beam from the modulating means. In a further aspect of this invention, the optical means includes means for changing the diameter of a scanned light beam, and according to the change of the diameter of the scanned light beam, the control means changes the feeding of image signals to the optical means and also changes the moving speed of an electrostatographic material in an image-forming means. The above and other objects of the invention will become apparent from the following description. Figure 1 is a perspective view showing one embodiment of the image-forming apparatus constructed in accordance with this invention; Figure 2 is a perspective view showing the image-forming apparatus of Figure 1 in a condition in which the cover is held at an open position; Figure 3 is a simplified sectional view of the image-forming apparatus of Figure 1; Figure 4 is a perspective view, partly broken away, of an optical means in the image-forming apparatus of Figure 1; Figure 5 is a top plan view of the optical means of Figure 4 with the upper wall of its housing detached; Figure 6 is a simplified schematic view showing a second lens assembly in the optical means of Figure 4; Figure 7 is a perspective view, partly broken away, of a casing and its surrounding structure in the optical means of Figure 4; Figure 8 is a simplified schematic view of the path of a scanned light beam in the optical means of Figure 4; Figure 9 is a top plan view showing a light shielding means and a light beam detecting means in the optical means of Figure 4; Figure 10 is a perspective view showing the light shielding means and light beam detecting means of Figure 9; Figure 11 is a perspective view, partly broken away, of a modified embodiment of the casing and its surrounding structure in the optical means; Figure 12 is a block diagram showing a control means in the image-forming apparatus of Figure 1; Figure 13 is a simplified schematic view of one example of a character produced in the image-forming apparatus of Figure 1; and Figure 14 is a simplified schematic view of one example of a character produced in the image-forming apparatus of Figure 1 when the speed of image formation is doubled. One specific embodiment of the image-forming apparatus constructed in accordance with this invention will be described below in detail with reference to the accompanying drawings. With reference to Figures 1, 2 and 3, the general construction of the image-forming apparatus shown generally at 2 is described. The image-forming apparatus 2 shown includes an electrostatic image-forming means shown generally at 4, an optical means shown generally at 6 and a control means shown generally at 8. The image-forming means 4 is composed of an image-forming unit having a first housing 10 which is relatively large and nearly rectangular-parallelpipedal. The optical means 6 is composed of an optical unit having a second housing 12 which is relatively small and nearly rectangular-parallelpipedal. The control means 8 is formed of a control unit having a third housing 14 which is relatively small and nearly rectangular-paralleipipedal. As will be described in detail hereinafter, the optical means 6 projects a scanned light beam to be controlled according to an image to be formed. The image-forming means 4 receives the scanned light beam projected from the optical means 6 and forms an image corresponding to the scanned light beam. The control means 8 controls the optical means 6 and the image-forming means 4 as required. The first housing 10 of the image-forming means 4 has legs 16 at its lower surface, and is placed on a suitable supporting base stand such as a floor surface. The second housing 12 of the optical means 6 is fixed to one side portion (the left side portion in Figure 3) of a substantially horizontal upper wall 18 of the first housing 10 by suitable means (not shown) such as screwing. The image-forming apparatus 2 has a cover shown generally at 20. The cover 20 has a nearly rectangular upper wall 22 and four side walls 24, 26, 28 and 30 extending downwardly from the upper wall 22. As shown in Figures 2 and 3, a supporting plate 32, nearly rectangular in shape, is fixed to the inside surface of the upper wall 22 of the cover 20 by suitable means (not shown) such as screwing. That end of the supporting plate 32 which is located on the left in Figure 1 is pivotally linked to one side wall of the second housing 12, i.e. the upper end of the outside surface of that side wall which is located on the left in Figure 1, by a pair of hinge linking means 34. Thus, the cover 20 is mounted such that it can be opened counterclockwise in Figure 3 from its closed position shown in Figures 1 and 3 about a hinge pin of the hinge linking means 34 as a center. The supporting plate 32 has integrally formed therein a downwardly extending portion 36 which extends downwardly from its one side edge near its left end portion in Figure 1. To the downwardly extending portion 36 is pivotally linked by means of a pin 40 one end of a gas spring means 38 known per se . The other end of the gas spring means 38 extends to the inside of the first housing 10 through an opening 42 formed in the upper wall 18 of the first housing 10, and is pivotally linked by means of a pin 46 to a bracket 44 (Figure 2) fixed within the first housing 10. The gas spring means 38 biases the cover 20 counterclockwise in Figure 3. There is also provided a locking mechanism 48 (Figure 3) for locking the cover 20 in the closed position against the biasing action of the gas spring means 38. As illustrated in Figure 3, the locking mechanism 48 includes a locking member 52 mounted pivotally on a pin 50 projecting rearwardly from the front side wall 24 of the cover 20. A spring 54 is provided taut between the upper end portion of the locking member 52 and the right side wall 26 of the cover 20. The spring 54 biases the locking member 52 clockwise in Figure 3 and elastically holds it at a locking position shown in Figure 3. A pressing member 56 is fixed to the locking member 52, and the free end of the pressing member 56 is located within an opening 58 formed in the right side wall 26 of the cover 20. On the other hand, an opening 60 is formed at that portion of the upper wall 18 of the first housing 10 which corresponds to the locking member 52, and an engaging pin 62 is disposed below the opening 60. When the cover 20 is pivoted clockwise in Figure 3 from its open position shown in Figure 2 and brought to a point near its closed position shown in Figure 3, the hook-like lower end portion of the locking member 52 advances into the first housing 10 through the opening 60 and its undersurface abuts against the engaging pin 62. Hence, when pivoting of the cover 20 is continued, the action of the engaging pin 62 causes the locking member 52 to pivot counterclockwise in Figure 3 against the elastic biasing action of the spring 54. When the cover 20 is pivoted to the closed position shown in Figure 3, the hook-like lower end portion of the locking member 52 goes beyond the engaging pin 62. As a result, the locking member 52 is pivoted clockwise and returned to the locking position by the elastic biasing action of the spring 54. Thus, an shown in Figure 3, the hook-like lower end portion of the locking member 52 comes into engagement with the engaging pin 62 whereby the cover 20 is locked in the closed position shown in Figure 3. To open the cover 20, the hook-like lower end portion of the locking member 52 is detached from the engaging pin 62 by pushing with a finger the free end of the pressing member 56 fixed to the locking member 52 and thereby pivoting the locking member 52 counterclockwise. Consequently, the cover 20 is pivoted counterclockwise in Figure 3 by the biasing action of the gas spring means 38. When the cover 20 has been pivoted to its open position shown in Figure 2, the biasing action of the gas spring means 38 comes into equilibrium with the moment ascribable to the weight of the cover 20 (as will be clear from the following description, the weight also includes the weight of the control means 8 mounted on the cover 20). The cover 20 is thus maintained stably at the open position, shown in Figure 2. In the illustrated image-forming apparatus 2, the housing of the control means 8, namely the third housing 14, is mounted in the cover 20 by fixing its upper surface to the supporting plate by suitable means (not shown) such as a setscrew. Since the control means 8 is relatively light in weight, mounting of the third housing 14 in the cover 20 does not cause any inconvenience. As is clearly seen from Figure 3, when the cover 20 is at its closed position at which it covers the upper surface of the first housing 10 of the image-forming means 4 and the second housing 12 of the optical means 6, the third housing 14 mounted in the cover 20 and moving together with the cover 20 is at its operating position at which it is located adjacent to the second housing 12 and above the first housing 10 with some distance from the upper wall 18 of the first housing 10. On the other hand, when the cover 20 is at its open position at which it exposes the upper surface of the first housing 10 and the second housing 12 to view, the third housing 14 is brought to its non-operating position at which it is apart from the upper surface of the first housing 10, as is clear from Figure 2. As shown in Figures 2 and 3, a relatively large opening 64 which enables access to the inside of the first housing 10 is formed is at least a part of that area of the upper wall 18 of the first housing 10 which is exposed to view when the cover 20 is held at the open position to hold the third housing 14 at the non-operating position. The opening 64 is closed by a freely openable and closable closure member 68 whose one end edge (the left end edge in Figure 3) is pivotally connected to the upper wall 18 by a pair of hinge linking means 66. Now, the electrostatic image-forming means 4 composed of the image-forming unit having the first housing 10 will be described in detail. With reference to Figure 3, a cylindrical rotating drum 70 is rotatably mounted nearly centrally in the first housing 10. A suitable electrostatographic material 72 is disposed on the peripheral surface of the rotating drum 70. Around the rotating drum 70 to be rotated in the direction of an arrow 74 are disposed a charging corona discharge device 76, a developing means generally shown at 78, a transfer corona discharge device 80, a peeling corona discharge device 82 and a cleaning means shown generally at 84 in this order in the rotating direction of the drum 70. The charging corona discharge device 76 substantially uniformly charges the electrostatographic material 72 to a specified polarity. An exposure area 86 exists between the charging corona discharge device 76 and the developing means 78. In the exposure zone 86, a scanned light beam advancing from the optical means 6 to the inside of the first housing 10 through an opening 90 formed in the upper wall 18 of the first housing 10 is projected onto the electrostatographic material 72, thereby to form a latent electrostatic image corresponding to the scanned light beam on the electrostatographic material 72. The opening 90 is of an elongated shape extending axially of the rotating drum 70, namely in a direction perpendicular to the sheet surface in Figure 3. The inner surface of the upper wall 18 which is near the opening 90 and the surface of frame materials of the charging corona discharge device 76 and the developing means 78 which face the path of the scanned light beam shown by an arrow 88 are conveniently coated in a dark color or covered with a felt or the like of a dark color in order to re-reflect the scanned light beam reflected from the eletrostatographic material 72 without absorption. This prevents the scanned light beam reflected by the electrostatographic material 72 from being reflected again toward the electrostatographic material 72 and thus disturbing the latent electrostatic image formed on it. The illustrated developing means 78 includes a main developing portion 92 having a casing 94 holding a two-component developer 96 composed of carrier particles and toner and a developer applicator means 98 disposed within the casing 94, and a toner supplying portion 100 for supplying toner 102 therein to the casing 94 as required. The developer applicator means 98 of the main developing portion 92 is rotated in the direction of an arrow 104 and magnetically attracts a developer 96 in the casing 94 to its surface to form a so-called "magnetic brush", and applies the magnetic brush to the electrostatographic material 72 through an opening 106 formed in the casing 94. Thus, the toner is applied to the latent electrostatic image formed on the electrostatographic material 72 to develop it to a toner image. The transfer corona discharge device 80 applies a corona discharge to the back surface of a copying paper to be contacted with the surface of the electrostatographic material 72 in a transfer zone 108 to transfer the toner image on the electrostatographic material 72 to the copying paper. The peeling corona discharge device 82 applies corona discharge to the back surface of the copying paper immediately downstream of the transfer zone 108 to peel the electrostatically adhering copying paper from the surface of the electrostatographic material 72. The cleaning means 84 has a blade 110 made of an elastic material to be pressed against the surface of the electrostatographic material 72, and by the action of the blade 110, removes residual toner particles from the electrostatographic material 72. The image-forming means 4 further includes a copying paper feed means shown generally at 112, and a copying paper conveying means shown generally at 114 for conveying a copying paper fed from the copying paper feed means 112 through the transfer zone 108. The paper feed means 112 has a cassette receiving stand 116, a copying paper cassette 118 mounted detachably on the cassette receiving stand 116 through an opening 110 formed on the right side wall 117 of the first housing 10, and a feed roller 120. The feed roller 120 is rotated selectively in the direction shown by an arrow and delivers a plurality of copying paper sheets stacked in the copying paper cassette 118 one by one. The copying paper conveying means 114 includes a pair of conveying rollers for conveying a copying paper fed from the paper feed means 112, guide plate means 124, 126 and 128 for guiding the copying paper conveyed by the pair of conveying rollers to the transfer zone 108, a sucking-type conveying and guiding mechanism 130 for conveying and guiding the copying paper peeled from the electrostatographic material 72, a pair of fixing rollers 132, guide plate means 134 and 136, a pair of delivery rollers 138, and a receiving tray 114 for receiving the copying paper discharged from the pair of delivery rollers 138 through an opening 142 formed in the left side wall 140 of the first housing 10. One of the pair of fixing rollers 132, i.e. the roller 132 located above, has formed therein a heating element. The pair of fixing rollers 132 press and heat the surface of the copying paper having a toner image transferred thereto from the electrostatographic material 72, and thereby fix the toner image onto the copying paper. The aforesaid construction itself of the image-forming means 4 is conventional and does not constitute any novel feature of the image-forming apparatus 2 constructed in accordance with this invention. Accordingly, a detailed description of the aforesaid construction in the image-forming means 4 will be omitted in the present specification. The optical means 6 composed of the optical unit having the second housing 12 will now be described in detail. With reference to Figure 4, the housing of the optical means 6, i.e. the second housing 12, is defined by a bottom wall 146, four side walls 148, 150, 152 and 154 and an upper wall 156. With reference to Figures 5 and 6 as well as Figure 4, a light beam generating means 158, a first reflecting mirror assembly 160, a first lens assembly 162, a second reflecting mirror assembly 164, a modulating means 166, a second lens assembly 168 (Figure 5), a scanning means 170 (Figure 5), a third lens assembly 172, a third reflecting mirror assembly 174, a fourth reflecting mirror assembly 176 and a fifth reflecting mirror assembly 178 are mounted on the bottom wall 146 of the second housing 112. The light beam generating means 158 is constructed of a gas laser tube having a cylindrical casing 180. The light beam generating means 158 is mounted on the bottom wall 148 by a pair of supporting means 182 such that its is adjacent to the side wall 148 and extends substantially parallel to the bottom wall 146 and the side wall 148. Each of the pair of supporting means 182 consists of a supporting stand 184 fixed to the bottom wall 146 and a nearly semicircular arcuate holding member 186 comperating with the supporting stand 184. A nearly semicircular recess is formed on the upper surface of the supporting stand 184. The light beam generating means 158 is positioned in the recess of the support stand 184 and then the holding member 186 is provided astride the light beam generating means 158. Then, the flange portions at both ends of the holding member 186 are fixed to the supporting stand 184 by a setscrew 188, thereby mounting the light beam generating means 158 on the bottom wall 146. The first reflecting mirror assembly 160 is comprised of a supporting stand 190 fixed to the bottom plate 146, a nearly U-shaped holding member 192 mounted on the support stand 190, a supporting member 194 mounted on the holding member 192, and a mirror 196 secured to the supporting member 194. The holding member 192 is mounted on the supporting stand 190 so that its rotating angular position about a substantially perpendicular axis can be freely adjusted. The supporting member 194 is mounted on the holding member 192 so that its rotating angular position about a substantially horizontal axis can be freely adjusted. The first lens assembly 162 is comprised of a supporting member 198 fixed to the bottom wall 146 so that its position can be freely adjusted, and a condensing lens 200 fixed within a through hole formed in the upper portion of the supporting member 198. The second reflecting mirror assembly 164 is substantially the same as the first reflecting mirror assembly 160 described above, and is comprised of a supporting stand 202 fixed to the bottom wall 146, a nearly U-shaped holding member 204 mounted on the supporting stand 202, a supporting member 206 mounted on the holding member 204 and a mirror 208 fixed to the supporting member 206. The holding member 204 is mounted on the supporting stand 202 such that its rotating angular position about a substantially perpendicular axis can be freely adjusted, and the supporting member 206 is mounted on the holding member 204 such that its rotating angular position about a substantially horizontal axis can be freely adjusted. The modulating means 166 may be constructed of a suitable modulator such as a known acousto-optical modulator, and is fixed to a supporting stand 210 which is fixed to the bottom plate 146 in such a manner that its position can be freely adjusted. The second lens assembly 168 has a cylindrical lens frame 212. As schematically shown in Figure 6, one collimator lens 214 and a plurality of (two in the drawing) expander lenses 216 are disposed within the lens frame 212. The lens frame 212 is mounted on the bottom wall 146 by a conventional supporting means 218 composed of a pair of supporting members so that its longitudinal position can be freely adjusted. One end portion of the lens frame 212 extends through the side wall of a casing shown generally at 220 and described in detail hereinafter, and is mounted on the casing 220 such that it can move freely in the longitudinal direction. With reference to Figure 7 also, the scanning means 170 is constructed of a rotating polygon mirror assembly known per se which has a nearly cylindrical case 222. The case 222 of the rotating polygon mirror assembly has accommodated therein a plurality of (8 in the drawing) scanning mirrors 224 (Figures 5 and 8) arranged in a polygonal shape and an electric motor (not shown) for rotating the scanning mirrors 224 at high speeds. An opening 226 is formed in the case 222 so as to permit falling of a light beam upon the scanning mirrors 224 and outgoing of a scanned light beam from the scanning mirrors 204. An annular holding member 230 is fixed around the case 222. The annular holding member 230 is fixed to a supporting stand 232 secured to bottom wall 146. Thus, the scanning means 170 composed of the rotating polygon mirror assembly is fixed to a predetermined position on the bottom wall 146. With reference to Figures 5 and 7, the scanning means 170 is surrounded by the casing 220. The casing 220 has a nearly arcuate upstanding side wall 234 extending along the contour of the supporting stand 232, an upstanding front wall 236 fixed between the two ends of the side plate 234 and an upper wall 238 closing the upper surface of the casing 220. The lower surface of the casing 220 is closed by the supporting stand 232 and the bottom wall 146. The side wall 234 of the casing 220 has formed therein a circular opening for incoming of a light beam through which one end portion of the lens frame 212 of the second lens assembly 168 extends. Preferably, an annular sealing material 242 made of a flexible material such as rubber is disposed in the opening 240 so as to seal substantially the space between the side wall 234 and the lens frame 212 while permitting movement of the lens frame 212 in the longitudinal direction. A nearly circular opening for outgoing of a light beam is formed in the front wall 236 of the casing 220. This opening, in cooperation with an arcuate recessed portion 244 (figure 7) formed in the front end of the supporting stand 232, defines a mounting portion for the third lens assembly 172. The third lens assembly 172 has a short cylindrical lens frame 246, a socalled f-ϑ lens 248 fixed within the lens frame 246, and a linking plate 250 fixed to the peripheral surface of the lens frame 246. The third lens assembly 172 is mounted at a predetermined position by inserting one end portion of the lens frame 246 in a circular mounting portion defined by the opening formed in the front wall 236 and the arcuate recessed portion 244 formed in the supporting stand 232, and fixing the linking plate 250 to the front wall 236. It will be understood from the above description of the casing 220 that the scanning means 170 constructed of the rotating polygon mirror assembly is substantially closed by the casing 220 and the two lens assemblies mounted on the casing 220, i.e. the second lens assembly 168 located upstream of the scanning means 170 as viewed in the advancing direction of the light beam and the third lens assembly 172 located downstream of the scanning means 170 as viewed in the advancing direction of the light beam. As is well known, in the rotating polygon mirror assembly, the scanning mirrors 224 are rotated at high speeds, and therefore considerable noises are produced. Propagation of these noises can be considerably reduced by substantially closing the rotating polygon mirror assembly as described above. With reference to Figures 4 and 5, a pair of supporting stands 252 spaced from each other a predetermined distance are fixed to the bottom wall 146. The third reflecting mirror assembly 174 and the fifth reflecting mirror assembly 178 are mounted between the pair of supporting stands 252. The third reflecting mirror assembly 174 has a supporting plate 254 both ends of which are fixed respectively to the side edges of the supporting stand 252. A mirror 358 is fixed to the back surface of the supporting plate 254 by means of a pair of fixing devices 256. The supporting plate 254 has formed therein an elongated opening extending in its longitudinal direction, and the main front surface portion of the mirror 258 is exposed to view through this opening. The fifth reflecting mirror assembly 178 has a supporting plate 262 having short shafts 260 formed at its opposite ends. By rotatably mounting the short shafts 260 formed at the opposite ends of the supporting plate 262 in holes formed at the upper end portions of the supporting stand 252 and hampering the rotation of the short shafts 260 by a setscrew 264 which advances into each of the holes from the side edge of each of the supporting stands 252 and abuts against each short shaft 260, the supporting plate 262 is mounted between the pair of supporting stand 252 such that its rotating angular position about its longitudinal axis can be freely adjusted. A mirror 268 is fixed to the back surface of the supporting plate 262 by a pair of fixing devices 266. The supporting plate 262 has formed therein an elongated opening which extends longitudinally thereof, and the main front surface portion of the mirror 268 is exposed to view through this opening. With reference to Figures 5 and 8, an elongated recessed portion 270 having a nearly triangular cross section is formed in the bottom wall 146, and the fourth reflecting mirror assembly 176 has a mirror 272 fixed to the recessed portion 270 by a pair of fixing devices 273. In the optical means 6 described above, as shown by the one-dot chain lines in Figures 4 and 5, a laser light beam generated by the light beam generating means 158 arrives at the mirror 196 of the first reflecting mirror assembly 160 and is reflected by the mirror 196. The reflected light beam passes through the condenser lens 200 of the first lens assembly 162, and arrives at the mirror 208 of the second reflecting mirror assembly 164 where it is reflected by the mirror 208. The reflected mirror then falls upon the modulating means 166. As will be described hereinafter, the modulating means 166 modulates the laser light beam according to image signals fed from the control means 8 (Figure 3). As will be clear from the description made hereinafter, only a modulated primary light beam among the laser light beams coming from the modulating means 166 falls upon the second lens assembly 168. The laser light beam incident to the second lens assembly 168 is conveyed to a parallel light by being passed through the collimator lens 214 (Figure 6), and then by passing it through a plurality of expander lenses 216 (Figure 6), its beam diameter is adjusted to the desired value. The laser light beam which has passed through the second lens assembly 168 arrives at the scanning mirrors 224 of the scanning means 170, and by the action of the scanning mirrors 224 rotating at high speeds, it is projected as a scanned laser light beam reciprocally scanned in the left-right directions in Figure 5. The scanned laser light beam projected from the scanning means 170 passes through the f-ϑ lens 248 of the third lens assembly 172, and thereafter is reflected successively by the mirror 258 of the third reflecting mirror assembly 174, the mirror 274 of the fourth reflecting mirror assembly 176 and the mirror 268 of the fifth reflecting mirror assembly 178. The laser light beam reflected by the fifth reflecting mirror assembly 178 goes out from the second housing 12 after its has passed through the light beam passing opening 276 formed in the bottom wall 146, advances into the first housing 10 through the opening 90 formed in the upper wall 18 of the first housing 10, and arrives at the electrostatographic material 72 disposed on the peripheral surface of the rotating drum 70 in the exposure zone 86 as stated hereinabove. It is necessary that the mirrors 258, 274 and 268 of the third to fifth reflecting mirror assemblies 174, 176 and 178 are disposed substantially parallel to the central axis of the rotating drum 70 (Figure 3), and the opening 276 formed in the bottom wall 146 should be in alignment with the opening 90 formed in the upper wall 18 of the first housing 10 and extends substantially parallel to the central axis of the rotating drum 70. On the other hand, it is preferred that the angle of the mirror 268 of the fifth reflecting mirror assembly 178 is prescribed so that the path of the laser light beam ranging from the mirror 268 of the fifth reflecting mirror assembly 178 to the electrostatographic material 72 slightly deviates with respect to the central axis of the rotating drum 70. If the path of the laser light beam ranging from the mirror 268 to the electrostatographic material 72 is in alignment with the central axis of the rotating drum 70, a part of the laser light beam arriving at the electrostatographic material 72 is reflected by the electrostatographic material 72 and then by being reflected by the mirrors 268, 274 and 258, returns to the scanning mirrors 224 of the scanning means 170, and thereafter is again projected onto the electrostatographic material 72. This may possibly result in disturbance of the latent electrostatic image formed on the electrostatographic material 72. In the illustrated optical means 6, a shielding means is provided which causes only a modulated primary light beam among the laser light beams from the modulating means 166 to fall upon the second lens assembly 168. With reference to Figures 9 and 10 together with Figure 5, an annular flange 278 is formed integrally at the upstream end, viewed in the advancing direction of the light beam, of the cylindrical lens frame 212 of the second lens assembly 168, and a light shielding means 280 is fixed to the flange 278. The light shielding means 280 is comprised of a pair of light shielding plates 282 and 284 spaced from each other some distance and fixed to the flange 278. As can be seen from Figure 10, the distance between the pair of light shielding plates 282 and 284 is substantially equal to the diameter of a modulated primary light beam M1 existing when image signals, i.e. modulation signals, have been fed to the modulating means 166, and the light shielding plates are positioned so that they permit passage of only the modulated primary light beam M1 from the modulating means 166. A non-modulated light beam NM existing when no image signal is fed to the modulating means 166 is shut off by the light shielding plate 282 and cannot fall upon the second lens assembly 168. As is well known to those skilled in the art, when image signals have been fed to the modulating means 166, not only the modulated primary light beam M, but also weak secondary, tertiary...light beams having a larger angle of deflection to the non-modulated light beam NM than the angle of deflection of the modulated primary light beam M1 exist. These secondary, tertiary,... light beams are shut off by the light shielding plate 284 and cannot fall upon the second lens assembly 168. A light beam detector 286 which may be constructed of a photocell, for example, is disposed in that area of the light shielding plate 282 upon which a non-modulated light beam NM falls. The light beam detector 286 produces a signal corresponding to the intensity of the projected non-modulated light beam NM and feeds it oto the control means 8 to be described in detail hereinafter. The control means 8 compares the signal fed from the light beam detector 286 with a predetermined standard value, and produces a warning signal when the intensity of the non-modulated light beam NM falls below a predetermined value as a result of the degradation of the light beam generating means 158. When the warning signal is produced, a warning lamp (not shown) provided in a panel 288 (Figure 1) disposed on the surface of the cover 20 is turned on. Since the light beam detector 286 detects the non-modulated light beam NM not used for the formation of a latent electrostatic image in the image-forming means 4, it does not adversly affect the image-forming action of the means 4. Furthermore, because the non-modulated light beam NM can exist for a relatively long period of time, a signal which the light beam detector 286 produces according to the intensity of the non-modulated light beam NM also exists over a relatively long period of time, and comparison, etc. of the signal can be easily and accurately effected. Furthermore, since the light beam detector 286 detects the non-modulated light beam NM, it can directly examine a light beam generated by the light beam generating means without being affected by the function of the modulating means 166. In the optical means 6 described above, it is important that the beam diameter of the scanned light beam projected should be prescribed at a predetermined value. The diameter of the the scanned light beam depends upon the characteristics and positions of the condensing lens 200, the collimator lens 214 and expander lenses 216 (Figure 6). Even when these lenses 200, 214 and 216 are precisely held at predetermined positions, the beam diameter of the scanned light beam may often deviate from a predetermined value owing to errors of the lenses 200, 214 and 216 in characteristics caused by production errors. In the illustrated embodiment constructed in accordance with the present invention, the lens frame 212 of the second lens assembly 168 having the collimator lens 214 and expander lenses 216 (Figure 6) is mounted so that its position in the longitudinal direction can be freely adjusted. By adjusting the lengthwise position of the lens frame 212 (i.e., the positions of the collimator lens 214 and the expander lenses 216), the diameter of the scanned light beam can be corrected to the predetermined value. In order to perform the position adjustment of the lens frame 212 easily, it is possible, for example as shown in Figure 11, to form an internal thread on the inner circumferential surface of the light beam incoming opening 240 formed on the side wall 234 of the casing 220 and an external thread 320 cooperating with the internal thread at one end portion of the lens frame 212, and to adjust the longitudinal position of the lens frame 212 by rotating the lens frame 212 about its central axis as a center and thus varying the degree of screwing between the internal thread and the external thread 320. If desired, instead of, or in addition to, the adjustment of the diameter of the scanned light beam by adjusting the longitudinal position of the lens frame 212, the diameter of the scanned light beam may be adjusted by mounting the collimator lens 214 (Figure 6) within the lens frame 212 by a suitable means so that its position can be freely adjusted and adjusting the position of the collimator lens 214 in the longitudinal direction of the lens frame 212. Alternatively, the diameter of the scanned light beam can be adjusted by constructing the expander lenses 216 (Figure 6) from zoom lenses known in the field of cameras and operating the zoom lenses. The control means 8 will now be described. With reference to Figure 12, the control means 8 includes a central processing means 348, a memory means 350 and a sequence control means 352 and a panel 288 (see Figure 1 also) disposed in the cover 20. Various operating switch means and display means are provided in the panel 288. Signals are fed from the various operating switch means provided in the panel 288 into the central processing means 348 which can be constructed of a microprocessor, and a signal is also fed into the central processing means 348 from a suitable device 354 such as a host computer to be combined with the image-forming apparatus 2. The memory means 350 is composed of a suitable memory such as a random access memory (RAM) and memorizes given image signals such as characters or numerical figures. The memory means 350 feeds image signals to the sequence control means 352 according to the signal fed from the central processing means 348. The sequence control means 352 feeds the image signals from the memory means 350 to the modulating means 166 of the optical means, and controls the various constituent elements of the image-forming means 4 as required correspondingly to the feeding of the image signal to the modulating means 166. When an image signal is fed from the sequence control means to the modulating means 166 of the optical means 6, the modulating means 166 modulates light beams, and therefore, a scanned light beam is projected onto the electrostatographic material 72 (Figure 3) of the image-forming means 4. On the other hand, when no image signal is fed to the modulating means 166 from the sequence control means 352, the modulating means 166 does not modulate light beams, and no scanned light beam is projected onto the electrostatographic material 72 (Figure 3). Thus, a required latent electrostatic image is formed as a so-called dot pattern on the electrostatographic material 72 (Figure 3). Figure 13 exemplifies letter C formed in a dot pattern. This letter C is expressed by 12 × 12 = 144 dots. In the white dots in Figure 13, the scanned light beam has been projected onto the electrostatographic material 72, and in the black dots, no scanned light beam has been projected onto the electrostatographic material 72. Since the above construction and operation of the control means 8 are known to those skilled in the art, a detailed description of these is omitted in the present specification. Frequently, it is desired in the image-forming device 2 to increase the speed of image formation even if this results in some reduction in the quality of the resulting image. To meet this desire, the speed of image formation is increased as shown below in one specific embodiment of the present invention. When it is desired to increase the speed of image formation, the diameter of the scanned light beam to be projected from the optical means 6 is increased to n times. The increasing of the diameter of the scanned light beam can be achieved by, for example, constructing the expander lenses 216 (Figure 6) of the optical means 6 from zoom lenses well known in the field of cameras, and properly operating the zoom lenses. The zoom lenses can be operated automatically by attaching a suitable operating means and feeding an operating signal to the operating means from the central processing means 348. In addition to increasing the diameter of the scanned light beam to n times the speed of feeding the image signals to the modulating means 166 of the optical means 6 from the memory means 350 through the sequence control means 352 is decreased to 1/n times, and the moving speed of the electrostatographic material 72 in the image-forming means 4, and therefore the rotating speed of the rotating drum 70 (Figure 3), are increased to n times (in which case the conveying speed of the copying paper is naturally increased to n times). It is easily appreciated that as a result, the number of image signals fed per predetermined area of the electrostatographic material 72 (Figure 3) is decreased to 1/2n times. By this procedure, the image forming speed can be increased to n times. Figure 14 exemplifies letter C formed in a dot pattern when the image forming speed has been doubled, namely when the diameter of the scanned light beam projected from the optical means 6 is doubled, the speed of feeding image signals to the modulating means through the sequence control means 352 from the memory means 350 is decreased to 1/2, and the moving speed of the electrostatographic material 72 in the image-forming means 4 is doubled. It will be easily seen by comparing Figures 13 and 14 that when the image-forming speed is doubled, the letter C as shown in Figure 14 is expressed by 6 × 6 = 36 dots which is 1/4 of the number of dots in Figure 13 (12 × 12 = 144). Since the display area of the letter C is the same for both, the letter C in Figure 14 has an inferior image quality to that in Figure 13 because of the decrease of the number of dots.

Claims: 1. An image-forming apparatus (2) comprising an optical means (6) for projecting a scanning light beam, which optical means (6) is controlled according to an image to be formed, said optical means (6) including a light beam generating means (158), scanning means in the form of a rotating polygon mirror assembly (170) for scanning the light beam from the light beam generating means (158), an upstream side lens assembly (168) located upstream of the polygon mirror assembly (170) as viewed in the advancing direction of the light beam and a downstream side lens assembly (172) located downstream of the polygon mirror assembly (170) as viewed in the advancing direction of the light beam; said scanning means (170) being incorporated in a nearly cylindrical first casing (222) having an opening (226) for the incident and reflected scanning light beam; an electrostatic image-forming means (4) for forming an image corresponding to the scanning light beam including an electrostatographic material (72) moved through an exposure zone (86) and irradiated with the scanning light beam by the optical means (6), and a control means (8) for controlling the optical means (6) and the image-forming means (4), said control means (8) being adapted to control the optical means (6) by feeding to the optical means (6) image signals for modulating the scanning light beam according to the image to be formed and to control the image-forming means (4) by controlling the speed of movement of the electrostatographic material (72) in the image-forming means (4), characterized in that the optical means (6) includes means for changing the diameter of the scanning light beam, and further a second casing (220) having an opening for the incoming light beam and an opening for the outgoing light beam and surrounding the rotating polygon mirror assembly (170) incorporated in said first casing (222), the upstream side lens assembly (168) being mounted in the opening for the incoming light beam in the casing (220), the downstream side lens assembly (172) being mounted in the opening for the outgoing light beam in the casing (220) and the rotating polygon mirror assembly (170) being substantially enclosed by the casing (220), the upstream side lens assembly (168) and the downstream side lens assembly (172), and in that the control means (8) changes the feeding of the image signal to the optical means (6) according to the change of the diameter of the scanning light beam. 2. The apparatus of claim 1, wherein when the diameter of the light beam to be scanned is increased n times, the control means (8) decreases the speed of feeding the image signal to be optical means (6) 1/n times, and increasees the moving speed of the electrostatographic material (72) in the image-forming means (4) n times. 3. The apparatus of claim 1, wherein the optical means (6) includes a zoom lens (216) disposed between a light beam generating means (158) and the scanning means (170), and the means for changing the beam diameter operates the zoom lens (216). 4. The apparatus of claim 3, wherein the optical means (6) includes a modulating means (166) disposed between the light beam generating means (158) and the zoom lens (216). 5. The apparatus of Claim 1, wherein the uptstream side lens assembly (168) is comprised of a cylindrical lens frame (212) mounted in the opening for the incoming light beam in the casing (220) and at least one lens (214, 216) mounted in the cylindrical lens frame (212). 6. The apparatus of claim 5, wherein one colimator lens (214) and a plurality of expander lenses (216) are mounted in the cylindrical lens frame (212). 7. The apparatus of claim 1, wherein the upstream side lens assembly (168) is mounted in the opening for the incoming light beam in the casing (220) so that its position can be adjusted freely in the advancing direction of the light beam. 8. The apparatus of claim 1, wherein the down-stream side lens assembly (172) is comprised of a cylindrical lens frame (246) mounted of the opening for the outgoing light beam in the casing (220) and a lens (248) mounted within the cylindrical lens frame. 9. The apparatus of claim 8, wherein said lens (248) is a f-ϑ lens. 10. An image-forming apparatus (2) according to claim 1, characterized in that said optical means (6) includes modulating means (166) into which image signals corresponding to the image to be formed are fed from the control means (8) and which modulates the light beam received from the light beam generating means (158) according to the image signals; in that scanning means (170) receive the light beam from the modulating means (166), and in that the optical means (6) further includes a light beam detector (286) disposed between the modulating means (166) and the scanning means (170) so as to monitor the intensity of the non-modulated light beam. 11. The apparatus of claim 10, wherein the optical means (6) inclusdes a light shielding means (280) disposed between the modulating means (166) and the scanning means (170) so as to permit only the modulated primary light beam from the modulating means (166) to pass therethrough and reach the scanning means (170), and the light beam detector (286) is disposed above the light shielding means (280) at an area to which the non-modulated light beam is projected from the modulating means (166). 12. The apparatus of claim 11, wherein the lens assembly (168) is disposed between the modulating means (166) and the scanning means (170), and the light shielding means (280) is disposed on the upstream end of the cylindrical lens frame (212) as viewed in the advancing direction of the light beam. 13. The apparatus of claim 12, wherein the light shielding means (280) is composed of a pair of light shielding plates (282, 284) spaced from each other and fixed to the upstream end of the cylindrical lens frame (212) so as to permit passage of only the modulated primary light beam from the modulating means (166).

Inventors: HAYASHI, KIYOSHI, HIRATA, NOBUYUKI, HORI, SETSUO, KOBAYASHI, KAZUHIRO, MIZUNO, MASAYUKI
Applicants: MITA INDUSTRIAL CO. LTD.
Application year: 1983
Main classification: G03G15
IPC codes: G06K15, G03G21, G03G15