Patent Publication Number: US-2004056185-A1

Title: Light scanning device of laser printer

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001] This application claims the benefit of Korean Application No. 2002-57243, filed Sep. 19, 2002, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002] 1. Field of the Invention  
       [0003] The present invention relates to a laser printer, and more particularly, to a light scanning device for scanning light to form a latent image on a photosensitive drum according to image data.  
       [0004] 2. Description of the Related Art  
       [0005] Generally, a laser printer is an apparatus in which toner is absorbed on a photosensitive drum according to image data, and the toner absorbed on the photosensitive drum is then transferred to a print medium, and thus an image can be reproduced on the print medium. At this time, light is scanned on the photosensitive drum to form a latent image according to the image data, and the toner is absorbed to the latent image formed by the light. The laser printer is a non-impact type which has the advantage of rapid speed and less noise, and thus it is widely used in various fields.  
       [0006] In such a laser printer, a monochrome laser beam printer (LBP) is provided with one light scanning device which linearly scans the light on the photosensitive drum, and a color LBP is provided with four light scanning devices corresponding to each of cyan, yellow, magenta and black.  
       [0007] However, in order to simplify construction of the color LBP, a tandem type light scanning device by which only one light scanning device can be applied to the color LBP has been recently developed. An example of the tandem type light scanning device is disclosed in Korean Laid-Open Publication No. 2000-0047538 (Jul. 25, 2000).  
       [0008]FIG. 1 is a schematic view showing a structure of a conventional tandem type light scanning device  10 . The conventional tandem type light scanning device includes an optical case  12 , light sources  14 A,  14 B,  14 C,  14 D, first reflecting mirrors  17 A and  17 B, second reflecting mirrors  18 A,  18 B,  18 C, and  18 D, image forming lens systems  20 A and  20 B, cylinder mirrors  24 A,  24 B,  24 C, and  24 D and a scanning unit  22 . The conventional tandem type light scanning device further includes a plurality of synchronization detecting units (not shown) to control horizontal synchronization of a scanning line.  
       [0009] Each of the light sources  14 A,  14 B,  14 C,  14 D has a laser diode (not shown) and a collimator lens. The scanning unit  22  has a polyhedral mirror  26  and a motor  28  to drive the polyhedral mirror  26 . In addition, the image forming lens systems  20 A and  20 B that are positioned at an upper and lower side, with respect to the scanning unit  22 , are provided with a pair of image forming lenses  21  and  23 .  
       [0010] In the conventional tandem type light scanning device, laser beams  14 Aa˜ 14 Da emitted from the laser diode are transformed into parallel light by a collimating lens, and then reflected by the first reflecting mirror  17 A. And, the laser beam reflected from the first reflecting mirror  17 A is focused on a deflecting surface  26 A of the polyhedral mirror  26  through the image forming lens systems  20 A and  20 B. The laser beam focused on the deflecting surface  26 A of the polyhedral mirror  26  is output to the second reflecting mirrors  18 A˜ 18 D through the image forming lens systems  20 A and  20 B, and then output to the corresponding cylinder mirrors  24 A˜ 24 D. Sequentially, the laser beam is reflected from the cylinder mirrors  24 A˜ 24 D, and then scanned to corresponding photosensitive drums  38 A˜ 38 D. At this time, the laser beams  14   a ˜ 14   d  pass through the image forming lens systems  20 A and  20 B, while forming a predetermined angle α with respect to a scanning plane S before and after being reflected by the scanning unit  22 .  
       [0011] When the laser beam is scanned from the light scanning device as described above, a latent image is formed on the corresponding photosensitive drums  38 A˜ 38 D, and then, an image is reproduced through a typical printing process on a print medium.  
       [0012] In the conventional tandem type light scanning device as described above, since two pairs of image forming lens systems  20 A and  20 B are disposed to be opposite to each other, and the reflecting mirror for forming an optical path and the synchronization detecting unit for the horizontal synchronization are provided to each light source  14 A- 14 D, the structure of the light scanning device is complicated and difficult to control.  
       [0013] Further, since the number of components of the light scanning device is increased, there is the problem of increased fabrication costs.  
       [0014] Further, since the laser beam which is incident/output to/from the image forming lens system has the predetermined angle with respect to the scanning plane, there is another problem of a bow of the scanning line. In order to solve this problem, a separate compensating unit is required.  
       [0015] In addition, since the image forming lens systems are arranged to be opposite to each other with the scanning unit in the center, two signals of four color image signals are scanned from the left to the right on the basis of a transferring direction of the print medium. Therefore, since two pairs of scanning directions with respect to the four color image signals are opposite to each other, it is impossible to obtain a correct color image. In other words, in order to obtain the correct color image, at least two image signals have to be scanned in a status that the left and right order is inverted. Therefore, there is yet another problem that additional devices for controlling the image signal are required.  
       SUMMARY OF THE INVENTION  
       [0016] Therefore, it is an aspect of the present invention to provide a tandem type light scanning device which has a simple structure, and which can be simply controlled without a separate compensating process with respect to a scanning line.  
       [0017] Additional aspects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.  
       [0018] The foregoing and/or other aspects are achieved by providing a light scanning device including a plurality of laser beam sources disposed to be parallel with each other, to emit laser beams having various wavelengths according to image data; a plurality of photosensitive drums corresponding to the plurality of laser beam sources; a plurality of collimator lenses to transform the laser beams emitted from the laser beam sources into parallel light; a first optical output portion having a plurality of first optical members to reflect or transmit the parallel light through a first surface thereof, and reflect or transmit the reflected light through a second surface thereof according to the wavelengths, and thus output the laser beams of the various wavelengths along a single output path at an output portion thereof; a scanning unit to reflect the light output from the first optical output portion; and a second optical output portion having a plurality of second optical members disposed in a row so as to receive the light reflected from the scanning unit at an input portion thereof and selectively reflect or transmit the received light according to the wavelengths, and scan the reflected/transmitted light to the corresponding photosensitive drum.  
       [0019] The light scanning device may further include a focusing lens to focus the light output from the first optical output portion on a reflecting surface of the scanning unit, and an image forming lens to focus the light reflected by the scanning unit on the front-most optical member of the second optical output portion.  
       [0020] Further, the light scanning device may further include a synchronization detecting unit to detect a synchronization signal and control a horizontal synchronization of an image in the light reflected by the scanning unit, and a third optical member to selectively transmit or reflect the light which is incident to the synchronization detecting unit.  
       [0021] Meanwhile, the plurality of optical members may be disposed in various manners so as to have a single output path. The optical members may be disposed in a vertical direction or disposed to be inclined in a row at a desired angle. At this time, the plurality of optical members of the first and second optical output portions respectively have a different reflection and transmission region according to the wavelength of the incident light.  
       [0022] The plurality of optical members may be dichroic mirrors or transparent glasses with a dichroic coating. Each of the dichroic mirrors has a different reflection and transmission wavelength region at the first or second optical output portion. Furthermore, the plurality of optical members may be band pass filters.  
       [0023] In the light scanning device, the light having various wavelengths is transmitted through a single output path to the scanning unit, and the light corresponding to each color signal is output from the scanning unit to the signal path, and then scanned through the optical members on each photosensitive drum corresponding to each color signal, whereby a simple structure and a small size of the light scanning device can be obtained. Further, since the scanning unit also has the single output path and thus only one synchronization detecting unit is required, a separate compensating unit is not needed to compensate for the bow of the scanning line according to the synchronization detection adjusting unit or the image data. Therefore, fabricating costs are reduced. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0024] These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings of which:  
     [0025]FIG. 1 is a schematic view of an internal structure of a conventional tandem type light scanning device;  
     [0026]FIGS. 2 and 3 are a plan view and a side view, respectively, of an internal structure of a light scanning device according to an embodiment of the present invention;  
     [0027]FIG. 4 is a distribution chart of a reflectance and a transmittance with respect to light of a second and third dichroic mirror of FIG. 3; and  
     [0028]FIG. 5 is a view showing reflection and transmission of an optical member with respect to the light. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
     [0029] Reference will now be made in detail to the present preferred embodiment of the present invention, an example of which is illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout.  
     [0030]FIGS. 2 and 3 are a plan view and a side view of an internal structure of a light scanning device according to an embodiment of the present invention. A light scanning device includes a light source portion  101  having first through fourth laser diodes L 1 ˜L 4 , a collimator lens portion  102  having first through fourth collimator lenses, a first optical output portion  103  having first through fourth dichroic mirrors M 1 - 1 ˜M 1 - 4 , a cylinder lens  104 , a scanning unit  105  including a polygonal reflecting mirror  106 , an image forming lens  107 , a second optical output portion  108  including fifth to eighth dichroic mirrors M 2 - 1 ˜M 2 - 4 , a synchronization detecting unit  110 , and a third optical member  111 .  
     [0031] The first through fourth diodes L 1 ˜L 4  of the light source portion  101  are disposed to be parallel to each other in a row.  
     [0032] The first through fourth collimator lenses C 1 ˜C 4  are disposed in a row corresponding to respective laser diodes L 1 ˜L 4  so as to transform laser beams emitted from each laser diode L 1 ˜L 4  into parallel light.  
     [0033] The first through fourth dichroic mirrors M 1 - 1 ˜M 1 - 4  of the first optical output portion  103  are disposed to reflect or transmit the light, which is transmitted through each of the collimator lenses C 1 ˜C 4  and input through each surface, and form a single output path at its end-most portion. As shown in FIG. 2, each of the first to fourth dichroic mirrors M 1 - 1 ˜M 1 - 4  is inclined and thus reflects the incident beam at a right angle. The first through fourth dichroic mirrors M 1 - 1 ˜M 1 - 4  are entirely disposed in a row. However, as shown in FIG. 2, the first to fourth laser diodes L 1 ˜L 4  and the first through fourth collimator lenses C 1 ˜C 4  need not to be disposed in a row, and may be disposed at different distances according to wavelengths. Further, herein, although the first through fourth dichroic mirrors M 1 - 1 ˜M 1 - 4  are vertically disposed, this should not be considered as limiting. For example, these elements may be inclined so that the incident light can be reflected at a desired constant angle. In addition, all of the first through fourth dichroic mirrors M 1 - 1 ˜M 1 - 4  need not be dichroic mirrors. For example, in the case of the first dichroic mirror M 1 - 1 , a general reflecting mirror may be used.  
     [0034] The cylinder lens  104  focuses the light output from the first optical output portion  103  on a polyhedral mirror  106  of the scanning unit  105 .  
     [0035] The scanning mirror  105  includes a motor (not shown) and the polygonal mirror  106 , and reflects the light transmitted through the cylinder lens  104  at a predetermined angle.  
     [0036] The image forming lens  107  transmits the light reflected by the scanning unit  105  to the second optical output portion  108 .  
     [0037] The second optical output portion  108  receives the light transmitted through the image forming lens  107  at its front-most portion, and reflects or transmits the light at the fifth through eighth dichroic mirrors M 2 - 1 ˜M 2 - 4  according to a wavelength of the light, and then scans the light to corresponding photosensitive drums D 1 ˜D 4 .  
     [0038] The second optical output portion  108  and the first optical output portion  103  differ in that, in case of the first optical output portion  103 , the light is output from its end-most portion but, in the case of the second optical output portion  108 , the light is incident to its front-most portion. However, these elements have the same or similar structures with respect to reflection and transmission of the light according to wavelength. Therefore, a detailed description thereof will be omitted.  
     [0039] In the light scanning device as described above, the light, which is emitted from the first to fourth laser diodes L 1 ˜L 4  of the light source portion  101  according to an image to be printed, is transformed through the corresponding capacitor C 1 ˜C 4  of the collimator lens  102  into the parallel light, and then arrives at the corresponding first to fourth dichroic mirrors M 1 - 1 ˜M 1 - 4 . At this time, the lights output from the laser diodes L 1 ˜L 4  of the light source portion  101  have the different wavelengths, respectively. For example, the first laser diode L 1  emits light having a wavelength of 650 nm, and the second, third and fourth laser diodes emit laser beams having respective wavelengths of 780 nm, 850 nm and 950 nm.  
     [0040] If the light having the different wavelengths is incident to the first optical output portion  103 , the first through fourth dichroic mirrors M 1 - 1 ˜M 1 - 4  selectively reflect or transmit the laser beams through both surfaces thereof, and then output the laser beams from the end-most portions through the signal output path to the scanning unit. That is, the fourth dichroic mirror M 1 - 4  transmits the light which is incident through the fourth collimator lens C 4 , but the first to third dichroic mirrors M 1 - 1 ˜M 1 - 3  reflect the light, which is incident through corresponding collimator lenses C 1 ˜C 3 , in a vertical direction. And the second through fourth dichroic mirrors M 1 - 2 ˜M 1 - 4  selectively reflect or transmit the light reflected from the first through third dichroic mirrors M 1 - 1 ˜M 1 - 3  according to the wavelength, and then the light is reflected from the last dichroic mirror M 1 - 4  to the scanning unit  105 . FIG. 4 is a view showing a distribution of a transmittance of the second, third and fourth dichroic mirrors M 1 - 2 ˜M 1 - 4  according to wavelength. FIG. 5 is a view showing reflection and transmission of an optical member with respect to the incident light. A boundary of the reflection and the transmission is ‘λ’. If an incident wavelength is smaller than a wavelength λ1 of a reflection region, the incident light is reflected from a surface of the optical member, and if the incident wavelength is greater than a wavelength λ2 of a transmission region, the incident light is transmitted through the optical member.  
     [0041] On the basis of the reflection and transmission of the optical member with respect to the incident light, reflection and transmission characteristics of the first through fourth dichroic mirror M 1 - 1 ˜M 1 - 4  will be described. The first dichroic mirror M 1 - 1  is a typical reflecting mirror, to reflect the light emitted from the first to fourth laser diodes L 1 ˜L 4 . The second dichroic mirror M 1 - 2  transmits the light emitted from the first laser diode L 1  but reflect the light emitted from the second to fourth laser diodes L 2 ˜L 3 . The third dichroic mirror M 1 - 3  transmits the light emitted from the first and second laser diodes L 1 , L 2  but reflects the light emitted from the third and fourth laser diodes L 3 , L 4 . The fourth dichroic mirror M 1 - 4  reflects the light emitted from the first to third laser diodes L 1 ˜L 3  but transmits the light emitted from the fourth laser diode L 4 .  
     [0042] Due to the characteristics described above, the light emitted from the first laser diode L 1  is reflected from a surface of the first dichroic mirror M 1 - 1 , and transmitted through the second and third dichroic mirrors M 1 - 2 , M 1 - 3 , and reflected by the fourth dichroic mirror M 1 - 4 , and then incident to the cylinder lens  104 . The light emitted from the second laser diode L 2  is reflected by a surface of the second dichroic mirror M 1 - 2 , and transmitted through the third dichroic mirror M 1 - 3 , and reflected by the fourth dichroic mirror M 1 - 4 , and then incident to the cylinder lens  104 . The light emitted from the third laser diode L 3  is reflected by the third dichroic mirror M 1 - 3 , and reflected again by the fourth dichroic mirror M 1 - 4 , and then incident to the cylinder lens  104 . The light emitted from the fourth laser diode L 4  is transmitted through the fourth dichroic mirror M 1 - 4 , and then incident to the cylinder lens  104 . That is, since the laser beam which is incident to the cylinder lens  104  has the same output path regardless of the wavelength, in a region from the cylinder lens  104  to the image forming lens  107 , the light scanning device can obtain the same effect as a mono laser beam printer using a single laser diode.  
     [0043] Sequentially, the laser beam, which is mixed with a plurality of wavelengths transmitting through the image forming lens  107 , is selectively reflected by or transmitted through the fifth through eighth dichroic mirrors M 2 - 1 ˜M 2 - 4  of the second optical output portion  108 , and then scanned to the corresponding photosensitive drums D 1 ˜D 4 . Here, the arrangement structure of each of the dichroic mirrors M 2 - 1 ˜M 2 - 4  of the second optical output portion  108  is applied in the same way as that of each dichroic mirror M 1 - 1 ˜M 1 - 4  of the first optical output portion  103 . In other words, the laser beam emitted from the fourth laser diode L 4  transmits through the eighth dichroic mirror M 2 - 4 , and is then scanned to the fourth photosensitive drum D 4 . The laser beam emitted from the third laser diode L 3  is reflected by the eighth dichroic mirror M 2 - 4 , and is reflected again by the seventh dichroic mirror M 2 - 3 , and is then scanned to the third photosensitive drum D 3 . The light emitted from the second dichroic mirror L 2  is reflected by the eighth dichroic mirror M 2 - 4 , and is transmitted through the seventh dichroic mirror M 2 - 3 , and reflected again by the sixth dichroic mirror M 2 - 2 , and then scanned to the second photosensitive drum D 2 . The light emitted from the first laser diode L 1  is reflected by the eighth dichroic mirror M 2 - 4 , and transmitted through the sixth and seventh dichroic mirrors M 2 - 3  and M 2 - 2  in turn, and reflected again by the fifth dichroic mirror M 2 - 1 , and then scanned to the first photosensitive drum D 1 .  
     [0044] Meanwhile, in order to control horizontal synchronization of a scanning line scanned on each photosensitive drum D 1 -D 4 , the synchronization detecting unit  110  detects the laser beam reflected at a desired position of the scanning unit  105  and then transferred to a control unit (not shown) of the system. At this time, the third optical member  111  to selectively reflect or transmit the laser beam may be disposed or omitted at an incident path through which the light is incident to the synchronization detecting unit  110 . Herein, since the synchronization detecting units  110  detect the laser beam which is scanned through the signal incident path to each photosensitive drum D 1 ˜D 4 , it is possible to synchronize the four scanning lines using only one synchronization detecting unit  110  without separate synchronization detecting units corresponding to each photosensitive drum D 1 -D 4 .  
     [0045] According to the tandem type light scanning device of the present embodiment, as described above, by using a proper arrangement structure of the light source having a plurality of wavelengths and the optical members having various light reflection and transmission regions corresponding to the wavelengths, the light having various wavelengths is transmitted through a single output path to the scanning unit  105 . The light corresponding to each color signal is output from the scanning unit  105  to the signal path, and then scanned through the optical members on each photosensitive drum corresponding to each color signal, whereby a simple structure and a small size of the light scanning device can be obtained.  
     [0046] Further, since it is possible to control the horizontal synchronization of the light scanned on each photosensitive drum using only one synchronization detecting unit, separate compensating units are not needed to compensate for the bow of the scanning line according to the synchronization detection adjusting unit or the image data. Therefore, fabricating costs are reduced.  
     [0047] Although a few preferred embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.