Patent Publication Number: US-2007103539-A1

Title: Print head and image forming apparatus employing the print head

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION  
      This application claims the benefit of Korean Patent Application No. 10-2005-0105472, filed on Nov. 4, 2005, in the Korean Intellectual Property Office, the disclosure of which is hereby incorporated by reference in its entirety.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a print head and an image forming apparatus employing the print head. More particularly, the invention is directed to a print head using a liquid crystal microlens array and to an image forming apparatus employing the print head.  
      2. Description of the Related Art  
      Generally, an electrophotographic image forming apparatus scans a photoconductor with a laser beam to expose an image forming part of the photoconductor to form an electrostatic latent image. Toner is supplied between the photoconductor and a developing roller disposed beside the photoconductor a predetermined distance in order to selectively attach the supplied toner to the image forming part by utilizing the electrical property.  
      Such an electrophotographic image forming apparatus utilizes a laser beam, which requires a laser scanning unit to project a laser beam. However, the laser scanning unit requires a precise and very expensive optical arrangement.  
      As a way of configuring the image forming apparatus without the laser scanning unit, a print head having a structure as shown in  FIG. 1  is provided in prior devices.  
      Referring to  FIG. 1 , the conventional print head includes a semiconductor light emitting device array (hereinafter, referred to as an LED array  1 ) having a plurality of LEDs, and a SELFOC lens array  5  condensing light emitted from the respective LEDs of the LED array  1  to image the light corresponding to the respective LEDs on a photoconductor. Here, the SELFOC lens is a kind of gradient index (GRIN) lens operating by an ion exchange method, for example, an ion exchange between SiO 2  and Ag.  
      According to an image signal from a main controller, the print head turns on/off the respective LEDs of the LED array  1  independently to a predetermined current level by means of driving chips  3 . Here, light emitted from on-state LEDs are condensed by the SELFOC lens array  5  and projected to the photoconductor to form a latent image  7 .  
      Meanwhile, when the print head forms the latent image  7  on the photoconductor by turning on/off the LEDs according to the input image signal, the amount of light emitted from a light emitting point of the respective LEDs deviate. To compensate for the light output deviation, each time a line is scanned in a main scanning direction, each light emitting point is on/off operated with reference to a preset current level corresponding to the light emitting point, making the amount of light of each light emitting point uniform. However, this complicates the configuration of a driving circuit. Further, if a current consumption difference increases suddenly according to an input image signal, like the case when white lines are scanned after black lines are scanned, a surge effect rises to cause damages to the circuitry.  
      An alternative way of configuring the image forming apparatus without the laser scanning unit is disclosed in U.S. Pat. No. 6,825,865, entitled “PRINT HEAD WITH LIQUID CRYSTAL SHUTTER.” 
      The disclosed print head, which uses a white light source, or red, blue, and green light sources and includes a liquid crystal shutter for each light source, is configured to pass red, blue, and green light through corresponding regions of a photoconductive film according to a voltage. Light transmitted through the liquid crystal shutter is transmitted through a SELFOC lens array via a reflector and then transmitted through a prism to form an image on the photosensitive film.  
      Such a print head with the liquid crystal shutter uses a SELFOC lens array and a prism for securing an optical path and focusing complicating mechanical and optical structures.  
     SUMMARY OF THE INVENTION  
      The present invention provides a print head that obviates a problem of compensating for light output deviation. The invention also obviates the surge problem rising in an LED print head. The invention is also directed to an image forming apparatus employing the print head.  
      According to an aspect of the present invention, a print head is provided to form a latent image by selectively emitting light to respective image points of a photoconductor. The print head includes: an illumination unit to emit the light; and a liquid crystal microlens array interposed between the illumination unit and the photoconductor. The liquid crystal microlens array selectively condenses a portion of the light emitted from the illumination unit and directed to image points corresponding to the latent image, onto the photoconductor.  
      According to another aspect of the present invention, an image forming apparatus is provided including: a photoconductor to form a latent image thereon; a print head to form the latent image by selectively emitting light to respective image points of the photoconductor. The print head includes an illumination unit to emit the light, and a liquid crystal microlens array interposed between the illumination unit and the photoconductor to selectively condense a portion of the light emitted from the illumination unit and directed to image points corresponding to the latent image, onto the photoconductor. The apparatus includes a developing unit to supply developer to the photoconductor to form a developer image corresponding to the latent image; a transfer unit to transfer the developer image formed on the photoconductor to a printing medium; and a fusing unit to fuse the developer image to the printing medium.  
      These and other aspects of the invention will become apparent from the following detailed description of the invention, which taken in conjunction with the annexed drawings, disclose embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:  
       FIG. 1  is a perspective view of a conventional LED print head utilizing a SELFOC lens array;  
       FIG. 2  is a schematic perspective view of a print head according to an embodiment of the present invention;  
       FIG. 3  is a partial cross-sectional view schematically showing a print head according to an embodiment of the present invention;  
       FIG. 4  is a partial cross-sectional view schematically showing a print head according to another embodiment of the present invention; and  
       FIG. 5  is a schematic view of an image forming apparatus employing a print head according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       FIG. 2  is a schematic perspective view of a print head according to an embodiment of the present invention.  FIGS. 3 and 4  are schematic partial cross-sectional views of  FIG. 2 , respectively.  
      Referring to  FIGS. 2 through 4 , the print head of an embodiment of the present invention forms a latent image by selectively project light onto image points of a photoconductor  10 . The print head includes an illumination unit  30  to emit light continuously during printing, and a liquid crystal microlens array  50  interposed between the illumination unit  30  and the photoconductor  10 . The liquid crystal microlens array  50  selectively condenses incident light from the illumination unit  30  onto the photoconductor  10  such that image points of the photoconductor  10  corresponding to the latent image can be selectively scanned.  
      Unlike the conventional LED array of the print head, the illumination unit  30  continuously emits light having a predetermined wavelength onto the photoconductor  10  regardless of an input image signal during printing.  
      Referring to  FIG. 3 , an illumination unit  30  of an embodiment includes a white light source  31  to emit white light, and a color filter  33  to selectively transmit the white light emitted from the white light source  31  in a manner such that light is transmitted having a particular wavelength to which the photoconductor  10  is sensitive. The white light source  31  may be formed using an LED array configured with LEDs or organic LEDs (OLEDs), a fluorescent lamp such as a cold cathode fluorescent lamp (CCFL), or a xenon lamp.  
      Referring to  FIG. 4 , an illumination unit  30  of another embodiment may include a light source  35  to continuously emit a high intensity light and a predetermined wavelength to which the photoconductor  10  is sensitive during printing, and a guide plate  37  to guide the light emitted from the light source  35  to the liquid crystal microlens array  50 .  
      As shown in  FIG. 4 , the liquid crystal microlens array  50  includes a transparent substrate  52 , a plurality of liquid crystal microlenses  60  arranged above the transparent substrate  52 , an alignment layer  54  formed on a side of the liquid crystal microlenses  60 , first and second transparent electrodes  53  and  55 , and first and second polarizers  51  and  57 .  
      The liquid crystal microlenses  60  are arranged in a width direction D w  of the photoconductor  10 , making up a lens array  70  as shown in  FIG. 2 . The lens array  70  can simultaneously form a line of a latent image on the photoconductor  10  in the width direction D w , and then, sequentially form the next line of the latent image when the photoconductor  10  is relatively advanced with respect to the lens array  70 . Meanwhile, as shown in  FIG. 2 , a plurality of lens arrays  70  may be provided in an advancing direction of the photoconductor  10 . In this case, a plurality of width lines of a latent image (three lines in case of  FIG. 2 ) can be simultaneously formed along the advancing direction of the photoconductor  10 .  
      Referring to  FIGS. 3 and 4 , each of the liquid crystal microlenses  60  includes a lens portion  63  to condense incident light and liquid crystals  65  and  67  filled in the lens portion  63 . The liquid crystal may be formed of nematic liquid crystal  65  or ferroelectric liquid crystal  67  as shown in  FIGS. 3 and 4 , respectively.  
      Referring to  FIG. 3 , the nematic liquid crystal  65  is formed in the lens portion  63  in a multi-layer fashion. Dipoles of the nematic liquid crystal  65  are arranged between a vertical direction  65   a  and a horizontal direction  65   b  according to a voltage applied through the first and second transparent electrodes  53  and  55 . Here, the response time of the nematic liquid crystal  65  ranges on the order of several hundreds of micro seconds to several milliseconds. On the contrary, in the image forming apparatus, a time required for each light emitting point to scan an image point of the photoconductor  10  is several tens to several micro seconds. Therefore, since the response time of the nematic liquid crystal  65  is greater than the required scanning time, it is difficult to use the nematic liquid crystal  65  for a print head configured to simultaneously form a single latent image line. Meanwhile, as explained above, by configuring the print head to simultaneously form a plurality of latent image lines, the scanning time required in the image forming apparatus can be satisfied even when the nematic liquid crystal  65  having a relatively slow operating characteristic is used.  
      Referring to  FIG. 4 , molecules of the ferroelectric liquid crystal  67  are arranged in a vertically erected layer fashion, and when a voltage is applied, corresponding molecular dipoles of each layer are spun in cone shapes  67   a  and  67   b  to change the polarizing direction. Therefore, refraction index of the liquid crystal microlens  60  changes. In this case, since on/off operation of the ferroelectric liquid crystal  67  is performed on the order of several micro seconds, the required scanning time of the image forming apparatus can be satisfied by configuring the print head to simultaneously form only a single latent image line.  
      Meanwhile, the liquid crystal microlenses  60  can be formed in desired outer shapes by adjusting deviation of the amount of ultraviolet light expose by using anisotropic separation, polymer dispersion, or polymer stabilization that exposes photocurable polymer and liquid crystal solution to ultraviolet light. For example, the structure of a liquid crystal microlens is disclosed in “Fabrication of Electrically Controllable Array Using Liquid Crystals”, Jae-Hoon Kim and Satyendra Kumar, Journal Of Lightwave Technology, Vol. 23, No. 2, pp. 628-632(February 2005), and “Fast Switchable and Bistable Microlens Array Using Ferroelectric Liquid Crystals”, Jae-Hoon Kim and Satyendra Kumar, Japanese Journal of Applied Physics, Vol. 43, No. 10, pp. 7050-7053(2004). Thus, a detailed description thereof will be omitted.  
      The alignment layer  54 , which is formed on one side of the liquid crystal microlenses  60 , determines the alignment direction of the liquid crystals  65  and  67 . Therefore, when the liquid crystal microlens  60  is not operated, the liquid crystals  65  and  67  is arranged in a direction determined by the alignment layer  54 .  
      The first and second transparent electrodes  53  and  55  are provided under and above the liquid crystal microlenses  60 , respectively. The first and second transparent electrodes  53  and  55  apply power to the plurality of liquid crystal microlenses  60  independently, such that liquid crystal alignment can be selectively changed with respect to image points of the photoconductor  10  where a latent image is formed.  
      The first and second polarizers  51  and  57  are provided under and above the liquid crystal microlenses  60 , respectively. The first and second polarizers  51  and  57  transmit incident light only having a particular polarization. Here, if the illumination unit  30  emits light having a particular polarization, the first polarizer  51  can be omitted.  
      An operation of the print head having the above-described structure will now be described.  
      During printing, the illumination unit  30  continuously emits lights having a predetermined wavelength toward the liquid crystal microlens array  50  regardless of an input image signal. The first polarizer  51  transmits only a particular polarization component of the light incident from the illumination unit  30  and blocks the other polarization components of the incident light.  
      The liquid crystal dipoles of the respective liquid crystal microlenses  60  are independently aligned by power applied to the first and second transparent electrodes  53  and  55 . Here, the polarization direction and refraction index of the respective lens portions  63  are changed according to the dipole alignment direction of the liquid crystal  65  and  67 . That is, since the liquid crystals  65  and  67  is characterized by the fact that refraction index of the liquid crystals  65  and  67  in an ordinary ray axis is different from that in an extra-ordinary ray axis, the refraction index varies between the two refraction index values according to the degree of applied voltage. Therefore, the respective liquid crystal microlenses  60  transmit incident diffusion light while condensing the diffusion light to different degrees depending on whether a voltage is applied or not. For example, when power is applied, the liquid crystal of the liquid crystal microlens  60  is aligned as shown with reference numerals  65   b  and  67   b , such that incident light can be condensed much more while passing through the liquid crystals  65   b  and  67   b . Meanwhile, when power is not applied, the liquid crystal of the liquid crystal microlens  60  is aligned as shown by reference numerals  65   a  and  67   a , such that the incident light can be straightly transmitted straight through the liquid crystals  65   a  and  67   a  or slightly condensed while transmitted through the liquid crystals  65   a  and  67   a.    
      The second polarizer  57  transmits only a particular polarization component of the light transmitted through the liquid crystal microlenses  60  toward the photoconductor  10 . Therefore, condensed light can be selectively projected to image points of the photoconductor  10  where a latent image to be formed, and light emitted to other regions of the photoconductor  10  can be excluded.  
       FIG. 5  is a schematic view of an image forming apparatus employing a print head according to the present invention.  
      Referring to  FIG. 5 , the image forming apparatus employing the print head according to an embodiment of the present invention includes a cabinet  110 , a photoconductor  150  provided in the cabinet  110 , a print head  160 , a developing unit  120 , a transfer roller  117 , and a fusing roller  119 .  
      The print head  160  forms a latent image on image points of the photoconductor  150  according to an image to be printed. Here, the print head  160  is constructed substantially in the same way as shown in  FIGS. 2 through 4 . Thus, a detailed-description will not be repeated.  
      The developing unit  120  contains developer (T) in a container  125 , and supplies the developer (T) to the photoconductor  150  through an agitator  127 , a supplying roller  124 , and a developer roller  121  to develop the latent image of the photoconductor  150 . A doctor blade  123  is provided on a circumference of the developer roller  121  to regulate the developer (T) supplied to the developer roller  121 . In the developing unit  120  configured as described above, the developer (T) forms a developer layer having a constant thickness as it passes between the doctor blade  123  and the developer roller  121 . A waste developer container  129  is provided in the developer unit  120  to store waste toner (W) collected by a cleaning blade  112  after developing.  
      As explained above, the developer image formed on the photoconductor  150  by the developing unit  120  is transferred to a printing medium (S) fed between the photoconductor  150  and the transfer roller  117 , and the transferred developer image is fused to the printing medium (S) by the fusing rollers  119 .  
      Further, the image forming apparatus, which prints an image on a printing medium (S) fed from a first cassette  131  or a second cassette  135 , includes a printing media feeding passage  141  and a printing media output passage  145 . Along the printing media feeding passage  141 , the image forming apparatus includes pick-up rollers  132  and  136  picking up printing media (S) one by one, feed rollers  133  guiding the feeding of the pick-up printing media (S), and registration rollers  142  to print an image on the printing medium (S) at a desired region. Along the printing media output passage  145 , the image forming apparatus includes the fusing rollers  119  and a plurality of eject rollers  147 .  
      Therefore, the developer image formed on the photoconductor  150  is transferred to the printing media (S), which is supplied from the first cassette  131  or the second cassette  135  and fed along the printing media feeding passage  141 , and then the transferred developer image is fused to the printing medium (S) by the fusing roller  119 . After an image is completely formed on the printing medium (S) in this way, the printing medium (S) is stacked in an output tray  149  formed on a top of the cabinet  110  through the printing media output passage  145 , completing the printing process.  
      The print head configured as described above according to the present invention and the image forming apparatus employing the print head can fundamentally solve the surge problem of the conventional LED print head caused by sudden current change by continuously operating the illumination unit during printing. Further, since a region on which an image is to be formed can be selectively illuminated by on/off controlling the first and second electrodes using a driving method used for a typical liquid crystal display, designing an additional driving circuit is not necessary for controlling the operation of the illumination unit.  
      Furthermore, since the print head utilizes the liquid crystal microlens array that has a liquid crystal shutter function for selectively transmitting incident light and an incident light condensing function, the print head can have a more compact structure than the conventional print head that utilizes a combination of a SELFOC lens array and a prism for condensing light and securing an optical path.  
      While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.