Abstract:
An optical device includes an optical element that transmits or reflects light, and scans the light onto a target surface to form a latent image on the target surface. The optical element includes a conducting portion that is located on at least any one of a light transmitting surface and a light reflecting surface, and a bias applying unit that applies a predetermined bias to the conducting portion.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present document incorporates by reference the entire contents of Japanese priority document, 2006-132997 filed in Japan on May 11, 2006. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an optical device, and an image forming apparatus. 
         [0004]    2. Description of the Related Art 
         [0005]    There have been utilized optical devices that include a light emitting element such as laser diode (LD) or light emitting diode (LED) to irradiate a target surface, and project an image from the target surface on various positions. Some image forming apparatuses, for example, copiers, printers, scanners, and facsimile machines, include such an optical device. In the optical devices and image forming apparatuses, if the performance is deteriorated by contamination of the surface of the optical element, treatments are usually performed to prevent contamination, to provide the optical element surface with contamination resistance, or to minimize performance deterioration even if the optical element is contaminated. If such treatments do not work well, contamination is removed or the optical element is replaced. 
         [0006]    To prevent the contamination of the optical element surface or provide the optical element with the contamination resistance, a process is shut down that causes the contamination, a layout is provided that minimizes influences of gravity (a method of preventing horizontal layout), or a coating is applied that hardly attaches matters that can contaminate the surface. 
         [0007]    To minimize the performance deterioration even if the optical element is contaminated, an image is enlarged on an optical path and focused on an irradiated surface, the amount of light is incleased depending on the degree of contamination, or image processing is performed. Although there are many methods for removing contamination, such methods are not necessary if contamination is suppressed to a degree that the performance deterioration does not occur. 
         [0008]    Meanwhile, electrophotographic devices with superior productivity and handling property become widely used as image forming apparatuses. Minimization of toner particles that influences greatly improvements in image quality has progressed. As toner particles have electric charges with a predetermined polarity because of their image forming function, they are easily attracted electrostatically. Therefore, measures against contamination of optical elements caused by toner are required. 
         [0009]    For example, Japanese Patent Application Laid-Open No. H8-244277 discloses a conventional technology in which contamination of an optical system is prevented through the use of a power source that is effective for preventing contamination of charging wires and electrostatic absorption for attracting floating toners. 
         [0010]    According to the conventional technology, contamination of an optical system is prevented by attracting floating toner around an optical element electrostatically. Toner that is actually floating in the image forming apparatus is usually moving not slowly but quickly to some extent because of airflow generated in the image forming apparatus. 
         [0011]    Toner does not remain around the optical element but collides with the optical element to be attached thereto or removed therefrom. A few kilovolts of voltage must be applied to generate efficiently, on an electric field, an attraction force that is sufficient to detach once electrostatically attracted toner from the optical element by a predetermined spatial distance. Measures against leakage to the vicinity need to be also considered. 
       SUMMARY OF THE INVENTION 
       [0012]    It is an object of the present invention to at least partially solve the problems in the conventional technology. 
         [0013]    According to an aspect of the present invention, an optical device that scans light onto a target surface to form a latent image on the target surface, includes an optical element that transmits or reflects light. The optical element includes a conducting portion that is located on at least any one of a light transmitting surface and a light reflecting surface, and a bias applying unit that applies a predetermined bias voltage to the conducting portion to form an electric field that repels floating matters. 
         [0014]    According to another aspect of the present invention, an image forming apparatus includes a developing device, an optical device that scans light onto a target surface to form a latent image on the target surface and includes an optical element that transmits or reflects light, and a power source. The optical element includes a conducting portion that is located on at least any one of a light transmitting surface and a light reflecting surface, and a bias applying unit that applies a predetermined bias voltage to the conducting portion to form an electric field that repels floating matters. The power source supplies the predetermined bias voltage to be applied to the conducting portion and a developing bias voltage to be applied to the developing device. 
         [0015]    The above and other objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a cut-out schematic perspective view of a general laser diode unit; 
           [0017]      FIG. 2  is a schematic diagram of an image forming system according to an embodiment of the present invention; 
           [0018]      FIG. 3  is a schematic perspective view of an optical scanning device in an image forming apparatus shown in  FIG. 2 ; 
           [0019]      FIG. 4  is a schematic diagram of an image forming unit in the image forming apparatus shown in  FIG. 2 ; 
           [0020]      FIG. 5  is a schematic diagram of an image forming apparatus that utilizes an LED printer head; 
           [0021]      FIG. 6  is a schematic perspective view of the LED printer head serving as an optical scanning device in the image forming apparatus shown in  FIG. 5 ; 
           [0022]      FIG. 7  is a schematic partial perspective view of an example of a SELFOC lens in the image forming apparatus shown in  FIG. 5 ; 
           [0023]      FIG. 8  is a schematic diagram of an image forming apparatus that includes an image reading device utilizing a reduced optical system; 
           [0024]      FIG. 9  is a schematic diagram of an image reading device utilizing an equi-magnification optical system that is incorporated in an image forming apparatus; 
           [0025]      FIG. 10  is a schematic perspective view of a part of a mirror of an optical device; 
           [0026]      FIG. 11  is a schematic circuit diagram of a bias load to the mirror of the optical device; 
           [0027]      FIG. 12  is a schematic diagram for explaining airflow on an optical element; 
           [0028]      FIG. 13  is a schematic diagram of a display unit that informs performance deterioration of a device over time; and 
           [0029]      FIG. 14  is a schematic diagram of another display unit that informs performance deterioration of a device over time. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0030]    Exemplary embodiments of the present invention are explained in detail below with reference to the accompanying drawings. 
         [0031]      FIG. 1  is a cut-out schematic perspective view of a general laser diode unit. In the laser diode unit A, a glass plate  2  is placed on the top surface of a cap  1 , a laser (LD) chip  4  is placed on a heat sink  3 , and a PIN chip  6  is placed on a stem  5  surrounded by the cap  1 . 
         [0032]    As shown in  FIG. 1 , the LD unit A is formed of a cladding layer and an active layer. Laser light emits from the LD chip through the glass plate  2  by electric charges from electrodes connected to the cladding layer (hereinafter, the light is referred to as “front beam a”). 
         [0033]    A back beam b which makes a pair with the front beam a is emitted to a photodiode (not shown, hereinafter, PD) in the LD unit A, so that PD outputs are obtained from the electrodes. The amount of beam light is thus monitored and control depending on monitor outputs is performed. Stable outputs are achieved with predetermined optical power. 
         [0034]    In an image forming apparatus, toner is a primary floating matter that causes contamination. Therefore, among optical systems used in the image forming apparatus, structural examples of four representative optical devices are described. 
         [0035]      FIG. 2  is a schematic perspective view of an image forming system B according to an embodiment of the present invention.  FIG. 3  is a schematic perspective view of an optical scanning device C in an image forming apparatus  7  shown in  FIG. 2 .  FIG. 4  is a schematic diagram of an image forming unit D in the image forming apparatus  7 . In the image forming system B, a personal computer (PC)  8  and a facsimile machine  9  are connected to the image forming apparatus  7 . 
         [0036]    The structural example of the optical scanning device which utilizes an LD is described with reference to  FIG. 3 . The optical scanning device C is formed of the LD unit A, an optical element that shapes a beam diameter for laser light emitted from the LD unit A before guiding to the scanned surface, and a control unit that controls LD light. 
         [0037]    With reference to  FIG. 3 , a writing unit of an exposure unit in the optical scanning device C includes the light emitting element A, a polygon mirror  10  serving as the optical element, an fθ lens  11 , a fold mirror  12 , mirrors  13  and  13   a , a cylindrical lens  14 , a synchronization detection sensor  15  that performs synchronization detection for generating LD light emitting timing, and a scanning optical system that exposes a photosensitive drum (photoconductor)  16  having the scanned surface to a plurality of beams with predetermined distances therebetween. 
         [0038]    The image forming unit D in the image forming apparatus  7  includes a charging device  17 , a developing device  18 , a transfer device  19 , a cleaning device  20 , and a neutralization device  21  around the photosensitive drum  16  on which a latent image is formed for performing charging, exposure, development, transfer, and cleaning processes. 
         [0039]    The image forming apparatus  7  includes, although not shown, a feeding unit that feeds a transfer sheet with an image formed thereon, a fixing unit that fixes the transferred image, and a discharge unit that discharges the fixed transfer sheet. 
         [0040]    To form images from image data read by a scanner and a digital camera, image data prepared on the PC  8 , and image data for the facsimile machine  9  received by a telephone line, the image data is passed through an image I/F unit to which the image data is inputted and an image processor for storing or editing/image processing the inputted image data. A latent image is then formed by the LD control unit. As an image forming apparatus with a plurality of light emitting elements, there provided apparatuses with a plurality of LD units and apparatuses configured by LD arrays including LD units. 
         [0041]      FIG. 5  is a schematic diagram of an image forming apparatus that utilizes an LED printer head.  FIG. 6  is a schematic perspective view of the LED printer head serving as the optical scanning device incorporated in the image forming apparatus shown in  FIG. 5 .  FIG. 7  is a schematic partial perspective view of an example of a SELFOC lens incorporated in the image forming apparatus shown in  FIG. 5 . 
         [0042]    A structural example of the image forming apparatus that includes an optical writing device utilizing the LED printer head (hereinafter, LPH) is described. With reference to  FIGS. 5 to 7 , the image forming apparatus  7  includes the charging device  17 , the developing device  18 , the transfer device  19 , the cleaning device  20 , the neutralization device  21 , toner  22  within the developing device  18 , a separating device  23 , a fixing device  24 , and a stacker  25 . 
         [0043]    The image forming apparatus  7  includes, as shown in  FIG. 6 , an LPH  26  made by arranging a plurality of integrated LED chips. The LPH  26  is configured by LED array elements  29  on the back surface of a substrate  28 , an equi-magnification lens  30 , a heatsink  31 , driver elements  32 , and an I/F unit  33  on the top surface of the substrate  28 . 
         [0044]    The image forming apparatus  7  includes a SELFOC lens array  27  with the configuration shown in  FIG. 7 . The LPH of the image forming apparatus  7  shown in  FIG. 5  is formed by arranging a plurality of integrated LED chips and performs imaging by the SELFOC lens array  27 . As compared to the optical scanning device shown in  FIG. 3 , the configuration of the LPH is simpler and space saving is easily realized. The LPH has been used conventionally for facsimile machines and printers. 
         [0045]    However, because each of LEDs in the LPH has a little light amount and its integration degree is hard to be improved, the LPH seems to be behind recent density growth. As LEDs vary with each other, correction for light amount is required. 
         [0046]    The light emission mechanism for LED is roughly classified into a strobe mechanism and a dynamic mechanism. In the strobe mechanism, light emission data is transferred to each LED and all LEDs are turned on by strobe signals. 
         [0047]    The strobe mechanism is usually performed in a divided manner to reduce a data transfer rate and prevent a great change in input current at the time of turning on LEDs. Although the dynamic mechanism requires a complicated control circuit, it has an advantage of small change in input current because the respective LEDs are turned on in a dynamic manner. 
         [0048]      FIG. 8  is a schematic diagram of the image forming apparatus  7  that includes an image reading device utilizing a reduced optical system. An image reading unit of the image forming apparatus  7  includes, in its scanning unit, a light source  35  and a first mirror  38  that irradiates light from a reflecting mirror which converges light from the light source on an original  37  placed on a contact glass  36  and that reflexes a reflected light image from the original  37 . 
         [0049]    The image reading unit reflexes an image from the first mirror  38  via a second mirror  39  and a third mirror  40  to allow the image to transmit thorough a lens  41  and image on a reading sensor  42 . 
         [0050]    A charge-coupled device (CCD) sensor is usually used for the reading sensor  42 . A one-line sensor used for monochrome images and a three-line sensor used for color images are provided. Color images are sometimes read with the one-line sensor in a manner that scanning is performed for several times with different wavelengths of the light source  35 . Since applications of downloading images in PCs have been improved recently, images are read as red-green-blue (RGB) information. A xenon or fluorescent lamp is used for the light source  35 . 
         [0051]    The three-line sensor requires a memory (not shown) for correcting the distance between the sensors in an image processor  43 . On basic reading conditions (scanning rate and reading density), the distance between the sensors is made to be a predetermined distance (integer multiplication of pixel size on the sensor). 
         [0052]    When color imaging is performed by switching the light source  35  in the one-line sensor, the distance between the sensors needs not to be corrected. An image memory that is capable of storing the entire reading area at least twice is required. 
         [0053]    In the image forming apparatus of  FIG. 8 , an electric signal from the image processor  43  is inputted to an writing optical system  44  in the printer unit. The photosensitive drum  16  is then exposed with light. An electrostatic latent image on the photosensitive drum  16  is developed by the developing device  18 . The developed image is transferred to a transfer sheet by the transfer device  19 . The transfer sheet with the transferred image is thermally fixed by the fixing device  24  and discharged outside the machine. 
         [0054]      FIG. 9  is a schematic diagram of an image reading device utilizing an equi-magnification optical system that is incorporated in the image forming apparatus. The image reading device includes a white light source  35  for irradiating the original  37  on the contact glass  36 , an equi-magnification sensor  45 , and an equi-magnification lens  46 . A SELFOC lens is usually utilized for the equi-magnification lens. 
         [0055]    The image reading device with such a configuration has a larger pixel size for the sensor as compared to the reduced optical system. In addition to CCD sensors, metal-oxide semiconductor (MOS) sensors are usually utilized. Larger pixel size improves the sensitivity of the sensor. Shorter optical path due to the equi-magnification lens  46  enables a decreased amount of light from the light source  35 . In addition to xenon and fluorescent lamps, LEDs and organic ELs are utilized for the light source  35 . 
         [0056]      FIG. 10  is a schematic perspective view of a part of a mirror of an optical device.  FIG. 11  is a schematic circuit diagram of a bias load to the mirror of the optical device. A method for applying a bias to optical elements of the optical devices exemplified above is described with reference to  FIGS. 10 and 11 . A mirror of the optical device, e.g., the first mirror  38  shown in  FIG. 8  has an aluminum evaporation layer (conducting portion)  47  made by evaporating aluminum on its mirror surface. 
         [0057]    The aluminum evaporation layer  47  which is the mirror surface made by aluminum evaporation is conductive. An electrode is thus formed with a leaf spring  48  serving as a retainer. Insulation to a member (not shown) connected to GND by a case (not shown) is maintained. The desired bias is applied to the surface by receiving bias from a power source (not shown). 
         [0058]    As shown in  FIG. 11 , the optical element (first mirror)  38  is connected to GND by the case (not shown). The desired bias is applied from a bias source (bias applying unit)  51  to the surface, i.e., the aluminum evaporation layer  47  ( FIG. 10 ). The bias circuit includes a display unit  49  for informing performance deterioration over time, which is described below. 
         [0059]    Films (conducting portions) made of conductive polymers (polyethylenedioxythiophene or the like) are coated on surfaces of lenses made of glass and plastic. The bias is similarly applied to the surface and an electric field for repelling floating matters is formed on the surface of the optical element. 
         [0060]    Impedance on the surface of the conducting portion varies depending on thicknesses and materials of the coating portion (conducting portion). According to the embodiment, the conducting portion is basically configured to be insulated from the environment unless there are specific reasons with respect to the optical element and the configured system, the load impedance is large. Differences in effects due to impedance characteristics need not to be considered. 
         [0061]    A power source for supplying a bias is explained below. The bias is supplied from a converter power source that utilizes ordinary transformers (not shown). Because of large load impedance, a converter power source that utilizes piezoelectric elements is also preferable. 
         [0062]    Optimal values of voltage required for applying bias vary depending on image forming apparatuses. The voltage is generally the same as that applied to toner in the developing unit and is approximately −600 Volts. 
         [0063]    According to the embodiment described above, by applying a bias for forming an electric field which repels floating matters against the desired optical element to the conducting portion on the surface of the optical element, floating matters with electric charges are prevented from attracted to the optical element. 
         [0064]    Accordingly, in the optical device that requires regular maintenances, the interval between the maintenances is significantly extended, as well as making the maintenances and cleaning mechanisms unnecessary. This technology is especially effective for configurations that are difficult to perform maintenance because of space saving and compactness and that cannot incorporate the cleaning mechanism. 
         [0065]    If the embodiment is applied to a system, which is usually problem-free but suffers from contamination due to installation environment or application, as a function of preventing contamination, quick actions are easily taken because of its simple configuration, and downtime for users is reduced. 
         [0066]    The above description has been made by taking toner as an example of floating matters. Toner generally has electric charge to form a visible image on a photoconductor, and the electric charge of toner is identified. Contamination prevention bias for floating toner is thus easily set. 
         [0067]    However, if electric charges of floating matters are not identified or the floating matters do not have electric charges, the contamination prevention bias can attract the floating matters electrostatically. In such a case, it is effective to supply a bipolar bias in a switched manner. 
         [0068]    In practice, voltage waveforms including sine waveform, rectangular waveform, and triangular waveform are made at a predetermined period. The bias is then supplied by controlling offsets at a DC output if necessary. Attraction force to the optical element is effectively reduced. 
         [0069]    Repelling of floating matters is realized by the contamination prevention bias and attraction of the floating matters to the optical element is prevented by reducing the attraction force. However, the floating matters can exist around the optical element. When the floating matters are accumulated even if they do not have any attraction force, the matters can affect the system optically. 
         [0070]    In this case, it is effective to remove the floating matters using brushes and blades. However, at the time of removal, if some of the floating matters are not removed and remains, such remained matters can form lines or dots and exert another influence on the system. Therefore, it is more effective to move the floating matters by air. 
         [0071]      FIG. 12  is a schematic diagram for explaining airflow on an optical element. With reference to  FIG. 12 , if airflow exists on the optical element (writing SELFOC lens)  27 , floating matters with no attraction force are moved downstream and hardly accumulated. The moved floating matters are removed by filters or attraction elements at a downstream position that is apart from the optical element  27 , which reduces adverse affects on other units. 
         [0072]    The intensity of the airflow on the optical element  27  is set not to accelerate accumulation on the optical element  27 . For example, around the photosensitive drum  16 , the airflow generated by the operation of the photosensitive drum  16  works sufficiently. 
         [0073]    The same voltage as that applied to toner is applied to the optical element. The voltage is supplied to the optical element by diverging from the power source for supplying developing bias for the developing device  18  (power source for supplying developing bias for developing roller  18   a ). This configuration achieves cost reduction as compared to a case of providing a power source only for contamination prevention. Further, the same potential as the potential of floating toner is always supplied by following changes in the developing bias. 
         [0074]    If the period during which toner is floating is not always the same as the timing for applying the developing bias, it can be optimized by providing a switch element for switching. If the contamination prevention bias needs to be applied despite the developing bias being turned off, the switch element is provided on a line for supplying to the developing bias to perform on-off control. 
         [0075]    In the technology for supplying voltage from not the power source for contamination prevention but other power source, when higher output voltage than the contamination prevention bias is supplied, as described in the structural examples, the contamination prevention bias is generated by resistance dividing because of its large load impedance. A simple configuration is thus realized. 
         [0076]    The contamination prevention bias is proved to be effective for undulating voltage and pulse voltage as well as direct current (DC) voltage. The scope of selection for power sources utilized commonly among image forming apparatuses is widened. The power source for applying bipolar bias is particularly cost advantageous. 
         [0077]    The relationship between the contamination prevention bias and the airflow for moving the floating matters needs to be controlled depending on the state of the floating matters near the optical element. On the periphery of the photoconductor of the image forming apparatus, for example, when the photoconductor is driven to rotate so that an image is formed thereon, the airflow is generated. When the photoconductor is stopped, the airflow is not generated. 
         [0078]    Generally, the photoconductor is driven and the developing bias is then applied. The photoconductor stops driving after the developing bias is stopped. If floating matters are toner, by applying the contamination prevention bias at least during the period when the photoconductor is driven and the airflow is generated, attraction of the floating matters near the optical element is prevented when the largest amount of floating matters exist. 
         [0079]    If there is no airflow, new floating matters are not attracted around the optical element, and application of the contamination prevention bias is stopped. Depending on influences on the optical element, the contamination prevention bias always needs to be applied. However, because toner holds electric charge during a finite period, the period of driving the photoconductor after application of the developing bias is stopped is extended to minimize the influences. 
         [0080]      FIG. 13  is a schematic diagram of a display unit that informs performance deterioration of a device over time.  FIG. 14  is a schematic diagram of another display unit that informs performance deterioration of a device over time. 
         [0081]    The technology related to the contamination prevention bias is aimed at reducing influence with respect to contamination of the optical system. If the contamination prevention bias is not applied because of its leakage, the performance of the device over time can be deteriorated. The performance deterioration does not affect functions of the device immediately, and these functions can be recovered by cleaning. 
         [0082]    Because functional problems are not presented immediately as described above, instead of stopping such functions, the lamp (display unit)  49  shown in  FIG. 13  is turned on. Alternatively, a comment  50  such as “optical system can be contaminated” is displayed on a display screen shown in  FIG. 14  to inform a user or a serviceperson of an appropriate operation such as repair. 
         [0083]    As set forth hereinabove, according to an embodiment of the present invention, an aluminum evaporation layer which is a mirror surface made by aluminum evaporation is conductive, and an electrode is formed with a leaf spring serving as a retainer. Isolation to a member connected to GND by a case is maintained. The desired bias is applied to the surface by supplying bias from a power source. The bias for forming an electric field for repelling floating matters against the desired conductive optical element is formed. The floating matters are prevented from being attracted to the optical element. 
         [0084]    Although the invention has been described with respect to a specific embodiment for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth.