Patent Document

BACKGROUND 
   Latent image ghosting in photoreceptors is a common problem. Such ghosts have generally been identified as resulting either directly from image-wise exposure or from different amounts of positive charge injected in non-toned and toned image areas. Both result in an arrangement of trapped positive charges that release during re-charging on a subsequent cycle, leading to a pattern of differential charge level that manifests itself in subsequent prints as an image-wise pattern of development, or ghost, of a prior image cycle. Massive erase light is conventionally used to flood the image-wise exposed photoreceptor to remove trapped charges. Such methods generally help, but sometimes prove to be insufficient to suppress ghosting. 
   The photo-discharge resulting from the erase light, often one or two orders of magnitude greater than the largest exposure utilized during the exposure step of the printing cycle, is always greater when following an image exposure than that observed had the image exposure not preceded the erase, and the magnitude of the increased erase discharge increases with increasing image exposure. One reason for this phenomenon is that not all charge pairs that are photo-generated during image exposure separate, release, and transport out of the photoreceptor layers prior to the erase step. Some portion of these remaining nascent charge pairs is released and transported over time following the initial rapid discharge. These charges are the “delayed release” charges, and manifest themselves as increased dark decay of partially discharged photoreceptors. The remaining portion of these nascent charge pairs continue to be released during the period prior to erase and, depending on their distribution within the generator layer, may contribute to an enhanced local field within the generator, thus augmenting the erase-photogenerated charge yield, resulting in the observed enhanced erase discharge following exposure. 
   Depending on the physical distribution within the generator layer of these subsequently erase-generated charge pairs, an image-wise population of nascent charges may persist after the first ghost generation cycle charge step, during which they release under the charging field, appearing as increased “depletion” charge, or dark decay, leading to lower charge potential in an image-wise sense. Whether this increased depletion results in positive or negative ghosting depends significantly on the rate of delayed release, the time between exposure and erase, and on the time between erase and charge. If the number density of erase-generated charge in the background is comparable with the image areas, then ghosting may be suppressed. This has traditionally been the rationale behind “flood erase” to suppress image ghosting, in order to minimize the difference in dark decay or depletion between image and background areas going into the first ghost generation. Unfortunately, this traditional approach to suppressing image ghosting has limited success. 
   SUMMARY 
   To aid in understanding both the mechanism and the phenomenology of ghosting, the original image is referred to as a generation “0” image, or the first exposure, development, and transfer of the original image on a rested or “virgin” photoreceptor. Also, the first ghost generation is part of a subsequent imaging event that utilizes the same physical part of the photoreceptor as the original image. Thus, the first ghost generation is the second xerographic cycle. This first generation ghost may appear on the same or a subsequent print, depending on how many xerographic cycles are required to image one print. Similarly, one can describe second and multiple generation ghost images. 
   Often, in order to print a latent ghost image, the generation print must itself have some structured image other than background. Thus, mid-density half-tones are used as test images. In such mid-density half-tones, the ghost of an original image that appears darker than its surrounding medium is identified as a “positive ghost,” and as a “negative ghost” if it appears lighter than its surrounding medium. Consequently, only positive ghosts can print in background areas as first generation ghosts if the background is devoid of toner. Positive ghosts generally result from the image-wise release of trapped positive charge remaining after image-wise exposure following the charge step in the first ghost generation. These charges might be more deeply trapped delayed release photogenerated charges, or simply deeply trapped holes not adequately compensated by negative charges generated during the erase step. 
   On the other hand, transfer-induced negative ghosting results from the release of trapped injected positive charge during the first ghost generation charging step. The image-wise developed toner blocks the injection of positive charges applied during the transfer step of the original image cycle, resulting in more trapped injected positive charge in the background compared with the exposed areas of the original image cycle. During the charging step of the first ghost generation cycle, the original background areas both charge and discharge at a lower voltage than original image areas, resulting in a negative ghost of the original image embedded in a half-tone surrounding medium. 
   In light of the above-discussed problems, various exemplary embodiments effectively neutralize the trapped positive charges associated with the latent image ghost by flooding the photoreceptor with sufficient electron-hole pairs to compensate for the trapped positive charges with free electrons, while simultaneously providing an electric field of sufficient strength to sweep out the free holes released by the electrons used in the compensation. This goal is achieved by arranging for the erase exposure and the negative charging to occur simultaneously, or sufficiently close to one another, such that the charge pairs that are subject to delayed release are sufficient to provide compensation for the ghost-trapped positive charges. 
   Various exemplary embodiments provide a ghost removing device for a marking apparatus that includes a photoreceptor, a primary charging device, an erase light generating device configured to generate an erase light when activated, at least one voltage source configured to supply a charge voltage to the primary charging device, and at least one controller configured to control the at least one voltage source to supply the charge voltage to the primary charging device and to activate the erase light generating device substantially simultaneously after transfer of a toned image within a marking cycle. 
   Various exemplary embodiments also provide a ghost removing method for a marking device with a photoreceptor, that includes performing a partial print cycle, applying an erase light to the photoreceptor via an erase light generating device, and applying a charge voltage to the photoreceptor via a charging device, wherein the erase light and the charge voltage are applied to the photoreceptor substantially simultaneously after completion of the partial print cycle. 
   Finally, various exemplary embodiments provide a xerographic device with a photoreceptor, means for performing a partial print cycle, means for applying an erase light to the photoreceptor via an erase light generating device, and means for applying a charge voltage to the photoreceptor via a charging device, wherein the erase light and the charge voltage are applied to the photoreceptor substantially simultaneously after completion of the partial print cycle. 
   It should be noted that the operating principles described herein also apply to other charging systems as well, such as BCR (Bias Charge Roll), whether in contact with a photoreceptor or not, and, for example, other charging devices such as the “microtron,” or the “dicorotron”. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Various exemplary embodiments of systems and methods will be described in detail, with reference to the following figures, wherein: 
       FIG. 1  is an illustration of an exemplary ghost removing device; 
       FIG. 2  is an illustration of an exemplary ghost removing device; and 
       FIG. 3  is a flow chart illustrating an exemplary ghost removing method. 
   

   DETAILED DESCRIPTION OF EMBODIMENTS 
   These and other features and advantages of this invention are described in, or are apparent from, the following detailed description of various exemplary embodiments of systems and methods. 
   The xerographic cycle is well recognized as comprising several steps, with the photoreceptor playing the central role of latent image-bearing member. These steps are i) charging, wherein one sign of charge is applied to the photoreceptor; ii) image-wise exposure, wherein an image wise pattern of light exposes and photo-discharges the photoreceptor; iii) development, wherein charged toner particles are presented to adhere to the discharged areas of the photoreceptor; iv) transfer, wherein an opposite signed charge is applied to the rear surface of a substrate to move the developed toner while retaining the image-wise pattern to the receiving substrate; v) detack, wherein some charge is applied to the substrate to facilitate stripping of the substrate from the photoreceptor; and vi) erase and cleaning, wherein the photoreceptor is flood exposed to uniformly discharge, and remove any residual toner from the photoreceptor prior to resuming the cycle with step i). 
     FIG. 1  is an illustration of an exemplary ghost removing device. In  FIG. 1 , a marking device  100  that includes a photoreceptor  160 , a cleaning device  120 , a developer  150 , a transfer  140 , detack  145 , and two charging devices  110  and  115  such as, for example, scorotrons, also includes a controller  170  and an erase light generating device such as, for example, an erase lamp  130 . According to various exemplary embodiments, the erase light generating device  130  is located over the scorotron  115  such that the scorotron  115  is sandwiched between the erase light generating device  130  and the photoreceptor  160 . According to various exemplary embodiments, the controller  170  controls the two scorotrons  110  and  115 , the photoreceptor  160  and the erase light from the erase light generating device  130  mounted in the marking device  100  so that the erase light generating device  130  shines through the grid of the scorotron  115  and onto the photoreceptor  160 . It should be noted that each of the scorotrons  110  and  115  may be replaced by a corotron or other charging device such as, for example, a BCR, a microtron or a dicotron. According to various exemplary embodiments, the erase light generating device may be replaced by any device that is capable of providing an erase light when activated. 
   According to various exemplary embodiments, the controller  170  controls a charge being applied to the photoreceptor  160  by the scorotron  110 , then an image-wise pattern of light exposes and photo-discharges the photoreceptor  160 . Subsequently, charged toner particles are provided to adhere to the discharged areas of the photoreceptor  160 , then the controller  170  controls the application of a charge, with a sign opposite to the charge applied to the photoreceptor  160 , to the receiving substrate at the transfer device  140  to remove the developed toner while retaining the image-wise pattern, and some additional charge is applied via the detack  145  to the substrate to facilitate stripping of the substrate from the photoreceptor  160 . The photoreceptor  160  is then flood-exposed to uniformly discharge by an erase light generating device  130 , and the cleaning device  120  removes any residual toner from the photoreceptor  160  prior to resuming another print cycle. According to various exemplary embodiments, the erase light generated by the erase light generating device  130  and controlled by the controller  170  shines through the scorotron  115  to reach the photoreceptor  160 , and the charge voltage applied to the scorotron  115  is applied simultaneously or near-simultaneously with the erase light generated by the erase light generating device  130 . Also, the controller  170  may control any device for supplying an erase light, and may control any device for supplying a charge voltage to the scorotrons  110  and  115 . 
   Various exemplary embodiments combine a higher field with the erase generated charges, and positive charges are swept out more efficiently, leaving negative erase-generated charges to compensate for any deeply trapped positive charges remaining on the photoreceptor  160  due either to image-wise exposure or to transfer charge injection in background areas. Furthermore, by discharging and charging near simultaneously, the image-wise structure of residual charge pairs, either those destined for delayed release or those more deeply trapped, is removed. In other words, by combining erase light and simultaneous charging, electron-hole pairs are generated in the generator layer of the photoreceptor in such an amount that the trapped charges are compensated, and any excess charge carriers are swept out such as to make spatially uniform any residual charge distribution that may remain on the photoreceptor  160 . 
   For example, using a photoreceptor with a known history of severe ghosting as large as, for example, Grade 5 on a scale of 0 to 5, where 0 is no observed ghost, and 5 is the worst, strong transfer ghosting is observed with no current applied to the scorotron  115 . With a voltage of about −4.5 KV applied to the scorotron  115  and −500 V applied to the screen of the scorotron  115 , the ghost grade drops to Grade 0. In subsequent operation, ghosting can be made to appear or disappear simply by turning the scorotron off or on, respectively. Table 1 illustrates the exposure of a detector diode located under the erase light generating device  130  in this exemplary embodiment. In Table 1, various measurements of exposure under the erase light generating device  130  are reported. 
   It should be noted that neither the positive charge from the scorotron, nor the negative charge without erase present, has an effect in reducing the ghost signal. In fact, positive charge with the erase light generating device  130  on results in darker prints and a stronger ghost grade. Also, applying a negative charge alone from the scorotron  115  without energizing the erase light  130  has no measurable effect on the strength of the ghost. Accordingly, simultaneous or near simultaneous charge and exposure is required to eliminate the ghost. According to various exemplary embodiments, the erase light generating device  130  may be an erase lamp. 
   
     
       
             
           
             
             
             
           
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               Measurement of Exposure under the Erase Lamp 
             
           
        
         
             
                 
               Detector Diode 
                 
             
             
               Erase Lamp Voltage 
               [ua] 
               Exposure [erg/cm{circumflex over ( )}2] 
             
             
                 
             
           
        
         
             
               12.5 
               0.56 
               1.6 
             
             
               13 
               2.31 
               6.5 
             
             
               14 
               9.45 
               27 
             
             
               15 
               20.6 
               58 
             
             
               16 
               31.14 
               88 
             
             
               17 
               43.12 
               122 
             
             
               18 
               53.6 
               152 
             
             
               19 
               64.31 
               182 
             
             
               20 
               74.46 
               211 
             
             
                 
             
           
        
       
     
   
     FIG. 2  is an illustration of another exemplary ghost removing device. In  FIG. 2 , a marking device  200  includes a controller  270 , cleaning device  220 , a developer  250 , a transfer  240  detack  245 , and an erase light generating device  230  located adjacent to a charging device such as, for example, a scorotron  210 . According to various exemplary embodiments, the controller  270  controls the application of a charge to the photoreceptor  260  via the scorotron  210 , then an image-wise pattern of light exposes and photo-discharges the photoreceptor  260 . Subsequently, charged toner particles are presented to adhere to the discharged areas of the photoreceptor  260 , then the controller  270  controls the application of a charge via the transfer  240 , with a sign opposite to the charge applied to the photoreceptor  260 , to move the developed toner while retaining the image-wise pattern to the receiving substrate, and some additional charge is then applied via the detack  245  to the substrate to facilitate stripping of the substrate from the photoreceptor  260 . Then, in the xerographic cycle, residual toner may be removed from the photoreceptor  260  at the cleaning station  220 . The photoreceptor  260  is then flood-exposed under control of the controller  270  to uniformly discharge by the exposure to erase light generated by the erase light generating device  230  that is located adjacent to the scorotron  210 , prior to resuming another print cycle. Simultaneously or near-simultaneously with the application of the erase light by the erase light generating device  230 , a charge voltage is applied under control of the controller  270  to the scorotron  210  in order to compensate for positive trapped charge and sweep out any excess photogenerated charge more efficiently. According to various exemplary embodiments, the erase light generating device  230  may be, for example, an erase lamp. 
   Table 2 illustrates the exposure of a detector diode located under the scorotron  210  in this exemplary embodiment. In Table 2, the erase lamp voltages that produce the best ghost suppression in this illustrated example are voltages of 13.2-13.6 V, resulting in erase light exposures of 0.5-0.9 erg/cm 2 , respectively. In other exemplary cases, where the erase light is located apart from the scorotron, but where the photoreceptor is exposed through a second charging device  115 , the ghost signal can be suppressed using a low value of erase exposure such as, for example, approximately 10% of the normal erase energy of 80 erg/cm 2 , or about 8 erg/cm 2 . Measurements of flare light under the scorotron  210  shows a very low exposure, of about 0.8 erg/cm 2 . Under these conditions, prints generally appear ghost free and of good print quality, despite the absence of a strong erase in the usual pre-clean position. In other words, according to various exemplary embodiments, some erase light is combined with some charging just prior to the primary charge action. Thus, the effect of this combination of erase light with some charge is also a photogeneration of compensating charge and efficient sweep out of any excess photo-generated carriers from the photoreceptor. 
   
     
       
             
           
             
             
             
           
             
             
             
           
         
             
               TABLE 2 
             
           
           
             
                 
             
             
               Measurement of Exposure under the Primary Charge Scorotron 
             
           
        
         
             
               Erase Lamp 
                 
                 
             
             
               Voltage 
               Detector Diode [ua] 
               Exposure [erg/cm{circumflex over ( )}2] 
             
             
                 
             
           
        
         
             
               12 
               0 
               0.0 
             
             
               12.5 
               0.02 
               0.1 
             
             
               13 
               0.1 
               0.3 
             
             
               13.2 
               0.17 
               0.5 
             
             
               13.3 
               0.19 
               0.5 
             
             
               13.4 
               0.26 
               0.7 
             
             
               13.5 
               0.29 
               0.8 
             
             
               13.6 
               0.31 
               0.9 
             
             
               13.7 
               0.36 
               1.0 
             
             
               13.8 
               0.4 
               1.1 
             
             
               13.9 
               0.45 
               1.3 
             
             
               14 
               0.52 
               1.5 
             
             
                 
             
           
        
       
     
   
   The above-described examples suggest that the mechanism by which the positive charge-induced negative ghosting is suppressed involves the generation and sweeping out of holes in the photoreceptor in such density as to provide adequate negative counter charge in the photoreceptor to compensate for the trapped holes that underlie the ghost phenomenon. In the example illustrated in  FIG. 1 , the generation and sweep out occurs at a different time than the re-charging of the photoreceptor. In the example illustrated in  FIG. 2 , the generation and sweep out occurs as part of the re-charging process, partly due to the flare light under the scorotron  210  and partly from the adjacent pre-charge exposure. 
     FIG. 3  is a flow chart illustrating an exemplary ghost removing method. In  FIG. 3 , the method starts in step S 100 , and continues to step S 110 , during which a partial print cycle is performed in a marking device. According to various exemplary embodiments, the partial print cycle includes charge of the photoreceptor, image-wise exposed, image-wise development, and toned image transfer. The partial print cycle may also include cleaning of the photoreceptor by a cleaning device after toned image transfer. According to various exemplary embodiments, the charging of the photoreceptor during step S 110  is performed using a primary charging device such as, for example, a scorotron, a corotron, a BCR, or a dicotron. Next, control continues to step S 120 , during which an erase light, provided by an erase light generating device, is applied to the photoreceptor. According to various exemplary embodiments, the erase light generating device such as, for example, an erase lamp, may be provided over a secondary charging device, such as a scorotron, such that the secondary charging device is sandwiched between the erase lamp and the photoreceptor. According to various exemplary embodiments, the erase light applied to the photoreceptor via the erase lamp uniformly discharges the photoreceptor. 
   Moreover, according to other exemplary embodiments, the erase lamp may be provided adjacent to the primary charging device, over the photoreceptor, and is sufficiently close to the primary charging device to allow some erase exposure to take place simultaneously or near-simultaneously to the charging applied by the primary charging device without completely filling the primary charging device with erase light. In this case, the erase light exposes only a small fraction of the primary charging device. 
   Next, control continues to step S 130 , during which a charge voltage is applied to the photoreceptor via the secondary charging device of the marking device. According to various exemplary embodiments, the charge voltage creates an electric field that sweeps out any remaining positive trapped charges from the photoreceptor more efficiently. During step S 130 , the charge voltage applied to the photoreceptor by the secondary charging device is applied simultaneously or near simultaneously to the erase exposure applied during step S 120 . According to various exemplary embodiments, the charge voltage applied by the secondary charging device during step S 130  is comparable in value to, and of the same sign as, the charge applied to the photoreceptor by the primary charging device during step S 110 . Thus, ghost removal is achieved when the erase light is applied during step S 120  and then the charge voltage is applied to the photoreceptor by the secondary charging device during step S 130  simultaneously or near-simultaneously to the application of the erase light. Next, control continues to step S 140 , where method ends. 
   It will be appreciated that various of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also, various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art, and are also intended to be encompassed by the following claims.

Technology Category: 3