Electrophotographic method

Abstract: A reproduction device forms a two-color copy in a single-pass mode. An original document (30), bearing fluorescent material on selected portions thereof, is illuminated by a light source (39). The light reflected from the document is transmitted through an colored filter (45) and is projected onto the surface of a monopolar photoreceptor (12). Light incident on the fluorescent material is absorbed over a specific wavelength range and is re-emitted at a different wavelength range. This light, and light reflected from the white background, are transmitted through the filter of a color associated with the re-emitted radiation. Light reaching the photoreceptor discharges charged areas thereon at two energy levels. The resulting latent image incorporates three separate discharge levels, corresponding to the dark image information, colored fluorescent areas, and background areas. The dark and colored areas are developed with appropriate colored toner by developer units (55, 56) biased at the appropriate levels.

Description: This invention relates generally to an electrophotographic reproduction method, and, more particularly, to a reproduction method in which information is reproduced in two different colors (of which one may be black). In a business office environment it is frequently desirable to reproduce at high speeds original documents containing highlighted portions. Typically the original document will have information in red and black. It is well known to produce two-color output copies using a multi-pass system, a present commercial example being the Canon 3625 copier. For this type of system, an operator utilizes a mimic-type electronic edit pad to delineate areas of an original it is desired to highlight. The coordinates of a selected area are entered into machine memory. During a first exposure cycle all areas on the photoreceptor are erased save the selected highlight color area, which is then subsequently developed with the appropriate color toner. The resulting image is transferred to a copy sheet, fused and returned to the developer station entrance zone where it is re-registered. A second exposure of the original is made and the highlight area only is erased. The resulting latent image is developed with conventional black toner, and transferred to the copy sheet which is then fused and the copy sheet conveyed to an output tray. The type of system exemplified by the Canon 3625 has several disadvantages; it requires an expensive electronic component, the edit pad as well as additional memory. Registration following the first exposure is difficult to achieve. Since the system is two-cycle (two-pass) the productivity is limited. It is therefore highly desirable for an electrophotographic reproduction machine to reproduce both the black and the color (red for most highlighting purposes) information in a single pass. By 'single-pass' is meant that a composite electrostatic latent image having regions corresponding to the red information and black information is recorded on the photoconductive surface. This composite electrostatic latent image is developed with black and red toner particles to produce a two-color toner powder image. This two-color powder image is subsequently transferred to the copy sheet and permanently affixed thereto. In this way, a highlighted color copy of the original document may be readily produced at relatively high speeds, automatically and in perfect registration. Numerous other approaches have been devised for producing highlight color copies. US-A-3,832,170 describes a photosensitive member having an insulating layer acting as a color filter. The photoconductive drum is divided into three segments, each segment corresponding to a different-colored electrostatic latent image. These latent images are then developed by toner particles complementary in color thereto. The toner powder images are then transferred to a copy sheet in superimposed registration to form a multicolor copy corresponding to the original document. US-A-4 078 929 discloses a reproduction machine which can form a two-color copy of an original document either using conventional light lens exposure techniques, or electronically. A charge pattern of a single polarity and having at least three different levels of potential is formed on a photoreceptor and developed in two colors by utilizing relatively negatively charged toner particles of one color and relatively positively charged toner particles of a second color. The exposure system requires the use of black and white images on an original document having a intermediate (grey) color. US-A-4 189 224 discloses a photoconductive drum formed with first and second photoconductive layers of different spectral sensitivities. The photoconductive drum is charged and exposed causing electrostatic latent images to be formed on the respective layers according to the color within the original document. The charges of the latent images are of opposite polarity. Toner particles, similarly of opposite polarity, are used to develop the respective latent images. The toner particles are of different colors. In this way, a two-color copy is formed. US-A-4 264 185 describes an electrophotographic printing machine employing a photoconductive drum formed with at least two photoconductive layers of different spectral sensitivities. One layer may be panchromatic, with the other layer being insensitive to red light. The drum is charged, at least twice, with opposite polarities to produce the charge pattern. A light image of the original document then exposes the charged regions of the drum. This results in positive and negative electrostatic latent images being recorded thereon. The latent images are developed with black and red toner particles of opposite polarity to form a two-color copy. US-A-4 335 194, discloses a photoconductive member comprising a red-sensitive photoconductive layer and a red-insensitive photoconductive layer. Two colors are printed by charging and exposing to white light, irradiating with red light and charging to an opposite polarity, charging to the same polarity as an opposite polarity, charging to the same polarity as the first polarity, and developing with red and black toners of opposite polarity. US-A-4 509 850 teaches an electrophotographic printing machine capable of reproducing both black information and red information in a single pass. A continuously charged area and a modulated charged area are recorded on to a photoconductive surface. The modulated charged area is developed with polar or polarizable marking particles of a first color, while the continuously charged area is developed with charged marking particles of a second color. US-A-4 479 242 discloses a dichromatic reading device capable of separating a specific color from other colors of an original document. An electric circuit is provided which obtains the difference between the level of an image signal readout without a filter, and the level of a signal readout through a complementary filter for the specific color. Subsequently, an image signal for the specific color is produced from the difference. US-A-4 068 938 teaches an electrophotographic printing machine capable of reproducing two-color copies from a two-color original document. An electrostatic latent image having three discrete potential levels is recorded onto a photoconductive drum. A high level is developed by particles of a first color corresponding to a dark color of the original document, and a low level is developed by particles of a second color. The underdeveloped portion remains the color of the sheet of support material. US-A-4 562 129 discloses a bipolar photoreceptor which permits three different potential levels to be formed thereon. From the discussion above it can be seen that those systems which utilize a single-pass highlight color system require the formation of separate levels of photoconductor discharge. One level (high) corresponds to black information, an intermediate level corresponds to white background, and a third level (low) corresponds to the highlight color (red). The majority of the references cited above disclose some kind of bi-polar photoreceptor comprising multiple layers, each layer sensitive to a different color. The black and red discharge areas are then developed by developer units biased to appropriate levels. It would be desirable to provide a single-pass color highlight system which does not require a bi-polar photoreceptor. It would also be desirable for the system to utilize a conventional light lens scanning system to make highlight color copies of originals having normal red and black images on a white background (unlike, for example, US-A-4,078,929 which makes two-color copies either from a CRT display or from an original with a grey background). According to the present invention a multi-level electrostatic potential image is derived from an original which has been modified by applying a fluorescent dye or pigment of a desired color and in a selected pattern onto a black and white original document. For a preferred embodiment a red fluorescent pigment is applied to the original. The original is illuminated by a lamp source with a white light emission, and a red filter is placed in the imaging path. Light reflected from the original document and passing through the filter will create a three charge level pattern at the photoreceptor. Light reflected from black areas will leave undischarged areas (high level); light reflected from white background areas will result in a lower discharge level. Light incident on the fluorescent pigment excites the pigment causing the low level incident light to be re-emitted as red light. This, plus the red reflected in the normal manner, generates a higher apparent reflectivity than the white paper, (a "whiter-than-white" effect). This red-enhanced emission discharges the photoreceptor to a third level lower than the white background level. This three-level charge pattern is then developed and an output two-color copy is produced by known xerographic techniques The present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic elevational view showing an electrophotographic printing machine incorporating the highlight color system of the present invention; Figure 2 represents an original document, showing an area formed by application of a fluorescent dye or pigment of a selected color; Figure 3 shows a plot of lamp emission output over a selected wavelength range; Figures 4, 4a, 4b, 4c show light absorbance and reflection from the black, white background and fluorescent areas of the original document of Figure 2; Figures 5a, Sb and Sc show the relative emission level over the selected wavelength range; Figure 6 shows relative emission over the selected wavelength after a filtering step, and Figure 7 shows three separate discharge levels at the photoreceptor following a document scan cycle. Figure 1 illustrates a preferred embodiment of an imaging system which is used to produce a two-color ouput copy, in a single-cycle scan (pass) of an original document Turning now to Figure 1, the electrophotographic printing machine uses a monopolar photoreceptor belt 10 having a photoconductive surface 12 formed on a conductive substrate. Belt 10 moves in the indicated direction, advancing sequentially through the various xerographic process stations. The belt is entrained about a drive roller 14 and two tension rollers. In operation, the operator may select the number of copies being reproduced as well as adjusting any of the parameters within the various processing stations. This is achieved by keying in the required adjustment and number of copies at the control panel 16. Control panel 16 is electrically coupled to a centralized processing unit 18, preferably a microprocessor made by Intel Corporation under Model No. 8086. CPU 18 is electrically connected to the various processing stations within the electrophotographic printing machine so as to control their operation. A portion of belt 10 passes through charging station A where a corona-generating device 22 charges photoconductive surface 12 to a relatively-high, substantially-uniform, negative potential. Next, the charged portion of photoconductive surface 12 is advanced through an imaging station B including an exposure system, 24. An original document 30, modified according to one aspect of the invention, is positioned face down on a transparent platen 34. An example of an original document 30 is shown in Figure 2. The document has black informational text areas 30A, white background areas 308, and a second informational area 30C formed, in this embodiment, by applying a red fluorescent pigment through a stencil. At development station C, a magnetic brush development system 54 advances developer materials into contact with the electrostatic latent images. The development system 54 comprises first and second developer stations including housings 55 and 56. Preferably, each magnetic brush development housing includes a pair of magnetic brush developer rollers. Thus, the housing 55 contains a pair of rollers 57, 58, while the housing 56 contains a pair of magnetic brush rollers 59, 60. Each pair of rollers advances its respective developer material into contact with the latent image. Each developer roller pair forms a brush-like structure comprising toner particles which are attracted therefrom by the latent electrostatic images on the photoreceptor. For the sake of illustration, housing 55 contains developer with black toner having triboelectric properties such that the toner is driven to the most highly charged areas of the latent image. Housing 56 contains developer with colored (red) toner having triboelectric charge properties such that the toner is urged towards parts of the latent image area having a charge representative of the area correspondly to the area of the original covered with the red pigment Appropriate developer biasing is accomplished via programmable power controls 61 and 62 electrically connected to respective developer housings 55 and 60 and to CPU 18. An appropriate program stored in fixed memory of the controller and, applied through a digital-to-analog converter (not shown), will cause the developer rolls, at the appropriate times, to rotate in one direction to effect image development, or in the opposite direction for causing the developer to cease contact with the photoreceptor. Optics assembly 35 contains the optical components which incrementally scan-illuminate the document from left to right and project a reflected image onto surface 12 of belt 10, forming a latent image of the document thereon. Shown schematically, these optical components comprise an illumination lamp assembly 38, comprising an elongated fluorescent lamp 39 and associated reflector 40. (A fluorescent lamp is used for its white-light emission characteristics; its fluorescent properties do not have any specific relation with the fluorescing pigment. For example, for a flash system a xenon flash lamp provides a white-light emission). Assembly 38 and full-rate scan mirror 42 are mounted on a scan carriage (not shown) adapted to travel along a path parallel to, and beneath, the platen. Lamp 39, in conjunction with reflector 40, illuminates an incremental line portion of document 30. The reflected image is reflected by scan mirror 42 to corner mirror assembly 46 which is adapted to move at half the rate of carriage mirror 42. The document image is projected along optical path OP through a filter 45, and then through lens 47. For this embodiment, where informational area 30C is a red fluorescent dye, filter 45 will also be red. The image is then reflected by a second corner mirror assembly 48 and by belt mirror 50, onto surface 12 to form thereon an electrostatic latent image corresponding to the information areas contained within original document 30. The latent image is formed with three separate discharge levels as will be discussed below. After development, belt 10 advances the red and black toner powder image to the transfer station D At the transfer station, a sheet of support material is moved into contact with the powder image. The sheet of support material is advanced to transfer station D by a sheet-feeder 63. Preferably, sheet feeding apparatus 63 includes a feed roll 64 contacting the uppermost sheet of a stack of sheets. Feed roll 64 rotates in the direction of the arrow so as to advance the uppermost sheet into the nip defined byforwarding rollers 68. Forwarding rollers 68 rotate in the direction of arrow 70 to transport the sheet into contact with photoconductive surface 12 of belt 10 so that the toner powder image developed thereon contacts the advancing sheet at the transfer station. Because the composite image developed on the photoreceptor consists of both positive and negative toner, a pre-transfer corona discharge member 74 is provided to condition the toner for effective transfer to a substrate using corona discharge. After transfer, the sheet continues to move on to fusing station E. Fusing station E includes a fuser assembly 80 which permanently affixes the transferred toner powder image to the sheet. Preferably, fuser assembly 80 includes a heated fuser roller 82 and a back-up roller 84. The sheet passes between fuser roller 82 and back-up roller 84, with the powder image contacting fuser roller 82. In this manner, the powder image is permanently affixed to the sheet. After fusing, forwarding rollers 86 advance the sheet to a catch tray (not shown) for subsequent removal from the copier by the operator. After the red and black powder image is transferred from photoconductive surface 12 to the copy sheet, belt 10 rotates the photoconductive surface to cleaning station F. At the cleaning station a brush removes the residual particles adhering to photoconductive surface 12. A discharge lamp 92 is activated prior to charge. The formation of the three-charge level latent image at the photoreceptor is now described with reference to Figures 3-6. As mentioned above, lamp 39 is a fluorescent lamp having a characteristic, generally-white emission. Thus, the emission from lamp 39 can be considered as comprising components of light in the blue,green and red wavelengths. Figure 3 shows a plot of the emission level over the wavelength range of 400-700nm. This output from lamp 39 illuminates original document 30, and a reflected image of the document is incrementally transmitted, during a scan mode, along optical path OP. The reflected image at, for example point R, of figure 1, has three separate light components which have been formed as described with reference to Figures 4, 5 and 6. Figures 4A, 4B and 4C are simplified schematic representations showing how the light from lamp 39 is reflected from areas 30A, 30B and 30C respectively of Figure 2. Figures 5A, 5B and 5C show, respectively, relative light emission outputs W over the selected wavelength. Referring then to Figure 4A, light incident on areas 30A of Figure 2, is completely absorbed, since black absorbs all wavelengths. No light reaches filter 45 and hence, as shown in Figure 5A, the level of light reflected from area 30A is close to zero. Figure 4B shows that light incident on the white background areas 308 of the document is nearly uniformly reflected, since the white background reflects all wavelengths. Thus the light reaching filter 45 will have a light level (Figure 5B) approximately equal to the lamp exposure levels of Figure 3. Figure 46 shows the light reflected from red fluorescent area 30C of Figure 2 The red component of the incident white light is reflected as red light. However, the blue and green components are absorbed, and by virtue of the fluorescing properties of the dye, are re-emitted as light of a higher wavelength e.g. as red light. Thus, the light reaching filter 45 will be red (wavelength range of 600-700 nm) and have a level shown in Figure 5C. As the reflected image passes through the red filter 45, the red filter transmits only the light in the red area, resulting in the plots shown in Figure 6. Figure 6a, 6b, and 6c show the relative emission output, over the selected wavelengths, following a filtering slip. From Figure 4A no light was reflected from the black areas; hence no light is transmitted in the red wavelength of interest, as shown in Figure 6A. From Figure 4B only the red component of the light reflected from white background areas passes through filter 45. Thus, the level of light passing through the filter, shown in Figure 6B, is reduced from thereflected level shown in Figure 5B by the lower wavelength component (400-600 nm) blocked by the red filter. From Figure 46, it is seen that, in addition to the red component of light reflected from the red fluorescent area 306, the light components converted from blue and green into red also pass through filter 45. Thus, as shown by Figure 6C the level of light is greater than the reflected light level of Figure 6B. The resulting photoconductor discharge for the red image, white background and black image portions of the original document are shown in Figure 7. It will be appreciated that, with appropriate setting of the bias levels of developers 55 and 56, developer 55 will develop the black area and developer 56 will develop the red area. It will be appreciated that other color fluorescing materials can be used instead of the red in the preferred embodiment. One skilled in the art may match the absorption and re-emission characteristics of a fluorescent dye material with the emission characteristics of the light source, and select the appropriate color filter The fluorescent material may also be used to highlight black informational areas of a document selectively. For example, if the red fluorescent pigment is applied to the body of the memo shown in Figure 2, that area will be reproduced as black text on red background in the output copy. While the original in the preferred embodiment was modified by applying the fluorescent material manually, other methods are possible for depositing the material at the desired location. As one example, a modified original may be created by forming a latent image original and developing selected areas with a fluorescent impregnated toner. Alternatively, an ink jet printer can be modified to print an output copy using black and a colored fluorescent ink. With either method, or other methods, the resultant copy will then be used as the original for purposes of the invention.

Claims: A method for producing a two-color copy of an original document (30) in a single pass, including the steps of: creating an original document having dark image areas on a pale background, and at least one area covered with fluorescent material which absorbs radiation within a first wavelength range and reëmits radiation over a second and different wavelength range; illuminating the document from a source (39) which emits radiation at a wavelength within the first range; passing the radiation reflected from the document and the radiation reëmitted by the fluorescent material through a filter (45) so as to transmit radiation within the second range, while blocking radiation within the first range; projecting the filtered radiation on to the surface of a monopolar photoconductor (12) to form a latent electrostatic image thereon, the latent image having a first discharge level corresponding to the dark areas of the original; a second discharge level corresponding to the colored fluorescent areas of the original, and a third discharge level corresponding to the background areas of the original; developing the latent image with developer of two colors (55, 56) to cause development of the first and second discharge levels with two toners of contrasting colors; transferring the two-colored developed image to a copy medium (63), and fusing the transferred image (80) on to the copy medium. A method as claimed in claim 1, wherein the fluorescent material is a red dye or pigment which absorbs radiation at wavelengths below 600 nm, and reëmits radiation over the range of 600 to 700 nm. A method as claimed in claim 1 or 2, wherein the fluorescent area is formed by a development process which includes a toner incorporating a fluorescent dye. A method as claimed in claim 1 or 2, wherein the or each fluorescent area is formed by an ink jet printing process using a colored fluorescent ink. A method as claimed in any preceding claim, wherein the fluorescent area is coterminous with the background area. A method as claimed in any of claims 1 to 4, wherein the fluorescent area covers the information area containing dark indicia.

Inventors: LEHMAN RICHARD F, REES JAMES D, LEHMAN, RICHARD F., REES, JAMES D.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1989
Main classification: G03G15
IPC codes: G03G15, G03G13