Digital printer using two-dimensional, full frame light valve

Abstract: The present invention is a digital printer that comprises the novel combination of a two-dimensional light valve (34), a backlight (36) that illuminates the entire two dimensional light valve, a driver (32) coupled to the light valve (34), the backlight (36) and an outside source (33) of digital image data, an imaging system (38), and a photoreceptor belt (40). The digital data representing the desired image is fed into the driver (32) from an outside source (33). The light valve (34) is then set according to the digital data by the driver. Once the light valve is set, the driver signals the backlight to illuminate the light valve in the manner of a flash exposure. The image (42) produced is then focused by an imaging system onto the surface of a photoreceptor belt (40) - thus exposing and capturing a latent image. In another aspect of the present invention, very compact lenslet arrays are employed in lieu of traditional optics. This reduces the overall space requirements of the presently claimed digital printer - making it readily suitable for desktop application.

Description: The present invention relates in general to digital printers and, in particular, to digital printers that employ two-dimensional, full frame flash exposure of the photoreceptor surface. The demand for digital printing has swelled with the advent of computing. The development of low-cost, high-speed computing has fundamentally impacted on the way users make documents. Wordprocessing and graphics programs have greatly eased the time and amount of effort required for document creation. All of these electronic documents are stored digitally in the computer until the user desires a hardcopy of the document. Because the vast majority of created documents are stored in digital form, it was only natural to devise a method of printing hardcopy directly from digital data. One current means of digital printing is laser xerographic printing. Digital data is supplied to the laser printer which is used to modulate the laser beam. The beam is raster scanned onto a photoreceptor surface in a line-by-line fashion. As the laser beam sweeps across the surface in the "scan direction", the photoreceptor moves in a perpendicular direction (the "process direction"). Although laser printing has been largely successful in the marketplace, the technique of raster scanning has some inherent limitations. One such limitation is speed. The top speed of a raster scanning printer is bounded by the maximum speed that a laser beam may be swept across the surface of a photoreceptor. A number of factors contribute to that upper bound, one such factor includes the top rotational speed of the mirror-faceted polygon which deflects the beam to produce the scan sweep. Currently, polygons driven by ball-bearing motors have top speeds of approximately 25,000 RPM. Polygons having air bearings have higher rotational rates yet; but their cost is also much higher than the ball-bearing version. As a result of this limitation, the current top throughput of a single beam laser printer is approximately 200 pages per minute. Another limitation of laser printing is the need for precision. Laser printing must be accomplished with exacting mechanical precision. Perfect synchronization between the photoreceptor and the raster scanned beam must be kept throughout the entire image writing process. The slightest deviation becomes noticeable to the human eye which is highly sensitive for spotting defects (e.g. deviations in "parallelness") in line after line of written text, or in graphic images and pictorials. These deviations occur chiefly due to imperfect control of the relative motion between the photoreceptor and the raster scanned beam. Relative motion control problems manifest themselves over the course of printing a page, which typically occurs in as little as one half of a second and with a relative displacement of up to 432mm (17 inches). Another method of digital printing that eliminates relative motion control problems has been described in: "Hybrid ER Copier", "Printer Modification for Flash Based Copier", "Color Printer Modification for Flash Based Copier", and "Fax Modification for Flash Based Copier", all by Thomas Hammond, published in Xerox Disclosure Journal (XDJ) Vol 16, No. 5, September/October 1991, at pages 309, 311, 305 and 307 respectively. Hammond's basic digital printer, as described in "Printer Modification for Flash Based Copier", uses a reflective (i.e. no backlighting), full frame, liquid crystal display (LCD) as a front end image projector for a flash based copier. Hammond proposes positioning the LCD over the platen of the copier in such a manner as to allow the LCD to provide the input image into the copier. To print digital data, Hammond first loads digital data into the LCD. Once the full LCD page is displayed, the copier would flash expose the LCD, thus exposing the copier's photoreceptor with the image data displayed on the LCD. In effect, the photoreceptor would be exposed with the same image data as if the original document were on the platen as opposed to the LCD. Figure 1 shows how the LCD would be configured atop the copier to produce the digital printer. As compared with laser printing, Hammond's digital printing method has no relative motion control problems. As will be discussed below, if the flash exposure is of sufficiently short duration, then there is no such problem. Hammond's method of digital printing allows for the decoupling of the functions of printing and copying. Hammond describes that, by removing the LCD from atop the platen, the copier may function as it usually does - as a stand alone copier. However, merely combining an LCD and a separate copier together to produce a printer is not effective from both a design and user point of view for several reasons. First, to achieve this effective decoupling, Hammond uses a reflective LCD (as opposed to a transmissive LCD) to provide image data input. The copier then flash exposes the LCD whenever the user commands. To copy another full frame image, the user would wait for the next image to be set in the LCD before the copier is allowed to flash expose. Thus, without a tighter coupling of the LCD to the the copier, printer throughput suffers. Second, a separate, detachable LCD would need to be property registered every time it was reattached to the copier. This reattachment and registration requires that the user invest additional time in the proper usage of Hammond's digital printer. US 5,128,773 discloses a video printer for receiving a video signal of an image to record a visible image thereof on a photosensitive recording medium. A liquid crystal panel is tightly fixed onto the photosensitive recording medium so that the liquid crystal panel is driven by the video signal to display an image. US 5,122,831 describes a photographic printer having a color liquid crystal panel disposed between a light source and a photographic film. The color liquid crystal panel is used as light amount controlling means, a shutter, or as both of them. The light transmittance of each cell is individually adjusted to correct the exposure unevenness and/or adjust the gradation. Thus, there exists a need for a digital printer that tightly couples the function of the image generator with its printer engine. Additionally, there is a need for a digital printer that can synchronize the functions of the image generator with the printer engine at electronic speeds to increase printer throughput. It is thus an object of the present invention to provide a digital printer that couples and synchronizes the functions of image generating with printing. According to the present invention, there is provided a digital printer in accordance with claim 1 of the accompanying claims. In summary, the present invention is a digital printer that comprises the novel combination of a two-dimensional light valve, a backlight that illuminates the entire two dimensional light valve, a driver coupled to the light valve, the backlight and an outside source of digital image data, an imaging system, and a photoreceptor belt. The digital data representing the desired image is fed into the driver from an outside source. The light valve is then set according to the digital data by the driver. Once the light valve is set, the driver signals the backlight to illuminate the light valve in the manner of a flash exposure. The image produced is then focused by an imaging system onto the surface of a photoreceptor belt - thus exposing and capturing a latent image. In another aspect of the present invention, very compact lenslet arrays are employed in lieu of traditional optics. This reduces the overall space requirements of the presently claimed digital printer - making it readily suitable for desktop application. One advantage of the present invention is speed. Because the light valve is set according to electrical signals representing the digital image data, loading the light valve involves only a very short period of time. Similarly, flash exposing the image onto the photoreceptor involves a second, very short time period. Thus, the time for capturing a latent image from digital data with the present invention compares favorably with time required to capture a full-page latent image with a raster scanning printer. Likewise, the coupling of the LCD to the flash exposure allows for synchronous activity between the LCD and the backlight at electronic speeds. Thus, higher page throughput may be achieved than user-demand printing system that are not similarly synchronized. Another advantage of the present invention is absence of relative motion effects. The entire, full-page image is transferred in such a small time period to the belt so there is no problem with mechanical imprecision, such as found in a raster scanner that must maintain precision line-after-line. The purpose and advantages of the present invention will be apparent to those skilled in the art from the following detailed description in conjunction with the drawings, in which:- Figure 1 is a perspective view of the digital printer as described in the aforementioned XDJ article by Hammond. Figure 2 is a perspective view of the components comprising the present invention. Figure 3A-3B are cross sectional views of two separate embodiments of the backlight used in the present invention. Figure 4 is an alternative embodiment of the present invention employing an alternative imaging system provided by a lenslet array. Figure 5 is cross sectional view of a commercially available embodiment of lenslet array suitable for use in the present invention. Referring to Figure 1, a perspective view is shown of the digital printer 10 as described in the XDJ article by Hammond. Hammond describes a digital printer which comprises a stand-alone, reflective LCD 14 and a stand-alone, flash expose copier 12. To construct Hammond's digital printer, the LCD 14 is placed atop the platen 16 of the copier 12 such that the image created by flash exposing the reflective LCD is focused onto the surface of the photoreceptor (not shown). The copier produces hard copy (e.g. paper) output 18 from images displayed on the reflective LCD. Because Hammond allows for decoupling the print and copy functions of this configuration, the reflective LCD may be removed and reattached from the copier. Thus, it is important to properly re-register the LCD every time the LCD is reattached. Otherwise, print defects will show in the final hard copy (e.g. noticeable black borders, print lines not parallel with edges of the paper, etc.). Another drawback arises because of this decoupling - namely, asynchronous functioning of the LCD and the copier. Because the LCD is autonomous from the copier, the LCD must be set before the copier may flash expose the LCD (presumably in response to user demand). Without synchronous functioning, the throughput of the digital copier combination as described in Hammond may not be as fast as one that has a fully coupled and integrated digital LCD display with the copier. Another drawback not addressed by Hammond is space design. Hammond proposes using any commercially available flash exposure copier in combination with a reflective LCD by merely setting the LCD atop the platen of the copier. This does not necessarily lead to the optimal design from the standpoint of space requirements. The ordinary optics found in current copiers may require as much as a metre (three feet) to accommodate the optical path. By using high performance lenslet arrays, as described below, the present invention may use as little as 80mm (three inches) for the optical path. This allows the present invention to be small enough to practically sit on a desktop. Figure 2 is a perspective view of the components comprising the present invention. The presently claimed digital printer 30 comprises a transmissive LCD light valve 34, a uniform, full frame backlight 36, a driver 32, an imaging system 38, and a photoreceptor belt 40. As shown, driver 32 is coupled to both backlight 36 and light valve 32. The driver is the unit that synchronizes the actions of the light valve, backlight and any outside sources 33 of digital data. Driver 32 accepts digital image data from an outside source 33 such as a storage area (not shown), another computer (likewise not shown) or the like. Driver 32 then sends the digital data to light valve 34 which in turn forms a pattern according to the data. After the light valve 34 is set, driver 32 signals backlight 36 to illuminate the light valve. The light from the backlight 36 is a brief, intense illumination; thus, "flash" exposing the photoreceptor 40, whose belt is moving in the direction of arrow B. The image exposed on the photoreceptor is depicted as the image 42. It will be appreciated that the design of a suitable driver to accomplish the above functions is well known to those skilled in the art. It will also be appreciated that while the present invention contemplates the use of any light valve and backlighting, it is desirable to use light valves or displays of the highest resolution possible. One such suitable light valve is the 330mm (13 inch) diagonal, 6.3 million pixel light valve developed by the Xerox Corporation. Likewise, it will be appreciated that the backlight 36 and light valve 34 may operate through separate drivers. The only requirement is that the drivers for the backlight and the light valve should be suitably synchronized such that the illumination of the light valve occurs after the setting of the light valve. Figure 3A and 3B depict two separate embodiments of the backlight suitable for the purposes of the present invention. The only requirement common to any potential backlight is that it provide uniform lighting with sufficient illumination to expose the photoreceptor. Figure 3A shows, in cross section, the first embodiment of backlight 36. Backlight 36 consists of a number of lighting elements 54, each disposed within a cavity defined by reflector 52. In order to avoid a "hot spot" (i.e. a place where the light intensity is on average greater than other spots), filter 56 may be disposed directly between the lighting element 54 and the opening of the cavity. The filter 56 may itself be either a diffuser, an absorber, or a reflector (either partial or total). For any given particular application, the particular composition of the filter may be set in order to avoid the creation of a hot spot directly underneath the lighting element 54. Figure 3B is a cross sectional view of another backlight embodiment. Backlight 36 in Figure 3B is an edge-lit backlight that uses a solid, transparent dielectric light guide 66 that confines the light rays (emanating from lighting element 62 and reflected by reflector 64) by internal reflection. Light may be reflected by the guide 66 by a variety of mechanisms, such as narrowing the guide (which causes the light rays to diverge and escape), the controlled roughing of the surface of the guide as a function of distance from the edge light, or the incorporation of a prismatic structure in the surface. Generally, anything that disturbs the longitudinal symmetry of the structure will cause light to escape. Figure 3B depicts the narrowing of the guide and controlled roughing of the guide surface to cause uniform reflection. These and other techniques for edge-lit backlighting are well known in the art and are further discussed in the following articles which are hereby incorporated by reference: "23.1: Invited Address: Light Distribution Methods for Backlighting LCDs" by Whitehead et al, in SID 92 Digest at page 419; and "5.4: Bright and Thin LCD Backlights for Monochrome and Color LCDs", by Lu et al, in SID 93 Digest at page 33. It will be appreciated that for the purposes of the present invention, any backlight method that provides the necessary uniformity and sufficient intensity is suitable. Thus, the present invention should not be limited to the embodiment herein disclosed. The sufficiency of the light intensity may be calculated by knowing the sensitivity of the photoreceptor surface. Typically, the illumination required at the photoreceptor is 5-10 ergs/cm 2 . If the light valve is calculated at 30 per cent efficiency and the imaging system is .5 per cent efficient, then the exposure at the input to the light valve must be .667 x 10 -4 joules/cm 2 . For an 279 x 216mm (11 x 8.5 inch) exposure, .4 joules is required. For a flashlamp that has a coupling efficiency into the light valve of 1 per cent, the flashlamp should be about 40 joules, which is feasible in current technology. After the illumination of the digital pattern formed on the light valve, the resulting image passes through an imaging system 38 for focusing the image onto the surface of the photoreceptor 40. The exposed image is depicted in Figure 2 as image 42. It will be appreciated that the design and construction of a suitable imaging system (comprising a combination of lenses, mirrors, apertures, and the like) is well known to those skilled in the art. The imaging system focuses the image 42 onto the surface of the photoreceptor 40. In order to achieve high throughput, photoreceptor 40 may comprise a moving belt that is sufficiently long to hold multiple latent images and is processed by several stations (e.g. charging, imaging, toning, fusing, cleaning, and the like) simultaneously. The movement of the belt is indicated in Figure 2 by arrow B. As mentioned previously, the flash exposure of a full-page, two-dimensional image onto a photoreceptor is faster and eliminates the relative motion problem that arises with the use of raster scanning. The present invention, however, must account for one design consideration - blur. Blurring of the latent image occurs because the photoreceptor belt is moving. Since the time of the "flash" is non-zero, the belt 40 will have moved relative to image 42 over the course of the exposure; thus causing blur. It is generally accepted that one pixel width of blur is within adequate tolerances for human readability. To design the present invention within one pixel width of blur, the flash exposure time and the speed of the photoreceptor belt is adjusted accordingly. For example, supposing that a pixel has the width of 0.041mm (1.6 x 10 -3 inches), it can be shown then that for a flash exposure time of 60 µsecs allows for a photoreceptor belt speed of 160 pages per minute (assuming that 254mm (10 inches) are needed to define the length of a given page image on the belt). Likewise, it can be shown that a flash exposure time of 30 µsecs allows for printing speeds of 320 pages per minute. The flash exposure time is a function of the frame rate of the light valve. The faster the light valve can be patterned and exposed, the faster the belt can be moved to stay within the one pixel of blur tolerance. It will be appreciated that these sample throughput rates are much higher than the current throughput for raster scanning printers. Figure 4 is an alternative embodiment 70 of the present invention. An alternative imaging means is provided by a lenslet array 72. The advantage of using a lenslet array instead of a conventional imaging means is reduced space requirements. While the optical path required for the more conventional design of Figure 2 might take up a metre (3 feet), the present invention as embodied in Figure 3 may require only 80mm (3 inches) - a significant reduction in design space. It will be appreciated that lenslet arrays are well known to those skilled in the art and are commercially available. Figure 5 is a cross sectional view of a commercial available embodiment of the lenslet array, as depicted as array 72 in Figure 4. Figure 5 shows the cross section of a lenslet array 72 as commercially produced by Corning under the trademark SMILE. Lenslet array 72 is fabricated by subjecting a light-sensitive glass material to a photothermal process. The glass is exposed to ultraviolet light through a photomask which is then thermally treated, causing the area exposed to UV light to undergo crystallization and densification. During crystallization, the exposed glass physically contracts in three dimensions, forcing the unexposed glass to erupt into spherical shaped lenses 74. In addition to forming the lenses, the thermal process renders the area 76 surrounding the lenses optically opaque. This particular embodiment is described in the article entitled "Photothermal Technique Generates Lens Arrays", by Baranowski et al, published in the November 1989 edition of Laser World Focus magazine. A similarly suitable embodiment is also commercially available through Nippon Sheet Glass under the trademark SELFOC. In summary, the present invention is a digital printer comprising the novel combination of a two-dimensional, transmissive light valve, a two-dimensional backlight to flash expose a photoreceptor, a driver to synchronize the functions of the light valve and the backlight, an imaging system, and a photoreceptor. In another aspect of the present invention, the imaging system could comprise a compact lenslet array in lieu of a more traditional optical system. Use of the lenslet array reduces the overall space requirement for the design of the presently claimed digital printer. Because the activities of the light valve and the backlight are synchronized at electronic speeds, the printer throughput is much higher than a digital printer that decouple the functions of LCD light valve and copying. Additionally, the registration of the light valve is constantly set in the present invention as opposed to any digital printer combination that has a separate stand-alone LCD display and a separate stand-alone copier.

Claims: A digital printer for printing images in accordance with digital image data input from an outside source (33), said digital printer comprising: a two dimensional backlight (36) capable of producing full frame illumination; a photoreceptor (40) sensitive to light emanating from said backlight; a two-dimensional, transmissive light valve (34) disposed between said backlight and said photoreceptor so that light from said backlight is transmitted through said light valve to produce an image on said photoreceptor, said image corresponding to the pattern formed on said light valve and said light valve being responsive to said input digital data such that said light valve displays said pattern according to said digital data; a driver (32) coupled to said light valve (34), said backlight (36) and said outside source (33) such that said driver accepts digital data from said outside source, sets said light valve in accordance with said digital data, and synchronizes said backlight with said light valve such that said backlight flash exposes the photoreceptor after said light valve is set according to said digital data characterized in that an imaging system (38) is disposed between said light valve (34) and said photoreceptor (40), such that said imaging system produces said corresponding image (42) on said photoreceptor; and wherein said digital data are digital image data providing a full-frame image on said light valve, said light valve being arranged in a permanently fixed position in the digital printer, and the digital printer being adapted to produce said image on said photoreceptor in a single flash, while said photoreceptor is moving. The digital printer as recited in claim 1, wherein said driver (32) further comprises: a light valve driver such that said light valve is responsive to said digital data transmitted by said light valve driver; a backlight driver such that said backlight flash exposes in response to a driver signal sent by said backlight driver; and a means for synchronizing said light valve driver and said backlight driver such that said backlight flash exposes only when said light valve driver has set said light valve according to said digital data. The digital printer as recited in claim 1 or claim 2, wherein said imaging system comprises a lenslet array (72). The digital printer as recited in claim 3, wherein said lenslet array (72) comprises an array of spherical lenses (74) surrounded by areas (76) of opaque material. A digital printer as recited in claim 1, wherein said backlight (36) produces light for a period of time such that the blur of said image on said photoreceptor belt is within a predefined tolerance.

Inventors: FEDERICO ANTHONY M, TURNER WILLIAM D, VALENTINE WILLIAM L, FEDERICO, ANTHONY M., TURNER, WILLIAM D., VALENTINE, WILLIAM L.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1995
Main classification: H04N1
IPC codes: B41J2, G06K15, H04N1, G03G15