WO2000059207A1

WO2000059207A1 - Light distribution device for fiber optic-based imaging system

Info

Publication number: WO2000059207A1
Application number: PCT/US2000/007260
Authority: WO
Inventors: David J. Schoon
Original assignee: Schoonscan, Inc.
Priority date: 1999-03-25
Filing date: 2000-03-15
Publication date: 2000-10-05

Abstract

A digital color printer includes a device for distributing light from a light source to a number of different optical paths defined by fibers. The light distributing device includes a body that is rotatable about an axis, and that has a first face intersecting the axis. A light guide has an input end on the first face, approximately centered about the axis, and has at least one arm leading from the input end to a respective output port positioned on a perimeter edge of the body. At least one color filter is positioned at the at least one output port to filter light exiting from the at least one output port.

Description

LIGHT DISTRIBUTION DEVICE FOR FD3ER OPTIC-BASED IMAGING SYSTEM

FIELD OF THE INVENTION The invention is directed to digital color printers, and methods of forming images on photosensitive media and the like. More particularly, the invention is directed to a system and method of forming multiple pixel images simultaneously via many concurrently operating beams of light.

BACKGROUND OF THE INVENTION

Digital color printers, also known as imaging systems, form visually observable images on hard copy from electronic information. Examples include xerographic printers, ink jet printers, laser, LED and CRT imagers (including black and white or color, and imaging onto silver halide media), dye sublimation and wax transfer imagers, among others. With each type of system there is generally a computer file of electronic information which contains representations of photographic images, artwork, graphics and/or text, and there is a desire to obtain a paper or film hard copy from that data.

Electronic production and manipulation of images and text is highly efficient. It is becoming increasingly more common to store photographs as computer files rather than, or in addition to, pieces of film. The digital environment permits easy retouching and editing, addition of text and imposition of various photos into a layout. Moreover, in the case of color photographs, digital color lookup tables can compensate for deficiencies in the photographic media and in the exposure conditions. The existence of images in digital form creates a need for high quality imaging systems to create hard copies of these digital images.

Some imaging technologies require the use of light for the creation of a latent image on a xerographic drum or on silver halide media. One common way of doing this is to deflect a laser beam with a rotating polygon mirror. For exposure of color silver halide media, for example, three lasers are used, one each of (typically) red, green, and blue. A commonly sought objective is to obtain high imaging speed, for example more than two square feet per minute; high resolution, for example more than 400 continuous tone pixels/inch (more than about 160 pixels/cm); and large image size, for example images from rolls of paper of 20" (approx. 50 cm) or greater width. It is also desired to minimize the size of the equipment used to produce this image. However, speed, resolution, image size, and equipment size tend to be competing factors that must be balanced or compromised.

One particular application for imaging technologies is in point of sale advertisements or trade show displays, many of which may need to be as large as 50" x 100" (approximately 125 cm x 250 cm) or larger. In such cases, it is desired that the text be sharp, even at close viewing distances. It is also desired that the image be created in a short time, for example less than 10 minutes. These simultaneous objectives cannot be met or approached by conventional technologies. Another application is the "package printer" market which requires that photos, such as school portraits, be imaged at various sizes and with the addition of text and other graphics. To compete with other processes the imaging speed must be at least 0.25 lineal inch per second (approximately 0.6 cm per second), and text even as small as 4 point size must be clearly readable. Again, this is not currently possible with conventional technologies. Another application is the pre-press market, wherein proofs of information are desired in advance of the direct imaging of printing plates. The proofs should show true colors, should show the halftone dots, and they should be imaged quickly, in a few minutes or less. This is not possible with conventional technologies.

Many other applications exist for a digital color printer. Digital cameras are becoming available which bypass the use of film, but allow no alternative but that the hard copy be produced by a digital rather than film-based device.

Therefore, a substantial need has arisen for an imaging system offering high speed, high quality color, high resolution, large image size, small equipment size, and moderate equipment cost. This is not possible with ink jet technology, 3 -laser technology, CRT technology, xerographic technologies, LED technology, or other known conventional technologies. SUMMARY OF THE INVENTION

Generally, the present invention relates to color digital printers, particularly to devices for distributing light from a light source to a number of different positions. In one embodiment of the invention, an apparatus for sequentially distributing light to multiple destinations includes a body that is rotatable about an axis, and that has a first face intersecting the axis. A light guide has an input end on the first face, approximately centered about the axis, and has at least one arm leading from the input end to a respective output port positioned on a perimeter edge of the body. At least one color filter is positioned at the at least one output port to filter light exiting from the at least one output port.

In another embodiment, an apparatus for distributing light includes rotatable means for rotating about a first axis, light guiding means for guiding light from a first face of the rotatable means to at least one output port on a circumference of the rotatable means, and filter means for filtering light exiting from the at least one output port.

In another embodiment of the invention, an apparatus for printing color images from digital data includes a light source and a light distributing device having an input port coupled to receive light from light source. The light distributing device has at least two output ports provided with color filters to filter light passing therethrough from the light source. The light distributing device is rotatable about an axis. A plurality of fibers is optically coupled at input ends to receive light at regular intervals from the output ports as the light distributing device rotates about the axis. A plurality of light valves is coupled to receive light from the plurality of fibers and is controllable in response to which particular optical fibers are presently coupled to receive light from the light distributing device.

The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and detailed description that follow more particularly exemplify these embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which: FIG. 1 shows an embodiment of a light distribution device according to the present invention;

FIG. 2 shows a detailed view of the light distribution device of FIG. 1;

FIG. 3 shows an arrangement of stationary fibers surrounding the light distribution device; FIG. 4 illustrates locations of optical fibers in an interface between stationary fibers surrounding the light distribution device and individual light valves;

FIG. 5 shows the orientation and electroding of the light valves relative to the first ends of the optical fibers shown in FIG. 4;

FIG 6 shows an enlarged portion of FIG. 5; FIG. 7 shows a color digital printer in block schematic form;

FIG. 8 illustrates a paper path through a color digital printer;

FIG. 9 schematically illustrates one embodiment of a light distribution device coupled through optical fibers to light valve devices; and

FIG. 10 schematically illustrates one embodiment of control electronics to control operation of the light valves.

While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

A digital color printer described in U.S. Patent 5,684,620, incorporated herein by reference, shows one particular approach to accomplishing the characteristics of speed, color quality, resolution, image size, small equipment size and reasonable cost that are required of digital color printers. In general, the printer provides for simultaneous imaging of multiple pixels. Imaging is accomplished in modules, each module containing one array of solid state (for example a PLZT or Kerr Cell type) light valves, these light valves being on a single chip. The light produced by the array of light valves is imaged by a lens onto photographic paper or other light-sensitive media. A similar image is created by other, adjacent modules, and the modules are aligned to create a continuous band of image over the entire width of the photographic paper.

The printer 1200 is illustrated in schematic form in FIG. 7. The printer includes a control module, 1201, which includes a light source, and a mechanism for distributing light from the light source sequentially into stationary fiber bundles. Bundles of fibers are illustrated as conduits 1220, 1221, 1222, and 1223. Each imaging module 1202, 1204, 1206, and 1208, includes a light valve chip coupled to respective conduits 1220, 1221, 1222, and 1223, and includes an associated projection lens system 1224a-1224d. The projection lens system may include one or more separate lenses. Each module 1202, 1204, 1206, and 1208, projects an image onto the photographic paper in respective imaging regions 1203, 1205, 1207, and 1209. The imaging regions 1203, 1205, 1207, and 1209 at least abut each other, and may be aligned to slightly overlap. Only four imaging modules are shown, but any number may be used in a particular application. The optical arrangement of imaging modules and photosensitive paper may be better understood by looking at the paper path, illustrated in FIG. 8. Photosensitive paper 1310 is typically provided in lengths of 275 or 575 feet (approximately 84 m or 175 m) on a supply roll, 1301. A dancer roller, 1303, applies tension to the paper 1310 and is free to move from side to side to maintain constant paper tension. A slowly rotating capstan, 1305, controls the slow movement of the paper, typically at about 0.30"/second (approximately 0.76 cm/s) during imaging. Another dancer roller, 1304, also acts to maintain constant tension on the paper 1310. The paper 1310 is taken up by the takeup roll, 1302. The imaging modules 1202, 1204, 1206, and 1208 project a continuous line of image across the sheet of paper 1310 at the imaging region 1306.

The distribution of light from the light source to the imaging modules 1208 is described with reference to FIG. 9, which shows a simplified arrangement with only a single imaging module 1208. The imaging module 1208 includes a number of light valve cells 1220 for controlling the amount of light passing to the photosensitive paper 1310. Light 1230 is directed from the light source 1232 to a light distribution device 1234 that distributes the light sequentially to a number of distribution fibers 1236a - 1236z. Each distribution fiber 1236a - 1236z is coupled to a group of N pixel fibers 1238, where N is the number of light valve cells 1220 in the imaging module 1208. Each pixel fiber 1238 within in one group is coupled to a different one of the N light valve cells 1220, therefore, there is a different light valve cell 1220 associated with each pixel fiber 1238. The output ends 1240 of the pixel fibers 1238, at the opposite ends from the coupling to the distribution fibers 1236, are coupled to respective light valve cells 1220. The output ends 1240 may be arranged in a specific order, for example the output ends 1240 of the pixel fibers 1238 leading from the first distribution fiber 1236a are positioned first on the light valve cell 1220, followed by the output ends 1240 of the pixel fibers 1238 leading from the second distribution fiber 1236b, and so on. The last output end 1240 positioned on the light valve cell 1220 leads from the last distribution fiber 1236z. Typically, there are M distribution fibers 1236 coupled to the distribution device 1234, each coupled to direct light to M different positions of the light valve cells 1220, where each position of the light valve cell 1220 corresponds to a printed pixel. The distribution of light operates in the following manner. The light source

1232 transmits light to the distribution device 1234. At the point in time when the distribution device 1234 distributes light into the first distribution fiber 1236a, all the first positions, marked A, on the light valve cells 1220 are illuminated. At this time, each of the light valve cells 1220 is controlled to pass or block light for the pixels on the photosensitive paper corresponding to the first positions A of the light valve cells 1220. A short time later, the distribution device 1234 distributes light to the second distribution fiber 1236b, and so the positions B on the light valve cells 1220 are illuminated. At this time, each of light valve cells 1220 is controlled to pass or block light for the pixels corresponding to the second positions B of the light valve cells 1220. This process is repeated up to the time when the distribution device 1234 distributes light to the last distribution fiber 1236z, at which time the last positions Z on the light valve cells 1220 are illuminated. At this time, each of the light valve cells 1220 is controlled to pass or block light for the pixels corresponding to the last positions Z of the light valve cells 1220. The structure of the light valve cells 1220 is described hereinbelow.

After the distribution device 1234 has distributed light to the last position Z, it may return to distributing light at position A to the first distribution fiber 1236a, or it may continue to distribute light at other positions, for example at positions AA through AZ, and B A through BZ, to other distribution fibers associated with other imaging modules. The distribution device 1234 may be configured to distribute light to more than one distribution fiber 1236 at any one time. For example, it may be configured to simultaneously distribute light to three different distribution fibers at positions A, AA, and BA, respectively, followed by B, AB, BB, and so on until it reaches positions Z, AZ, and BZ.

Furthermore, there may be more than one distribution fiber located at each position of the distribution device 1234. For example, there may be two or more distribution fibers located at each of positions A-Z of the distribution device 1234, where the first set of distribution fibers 1236 feeds light to a first imaging module and a second set of distribution fibers feeds light to a second imaging module operating in parallel and simultaneously with the first imaging module.

It should be appreciated that the input ends of the pixel fibers 1238 may be bundled and positioned around the distribution device 1234 directly, without the need for the distribution fibers 1236. This avoids the optical losses encountered at the coupling between the distribution fibers 1236 and the pixel fibers 1238. On the other hand, this approach requires the placement of a large number of pixel fibers 1238 around the distribution device 1234. The pixel fibers 1238 are typically significantly smaller in diameter than the distribution fiber, and are typically fragile and easily broken. Therefore, it is convenient to protect the pixel fibers by mounting them in modules, and coupling them to the distribution device 1234 by the distribution fibers 1236. Furthermore, in large systems, where there is a large number of pixel fibers 1238 and/or the paper to be exposed is wide, manufacturing complexity, weight and cost may all be reduced by using distribution fibers 1236 to distribute light from the distribution device to a point physically close to the imaging modules. Each light valve cell 1220 is illuminated by multiple pixel fibers. An imaging module 1208 includes a number of light valve cells 1220 on one or more light valve chips. Several imaging modules 1208 operate in parallel to ensure that the entire width of photographic media is exposed by a more or less continuous band of light. As discussed above, the timing of the control data fed to each light valve cell is made to correspond with the particular fiber or fibers being illuminated at each instant in time. The color of light presented to the light valve cells 1220 changes in sequence, for example from red to green to blue and back to red, in order to print in color. Each light valve cell 1220 generates not just one but many pixels across the paper. For example, if each light valve cell 1220 is coupled with 32 pixel fibers, then each light valve generates 32 pixels. The number of light valves is selected to be small enough so that the requirements of the electronic control circuitry are reasonable, yet the number of light valves is high enough so that the speed of imaging is high. The selection of the number of imaging modules and pixels associated with each light valve cell 1220 generally represents a compromise between system complexity and printing speed.

While some imaging modules are imaging with red light, other modules may be imaging with green light or with blue light. Moreover, some pixel fibers create images at points further ahead in the direction of paper motion while other pixel fibers create images at points further behind in that same direction. This is due to the fact that the lines of fibers are not arranged perpendicular to the direction of paper motion, but are arranged at an angle relative to the motion direction. This angular arrangement results in the use of more fibers to cover the width of the paper, resulting in a higher resolution without requiring optical fibers of smaller diameter, which are harder to work with and more susceptible to breakage. The electronic data used to control the light valve chips is programmed to reflect the timing, position, and color of illumination of various pixels.

A simplified functional block diagram showing the organization of the electronic control system is given in FIG. 10. A clock oscillator, 1409, provides clock pulses which are counted by the transfer number counter, 1408. The counter 1408 counts, for each pixel line (once for each revolution of the light distribution device described below) one count for each of the pixel fibers in the imager. The fiber number count, provided by the counter 1408, serves as an address input to the memory offsets table 1407. This table 1407, which may be implemented as a memory, provides a read address offset for the buffer memory 1401. Memory 1401 may be large, for example, 400 megabytes in size, and contains the file to be imaged on to the paper. The address offsets provided by table 1407 include compensation for the angle of each line of fibers relative to the direction of paper motion. Compensation for slight overlaps between cells and between modules may also be provided. Moreover, the address offsets provided by table 1407 may also account for any mechanical mismatch between modules.

The output of buffer memory 1401 flows into the color table memory 1402 which provides color compensation for the characteristics of the color media, and also compensates for any nonlinearities of the light valves. Such compensation typically includes general compensation,, without providing compensation for individual light valves. The data output from the color table memory 1402 then flows into fiber strength memory 1403. The fiber strength memory 1403 also receives as an address input the transfer number address from the counter 1408. The fiber strength memory 1403 provides compensation on a fiber-by-fiber basis. The compensation covers, for example, intensity variations caused by variations in fiber- to-fiber horizontal spacing that may be caused primarily by pixel fibers not being arranged exactly in a straight line with respect to their respective light valve cells, overlap between the end pixel fiber(s) in one line with the first pixel fiber(s) in the next line (or cell), and variations in the polishing of the pixel fibers. The fiber strength memory 1403 ensures that at every fiber causes the same degree of exposure of the photographic paper as every other fiber. The image data passes out from the fiber strength memory 1403 into each of the module buffer memories 1404. Each imaging module 1202, 1204, 1206 and 1208 has its own module buffer memory 1404. Within each imaging module, the data flows from the buffer memory 1404 into a digital-to-analog (d/a) converter and high voltage driver circuit 1405 which supplies the various voltages needed by the light valves, 1406.

The module buffer memory 1404 typically receives data from the fiber strength memory 1403 in serial form, i.e. one pixel at a time. Changes in data transmitted to the light valves 1406 need to occur at specific points in time, and in a massively parallel format. The module buffer memory 1406 therefore stores the serially received data from the fiber strength tables 1403 to permit the data to be transmitted to the light valves 1406 at the correct time and in parallel. The module buffer memory 1406 receives data in a 24-bit bus, 8 bits for red, 8 for green, and 8 for blue, and delivers that data to the d/a converters and high voltage drivers, 1405, 8 bits at a time, selected for the color being imaged. The color and fiber # to be selected for delivery of data to the d/a converters and voltage drivers 1405 is determined by the fiber number counter, 1410, which determines not only the distribution fiber number (for example 1 to 54) being imaged by any module at any instant, but also determines the color of imaging. The fiber number counter 1410 may be a lookup table memory, and receives an address input from a phase locked loop (PLL) circuit 1411. The PLL circuit 1411 synchronizes to the synchronization sensor 1412 on the light distribution device. The synchronization sensor 1412 provides a sync pulse for every cycle of the light distribution device. The PLL circuit 1411 divides that time interval into as many equal time intervals as are necessary to complete one full revolution of the rotating body 104. For the example discussed below with respect to Table I, this is 248 equal time intervals.

One particular embodiment of a digital color printer is suitable for exposing standard RA4 processable color paper (e.g., Kodak Supra II or Konica QA) for making color prints from digital information. Such a printer may image paper having a width of 30" (approximately 76 cm), at 512 pixels/inch (approximately 200 pixels/cm) , with a lineal speed of 0.3"/second (approximately 0.76 cm/second). This embodiment uses eighteen imaging modules, each imaging module imaging approximately one eighteenth of the paper width. Each imaging module includes sixteen light valve cells with fifty four pixel fibers positioned in a linear array behind each light valve cell. Thus, each imaging module generates 16 x 54 = 864 pixels. Each of the fifty four pixel fibers is sequentially illuminated in typical successive cycles of red, green, and blue light. In order to increase the imaging capability of each light valve cell, each light valve is not illuminated uniformly or completely, but selectively by, at any instant in time, one or a small number of the pixel fibers located behind each cell. Each light valve cell is made, at any instant in time, optically opaque or transparent, or partly opaque, according to the magnitude of voltage applied to the light valve cell by the high voltage drivers 1405.

The total number of pixel fibers in the imager is 864 x 18 = 15,552. The diameter of the pixel fibers is determined by the required resolution and printing speed, and also by considerations of manufacturability. Where the pixel fibers are 0.003" (approximately 75μm) in diameter, they are fragile and easily subject to damage. Therefore, in order to minimize the likelihood of fiber breakage during manufacturing and servicing of the imager, light is distributed within the imager first by illuminating, at the distribution device 1234, relatively large distribution fibers, typically having a diameter of approximately 0.02", or 500 μm. The light may then be transported from the distribution device 1234 to each imaging module by an associated group of distribution fibers. Each 0.02" diameter distribution fiber illuminates the input ends of sixteen 0.003" (75 μm) diameter pixel fibers. The repeating sequence of illumination of the fifty four distribution fibers which are presented to each imaging module are as shown in Table I. Each cycle number takes 26.25 microseconds to complete, and the overall cycle repeats indefinitely.

Table I Illumination Sequence for Distribution Fibers

Cycle # Color Distribution Fiber

1 red #1

2 red #2

3 red #3

54 red #54

55-62 none —

63 green #1

64 green #2

65 green #3

116 green #54

117-124 none —

125 blue #1

126 blue #2

127 blue #3 178 blue #54

179-186 none —

187 red #1

188 red #2

189 red #3

240 red #54

241-248 none —

1 red #1 etc.

The complete cycle repeats every (248 x 26.25 =) 6510 microseconds. Therefore, there are 153.6 cycles per second. The vertical resolution on the photosensitive paper may be the same as the horizontal resolution, namely 512 pixels/inch (200 pixels/cm), so the vertical rate of imaging is 153.6 / 512, = 0.30" per second (0.76 cm/s). It should be appreciated that the vertical resolution on the photosensitive paper may be different from the horizontal resolution.

Two red exposures may be used for every single exposure of green or blue where the photosensitive paper, for example RA4 processable color negative paper, is less sensitive to red than to green or blue. The double exposure to red, together with some variation in the dwell time of the light distribution device 1234, compensates for this characteristic of the paper. It should be understood that "fiber #1" referred to above means all fibers #1 in each of many, e.g., 16, light valves per module and each of 18 modules, 288 fibers in all, are illuminated.

The invention described herein is directed to a new method of distributing light from the light source to the distribution fibers, and of controlling the color of light illuminating the distribution fibers at any one time. It is important that, of the many pixel fibers in each array gated by a single light valve, only one or two of the pixel fibers should be brightly illuminated while the others receive little, if any, light. This assures that there is no crosstalk among the different pixels of the image. Moreover, after all of the fibers associated with one imaging module have been successively illuminated with one color, the color is changed to be successively red, green, and blue.

The method of distributing light from the light source sequentially to multiple fibers that was disclosed in U.S. Patent 5,684,620 involved a rotating polygon mirror. In order to project a small spot onto the fibers, a highly collimated and well focused beam of light was needed, which required the use of a short-arc xenon lamp. Short-arc xenon lamps are difficult to operate due to the substantial electromagnetic interference (EMI) generated during starting, and the complicated driving circuit that is required. Moreover, synchronizing a color filter wheel to the rotating polygon mirror is complex.

Another concern is contrast ratio. In normal operation, each fiber is "off, i.e. is not illuminated, for a period of time substantially longer than it is "on", i.e. is illuminated. Therefore, any light scattered to the "off fibers is integrated by the light-sensitive media for a relatively long time. A realistic requirement for contrast ratio of light presented to the fibers is of the order of 1.6 x 10⁴, which is difficult to achieve with light distribution based on a rotating mirror.

According to one particular embodiment of the invention, and with reference to FIGs. 1 and 2, white light is generated by a lamp 101, for example a type ELC quartz halogen projector lamp, which contains an integral reflector (the figure shows the external surface of the reflector). The white light is projected via optical paths 102 to focus on one end 103 of a light guide such as a fiber optic bundle 110. One example of a fiber optic bundle 110 includes a plurality of individual fibers, each having a diameter of 0.002" (approximately 50 μm), where the diameter of the bundle input end 103 is 0.313" (approximately 0.8 cm). The output end 112 of the fiber optic bundle 110 is positioned close to the center of a rotating body 104, driven by a motor 107 to rotate about its axis at the speed which is needed to accomplish the fiber "on" times indicated above. Using the above example, this speed is 153.6 revolutions/second, or 9,216 RPM. The rotating body 104 may be, for example, disk-shaped. This shape may be referred to as a "puck," but it should be noted that other shapes may be used. It will also be appreciated that light may be focused on to the input face 116 directly, without the use of the fiber optic bundle 110.

A light guide 114 within the rotating body 104 has an input face 116 located at the center 108 of the face of the rotating body 104 opposing the bundle 110. The input face 116 receives the light from the bundle 110. The light guide 114 separates the incoming light along a number of different paths to output ports located on the outer perimeter 118 of the rotating body 104. In the particular embodiment illustrated, the light guide 114 separates the light along four different paths to output ports, of which two 105 and 106 may be seen in FIG. 1. The light guide 114 may also be formed from a plurality of fiber optic strands, starting at the center 108 of the face of the rotating body, and approximately one quarter of the strands are directed to each of the four output ports arranged around the perimeter 118. Each output port may advantageously include a rectangular arrangement of fibers. In other embodiments, the light guide 114 may be coupled to any suitable number of output ports, not just four. For example, the light guide may be coupled to one, or more, output ports.

Internal fiber optic pathways 215, 216, 217, and 218 lead from the center 108 of the face to the output ports 105, 106, 227, and 228. Each port is covered by a respective dichroic color filter 205, 206; 207 and 208. Here, filters 205 and 208 transmit red light, filter 207 transmits blue light and filter 206 transmits green light. The colors of the filters may be selected to optimize the exposure of the photosensitive medium, and need not necessarily be red, green or blue. Slightly more than one quarter of the fibers in the light guide 114 are used for each of the two red filtered ports 105 and 228, and slightly less than one quarter of the fibers are used for each of the green and blue filtered ports 106 and 227. The number of fibers directed to each port is selected in accordance with the spectral sensitivity of the photosensitive paper upon which an image is to be printed. The widths of the ports 105, 106, 227, and 228 (in the direction of the rotating body's circumference) are selected to correspond to the number of fibers which exit at each slit, which may be approximately 0.070" (around 0.175 cm). The height of each port 105, 106, 227, and 228 (in the direction parallel to the rotating body's rotation axis) may be approximately .150" (0.38 cm). It will, of course, be appreciated that the ports may have non-rectangular shapes and have dimensions different from those described here for this particular example. It should also be appreciated that the variation in exposure between ports may be accomplished additionally or alternatively by variously sized apertures placed over the dichroic filters.

Where the widths of the ports 105, 106, 227, and 228 are significantly less than 0.070", for example 0.035" wide, mostly only one distribution fiber is illuminated at any one time, having the effect of providing sharper images. A disadvantage of this approach is, however, is that there is a lower overall light efficiency, thus necessitating that the optical source is brighter in order to obtain the same imaging speed.

One particular arrangement of distribution fibers positioned around the rotating body 104 to receive light from the exit ports of the rotating body 104 is illustrated in FIG. 3. Various groups of distribution fibers, each including a specific number of large diameter fiber optics, are placed around the rotating body 104, the fibers being spaced at regular angular intervals so as to receive bursts of light as the rotating body 104 rotates around its axis. The distribution fibers may be mounted in a stationary block, or stator, (not illustrated) that contains slits to hold the fibers in position. Where there are fifty four distribution fibers for each imaging module, with 248 cycles, as described in the example above, the distribution fibers may be positioned around the rotating body 104 at regular spacings of 1.45° (1/248 of a circle).

In one particular embodiment, the distribution fibers have a diameter of 0.02" (approximately 0.05 cm), the rotating body 104 has a diameter of 3" (7.5 cm) and the spacing between distribution fibers close to the rotating body 104 is approximately 0.038" (approximately 0.096 cm). The slits in the stator that hold the fibers are 0.020" (approximately 0.05 cm) wide and 0.120" (approximately 0.3 cm) long. For clarity, only two distribution fibers are illustrated for each slit position, for example distribution fibers from the groups marked 307 and 308. Each of the ports 105, 106, 227, and 228 on the rotating body 104 projects light onto the large diameter fibers. If the ports 105, 106, 227, and 228 are narrow, and the spacing between the circumference 118 of the rotating body 104 and the surrounding stationary fiber bundles is small, approximately only two distribution fiber positions are illuminated by each port 105, 106, 227, and 228 at each instant in time. When the distribution fibers are not receiving light, they "see" the surface of the perimeter 118, which is advantageously black to reduce scattered light and, therefore, provide a high contrast between light levels when the fibers are "on" and "off.

The eight groups of distribution fibers 301, 302, 303, 304, 305, 306, 307, and 308 illustrated in FIG. 3 support eight imaging modules. The arrow 310 shows the direction of rotating body rotation. To support, as is desired, not eight but rather a total of eighteen imaging modules, a greater number of distribution fibers need to be placed at each position around the rotating body 104. For example, four distribution fibers may be located at each position for two groups of positions and five distribution fibers located per position for the remaining two groups of positions.

The circuitry used to control the light valve cells needs to "know" where the rotating body 104 is at each point in time. One method of deducing the instantaneous position of the rotating body 104 is to project light through a small hole 130 in the rotating body. A light source 132, such as a light emitting diode or a laser diode, is placed on one side of the rotating body 104 and a light detector 134, such a photodiode or phototransistor, is placed on the other side of the rotating body 104. The light detector 134 detects a light signal when the hole 130 is aligned between the light source 132 and the detector 134, i.e. once every revolution of the rotating body 104. This generates a "start of scan" pulse. It will be appreciated that electronic circuitry may be used to time the interval between the "start of scan" pulses and to divide this interval time into a number of equal time units, 248 time units in the current example. At the start of each such interval in time (each being 26.25 microseconds long), new data is used to control each of the various light valves according to which particular fibers are being illuminated at that time.

Within each imaging module is a light valve chip containing a number of light valve cells. In this particular embodiment, the light valve cells take the form of a narrow slit. Behind each of these slit-shaped light valve cells is situated a linear row of pixel fibers. A harness 400 of pixel fibers 410 for a single imaging module 1208 having sixteen light valve cells, where each light valve cell prints fifty four pixels, is illustrated in FIG. 4. The pixel fibers 410 may be, for example, 75 micron (0.003") diameter, fused silica fiber optic strands, as manufactured by Polymicro Technologies, Inc., type FDP-061-067-075 A. Each pixel fiber 410 has an input end 412 and an output end 414. The output ends 414 are in linear groups 401 and 402 positioned behind the light valve cells. The input ends 412 are in bundles 403 and 404 positioned to receive light from the distribution fibers 1236 leading from the distribution device 1234. In the present example, each imaging module has a group of fifty four 0.02" (500 μm) diameter distribution fibers 1236 leading from the distribution device 1234. The harness 400 has sixteen linear groups 401 and 402 of output ends, corresponding to the sixteen light valve cells of the imaging module. The harness 400 also has fifty-four bundles 403 and 404 of input ends, corresponding to the fifty-four distribution fibers 1236. Each of the fifty four pixel fibers 410 in each linear group 401 and 402 receives light from one of the fifty-four input bundles 403 and 404. For example, if the first input bundle 403 is illuminated with red light, the first pixel fiber 410 of each of the linear groups 401 and 402 is illuminated with red light. If the second input bundle 404 is illuminated with red light, then the second pixel fiber 410 in the linear groups 401 and 402 is illuminated with red light, and so on.

The light valve chip 501 and associated pixel fiber output ends 414 are illustrated in FIG. 5. The light valve chip 501 may be made of PLZT material, as is commonly used in solid state light valves, or may be another type of polarization modulator, including other Kerr-type modulators. The light valve chip 501 includes a number of separate light valve cells 510, in this case sixteen. Light is transmitted through each light valve cell 510 from an associated pixel fiber 410. The light valve cell 510 rotates the polarization of the light in response to an electric field, and is typically sandwiched between two crossed polarizers, as is described in U.S. patent no. 5,054,893. Rotation of the polarization of light passing through the light valve cell 510 results in light being transmitted through the pair of crossed polarizers. Each light valve cell 510 is positioned between electrode pairs, between which an applied electric potential controls the polarization rotating properties of the cell 510. In one particular embodiment, each cell 510 is formed between a common electrode 504 and an individual electrode. The common electrode 504 is connected to each of 16 comb-like fingers 505. Individual electrodes, such as electrode 503, are connected to respective connection tabs 502. The light valve chip 501 may be placed inside a cutout on a printed circuit board; conductive epoxy junctions may be made between traces on the printed circuit board and the connection tabs 502 and the common electrode 504. In this way, each light valve cell 510 may be addressed individually and separately from the other cells 510 on the chip 501.

The region between any common finger electrode 505, and any individual electrode 503 generates an electric field responsive to the voltage applied between the two electrodes. Except in the cases of the end electrodes, two regions of electric field are generated, one above and one below the individual electrode 503. Only one of these regions is used as a light valve cell 510 to gate light. In the region above the individual electrode 503 and below the common finger electrode 505, all of the pixel fibers 410 are configured in a linear array.

The light admitted through the various light valve cells 510 is projected by a lens system onto the photographic paper passing over a turning capstan. In one embodiment, each light valve array 501, having sixteen cells 510, has an active length of 1.4", so it is magnified by 1.205 times to produce an image length of 1.6875". Eighteen imaging modules are positioned every 1.68" to produce a total imaging length of 30". It should be appreciated that the amount of magnification by the lens system is dependent on several factors including, but not limited to, the desired resolution, the size of the light valve chip in the imaging module and the printing speed.

A portion of FIG. 5 is enlarged in FIG. 6 to better show the linear arrays of output ends 414 of the pixel fibers 410. A light valve cell 520 is formed between individual electrode 503a and common finger electrode 505a. Another light valve cell 522 is formed between another individual electrode 503b and common finger electrode 505b. Pixel fibers 506 - 510 form one linear fiber array 530 in the first light valve cell 520 and pixel fibers 511 - 512 form another linear array 532 in the second light valve cell 522. There are fifty four pixel fibers 410 in each array 530 and 532.

The direction of paper motion relative to the light valve cells 520 and 522 is illustrated by the arrow 516. The fiber arrays 530 and 532 form an angle, θ, relative to the direction of paper motion. The angle, θ, and the number of pixel fibers in each array 530 and 532, are selected so that, in the direction perpendicular to the direction of paper motion, the last fiber 510 in the first array 530, matches or slightly overlaps the position of the first fiber 511, in the second array 532. If there is overlap between the end pixel fibers of adjacent arrays, the brightness of overlapping fibers may be adjusted to compensate.

The voltage to each light valve cell 520 and 522 is programmed to correspond to the particular pixel fiber 410 or fibers illuminated at any instant in time. If more than one pixel fiber 410, presumably adjacent, is illuminated at any instant in time, the data may correspond to the brightest pixel fiber or the average position of the illuminated pixel fibers.

Various modifications, within the scope of the invention, may be made to the examples discussed above within the scope of the present invention. For example, a rotating body may be used having a different number of exit ports. Where a rotating body is provided with six exit ports, rather than four, the ports may be provided with sequential color filters of red, green, blue, red, green, and blue. This may require a larger rotating body diameter, for example 4" rather than 3". To support eighteen imaging modules using such a rotating body, only three distribution fibers are needed at each location around the rotating body. Moreover, the rotating body speed needed to support a given imaging speed is one half the number of lines imaged per second on the paper, since two scans are made per revolution of the rotating body. Other provisions (e.g., variations in slit width, variations in lookup tables, or adjustment of the color temperature of the illumination source) may be needed to compensate for the fact that there is only one, and not two, red exposures per cycle. In such a case, the rotating body motor speed may be increased to 12,000 RPM, or 200 revolutions/second, resulting in an imaging speed of 400 pixels/second. With a vertical resolution of 400 pixels/inch (157 pixels/cm), a 1 "/second (2.5 cm s) paper speed may be obtained. In another embodiment, there are eight imaging modules, and only one fiber group surrounds about ^lA of the rotating body. Therefore, only one color illuminates the fiber group at any one time. An advantage of this embodiment is that the electronic complexity, required to provide for simultaneous imaging of different colors, may be reduced. On the other hand, since the green and blue light is not used at the same time as the red light, a substantial fraction of the light is discarded in this case. This embodiment illustrates the compromises that may be made between the system complexity and light use efficiency This approach may be advantageous in applications such as imaging high sensitivity film, where the amount of light is not as important, and/or the image width need not be so wide. Different numbers of pixel fibers per light valve cell, and/or a different number of light valve cells per imaging module, may be used. For example, the number of pixel fibers per light valve cell may be fifty, with thirty two light valve cells per light valve chip. In this case, a single module, with 1,600 pixels (50 x 32), may be used to expose a roll of photographic paper 4" (10 cm) wide at a resolution of 400 pixels/inch (157 pixels/cm). The rate of scanning in the light distribution device primary scanner may be increased to 200 scan cycles (red, green, blue, red, of all 50 fibers each) per second. With a vertical resolution also of 400 pixels/inch (157 pixels/cm), the vertical imaging rate may be 0.50"/second (1.25 cm/s). Here, since the total number of pixel fibers is small, and the total width of paper to be covered is also small, it may be advantageous to avoid the use of distribution fibers, and to couple the pixel fibers directly to the distribution device. There are many other aspects of the digital color printer that may be different from the examples discussed above, without departing from the scope of the invention. For example, the number of imaging modules used across the width of the photosensitive paper may be selected depending on what particular resolution and speed are desired. Several design aspects of the printer are dependent on the width and nature of the photosensitive medium used, such as the amount of light required to expose the medium. This may affect the printing speed.

As noted above, the present invention is applicable to digital color printers and is believed to be particularly applicable to high speed, high resolution printers. Accordingly, the present invention should not be considered limited to the particular examples described above, but rather should be understood to cover all aspects of the invention as fairly set out in the attached claims. Various modifications, equivalent processes, as well as numerous structures to which the present invention may be applicable will be readily apparent to those of skill in the art to which the present invention is directed upon review of the present specification. The claims are intended to cover such modifications and devices.

Claims

What is claimed is:

1. An apparatus for sequentially distributing light to multiple destinations, comprising: a body rotatable about an axis, and having a first surface intersecting the axis; light guide having an input end on the first surface, approximately centered about the axis, the light guide having at least one arm leading from the input end to a respective output port positioned on a perimeter edge of the body; and at least one color filter positioned at the at least one output port to filter light exiting from the at least one output port.

2. An apparatus as described in claim 1, wherein the light guide has at least two arms leading from the input end to respective output ports positioned on a perimeter edge of the body, and at least one color filter is positioned at each of the output ports

3. An apparatus as described in claim 2, wherein the output ports are positioned approximately uniformly spaced around the perimeter edge of the body.

4. An apparatus as described in claim 1, wherein the light guide includes a first plurality of optical fibers, and the at least two arms each are formed from respective groups of optical fibers of the first plurality of optical fibers.

5. An apparatus as described in claim 1, further comprising a stationary second plurality of optical fibers positioned with input ends mounted around the perimeter edge of the rotatable body to receive light from the output ports.

6. An apparatus as described in claim 5, wherein the stationary second plurality of optical fibers includes pixel fibers optically coupled directly to at least one light valve cell.

7. An apparatus as described in claim 5, wherein the stationary second plurality of optical fibers includes distribution fibers, each distribution fiber optically coupled to respective pixel fibers, the pixel fibers being optically coupled to respective light valve cells.

8. An apparatus as described in claim 1, wherein the output ports have respective widths measured in a rotation direction, not all the widths of the output ports having a same value of width.

9. An apparatus as described in claim 1 , further comprising a light source optically coupled to the input end of the light guide.

10. An apparatus as described in claim 1, further comprising a motor coupled to the body to rotate the body about the axis.

11. An apparatus as described in claim 1 , further comprising a rotation detector coupled to detect rotation of the body.

12. An apparatus for distributing light, comprising: rotatable means for rotating about a first axis; light guiding means for guiding light from a first surface of the rotatable means to at least one output port on a circumference of the rotatable means; and filter means for filtering light exiting from the at least one output port.

13. An apparatus as recited in claim 12, wherein the rotatable means includes at least two output ports on the circumference and optically coupled to the first surface by the light guiding means

14. An apparatus for printing color images from digital data, comprising: a light source; a light distributing device having an input port coupled to receive light from light source and at least two output ports, the output ports being provided with color filters to filter light passing therethrough from the light source, the light distributing device being rotatable about an axis; a plurality of fibers optically coupled at input ends to receive light at regular intervals from the output ports as the light distributing device rotates about the axis; and a plurality of light valves coupled to receive light from the plurality of fibers and controllable in response to which particular optical fibers are presently coupled to receive light from the light distributing device.

15. An apparatus as recited in claim 14, further comprising a controller coupled to the plurality of light valves to control operation of the light valves.

16. An apparatus as recited in claim 14, wherein the light source includes a lamp, light from the lamp being coupled to the input port of the light distributing device through a light guide.

17. An apparatus as recited in claim 14, wherein the light distributing device includes a light guide having an input at the input port and having light guiding arms coupled to the output ports.

18. An apparatus as recited in claim 14, wherein the plurality of fibers are arranged radially about the axis with input ends facing the axis to couple light passing out from the output ports in a radial direction.

19. An apparatus as recited in claim 14, wherein the plurality of fibers includes pixel fibers coupled directly to the plurality of light valves

20. An apparatus as recited in claim 14, wherein the plurality of fibers includes distribution fibers, each distribution fiber being coupled to a plurality of pixel fibers having output ends coupled to different light valve cells in an imaging module.