FILM RECORDER LIGHT SOURCE BASED ON FIELD EMISSION CATHODE

Abstract: A print head utilizing a field emission CRT for an optical printer for printing on photosensitive surfaces is disclosed herein. A plurality of small electron sets consisting of cathode emitting cones (26) in an anode aperture form a gap which is less than the electron mean free path in ambient atmosphere and the sets are preferably closely spaced to form a substantially columnated beam. A third electrode preferably accelerates and cleans up the beam which is separated from the cathode (24). The beam is then incident upon a luminophor film (34) which is excited, thereby generating light. The light is transmitted through a transmissive face plate (36) such as a fiber optic face plate where it is incident upon the photosensitive material. Multiple transmissions of a single color is also described to speed printing by time delay integration.

Description: Film Recorder Light Source Based on Field Emission Cathode Background of the Invention The present invention relates generally to electrophotographic printing and film recording apparatus, such as light emitting optical printers. More particularly, the invention relates to apparatus for printing images by exposing a photosensitive recording medium to light generated by a field emission electron source. Optical printers make use of a variety of exposure devices such as lasers, light emitting diodes ("LED" 's), spacial light modulators ("SLM" 's), and cathode ray tubes ("CRT" 's). All have been applied to expose photographic film as the photosensitive recording medium. At slow printing speeds, a single emitter light source, the laser for example, or light shutter can be used. These exposure devices require two orthogonal mechanical motions to scan and expose the photosensitive recording medium. Increased mechanical motions require a control system with a precision to govern and synchronize the mechanical motions which, in turn, raise complexity and cost of the optical printer. For higher printing speeds, two exposure methods are commonly used. The first method is a line scan method which uses the LED's or the SLM's configured into linear arrays. These are made long enough to span the width of the photosensitive recording medium so that an entire line is exposed at once. The line scan method is more compact and faster than the single emitter light source but still requires mechanical motion to transport the photosensitive recording medium past the linear array. Line arrays using LED's to form an image on the recording media are useful for high speed printing but are further limited by available colors of the LED's. Blue LED's, for example, are not made in arrays and are expensive. The second method uses the CRT; a light spot is generated by focusing an electron beam which excites a cathode luminescent luminophor screen. Deflecting the beam with either electrostatic or magnetic fields allows scanning of the spot across a CRT face plate coated with a luminophor to translate absorbed electron beam energy into emitted luminescent radiation. The luminescent radiation is focused on the photosensitive recording medium to create an image thereon. In optical printing applications, CRT's can be used in two ways: area raster scan and line scan. Area raster scan is when the beam is deflected in two directions. Line scan is as previously described where the beam is moved across one line at a time. Line scan types are more compact but, again, need a mechanical scan motion along one direction to expose the film. The area raster type eliminates the mechanical scan motion but requires additional complex electronics to control the beam in two directions. Another problem encountered in conventional CRT exposures is printer resolution. Spot diameter formed on the CRT face plate primarily determines printer resolution. A size of the electron beam as well as the face plate determine the spot diameter. For a high quality CRT the beam is tightly focused so that the spot size is essentially dependent on the face plate and an amount of light piping, or internal light reflection introduced by the face plate. To minimize the spot on the face plate requires the luminophor to have particles which are small and have a uniform size. In addition, the thickness of the luminophor layer and the particular size distribution in the face plate must be extremely uniform to minimize spot intensity variations. Face plates fabricated from polycrystalline thin film fluorescent materials are used to improve spot size and reduce intensity variations, however this technique results in reduced optical output due to light piping in the thin film as well as higher manufacturing costs. Additionally, the beam in the CRT is formed by a high performance electron gun. To maintain a small diameter beam, the beam must be continuously refocused to insure that the spot size is kept constant as the beam scans across the screen. This is achieved by dynamic focusing electronics which add additional expense to the exposure device. In order to insure good spot position accuracy on the screen, highly stable deflection circuits as well as magnetic shielding of the CRT is also needed which increases the cost of the overall printing system. Recently, a number of structures have been proposed that employ microlithographically fabricated electron sources for charge pattern formation in various printing applications. Representative of this class of electron sources are U.S. Patent Numbers 4,259,678 and 4,858,062, 4,810,935, and 4,904,895. Such sources might, for example, be used to write a pattern on a cathode ray tube or on a liquid crystal display. Typically in proposed constructions of this type, the electrons must generally be accelerated to an extremely high energy often in the tens of kilovolts range to actuate the luminophor of the CRT screen or to pass through an electron transmissive face plate or window. Such high energy levels in an electrophotographic imaging apparatus can raise problems of cost, complexity, reliability, and even safety. Summary The aforementioned and other objects are achieved by the invention which provides a field emission print head for an optical printer. The print head projects light onto a photosensitive recording medium in the optical printer to record an image thereon. The print head comprises a plurality of field emission devices, each having a first and a second electrode separated by insulation means; illuminescent means; and a third electrode. The plurality of field emission devices are divided into groups where each group defines a single spot for printing purposes. Each of the groups is addressable to alternate between an operative mode and an inoperative mode. In the operative mode, the groups emit electrons therefrom. In the inoperative mode, the group remains inactive without emitting electrons. Each of the field emission devices have the first electrode which is adapted to have a first electrical potential applied thereto and has a protrusion extending therefrom. The first electrode is common to all of the field emission devices in the group. The insulation means is secured to a top surface of the first electrode and has a void overlying the protrusion. The insulation means has a high dielectric strength to inhibit electrical conduction from the first electrode to the second electrode. The second electrode is secured to the insulation means in a position opposed to the first electrode. The second electrode is adapted to have a second electrical potential applied thereto and is operable to switch the field emission device between the operative mode and the inoperative mode. The operative mode is achieved when a difference between the first electrical potential and the second electrical potential exceeds a threshold voltage thus causing protrusion to emit the electrons due to electron tunneling. Alternatively, the inoperative mode is achieved when the threshold voltage is not exceeded. The illuminescent means is separated from the field emission devices along a plane parallel to the field emission devices. It is laid out in spots corresponding to the aforementioned groups of field emission devices where the spots are fabricated from a luminophor which generates light upon excitement by the electrons. The third electrode is also spaced apart from said field emission devices providing a platform upon which the illuminescent means is fixed. The third electrode being light transmissive and adapted to have an electrical potential applied thereto which is selectable to greatly exceed said second electrical potential thus attracting said electrons into engagement with the illuminescent means. The illuminescent means then emits light to project the image onto the photosensitive recording medium. In further aspects, the invention provides methods in accord with the apparatus described above. The aforementioned and other aspects of the invention are evident in the drawings and in the description that follows. Brief Description of the Drawings The foregoing and other objects of this invention, the various features thereof, as well as the invention itself, may be more fully understood from the following description, when read together with the accompanying drawings in which: Figure 1 shows a printer in accordance with the present invention; Figure 2 shows a perspective view of a detail of a print head in the system of Figure 1; Figure 3A shows a view similar to that of Figure 2 of an electron emission structure also indicating trajectories and circuit structure; Figure 3B is a graph of a cathode current versus cathode gate voltage for the print head; Figure 4 shows a perspective and detail view of a print head in accordance with the invention as shown in Figure 1 ; Figure 5A shows a top view of a portion of a fiber optic phase plate for use with the invention; and Figure 5B shows a profile of the face plate of Figure 5A for use with the invention. Detailed Description While the present invention retains utility within a wide variety of printing devices and may be embodied in several different forms, it is advantageousl employed in connection with an optical printer for printing on photosensitive print media. Though this is the form of the preferred embodiment and will be described as such, this embodiment should be considered illustrative and not restrictive. Figure 1 shows an optical printer 10 according to the present invention which is for recording electronic images onto a photosensitive recording medium 14. In the preferred embodiment, the photosensitive recording medium 14 is an integral film, such as that described in commonly assigned United States Patent No. 4,563,411 entitled "Copolymeric Mordants and Photographic Products and Processes Containing Same" and issued to Irena Y. Bronstein-Bonte on January 7, 1986. Though this is the preferred embodiment, various other photosensitive recording media can be substituted without detriment to the invention. The photosensitive recording medium 14 is fed into the printer 10 by conveyancing systems well known in the art and is drawn through the printer 10 by rollers 16 driven by drive motor 18. As the photosensitive recording medium 14 is pulled through the printer 10, a processor 19 transfers image data to a print head 20. The print head 20 then projects light (indicated by arrows) onto the photosensitive recording medium 14 such that a coherent image is formed thereon. The print head 20 is a solid state thin film electron source which generates an electric field for luminescent excitation of luminophor materials thus producing light. The print head 20 is shown as a line array for imaging a single line on the photosensitive recording medium 14. An area device utilizing a plurality of line arrays can also be used and should be considered an alternative embodiment of the invention. The luminophor material is one which translates at least a portion of absorbed energy into emitted luminescent radiation. In the preferred embodiment, the luminophor is a phosphor based material. The light is transmitted to the photosensitive recording medium 14 by the print head 20. The light then passes through a lens 21. The lens 21 can be a conventional lens or as in the preferred embodiment can be an array of graded index rods, or grinrods. In the case of contact printing, the lens 21 would be removed and a light transmissive faceplate would be used. Such a faceplate is later herein described. Figure 2 illustrates a basic construction of a field emission cathode ray tube ("CRT") as applied to the print head 20 of the invention. A cathode tip, or cone 26, emits electrons as shown by the dashed line which impinge a face plate 36 coated with a luminophor film 34 so as to excite the luminophor film 34 and emit light. The print head 20 is fabricated with an array of the cones 26 deposited on a back plane 22. These operate at room temperature. Each cone 26 has a sharp radius at its apex such that electron tunneling is enabled when a positive voltage is applied to the gate film. The back plane 22 is generally a glass substrate upon which the field emission CRT is fabricated. A cathode 24 forms a next layer on the back plane 22 such that the cathode 24 extends over an entire surface of the back plane 22. The cathode 24 has the aforementioned array of cones 26 deposited thereon such that centers of the cones range in distance from each other from three micrometers to ten micrometers, and are electrically connected at their bases. The cones 26 are separated from each other by an insulator 28 which is deposited over all surfaces of the cathode 24 which do not have a cone 26 protruding therefrom. The insulator 28 is fabricated from any of numerous materials having a high dielectric strength. In the preferred embodiment, the insulator 28 is silicon dioxide. Spaced in a same plane with the apex of the cones 26 is a gate 30 which is separated from the cathode 24 by the insulator 28 and is made of an electrically conductive metal. The gate 30 is etched with an aperture 40 for each of the cones 26 such that the apex of the cone 26 protrudes through the gate 30. A positive potential at the gate 30 relative to the cathode 24 produces an intense electric field at the apex of the cone 26. The field strength is sufficient to initiate electron tunneling from the apex of the cone 26 to a space around the cone 26. Field strengths of 10 9 to 10 10 volts per meter are typical for gate voltages of 80-100 volts with respect to the cathode 24. An anode 34 is spaced apart from and above the cones 26 and has a potential significantly higher than that of the gate 30 such that once electronic tunneling is initiated, electrons are drawn up towards the anode 34 as shown by the dashed line. A thin film of luminophor 32 is coated on the anode 34 such that as the electrons impinge upon the luminophor 32, the luminophor 32 is excited and produces a light. The light can be varied in wavelength dependent upon the choice of luminophor 32 and is often one of red, green, or blue depending on a spot color required in the print head. In this embodiment of the invention, the luminophor film 32 is made of a broad spectrum phosphor which is used to produce an essentially white light. The white light is passed through the anode which is transmissive to light and is transmitted by a face plate 36. Color, in this embodiment, is produced by using a color filter 38. Referring now to Figures 3A and 3B, the circuit characteristics of the print head are shown in greater detail. Figure 3 A illustrates that as a voltage V g is placed across the gate 30 and the cathode 24 through a resistance R g , a current i-, is produced therebetween. Similarly, a voltage V a is placed between the anode 34 and the cathode 24 through resistance R a and a current -a is produced. By Kirchhoff s current law, ta + -g = - where - is current through the cathode 24. Due to a minimal attraction of electrons to the gate 30, the resistor R g is not actually required. The attraction is minimal because V g is small, approximately 50 volts, relative to V a which is on the order of 200 volts. In fact, the current i g is produced from leakage through the insulator 28 and from stray electrons from the cathode 24. Therefore, since - g is minimal compared to i * , - a can be ignored in the above equation thus leaving i, ≡ • * . This current flow causes an attraction of electrons emitted from the cone 26 due to the electron tunneling to be drawn towards the anode 34 as shown by the dashed lines. The choice of the gate voltage, V g , is not arbitrary but is instead used to switch sections of cones on and off. If V g is less than a threshold value, V TH , tunneling is not enabled and the electron flow is not created. But as V g rises, the threshold value is exceeded and tunneling in the cone 26 causes the emission of the electrons. This is graphically seen in Figure 3B where once the threshold voltage, V TH , is exceeded the cathode current increases exponentially with the electron field. This characteristic allows for easy switching of individual cones or sections of cones between on and off, thus multiplexing is enabled. Also indicated on the graph is an operating voltage, V OP , producing an operating cathode current, lop* The operating voltage is chosen to produce sufficient electron flux, to cause sufficient brightness levels in the phosphor while keeping the cathode current below an amount which will burn out the phosphor. Groups of these field emitting cones 26 work together to create a single spot for printing purposes. A single spot forming group is commonly on the order of 10 4 cones per square millimeter. This density ensures redundancy and uniformity of emission. Each spot forming group is composed of plural adjacent cones 26 placed sufficiently closely together so that outer portions of each electron beam overlap. This overlap has an additive effect, causing the fields from adjacent cones 26 to form an electron flux that is relatively uniform. This geometry ensures that multiple parallel beams are efficiently directed substantially along a normal to the anode plane and are non-diverging in the dot center region. For example, cones may be placed approximately five micrometers apart to form an extended array of electron emitters which collectively constitute a beam for depositing one charge spot. Figure 4 shows a print head 20 for multi-color printing that includes multiple arrays of spot forming groups as previously described. In the illustrated embodiment, the print head 20 consists of the back plane 22 which carries the gate 30 and the cathode 24 field emission structure. Above that is the face plate 36 with red, green, and blue luminophor stripes and a common anode 34. The gates are three electrically independent rows 42 of film which control the red, green, and blue luminophor stripes. Beneath the gate films running in an orthogonal direction are cathodes in columns 42 which are also electrically isolated. The cathodes consist of field emitters which are defined at the intersection of a gate and cathode structure and are shown in detail in Figure 4. Thus, electron flux is confined exclusively to the areas at the intersections and are addressable by rectangular coordinates to selectively activate individual areas. The areas define pixels and would be approximately 100 square micrometers per a 200 dot per inch printer. Each pixel consists of 600 or more cones 26. The pixels are addressed sequentially one row at a time using pulse width modulation. The gate-to-anode spacing is on the order of 200 to 2000 micrometers providing good proximity focusing of the electrons. This is accomplished in the preferred embodiment by utilizing thin film technology. The print head can be used as shown for imaging one line at a time or may be run together with additional single line print head structures to form an area printing device which conveys an entire image at once. Figure 5 shows a print head 20 for multi-color printing where like numbers represent like objects. In this embodiment, time delay multiplexing is utilized to increase print speeds. The print head 20', as before, consists of the back plane 22' which carries the gate 30' and the cathode 24' field emission structure. Above that is the face plate 36' with red, green, and blue luminophor stripes and a common anode 34'. The gates are three sets of electrically independent rows of film which control the red, green, and blue luminophor stripes. Beneath the gate films running in an orthogonal direction are cathodes in columns 42 which are also electrically isolated. The cathodes consist of field emitters which are defined at the intersection of a gate and cathode structure as previously described. Thus, electron flux is confined exclusively to the areas at the intersections and are addressable by rectangular coordinates to selectively activate individual areas. Unlike the previous embodiment, this embodiment repeats rows of colors such that there are two rows 50 for imaging red, two rows 52 for green, and two rows 54 for blue. A signal sent to the a first row of a single color would then be repeated in the second row at a time delayed in accordance with a speed that the photosensitive recording medium is being moved thereby intensifying the color associated with that row. In this way a faster transport system could be utilized since a single color need only be exposed for one half of the exposure time necessary for a single row of colors. Therefore, the print speed has been approximately doubled. In practice, a photosensitive recording medium is introduced into the optical printer where a first row of red is exposed. Since there are two consecutive rows 50 of red the exposure time as compared to a single row is decreased by one half. The second row having received the same print data as the first row, only delayed in time, then exposes the same line again for the color red. The photosensitive recording medium by its nature integrates the two exposures to form one line fully exposed in red. The same process is repeated for the rows of green 52 and blue 54 thus producing a fully exposed line. As a line is being exposed for green, a next line is being exposed in red to for a complete image upon the photosensitive recording medium. The number of rows of color may again be increased to three rows or more each time increasing possible print speed as compared to the single row embodiment. In the case of three rows the print speed is increased by a factor of three. Referring now to Figures 6 A and 6B, in order to increase light intensity on the photosensitive recording medium 14, commonly referred to as the film plane, a fiber optic face plate is used instead of clear glass for the face plate 36. The fiber optic face plate is particularly useful for direct contact printing on low ASA film. Fibers which have high numeric aperture are used to capture a large fraction of the light emitted by a top surface of the luminophor 32. Pixels 46 are defined by spots of luminophor 32 which are deposited on the faceplate 36 as a thin film material secured by a lithographic process. In the preferred embodiment, the pixels 46 are offset with respect to each other to increase spacing between the pixels 46 thereby reducing crosstalk between the pixels 46. An optical reflector 48 is then secured over the pixels 46 to prevent light piping. The optical reflector 48 is commonly a thin film layer of a reflective metal such as 0.25 micrometers of aluminum which is used in the preferred embodiment. The optical reflector 48 ensures that the light is directed to the top surface of the luminophor and coupled into the fibers. The aluminum must be thin enough so that energetic electrons reach the luminophor, yet it must be of sufficient thickness so that it acts as an efficient reflector of light. The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Claims: Claims 1. A print head for projecting an image onto an photosensitive recording medium in an optical printer, said print head comprising a plurality of field emission devices, each of said field emission devices having an operative mode and an inoperative mode, said operative mode for emitting electrons therefrom and said inoperative mode for remaining inactive, each of said plurality of field emission devices comprises a first electrode adapted to have a first electrical potential applied thereto and having a protrusion for emitting electrons extending therefrom; insulation means secured to a top surface of said first electrode and having a void overlying said protrusion, said insulation means for inhibiting electrical conduction from the first electrode; a second electrode secured to said insulation means opposed to said first electrode adapted to have a second electrical potential applied thereto which is operable to switch the field emission device between said operative mode and said inoperative mode where said operative mode is achieved when a difference between said first electrical potential and said second electrical potential exceeds a threshold voltage thus emitting said electrons from the conical protrusion, said inoperative mode is achieved when the threshold voltage is not exceeded; illuminescent means for generating light upon excitement by said electrons; and a third electrode spaced apart from said field emission device such that said illuminescent means is fixed therebetween, said third electrode being light transmissive and adapted to have an electrical potential applied thereto which is selectable to greatly exceed said second electrical potential thus attracting said electrons and drawing said electrons into engagement with said illuminescent means such that said illuminescent means emits said light to project said image onto the photosensitive recording medium. 2. The print head according to claim 1 wherein said plurality of field emission devices are deposited onto a conductive metal substrate. 3. The print head according to claim 2 wherein said conductive metal substrate is deposited on a non-conductive back plane. 4. The print head according to claim 3 wherein said non-conductive back plane is fabricated from glass. 5. The print head according to claim 1 wherein said third electrode is secured to a light transmissive face plate for conducting said light toward the photosensitive recording medium. 6. The print head according to claim 5 wherein said light transmissive face plate is fabricated from a fiber optic material having a polished surface and high aperture values. 7. The print head according to claim 1 wherein said illuminescent means is a layer of phosphorous which is excitable by said electrons to produce said light. 8. The print head according to claim 1 wherein said illuminescent means further comprises color optical filters for altering a color of the light produced by the illuminescent means. 9. The print head according to claim 1 wherein said insulation means is fabricated of silicon dioxide. 10. The print head according to claim 1 wherein said protrusion is sharply pointed at an apex and said apex extends outward away from a plane of the first electrode. 11. The print head according to claim 10 wherein said apex pierces a plane of said second electrode. 12. The print head according to claim 10 wherein said protrusion is conical in shape. 13. The print head according to claim 1 wherein said plurality of field emission devices and said third electrode are independently selectable to between operable and inoperable modes such that each of said field emission devices in said plurality of field emission devices is addressable. 14. The print head according to claim 1 further comprising optical reflector means secured to said illuminescent means for allowing transmission of electrons therethrough while being reflective of said light generated by said illuminescent means. 15. The print head according to claim 14 wherein said optical reflector means is a thin film of aluminum. 16. A print head for use in and optical printer where said print head receives a data signal representative of an image and translates the data signal into a light signal which is projected onto a photosensitive medium to reproduce the image, said print head comprising a matrix of field emission devices where said field emission devices are individually addressable to selectively emit individual electron beams; and a photo emissive layer disposed above the matrix of field emission devices which is excitable by the individual electron beams to produce a spot of light for each incidence of excitation by each of said individual electron beams such that the spot of light images one pixel onto the photosensitive medium. 17. A print head for an optical printer comprising an array of individually addressable field emission devices where said field emission devices are selectable to individually emit electron beams; and a photo emissive layer disposed in a path of said electron beams such that said photo emissive layer emits a localized spot of light for each excitation by each of said electron beams. light conduction means disposed above said array and in mechanical contact with said photo emissive layer for conducting the light to the photosensitive medium. 18. The print head according to claim 17 wherein said light conduction means is fiber optic to inhibit light piping. 19. A print head projecting image forming light onto a photosensitive medium comprising at least one row of field emission devices for projecting one of three colors; and a column substantially orthogonally to said at least one row such that selectively applying a voltage to the column and the at least one row causes the field emission device to emit an electron beam; and a faceplate coated with a luminophor which is excitable by incident electrons to project light onto the photosensitive medium. 20. The print head according to claim 19 wherein said at least one row is at least two rows for each of the three colors where each of the at least two rows projects less than all of the image forming light required for each of the three colors. 21. The print head according to claim 20 further comprising time delay integration means for passing a signal representative of a line of an image repetitively to each of said at least two rows of field emission devices with a time delay between transmission to each row subsequent to a first of the at least two rows.

Inventors: DELPICO JOSEPH, REISCH MICHAEL L, DELPICO, JOSEPH, REISCH, MICHAEL, L.
Applicants: POLAROID CORP, POLAROID CORPORATION
Application year: 1995
Main classification: H01J1
IPC codes: H01J1, H01J29, H01J31