High writing speed dark-trace tube with flood beam enhancement
United States Patent 1 1 1111 3,875,447  
 Kazan Apr. 1, 1975 [54] HIGH WRITING SPEED DARK-TRACE 3,700,791 10/1972 Bosomworth 313/91 X TUBE WITH FLOOD BEAM 3,796,909 3/1974 Chang et al 315/12 ENHANCEMENT E R b ts runary xammer- 0 er ega [75] Inventor: Kazan Mohegan Lake Attorney, Agent, or Firm-Sughrue, Rothwell, Mion,  
  Zinn &amp; Macpeak [73] Assignee: International Business Machines Corporation, Armonk, NY. [57] ABSTRACT [22] Filed: Dec. 12, 1972 A dark trace cathode ray tube comprising an evacuated envelope with a transparent faceplate, a cath- [211 Appl&#39; 314289 odochromic screen mounted within said envelope and adjacent said faceplate comprising a layer of a cath- [52] US. Cl. 313/465, 178/7.87 odochromic material on a transparent conductive sup- [51] Int. Cl. I-I0lj 29/14, HOlj 31/08 port, means for scanning&#39;selected areas of said cath- [58] Field of Search 313/91; 315/10, 12 (US. only) odochromic screen with electrons, means for flooding said cathodochromic screen with electrons of an en- [56] References Cited ergy appropriate to darken said scanned areas but of UNITED STATES PATENTS an energy which substantially is ineffective to darken 5 817 H1950 skcum 315/12 the unscanned area, a grounded collector mesh posi- 31111111214 9/1908 Hamanri 350/160 P ioned adjacent Said cathodOchromic Screen and the 3.413505 11/1968 Hart ct 111 313/91 P Of Said electrons from Said flooding means and 3.548.236 12/1970 said scanning means. 3.560.782 2/1971 Hamunn 313/91 3.683.358 8/1972 Eichclbcrgcr 315/169 TV 4 Clam, 2 D&#39;awmg Flgures HIGH WRITING SPEED DARK-TRACE TUBE WITH FLOOD BEAM ENHANCEMENT BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a cathode ray storage tube. More particularly, this invention relates to a dark trace cathode ray tube having a high speed writing ability and a high-contrast image generating ability.  
 2. Description of the Prior Art Known dark-trace or cathodochromic tubes are severely limited in that the speed with which a pattern can be generated thereon is low, in some cases a minute or longer being required to record an entire frame of information, using conventional dark-trace cathode ray tubes. This slow writing speed results from the fact that a high charge density is required on the screen to produce a sufficiently high-contrast image which does not fade under normal illumination.  
  Conventional dark-trace cathode ray tubes comprise an electron gun for providing a deflectable electron beam for scanning a screen containing thereon a light transducing material which comprises a scotophor or a mixture of scotophors. On irradiation of the scotophor layer with an electron beam, a dark-trace record in the scotophor layer is formed due to the darkening selectively of the area so irradiated. With these conventional dark trace cathode ray tubes, an image or a pattern can be generated on such a screen by selectively irradiating areas of the screen whereby those irradiated areas darken, forming the pattern. Erasure of the screen to remove the dark-trace record is generally accomplished by use of heat or by flooding the screen with light of a suitable&#39;wavelength.  
  US. Pat. No. 2,755,404 discloses a method of faster erasure of a pattern formed on a dark-trace cathode ray tube having a scotophor screen attached to a sheet of a light transluscent material with the scotophor layer being coated with a layer of microcrystalline aluminum. High velocity electrons from an electron gun are directed onto the scotophor screen to form the darktrace in the scotophor screen in accordance with the areas of the scotophor screen irradiated with the electrons. An infrared heating element is employed to uniformly heat the scotophor material for erasure of the image so formed with the aluminum layer absorbing the infrared radiant energy causing faster erasure of the dark-trace record.  
  US. Pat. No. 3,l48,28l discloses a cathode ray storage tube utilizing a dark-trace layer and a phosphor layer. The cathode ray storage tube has a faceplate containing successive layers of a dark-trace material and a phosphor, an electron gun to direct an electron beam onto the layers, the electron gun containing means for changing the&#39;energy of the electron beam generated, and a means for heating the dark-trace tube to render the dark-trace material transparent. The faceplate is irradiated with an electron beam, and an information pattern registered in the dark-trace mate rial, the energy of the electron beam being increased such that the electron beam penetrates the phosphor layer and renders selected areas of the dark trace material opaque. The energy of the electron beam from the electron gun is then reduced such that the electron beam is unable to penetrate the phosphor to reach the dark-trace material. The flying spot of light, however, produced by the fluorescence of the phosphor when viewed by a photocell external to the tubeface produces an electrical output corresponding to the information stored in the dark-trace layer because of the varying opaqueness of the dark-trace material.  
  US. Pat. No. 3,447,020 relates to a dark-trace storage tube containing a screen combination which utilizes radiant energy reflected to erase the screen. The dark-trace storage tube disclosed in this patent employs an electron gun for bombarding a screen with electrons to create a dark-trace record in a dark-trace screen supported on a transparent sheet. A black body light absorbent layer such as&#39;chromium oxide or Nichrome is contained on the surface of the faceplate opposite the direction of exposure from the electron gun. The arrangement also contains a source for generating radiant energy for uniformly heating the entire surface of the black body light absorbent layer so as to increase the temperature causing erasure of the dark-trace material.  
  US. Pat. No. 3,548,236 discloses a dark trace cathode ray tube which comprises an image screen containing a non-luminescent, photochromic material which forms a visible dark-trace image upon irradiation with an electron beam and which is erasable by photoinduced electron charge transfer transition, whereby light of a high intensity and appropriate wavelength can be used to erase the photochromic material and destroy the dark-trace record formed by the electron beam illumination.  
  Conventional dark-trace cathode ray tubes have definite limitations and disadvantages, an important one being their low writing speed. This low writing speed arises as a result of the necessity of providing a high charge density on the screen to obtain an image of sufficient contrast which will not fade under normal illumination. Although the devices discussed in detail above are all concerned with methods of erasing information written on a dark-trace cathode ray tube, no method is described for any of these devices which would allow an increase in the speed with which writing can be generated on the dark-trace screen.  
  Accordingly, an object of this invention is to&#39; provide a dark-trace tube of the cathode ray tube type having a high writing speed.  
  Another object of this invention is to provide a darktrace cathode ray tube upon which a high-contrast dark-trace image can be formed but also upon which the high-contrast dark-trace image can be formed at high speed.  
  It is another object of this invention to provide a dark trace cathode ray tube whereupon a charge pattern is generated on the surface of a cathodochromic screen at high speed with the screen being subsequently flooded with electrons of low energy to cause a highcontrast dark-trace pattern to develop.  
 SUMMARY OF THE INVENTION The above objects are accomplished by this invention which is a dark-trace cathode ray tube comprising an evacuated envelope with a transparent faceplate, a cathodochromic screen mounted within said envelope adjacent said faceplate and comprising a layer of a cathodochromic material (as used herein and in the appended claims, the term cathodochromic is intended to cover photochromic materials as well) on a transparent conductive support, a means for scanning a selected area of the cathodochromic screen with eleccathodochromic screen and in the path of the electrons from the electron flooding means and the electron scanning means.  
 BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS FIGS. 1 and 2 show embodiments of the high writing speed dark-trace tube according to this invention.  
 DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the device of this invention is shown in FIG. 1 in which I is an envelope, 2 is a transparent support layer, 3 is a backplate of a transparent electrically-conducting material and 4 is a cathodochromic screen comprising a layer of a cathodochromic material or scotophor, such as a photochromic sodalite;  
 5 is an electron gun whose electron beam can be appropriately deflected by a deflection means conventional in the art (not shown) such that the electron beam from the electron gun can be impinged upon selected areas of the cathodochromic screen 4 in response to an electrical signal, for example, from a character generator or T-V source; 6 is a second electron gun which can provide a low-energy beam of electrons for flooding uniformly the cathodochromic screen 4; 7 is a collector mesh or grid positioned adjacent the cathodochromic screen 4 and interposed in the path of the electron beam from the writing gun 5 and the flood beam from the flood gun 6 (instead of the collector mesh 7, a collector cylinder without the mesh may be used; the use of such a collector mesh or collector cylinder is wellknown. See, for example, B. Kazan and M. Knoll, Elec- Ironic Image Storage, Academic Press, N.Y., 1968); 8 is a resistance element which maintains the backplate 3 at ground potential. For erasure of a charge pattern, a pulse as shown is applied across resistance element 8. Erasure of the dark-trace pattern also may be achieved by heating elements 10 and 11. Alternatively, backplate 2 may be heated directly by passage of current therethrough via terminals 9 and 12. In general, the  
 . transparent support plate 2 is of mica or glass. Suitable transparent conductor materials which can be employed for the backplate 3 are tin oxide or indium oxide. Suitable cathodochromic materials which can be employed to prepare the cathodochromic screen 4 are (a) sodalites containing chlorine, bromine or iodine, (b) doped titanes such as SrTiO (Fe, M), or (c) alkali metalhalides such as potassium chloride, potassium bromide or potassium iodine. Any of the conventional electron guns whereby a beam of electron can be generated in a cathode ray tube and deflected appropriately can be employed as the electron beam source and any of the commonly employed electron guns which can generate a flood of a beam of electrons such as employed in conventional viewing storage tube can be employed as the electron gun 6. The collector mesh or grid 7 is generally of a conductive material such as nickel and the collector mesh is grounded.  
  In the operation of the embodiment shown and described in FIG. 1, the surface of the dark-trace screen is assumed to be uniformly charged to l00 volts, the  
 flood gun cathode being maintained at l00 volts. This uniform charging can be the result of previous erasure of the dark-trace cathode ray tube. To form a darktrace record on the dark-trace screen, selected areas of the screen are scanned by appropriately directing the beam of electrons from the writing electron gun 5, the cathode of electron gun 5 is being maintained at about -2 KV. During writing the flood beam gun 6 can be maintained either on or off since the current density of the electrons flooding the dark-trace screen is substantially lower than the current density of the writing beam. The areas bombarded by the writing beam undergo a positive potential shift since the secondary emission ratio is greater than unity. If this potential shift is greater than the first crossover potential of the material, the flood beam will shift these areas to the potential of the grounded collector, maintaining them at this potential. However, those screen areas which were not bombarded by the electron beam from the writing electron gun 5 will be maintained by the electron flood beam at -l00 volts. Due to the relatively insulating nature of the cathodochromic screen 4, very little of the flood beam of electrons will land on those areas not bombarded during writing and flood beam electrons reaching those areas will arrive at the surface with essentially no energy. However, the areas previously scanned by the writing beam will continue to be bombarded by the flood beam with substantial energy, thus becomes increasingly darker. After an exposure time of several seconds, for example, to electrons from the flood beam, the potential pattern created by the writing beam will result in a high-contrast visible image. The flood beam electron source 6 as well as the writing beam electron source 5 may now be turned off. The resultant image formed in the cathodochromic screen 4 can now be viewed with external illumination as long as necessary. To erase the stored image the cathodochromic screen 4 can be heated for several seconds using an internal heating element such as is incorporated in conventional darktrace storage tubes. To prepare the cathodochromic dark trace tube of this invention for the recording of a new image, a negative erasing voltage pulse is applied to the backplate 3 with the flood beam turned on. If the decay time of this pulse is not too rapid, e.g., a few tenths of a second, the entire surface of the cathodochromic screen 4 will be shifted to the -l00 volts potential required for the writing of a new image. Thus, the combination of initially creating a potential pattern on the cathodochromic screen 4 followed by the flooding of the screen with low energy electrons, causes a high contrast dark-trace image to develop in a short time on the cathodochromic screen 4. As already mentioned, erasure of the screen to remove the image or to change the darktrace image formed can be efiected by heating the cathodochromic screen 4 using, for example, an internal heating element 3.  
  In a preferred aspect of this embodiment described in FIG. 1, the speed of image development and the degree of darkening may be enhanced, if during irradiation of the screen with flood beam electrons after writing, a potential, for example, of +200 volts, is applied to backplate 3 rather than maintaining the backplate at ground potential. This results in a high field across the cathodochromic screen 4 causing the F-centers (i.e., regions of opacity generated near the surface of the material by the flood beam) to migrate toward the opposite surface of the cathodochromic layer 4, thus increasing the darkening or.light absorption of the layer which would otherwise be limited by the saturation of F-centers at the bombarded surface of the screen.  
  A second embodiment of the high speed dark-trace cathode ray tube of this invention is shown in FIG. 2. In FIG. 2, the dark-trace cathode ray tube 19 comprises source 23 of an electron beam which can be deflected using means conventional in the art, not shown, to bombard selected areas of the target screen 16, a source of electrons 13 for uniformly flooding the target, a collector grid 14 for the electrons and a faceplate 18. The target screen 16 comprises a UV-emitting phosphor layer and a layer of a photochromic material 20 and, a thin film heating element 17. In operation, an input signal is applied to electron beam source 23 to modulate the electron beam therefrom causing the electron beam generated to impinge upon selected areas of the phosphor layer 15, resulting in a charge pattern being formed on the phosphor layer 15. A stored luminescent pattern will then be produced at the phosphor layer corresponding to the charge pattern when the screen is flooded by the low-energy electrons from electron flood gun 13. Assuming that the phosphor emits ultraviolet light, the UV light pattern will cause the photochromic material in layer 20 to darken locally in accordance with the ultraviolet light pattern generated. The dark-trace pattern produced by ultraviolet light (unlike the dark-trace pattern produced by direct bombardment of the cathodochromic material in FIG. 1) will generally slowly fade as a result of illumination with ambient or other viewing light. The species UV phosphor and the specific photochromic material are so selected that the ultraviolet light generated is appropriate to cause the photochromic material in layer 20 to darken. When desired, a small amount of an insulating powder material such as magnesium oxide can be mixed with the UV phosphor of layer 15 to increase the secondary emission ratio, thus increasing the writing speed. The UV phosphor layer can also be applied as a mosaic of small elements or as a porous layer to prevent spreading or migration of the charge pattern. As in the case of FIG. 1, erasure of the dark-trace image thus formed can be accomplished by activation of the resistive heating element 17 adjacent the photochromic layer 20, for example, by passing an electric current through the terminals 21 and 22 or by illumination of the dark-trace screen with light of the appropriate wavelength.  
  In both embodiments of the dark-trace cathode ray storage tube of this invention, the screen can be scanned by the writing beam at very high speeds, since this beam is only required to establish a charge pattern, the desired contrast of the visible dark-trace image being obtained by flooding of the screen with electrons from the flood gun. In operation, the screen target can be maintained at a slightly elevated temperature, taking into consideration the rate of darkening of the image by flood beam irradiation and the bleaching rate due to heating or illumination with light to control the growth and decay time of a recorded image. In addition, as an additional advantage of the embodiments described herein, a dark-trace pattern may be recorded on the photochromic screen using a light pen which generates a spot of ultraviolet light. This pattern may be superimposed on an already existing pattern previously written on the screen.  
  Suitable photochromic materials which can be employed in this invention are photochromic sodalite such as Na AI Si O 2NaCl(S), doped titanates such as SrTiO (Fe, Mo), and fluorides such as CaF (La). In addition, suitable UV phosphors which can be employed in combination therewith are materials such as YAGzCe or CaMgSiO Ce.  
  While the invention has been described in detail and in terms of specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.  
 What is claimed is:  
  1. A dark-trace cathode ray tube comprising an evacuated envelope with a transparent faceplate,  
 screen means mounted within said envelope and adjacent said faceplate comprising a layer of a photochromic material on a transparent electrically conductive support, and a UV phosphor layer coated on said photochromic material layer on the side opposite said faceplate,  
 a writing means for scanning selected areas of said photochromic screen means with electrons and thus changing the electric potential of selected areas in said photochromic screen means, flooding means for flooding said photochromic screen means, with low energy electrons to maintain a bistable potential pattern and producing darkening of the more positive areas, whereby the electrons from said writing means form a charge pattern on said phosphor layer and the electrons from said flooding means maintain a bistable potential pattern and causes said phosphor layer to fluoresce in accordance with said charge pattern wherein said photochromic material darkens in accordance with the fluorescence,  
 a collector means positioned adjacent to said photochromic screen means and in the path of electrons from said writing means and said flooding means, the secondary emission ratio of said photochromic screen means being greater than unity and said collector means serving to collect the secondary emission electrons emitted from said photochromic screen means when selected areas are scanned by said writing means to establish said bistable potential pattern and further serving to collect the secondary emission electrons from said photochromic screen means when said selected areas are bombarded by electrons from said flooding means thereby shifting said selected areas to the potential of said collector means, said photochromic screen means being initially uniformly charged to the same potential as said flooding means, said potential being negative with respect to said collector means but substantially positive with respect to said writing means, and  
 erase means for erasing the dark-trace pattern on said photochromic screens means comprising terminal means connected to said transparent electrically conductive support and adapted to receive an erase pulse to develop a uniform negative charge pattern over said screen means.  
  2. The dark trace cathode ray tube of claim 1, wherein said photochromic material is selected from the group consisting of sodalite, alkali metal halides, rare earth doped fluorides and doped titanates.  
 vvherein the potential of said flooding means is -l00 volts and the potential of said writing means is about UNYI&#39;ED STATES PATENT AND TRADENIARK OFFICE CERTHEQATE Gt April 1, 1975 Benjamin KA ZAN PATENT NO.  
 DATED I: 1 EN&#39;I&#39;OR(S) I It is certified that error appears in the above -iden&#39;tifi-ed patent and that said Letters Patent are hereby corrected shown belsw:  
 IN THE SPE Cl FICA TION:  
 Column 3, line 4 line 45 line 55 line 56 Column 4, line 7 line 40 line 54 Column 5, line 32 IN THE C LAIMS: M  
 Column 6, line 34 [SEAL] delete &#34;electron&#34; (second occurrence) and insert electrons after &#34;gun 5&#34; delete &#34;is&#34; delete &#34;darktrace&#34; and insert dark trace delete &#34;darktrace&#34; and insert dark trace delete &#34;species&#34; and insert specific delete &#34;causes&#34; and insert cause Signed and Scaled this twenty-second Day of July 1975 A rtest:  
 RUTH C. MASON Arresting Officer C, MARSHALL DANN Commissioner of Parents and Trademarks