US 3548236 A
Description (OCR text may contain errors)
Z. J. KISS Dec. 15, 1970 DARK TRACE CATHODE RAY TUBE WITH PHOTOCHROMIC IMAGE SCREEN Filed Jan. 24, 1968 SWS@ x. .,m
.w Q 25m. h hmkowwwwwwwwnw conm. o www l' N .n NN QNSNE m Tx um QEN UQ l1 W mxmu m M l) -mw A 7 r E E H N :l N. Z Nu U. Q QQ .muwm E w mnm United States Patent Olhce DARK TRACE CATHODE RAY TUBE WITH PHOTOCHROMIC IMAGE SCREEN Zoltan I. Kiss, Belle Mead, NJ., assignor to RCA Corporation, a corporation of Delaware Filed Jan. 24, 1968, Ser. No. 700,148 Int. Cl. F21k 2/ 00; H01j 29/14, 29/26 U.S. Cl. 313-91 11 Claims ABSTRACT OF THE DISCLOSURE In a cathode ray tube, an image screen is comprised .of a non-luminescent, photochromic material characterized in that a stable, visible, dark trace image formed on this maten'al is completely erasable by a photon induced electron charge transfer transition.
BACKGROUND OF THE INVENTION Dark trace cathode ray tubes are known in the art. U.S. Pat. No. 2,432,908 issued to Humboldt W. Leverenz describes a dark trace cathode ray tube comprising an alkali halide target material. `In this tube, electron beam bombardment of the alkali halide material creates color centers in the material. These color centers form an image which can be viewed by transmitted or reflected light. In order to completely erase this image and dissipate the color centers, one either must wait for the normal thermal decay or provide heat to the alkali halide to accelerate erasure. Heat for erasure has been provided by various means including heating filaments, ultra violet and infra red light and high intensity illumination within the absorption band of the alkali halide target. However, erasure due to heat presents two problems. One problem is that the erasure time is often longer than desirable and the second problem is that a new image cannot be formed on the target until the target has lost a substantial proportion of the heat applied to it during erasure. Hence, it is desirable to have a cathode ray tube in which an image can be formed by an electron beam and can be erased by means other than thermal decay. For example, images that can be erased due to quantized charge transfer transitions would be superior to prior art dark trace cathode ray tubes in which the images are erased by heating.
In U.S. Pat. No. 2,563,472, issued to Humboldt W. Leverenz, a cathode ray tube having a scotophor target is disclosed. The cathode ray tube disclosed therein has inducible and eradicable absorption bands in the invisible regions of the spectrum. Images formed on this type of tube are invisible to the eye and must be used in conjunction with an image converter for the images to be seen by an observer. Some invisible trace cathode ray tubes as well as some visible dark trace cathode ray tubes have the disadvantage of luminescing when exposed to cathode rays. This luminescence is unwanted and distracting when used in the screen of a dark trace cathode ray tube.
SUMMARY OF THE INVENTION In a cathode ray tube, an image screen is comprised of a non-luminescent, photochromic material characterized in that a stable, visible dark trace image formed on said material by an electron beam is completely erasable by a photo-induced electron charge transfer transition.
BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1-3 are graphical representations of the absorption characteristics of severa1 novel screen materials before and after erasure.
3,548,236 Patented Dec. 15, 1970 A photochromic material as used herein is a material having photon inducible and photon eradicable absorption bands in the visible regions of the electromagnetic spectrum. In the particular inorganic crystalline photochromic materials disclosed herein, the absorption bands are also inducible by electron beam bombardment of the photochromic material. The mechanism of erasure of the absorption bands in these materials involves a photon-induced electron charge transfer transition. In this mechanism, absorption of a photon induces electron transfer from one trap site in the photochromic crystal to another site in the photochromic crystal. This electron transfer causes erasure of a previously induced absorption band. Generally, the photochromic materials useful in the novel dark trace cathode ray tubes disclosed herein are non-cathodoluminescent.
Examples of such photochromic materials are: a1- kaline earth titanates containing small quantities of transition metal ions, such as, strontium titanate doped with iron and/or molybdenum, and calcium titanate doped with iron and/or molybdenum; sodalite preferably containing small quantities of transition metal ions, such as sodalite doped with iron; alkaline earth uorides containing small amounts of divalent rare earth ions, such as, calcium fluoride doped with cerium, lanthanum, gadolium or terbium; and molybdenum trioxide.
FIG. l is a graphical representation of characteristic absorption properties of one millimeter thick calcium titanate crystal doped with 0.05% iron and 0.1% molybdenum. Curve 1 shows the absorption characteristics of this material before being colored by an electron beam. This curve is identical to the absorption characteristics obtained after erasure or bleaching of a previously colored crystal. Such erasure is accomplished by exposing the colored crystal to high intensity light in the absorption band. Preferably light of about 4300 A. is used. Curve 2 shows the absorption characteristics of the calcium titanate after being colored by an electron beam. The crystal colored by an electron beam appears almostblack to the eye while the uncolored or erased material as shown in Curve 1 appears transparent and relatively neutral in color to the eye.
FIG. 2 is a graphical representation of the characteristic absorption properties of a sodalite or hackmenite crystal doped with iron. This crystal in its uncolored or bleached state does not absorb light in the visible region of the spectrum, as indicated by Curve 3, and appears neutral and completely transparent to the eye. After electron beam bombardment, the crystal takes on a magenta color in the regions bombarded by the electron beam and these regions have an absorption characteristic as shown in Curve 4. Erasure of this magenta color is accomplished by exposing the photochromic to light anywhere in the induced absorption band. Preferably, light of about 5200 A. is used since absorption and erasure efficiency is greatest at about this wavelength. In all of the novel materials, the efficiency of the photon-induced electron transfer transition which causes erasure is substantially less than the efficiency of writing the image onto the crystal. Due to this fact, normal room light will not cause substantial erasure of the image and a high intensity light at the wavelength of greatest bleaching eiciency is preferred for bleaching.
FIG. 3 is a graphical representation of the absorption characteristic of calcium fluoride doped with divalent cerium. This material in the state prior to coloring with an electron beam (as shown in Curve 5) is relatively transparent to visible light and possesses an absorption band which peaks at about 4000 A. When either light in the band around 4000 A. or an electron beam impinges on the calcium fluoride crystal, the absorption characteristics change to that shown in Curve 6, leaving a visible image on the crystal due to an increase in absorption in a wavelength ban from about 4800 A. to about 6400 A. The absorption characteristic, as shown in Curve 6, makes the crystal appear green to the eye under white light conditions. This absorption characteristic can be erased by shining intense green light upon the crystal.
In FIG. 4 a cathode ray tube 10 having a screen 11 comprised of a photochromic material as disclosed herein, is shown. The cathode ray tube comprises an evacuated envelope 12 formed with a bulb portion 13 and a neck portion 14 extending at an angle to the axis of the bulb portion 13 as shown. Within the bulb portion 13 of the tube 10 is applied a crystalline film of a suitable photochromic material 11 such as calcium titanate doped with divalent iron and having the characteristics as described above.
The photochromic film or screen 11 is put down upon a flat optically transparent portion 15 of the bulb 13. The opposite wall 16 of the bulb 13 is also a at portion and optically transparent to permit light to pass undistorted therethrough. Within the neck portion 14 of the cathode ray tube 10, is an electron gun structure 17 for forming and focusing a cathode ray beam upon the photochromic screen 11. The electron gun structure 17 may be of any conventional design and is well known in the art. The electron beam formed by the gun structure may be scanned over the surface of the screen by horizontal deflection coils 18 and vertical deflection coils 19 to provide the horizonatl and vertical scansion. The horizontal and vertical deflection coils 18 and 19 are respectively connected to appropriate circuits as is well known in the art.
The various electrode terminals 21 of the gun are connected, as shown, to a D.C. voltage supply 22 to provide appropriate operating voltages to the gun structure. The cathode ray tube 10 is connected through a signal receiver 23 to the voltage supply 22 to provide an operating voltage for maintaining an appropriate cutoff voltage for the electron beam. The receiver may be of any type to modulate the cathode ray beam of the tube 10.
A source of radiation 31 provides an emission of visible radiation including radiation within the electron beaminduced absorption band of the screen. The radiation is projected through the transparent bulb wall upon the screen. This source of radiation can be a white light tungssten bulb. In this structure the photochromic screen 11 is preferably transparent to light in its unexcited state so that the radiation from the radiation source 31 will be transmitted through the screen to a viewer 32 positioned on the same side of the tube as the Screen. Such a screen can be made from single crystal photochromic material, transparent evaporated layers or transparent hot pressed layers of the photochromic material or by having the photochromic material imbedded in a glass or plastic having the same index of refraction as the photochromic material so as to prevent scattering of light from the surfaces of individual photochromic particles comprising the screen. Generally, the photochromic screen need not be made greater than the penetration depth of the electron beam. This depth is a function of beam voltage and density of the photochromic screen. In operation of the embodiment as shown in FIG. 4, a desired signal voltage applied by the receiver 23 to the electron gun 17 will cause the electron beam to create visible traces on the photochromic screen. Signal voltages which modulate the electron beam while the beam is scanned by the coils 18 and 19 can create a predetermined desired image on the-screen 11 by changing the absorption charactreistics of selected areas of the screen. The images thus formed can then be either selectively or completely erased by light in the absorption band of an intensity greater than that from the radiation source 31. For example, radiation of the desired frequency from a laser 33 may provide erasure. This radiation canv be scanned by means known in the art to provide selective erasure. Alternatively, a light source such as a high intensity flood light may be used to accomplish erasure. The structure of the cathode ray tube as shown in FIG. 4 may be termed the transmissive mode or structure of the device. However, the embodiment of a cathode ray tube, as shown in FIG. 5, is preferable for most purposes. In this tube 40, a photochromic screen 41 is supported by an optically transparent facepalte 42. A reective coating 43 may be desposited coextensively with the screen 41, as shown. The screen 41 is comprised of a finely-divided powdered photochromic material which reflects light due to Scattering of the light by the powder. When the screen is comprised of such a powdered photochromic material, a viewer observes traces or images on the screen by means of reflected light rather than by means of transmitted light as described above. The particle size of the powder should generally be less than about 5 microns and preferably be about 1 micron. The screen 41 thickness is preferably about l0 microns thick. The screen 41 can be fabricated in the same manner as phosphor screens for cathode ray tubes. Such methods are well known in the art and need not be discussed herein. With this tube, the light source should be on the same side of the screen as the viewer, namely in front of the screen.
The face 42 of the tube 40, which supports the rotochromic screen should be optically transparent to light in the absorption band of the excited photochromic material. An image formed on the screen 41 can be erased by shining light upon the screen within the absorption band of the photochromic material. Preferably, the light used for erasure is of high intensity due to the fact that the eficiency of erasing is less than that for writing. The image formed on the screen can either be totally erased or in the alternative selected portions of the image can be erased by, for example, means of a liber optic light pen 44 which directs the erasing light to such selected portions. One reason why a cathode ray tube having a powdered photochromic screen is preferable as compared with the tube type shown in FIG. 4 is that a higher contrast ratio and a darker appearing image can be formed on the powdered screen. This is due to the fact that internal reection of the light in the powder particles gives the light an effective longer absorption path and hence a greater optical density.
Some of the non-absorbed light is normally lost due to transmittance or scattering in a direction away from the viewer. This results in a loss of contrast ratio and brightness of the image screen. This loss can be substantially reduced by including the reflecting layer 43, such as an evaporated aluminum film behind the photochromic screen as shown in FIG. 5.
The photochromic materials having the highest contrast ratios between its bleached state and its image induced state are generally preferably for use as a cathode ray tube screen. Of the materials disclosed herein, sodalite containing from about to 2000 p.p.m. of iron and preferably about 1000 p.p.m iron, and calcium titanate containing from about 100 to 2000 p.p.m. of iron and molybdenum are preferred. The latter being preferred due to the black image formed by an electron beam thereby resulting in a black and white picture.
Referring to FIG. 6, the cathode ray tube 60 includes a layer 61 of a cathodoluminescent phosphor disposed on the photochromic layer 41, as shown. The phosphor layer 61 emits light in response to electron beam impingement of the phosphor layer 61 of a wavelength within the induced absorption or read band of the photochromic layer 41.
In operation of the tube 60, electron gun voltages are adjusted to produce a high voltage electron beam which penetrates the phosphor layer 61 and causes darkening on the photochromic layer 41 and a lower voltage electron beam which does not penetrate the phosphor layer' 41 but instead causes emission of the phosphor. The electron beam is made to scan both the photochromic layer 41 and the phosphor layer 61. The light emitted by the phosphor layer 61 is the light used to read the image recorded on the photochromic layer 41. This is the same wavelength light that causes erasure of the image.
Alternatively, electron beams for operating such a tube may be developed yby the use of two electron beam guns in the tube structure. Here, one gun would be adjusted at a relatively high voltage so as to develop a beam which would penetrate the phosphor layer and write images on the photochromic layer while the second gun would be adjusted to a voltage which would cause phosphor emission and would not significantly penetrate into the photochromic layer.
The phosphor in such a tube must be matched to the particular photochromic layer used. The phosphor layer is preferably relatively thin and generally is in the order of about 1 to 10 microns thick. In addition, it is ad vantageous for the phosphor to be of the fast decay type so as to reduce erasure of the image due to the read light and to prevent picture smearing. Preferably, the phosphor has a decay rate equivalent to or greater than the elemental image scan rate of the electron beam. Generally, this is in the order of about 1041 seconds or less. It is also preferable to back the phosphor layer with a reflective coating such as aluminum so as to efficiently utilize the light emitted by the phosphor. An example of one suitable phosphor-photochromic combination in a tube of this type is the combination of a gallium phosphide or cerium doped yttrium aluminum garnet phosphor with a sodalite photochromic layer.
As previously indicated, a cathode ray tube of this type can be used in conjunction with a detector 62 which in turn can feed information stored on the cathode ray tube back to a computer. The same computer can be used to up-date the information on the cathode ray tube.
What is claimed is:
1. An electron discharge device comprising an evacuated envelope, a photochromic screen mounted within said envelope, said screen formed of at least one photochromic material chosen from the group consisting of alkaline earth lluorides containing a rare earth ion impurity selected from Ce, La, Tb and Gd, and alkaline earth titanates containing transition metal ion impurities and having an electron beam inducible and photoninduced charge transfer transition eradicable absorption band in the visible regions of the spectrum, and electron beam means within said envelope for producing an absorption pattern on said screen.
2. The device recited in claim 1 wherein said photochromic screen comprises calcium fluoride containing an impurity ion selected from the group of divalent rare earth ions consisting of cerium, lanthanum, gadolinium and terbium.
3. The electron discharge device recited in claim 1 wherein the photochromic screen is comprised of a titanate selected from the group consisting of calcium titanate and strontium titanate and wherein said titanate contans small proportions of at least one transition metal ion.
4. The device described in claim 3 wherein the transition metal ion is at least one member of the group consisting of iron and molybdenum.
5v. The device described in claim 3 wherein the screen is comprised of powdered calcium titanate containing small proportions of iron ions.
6. The electron discharge device recited in claim 1 wherein said photochromic screen is comprised of nely divided photochromic powder.
7. The device recited in claim 6 wherein said powder particles have an average particle size of less than 5 microns and wherein said screen includes a reective layer behind said screen.
8. The device recited in claim 7 wherein said powder particles have an average particle size of about 1 micron and said screen formed from said powder has a thickness of about 10 microns.
9. The electron discharge device recited in claim 1 including a reflecting layer contiguous with and behind said photochromic screen.
10. In a dark trace cathode rayy tube Comprising (a) an evacuated envelope,
(b) a transparent screen support,
(c) a viewing screen on said support and within said envelope, said screen comprised of a layer of photochromic material which exhibits darkening thereon in response to electron impingement thereon and which dargening is eradicable by a photon induced electron charged transfer transition induced by light of a given frequency,
(d) means for projecting an electron beam onto said screen to produce a darkening of selected areas thereof, and
(e) means for reading images stored on said screen, said reading means being Iwithin said envelope, the improvement wherein said reading means comprises a cathode-luminescent phosphor layer having a decay time of less than 10"7 seconds and characterized in that it emits light in a visible read band of said photochromic screen, and electron beam means for causing said emission of said phosphor layer.
11. In combination, a dark trace cathode ray tube as described in claim 10 and an image detector coupled thereto.
References Cited UNITED STATES PATENTS 2,416,574 2/1947 Fonda 313-91 2,432,908 12/ 1947 Leverenz 313-91 3,253,497 5/1966 Dreyer S13-91X 3,452,332 6/1969 Bron et al 178-7.87X 3,453,604 7/ 1969 Geusic et al S13-92X ROBERT SEGAL, Primary Examiner V. LAFRANCHI, Assistant Examiner U.S. Cl. X.R.