US 2909667 A
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R. K. ORTHUBER ET A1. 2,909,667
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RICHARD K. ORTHUBER y CHRISTIAN C. LARSON ATTORNEY United States Patent O IMAGE DEVICE Richard K. Orthuber and Christian C. Larson, Fort Wayne, Ind., assignors to International Telephone and Telegraph Corporation Application August 16, 1955, Serial No. 528,700 s claims. (Cl. 25o- 213) The present invention relates to an image device, and more particularly 'to a solid-State laminated cell which is capable of reproducing in visible form a radiation image.
There is disclosed in Ullery application Ser. No. 362,204, tiled lune l7, 1953 and entitled Display Amplier and Method of Making Same, now Patent No. 2,773,992, a display-amplifying device which is capable of reproducing or intensifying a radiation image. This device is a laminated structure composed of electroluminescent phosphor and photosensitive layers sandwiched between two plate-like electrodes. A source of alternating voltage is coupled to these two electrodes, and the impedence of the photosensitive layer in the absence of radiation is designed to be so high that such voltage will not cause the phosphor layer to luminesce.
However, in the presence of exciting radiation the imf pedance of the photosensitive layer is reduced sufficiently to impress a greater amount of voltage over the phosphor layer, thereby causing it to luminesce.
In the Ullery application7 the photosensitive layer is particularly designed to provide the necessary impedance control for regulating the voltage applied to the phosphor layer. This photosensitivelayer in one form consisted of a base panel of quartz or photoform glass having a plurality of spaced transverse apertures which were lined respectively with evaporated photoconductive material such as cadmium sulphide. The photoconductive material lining the apertures constituted the impedancechanging portion of the display device and served to control the degree of excitation of the adjoining phosphor material.
This invention is intended to constitute an improvement over the device of the earlier application and to provide a slightly diferent mode of operation 4which adapts the display device to a cathode ray tube and in addition utilizes regeneration in the display device itself for intensifying the reproduced image.
It is an object of this invention to provide a method and apparatus for intensifying an image.
It is another object of this invention to provide a method for utilizing feedback radiation for intensifying a reproduced image. A
It is still another object to provide a solid-state display device which utilizes feedbackbetween the phosphor and vphotosensitive layers in such a manner as to provide a faithful reproduction of a given image, which reproduction includes shades of gray as well as extreme hi-ghlight and low-light levels.
The above-mentioned and other features Vand objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:
Fig. 1 is a cross-sectional view in diagram form of a display device of this invention;
se ICC Fig. 2 is a graph used in explaining the operation of this invention;
Fig. 3 is an enlarged fragmentary cross-section of one specific embodiment of this invention;
Fig. 4 is an enlarged section of the photosensitive layer of Fig. 3;
Fig. 5 is a cross-section of a cathode ray device utilizing the cell of Fig. 3;
Fig. 6 is a block ydiagram of a circuit which includes the tube of Fig. 5;
Fig. 7 is an illustration of wave forms used in explain-- ing the operation of the circuit of Fig. 6; and i e Fig. 8 is a diagrammatic illustration of a switching device used in conjunction with a laminated cell of slightly different construction than that shown in Fig. 3.
Referring to Fig. l of the drawings, the display device: or laminated cell is indicated by the reference numeral 1.. The laminations of this cell comprise a glass or the like supporting plate 2, a transparent film of conductive material 3, such as evaporated silver or a layer resulting, from a reaction of stannous chloride with the glass plate (NESA) applied to one side of plate v2, a lamina of photoconductive material 4 (cadmium sulphide, for example), applied to the film 3, a layer of electroluminescent phosphor material 5 mounted on the layer 4, another tilm 6 of conductive material on the phosphor lamina 5 and a supporting glass plate 7 which may be applied to the film 6. A light attenuating insulating layer (not shown in Fig. l) may be interposed between layers 4 and 5 for limiting light feedback therebetween.
A source of alternating voltage such as 600 volts at 400 cycles is connected to the films or plates 3 and 6. Assuming no incident radiation on the photosensitive layer 4, the impedance of this layer is designed to be so high that an insuicient portion of the applied voltage will appear over the phosphor layer 5 itself to cause excitation thereof. However, with incident radiation on the layer 4, the impedance is reduced suiciently to cause an increase over the phosphor layer 5 to an extent sufficient to cause luminescence thereof. By properly designing the impedances of the two layers 4 and 5, the phosphor layer 5 may be made to luminesce with a brightness corresponding to incident radiation falling on the photosensitive layer 4. The fundamentals of operation of a display device of this character are fully explained in the aforementioned Ullery application as well as in published literature.
When the phosphor layer 5 is luminescing a certain amount of this luminescence is returned to the photoconductive layer 4. This returned or feedback light causes a further change in the impedance of the layer 4, thereby causing further excitation of the phosphor layer 5. If this action is allowed to continue, a saturation condition will be reached at which the phosphor will continue to luminesce and the feedback light falling on the photoconductive layer will serve to hold this saturation condition. A
This feedback or regenerative action is readily understood by reference to the graph of Fig. 2. This graph presents the response curve of a typical display device or screen and plots light ux emitted per unit area by the phosphor as a function of photoconductor illumination. The curves are calculated by assuming (l) the brightness of the electroluminescent panel increases with the third power of the applied alternating current voltage; (24) the photoconductive lamina responds to the illumination in a linear fashion; (3) the capacitance of the photoconductive lamina is about 20% of the capacitance of anv photoconductor in millilumens per square centimeter. The ordinate represents phosphor brightness output also in terms of millilumens per square centimeter. 'The S-curve 8 is the display screen response cu-rve with no light-feedback while the straight line 9 represents the illumination of the photoconductive layer 4 caused by the phosphor radiation with approximately 10% of the light of the phosphor layer 5 being fed back thereto.
It is evident that once the phosphor 5 has been excited to approximately 8.5 units brightness (point which requires an illumination on the photoconductive layer of approximately .75 unit, this illumination can be provided by feedback between the phosphor and photoconductor, whereupon the phosphor will maintain this brightness without further incident radiation falling on the photoconductor. By means of this regeneration or feedback, the cell has driven itself to equilibrium brightness which is a stable condition. A similar stable condition or equilibrium brightness point can be seen to exist at point lll where the two curves 8 and 9 cross. Still a third equilibrium point is found at the intersection 12 of the feedback line 9 and the response curve 8. This point 12 however is unstable, but its importance resides in the fact that initial excitation to a point somewhat below three-tenths (0.3) unit will cause the phosphor brightness to drift down to the lower brightness point 11. Excitation to a point above point 12 will cause the brightness to build up until the upper stable equilibrium point 10 is reached.
Saturation brightness corresponding to point lit will be maintained indefinitely as long as the excitation voltage is applied to the screen. ln order to extinguish the screen itself, it is necessary to momentarily reduce or cut off the exciting voltage to such an extent that the gain of the screen drops so much, that the upper equilibrium points 12 and 1h disappear (dotted curve in Fig. 2).
These operating features as characterized by the graph of Fig. 2 can be utilized in different ways to obtain either images having no half-tones (shades of gray) or images which constitute faithful reproductions in gray scale of a given image.
In Figs. 3, 4 and 5 are shown one form of display screen which utilizes optical feedback for intensifying an image. With reference to these figures, the screen consists of a glass plate 13 which carries a transparent, conductive film 14 of any suitable material such as stannous oxide. On top of this film 14 is an electroluminescent phosphor lamina 15 which is in contact with a sheet of perforated glass 16. This glass preferably consists of conventional fotoform glass which can be etched photographically in closely controlled patterns to provide a plurality of spaced apertures 1S. T he upper surface of the perforated glass sheet carries a metallic coating 19 which may be applied by evaporation or the like. During this evaporation, care must be exercised to prevent metal from depositing on the inner walls of the apertures 13. This is done by lling the holes with a line powder of glass, chalk or magnesium oxide or similar finely comminuted powder just prior to evaporation of the metal. After evaporation, this powder is shaken or blown out of the apertures.
Photoconductive material 18a, such as cadmium sulphide, is evaporated from the underside of the glass sheet 16 onto the walls-of the apertures 18. The cadmium sulphide deposits on the underside of the glass sheet subsequently are removed by grinding or the like. Thus, each aperture r3 is provided with a photoconductive lining which is in contact at its upper end with the metallic coating i9 and at its lower end with the phosphor layer 15 ln one form of the invention, the glass sheet 16 is opaque to light which emanates from the phosphor layer 15. By doing this, the feedback path of the light between the phosphor and photoconductive layers is so restricted that light emitted from one phosphor elemental area can reach only the associated but not the laterally adjoining elements of the photoconductor.
It will now be apparent that each aperture 18, together with the adjacent elemental area of phosphor 15, forms an element of the display screen, which may, provided -the sensitivity or response of both the photoconductor and phosphor elements are high enough, provide gains higher than the light feedback between the phosphor and photoconductor. When these conditions prevail, the screen will reproduce a given image with brightness output corresponding to the saturation point 10 of Fig. 2.
The screen of Fig. 3 is mounted in the front end of the cathode ray tube 21 of Fig. 5 which is conventional as a television picture tube in every respect. The apertures 18 open inwardly so that the electron beam 22 may impinge the photoconductive linings 20. The beam 22 is scanned over the screen in the usual manner in accordance with conventional television standards, deliection coils 23 denoting the necessary deecting means for accomplishing this function. As the electron beam strikes, for example, the photoconductive lining 20 of one aperture 18, the impedance of this respective lining is reduced by bombardment-induced conductivity thereby causing an increase of exciting potential over the adjacent phosphor element 15. Thus a beam of uniform intensity scanned over the display screen at the usual frequencies will cause the entire panel to luminesce. By utilizing feedback from the phosphor, the panel will luminesce with saturation brightness.
It will be noted that the intensifier screen of Figs. 3 and 5 does not carry any kind of a supporting plate on top of the glass sheet 16. By so constructing the screen it can bel mounted in the evacuated envelope of a cathode ray tube as shown in Fig. 5, the individual photoconductive linings 2t) being thereby exposed to bombardment by a scanning beam 22.
Assuming for the moment that the time constant of the photoconductor 2t) is extremely short as compared to the frame time of the television picture being produced, the brightness of each element would adjust itself to the upper or lower equilibrium brightness levels 10 or 11, respectively, within a small fraction of the frametime. Therefore, the average brightness observed during a frame would be substantially either the upper or lower equilibrium levels 10 or 11 and intermediate tones (shades of gray) would be completely absent. This lack in gray scale-reproduction can be remedied if the photoconductor used responds to changes in illumination with a time-constant comparable to frame-time. This condition may be accomplished by either varying the chemical composition of the photoconductive material 20 to obtain the proper time-constant or by varying the scanning or frame-time to render the time constant of the photoconductor either equal to or longer than the frame-time. With the time constant so adjusted, the rate which any elemental area of the display screen approaches an equilibrium value will be so slow that such equilibrium brightness will never be quite reached during the period of one frame. The brightness average, therefore, over the period of one frame will depend on the level or initial excitation, or stating the same in other words, their image reproduced by the phosphor will contain shades of gray corresponding to shades of gray included in the exciting image.
Now having shown that it is possible to reproduce images without gray scale on the one hand or on the other hand images which do contain gray scale, attention is now directed to the problem of terminating regeneration just prior to the development of saturation brightness. It has already been mentioned that saturation brightness can be extinguished by interrupting or reducing the exciting voltage momentarily to a sufiiciently low level that will provide inadequate exciting voltage for the phosphor.
of Fig. 7, during the active period of frame-scanning indicated by the reference numeral 24,' the display screen ils building up toward saturation brightness but that during the' retrace period 25, the exciting voltage to the screen is momentarily substantially reduced or turned olf completely to return the screen to its unexcited or darkened state. This control of the screen may be accomplished by means of the apparatus shown -in Fig. 6 wherein like numerals indicate like parts. Suitable scanning circuits 26 of conventional design supply the necessary scanning signals (Fig. 7n) to the deflecting coils 23, these same scanning signals being fed to a conventional blanking circuit 27 which produces suitable blanking pulses 28 during each retrace 25. The exciting voltage for the display screen 1 is furnished by an audio frequency generator 29 which may be switched on and oi by means of the blanking pulse 28. A satisfactory mode of operation is to use the pulse 28 to bias the generator 29 to cut off, thereby eectively switching ot completely the exciting voltage to the display screen 1.
'Ihus it -is seen that for each active period of scan 24, which represents the period of one picture frame, exciting voltage is applied to the display screen 1, but that during the retrace intervals 25, the exciting voltage to the screen 1 is turned ol. During the active scan period 24 the screen 1 can approach saturation brightness through its regenerative action, but because of the relatively slow time constant of the photoconductor, this saturation brightness will never quite be reached.
While reproduction of an intensified television image is possible through the use of the system of Fig. 6, excessive vertical shading results since the lower parts of the screen have much less time available to drift or regenerate toward the upper stable brightness condition and would also be presented to the observer for a much shorter period of time, thus decreasing the apparent brightness in the lower parts of the picture. Therefore, the screen should not be de-energized simultaneously in its entire area but only in elemental areas such as single horizontal lines or a `group of horizontal lines simultaneously.
In Fig. 8 is illustrated a display screen which is substantially lidentical to that of Fig. 3 with the exception that the conductive lilm on the phosphor layer is actually composed of a plurality of horizontal, mutually insulated, parallel extending conductive strips 30 which are individually connected to a mechanical rotary switch or distributor 31 so that at any moment only one of the strips 30 is disconnected from the exciting voltage source 32. The switch consists of a plurality of stator contacts indicated by the arrows and a rotor contact 33 which makes one complete cycle in the same period of a television picture frame. Rotation of the rotor 33 is synchronized with the electron beam such that the disconnected strip 38 is just one step ahead of the strip being scanned by the beam.
By this switching means, exciting voltage to the elemental areas of the display screen 1 is sequentially interrupted, thereby preventing the vertical shading elect previously mentioned.
An alternative method ofv returning the display screen 1 to its unexcited condition consists in avoiding initial incident excitation above the unstable equilibrium point corresponding to numeral 12 of Fig. 2. This would mean that the incident illumination or excitation on the photosensitive layer 4 should be something less than .3 units. In this event, the regenerative effect will cause the brightness output of the phosphor to drift downwardly toward the lower equilibrium point 11. Therefore half-tone reproduction is possible without the necessity of de-energizing the display screen or parts thereof by a switching procedure as illustrated by Fig. 8.
' While we have described abovethe principles of our invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of our invention.
What is claimed is:
l. A feedback image-reproducing device comprising a panel of electroluminescent phosphor material, a panel of photosensitive material adjacent to and electrically coupled to said phosphor panel, said photosensitive panel comprising a base of insulating material having a plurality of transverse apertures, photosensitive material lining the walls of said apertures, a conductive material on the surface of said base remote from said phosphor panel, said conductive material making electrical contact with the adjacent portions of said photosensitive lining, said apertures with the lining and circumscribing conductive material being physically open and thereby accessible to electron bombardment, a conductive plate-like electrode on the phosphor panel surface which is remote from said photosensitive panel, and an electron gun disposed to direct a beam of -electrons into said base apertures for altering the impedance of said photosensitive material which causes said phosphor material to luminesce, said base material being opaque thereby limiting feedback radiation from discrete areas of said phosphor panel to corresponding apertures of said base.
2. A regenerative optical device comprising lirst means which luminesces in response to a variable electric field, second means which changes in impedance in response to varying incident radiation, said first and second means being electrically coupled together and disposed in proximity to each other whereby said second means will receive radiation from said irst means, said second means comprising a plurality of radiation-sensitive elements conductively interconnected together and which are accessible to electron bombardment, and neans for bombarding said elements with electrons.
3. A device of the character described comprising rst means which luminesces in response to a variable electric lield, second means which changes in impedance in response to varying incident radiation, said first and second means being electrically coupled together, said second means comprising a plurality of radiation-sensitive elements conductively interconnected together and which are accessible to electron bombardment, and means for bombarding said elements with electrons.
4. A device of the character described comprising first means which luminesces in response to a variable electric ield, second means which changes in impedance in response to varying incident radiation, said first and second means being electrically coupled together, said second means comprising a plurality of radiation-sensitive elements conductively interconnected together at one portion thereof and electrically coupled to said irst means at another portion thereof, said elements being open for electron bombardment, and means for bombarding said elements with electrons.
5. An image-reproducing device comprising a panel of electroluminescent phosphor material, a panel of photosensitive material adjacent to and electrically coupled to said phosphor panel, said photosensitive'panel comprising a base of insulating material having a plurality of transverse apertures, photosensitive material lining the walls of said apertures, a conductive material on the surface of said base remote from said phosphor panel, said conductive material making electrical contact with the adjacent portions of said photosensitive lining, said apertures with the lining and circumscribing conductive material being physically open and thereby accessible to electron bombardment, a conductive plate-like electrode on the phosphor panel surface which is remote from said photosensitive panel, and an electron gun disposed to direct a beam of electrons into said base apertures for a altering the` impedancel of said photosensitive material 2,728,815 Kalfaian Dec. 27, 1955r which causes said phosphor material to luminesce. 2,730,708 McNaney Jan. 10, 1956 2,773,992 Ullery Dec. V11, 19.56
References Citedv in the le of this patent UNITED STATES PATENTS 5 FOREIGN PATENTS 2,650,310 White Aug. 25, 1953 157,101 Australia lune` 16, 1954