US2908824A - Radiant energy translating device - Google Patents

Description

Oct. 13, 1959 v F. H. N|coL. L

RADIANT ENERGY TRANSLATING'DEVICE Filed s /PHoTovco/voucr/ VE ept. 17, 1954 PHOSPHOQ ...ar O 4Z ES/Sr/Ve 44 CONDUCT/ve Z/ #7 N @g/animar Hosp/4o f M13 PHOSPHOR a PHosPHoQ PHOTO co/vouc r/vE IN VEN TOR.

United States Patent RADIANT ENERGY TRANSLATING DEVICE Frederick H. Nicoll, Princeton, NJ., assignor to Radio Corporation of America, a corporation of Delaware Application September 17, 1954, Serial No. 456,785

9 Claims. (Cl. Z50-213) This invention relates to the art of translating radiant energy of one kind into radiant energy of the same or a different kind, and in particular, to I'the art of making visible radiant energy representative of an image. It relates to devices which amplify or'intensify light of a given wave length, such devices being known as light amplifiers, and to devices for converting radiation falling within a particular range of wave lengths into radiation falling within a different range of wave lengths, such as light converters, for example.

Certain materials are known which emit visible light when irradiated by energy of a wavelength outside the visible spectrum. ,'Ihese are known as luminescent materials or phosphors. Ultra-violet light is particularly effective in exciting phosphors to luminescence.

The spectral emission of phosphors is concentrated in bands of frequencies which are determined by theirV ci 2,908,824 f Patented Oct. 13, 1959 luminescence. However, it will become apparent that the two phenomena are quite different. On the one hand, electroluminescence involves the use of an electric eld to excite a phosphor to luminescence. The present invention, on the other hand, makes use of the property of electric fields to reduce or quench the light emission produced in a phosphor by a separate exciting source such as ultra-violet light, for example. More specifically this invention utilizes a photoconductor to control the changes in light level output produced from a quench-operated phosphor.

llt is an object of my invention to provide a novel arrangement and operation of radiation sensitive elements for the purpose of displaying light images.

-Ilt is a more specific object of this'invention to utilize the quenching eect of phosphors in the production or amplification of light and light images.

The above and related objects are achieved in accordance with the invention by providing a method of and means for controlling the changes in light level output of a body of phosphor material excited by ultra-violet radiation, said changes in light output being a function of the degree of quenching induced by an applied electric field.

The invention` contemplates the provision of a luminescent body, such as a body of phosphor material, means to continually irradiate the luminescent body with ultra-voilet radiation and thus cause light emission from the luminescent body, means for applying an electric field to the luminescent body to quench the light emission, and photoconductive means for varying the electric iield'acrossthe phosphor. According to one aspect of the invention, a luminescent body is arranged in series electrically with a resistive body and in parallel with a photoconductive body. Means are also provided for connecting a voltage source across the series parallel combination such that substantially all of the voltage appears across the luminescent body. Thus, with the ultra-violet radiation striking the luminescent body, the phosphor remains quenched and gives off no light. Radiation of a p second kind, for example visible light, striking the photoconductive body, lowers the impedance of the photo- Phosphors which exhibit thiseffect to a substantial degree are known as electroluminescent phosphors. An electroluminescent phosphor is described in U.S. Patent 2,- 566,349 to Mager. l

Photo control of electroluminescence has achieved with devices utilizing series operation of an electroluminescent element and a photoconductor and using an alternating field across the combination. Light on the photoconductor then causes an increase in the been alternating field across the electroluminescent element,

thus producing light. v

It has been found with certain phosphors that a differl. entv effect exists with the application of either A.C. or DJC. voltages. If a phosphor layer sandwiched between two conducting electrodes is irradiated with ultra-violet light, and no voltage is applied, normal emission ofl visible light occurs. -lf a voltage, D.C. or A.C., is then applied across the phosphor, the emitted visible light decreases in intensity in the presence of ythe resulting electric field, and remains quenchedf at this low level. The lremoval of the voltage and short circuiting of the electrodes to discharge the field produces a sudden flash of light followed by a decrease to the original steady state intensity produced by the steady ultra-violet excitation. Re-application of the voltage produces another flash followed by a return to the lower light level caused by the quenching action of the applied electric field.

The so-called quenching effect exists with phosphors [which exhibit electroluminescence to a substantial degree las well as those which exhibit little or no electroconductive body andcauses a decrease in the voltage appearing across the luminescent body, such that the luminescent body becomes unquenched and gives ofrr light.

According to another aspect of the invention a luminescent body is arranged in series electrically with a photoconductive body and a voltage source such that substantially all the voltage appears across the photoconductive body, and little or no voltage appears across the luminescent body. Thus, when short wave length radiation strikes the luminescent body, uniform light is emitted therefrom. When radiation of a second kind strikes the photoconductive body, however, the impedance of the latter is lowered and an increase in voltage appears across the luminescent body, thus causing a quenching or decrease in the light emitted from the luminescent body.

'The novel features which are believed to be characteristic of the invention are set forth with particularity in the appended claims. The invention itself however, both as to its organization and its method of operation, together with additional objects and advantages thereof will best be understood by reference to the following description when read in connection with the single sheet of accompanying drawings in which:

Fig. l illustrates schematically a circuit arrangement utilizing one form of a device constructed in accordance with the invention;

Fig. 2 is a graphical representation of the operation of the device of Fig. l; i

Fig. 3 illustrates a device constructed in accordance with the invention for displaying light images;

Figs. 4, 5, and 6 are modifications of Fig. 3 utilizing light shielding means; and

Fig. 7 illustrates a device constructed in accordance with the invention for displaying negative light images.

AReferring to Fig. l, there is shown a laminated structure 10 including in the order named a transparent foundation member 12, for example, a sheet of glass, a transparent conductive coating 14, for example, a thin lm of tin chloride, a layer of phosphor 16 and a conductive member 13. The conductive coating 14 and conductive member 18 serve as electrodes connecting the phosphor layer 16 into an electric circuit which comprises a photoconductive cell 20 in series with a resistor 22 and a voltage source 24, phosphor layer 16 being connected in parallel with the photoconductive cell 20. i The photoconductive cell 20 may be made of any material whose electrical conductivity is variable according to the intensity of the type of radiation to be employed. An example is cadmium sulfide, which, -When activated by denite amounts of copper or other metallic impurities may be made sensitive to visible light, X-rays, infra-red radiation, and other types of radiation. Cadmium selenide is a material which similarly can be made sensitive to various types of radiation.

The impedance of the phosphor 16, and that of the photoconductor 20 in the unexcited condition, is very high compared to the resistance of the resistor 22. Thus with no radiation of the photoconductor, there will be little or no current flow and substantially no voltage drop across the resistor 22; hence, substantially all the voltage of the source 24 will appear across the photoconductive cell 20 and the phosphor 16. With radiation on the photoconductive cell 20, however, its conductivity vwill increase allowing current to flow, and the resulting voltage drop across the resistor 22 will be suiicient to drop the voltage appearing across the phosphor 16.

The circuit of Fig. 1 is thus a convenient means of controlling the quenching voltage across a phosphor. To operate the device of Fig. 1 on the principle of light quenching, then, the phosphor 16 is allowed to be irradiated by ultra-violet rays indicated generally as 26, the radiation being maintained continuously. When the photoconductive cell is subjected to other radiation 28 which may, for example, be visible light, the impedance of the photoconductive cell 20 is lowered, causing current to be drawn through the resistor 22, which reduces to a small value the voltage appearing across the phosphor layer 16. Therefore, since there is little or no quenching voltage across the phosphor 16, light is emitted at substantially full brilliance from the phosphor 16 due to excitation by the ultra-violet light 26. The phosphor 16 is then in the unquenched condition. When the light 28 is removed, however, the photoconductive cell impedance rises to a high value, little or no voltage drop occurs across the resistor 22, and substantially full voltage appears across the phosphor 16. Thus the light emission from the phosphor 16 is reduced or quenched.

These results are shown in the graph of Fig. 2 which illustrates the changes in light emission from the phosphor 16 as voltage is applied to and removed from the phosphor, it being understood that the phosphor 16 is continuously being irradiated by the ultra-violet rays 26. It is seen that during the period indicated by T2-T1, which corresponds to the light olf condition of the photoconductor 20 `and the voltage on condition of the phosphor 16, the level of light emission from the phosphor 16 is of low amplitude A1. At the end of time T2 when the photoconductor 20 is irradiated with visible light 28 and the voltage thereby removed from the phosphor 16, there is a sudden iash of light to a high level A21 followed by a rapid decrease to a steady level A2 which is substantially equal to the undisturbed light emission caused by the ultra-violet light excitation alone. At the end of time T3 when the light is again removed from the photoconductor 20, there is a ah of light t .3,

4 level A31 followed by a rapid decrease to a steady low level equal to A1 again, whichis the level of light emission obtained by exciting the phosphor 1.6 with ultraviolet light and with substantially full voltage applied across the phosphor 16.

A device was built and operated Iaccording to Fig. 1 using a phosphor layer 16 about 2.0 mils thick in conjunction with a copper activated cadmium sulfide photo conductor 20. With the resistor 22 equal to l megohm and SQO volts D.C. applied, it was possible to control the light level of a luminescent panel 6 inches square with the light from a flashlight on the photoconductive cell 20. The light from the ashlight was of a level to cause a drop in the impedance of the photoconductor 2t) from ya value of many megohms in the dark to a value of the order of l megohm or less. The phosphor used comprised blue fluorescing zinc sulfide activated by copper. Zinc selenide activated by copper, which gives off yellow light, is another phosphor which exhibits quenching to a substantial degree. In the device illustrated in Figs. 1 and 2, light level changes differing by a factor of about l0 were :achieved between the quenched and unquenched condition. While these phosphors are given as specific illustrations of materials which have been used successfully, they are not intended to be limiting features of the present invention. Generally, all phosphor materials exhibit the quenching etect to a more or less degree, land the light gain of the device described and those devices to be described will be determined largely by the eiciency of the particular phosphor used and the sensitivity of the photoconductor.

It is also possible to produce light images by utilizing the principles of phosphor quenching. Such a device is illustrated in Fig. 3. In the drawing, a viewing screen 30 comprises a transparent support member 32 having on one surface an electrode in the form of a transparent conductive coating 34, and on the conductive coating 34 a mosaic made up of la multiplicity of spaced-apart dot groups or clusters. Each cluster consists of an elemental area or dot of fluorescent phosphor material 36, an elemental area or dot of photoconductive material 38, a thin layer of electrically conductive, light transmitting material 39 on the upper surface of the dots 36 and 38, and an elemental area or dot of resistive material 40, of appropriate resistivity, on the conductive layer 39. The other electrode comprises a conductive wire mesh 42, of copper for example, in electrical contact with the upper ends of the resistive dots 40, the connections being made by silver paste, for example.

The photoconductive material 38 is preferably a material which is not responsive to ultra-violet light. I have found some forms of cadmium sulde powdered material activated with copper to be rather insensitive to ultraviolet light. The conductive layer 39 may be a thin semi-transparent layer of silver, for example.

With a voltage 44 across the electrodes and ultraviolet light 46 tlooding the screen 30 from one side (indicated by the position of the observer O), and in the absence of a radiant image 48 on the screen, the screen will be in the dark condition, the phosphor elements 36 being quenched by the voltage from the source 44. When a radiant image 48 is focussed on the non-viewing side of the screen 30, the photoconductive elements 38 will become conducting to a degree depending on the intensity of the incident image radiations 48, and the corresponding phosphor elements 36 will become unquenched to a similar degree. In a dark area, for example, the photoconductor will not draw current and the phosphor dot will remain quenched and will give olf minimum light. In a fully lighted area the photoconductor will be fully conducting and the phosphor dot willl be completely unquenched and will give oil maximum light. In the gray areas the photoconductor will be conductive in an intermediate amount,` and; the phosphor dot will bef partially Thus it is seen that a visible image (visible to an obquenched and give olf Van intermediate amount of server O)` may' be produced on one side of thescreen of If the photoconductor is not shielded from. the luminescent light, `as shown in Fig. 3, it is possible to retain a picture even after the'radiant image is removed. In the device of Fig. 3, for example, it is apparent that some of the light emitted by a phosphor dot 36 will fall on an adjacent photoconductive dot 38. This will have an additive elect, over and above the elfect caused by the incident radiation 48, in lowering the impedance of the photoconductive material 38. Such feeding back of output light Vwill give the device added sensitivity. Moreover, if the amount of light fed back is equal to or greater than the incident light,the device will be a self-sustaining or storage device, and light will continue to be given off even after removal of the radiant image 48.

yReferring to Fig. 4, feedback may be suppressed, if desired, by properly shielding the photoconductor from the light emitted by the phosphor, as by `:providing a thin layer of light-opaque insulating material 41 over the phosphor dots 36. By extending the layer 4 1 so that it also cover the conductive coating 34, as shown in Fig. 4, it will shield the photoconductive dots 38 from the ultraviolet light 46 as well as from the luminescence from the phosphor dots 36. The layer 41 may be a thin layer of black lacquer, which is opaque to both the luminescent light and the ultra-violet light. Such a layer may be made thin enough to provide good conductivity through its thickness, while at the same time having high resistance in the lateral direction. In this Way the dots 36 and 38 will be tied together electrically at their upper surfaces to the semitransparent conductive layer 39; the dots 36 and 38 will also be tied together electrically at their lower surfaces to the transparent conductive coating 34; and there will be no leakage current between the layer 39 and coating 34 to shunt out either of the dots 36 or 38. Other modifications of a shielding layer are shown in Figs. 5 and 6. In Fig. 5 the shielding layer 43 covers only the phosphor dots 36 to shield the photoconductive dots 38 from the luminescent light, but not from the ultra-violet light 46. Such a construction of course presupposes the use of a photoconductive material which is not responsive to ultra-violet light. In Fig. 6 the shielding layer 45 is placed intermediate the conductive coating 34 and the photoconductive dot 38 to shield the photoconductive dot 38 from the ultra-violet light 46 only.

Feedback of the luminescent -light may also be prevented without the use of a shielding layer by utilizing a photoconductive material which is rather insensitive to the luminescent light but is responsive to the incident radiation. Por example, cadmium sullide powder ac'- tivated with copper is sensitive to light in the infrared region but rather insensitive in the blue and ultra-violet region. Hence, by using this type of photoconductor in combination with a blue emitting phosphor, for example, zinc sulfide activated with copper, not only will .feedback be suppressed but also excitation of the photoconductor from the ultra-violet light can be prevented.

1f the phosphor used is one which is electroluminescent, the device should be operated at a voltage below that at which the phosphor will electroluminesce, in order to 'amasar make the applied electric eld quench instead of cause luminescence from the phosphor. 1

The principles ofV quenching canbe' used torproduce negative images as well as the positive images already discussed. This can be accomplished by operating the elements in a series electrical circuit. Fig. 7 illustrates a `device of this character.v In this lembodiment a luminescent panel comprises a laminated structure containing in the order named a transparent support member 52, a conductive coating 54, a layer of phosphor 56, a layer of photoconductive material 58, a second conductive coating 60, and a second transparent support member 62. A source of voltage 64 is connected to the electrodes (conductive coatings 54 and 60) and when so connectedthe screen is made up of a multiplicity of elements of photoconductor and phosphor in series.

lf the photoconductor 58 when unexcited, has an impedance very high compared to that ofthe phosphor 56, substantially all of the voltage will appear across the photoconductor 58, and little or no voltage will appear across the phosphor 56. When the phosphor 56 is excited by ultraviolet light radiations 66, light is emitted uniformly over the surface of the phosphor 56. Then when a radiant image 68 is caused to impinge on the photoconductor 58, areas of the photoconductor will become conducting in degree according to the intensity of radia- Vtions 68, and corresponding areas of the phosphor 56 directly behind the photoconductor 58 will have corresponding quenching voltages applied thereacross,l the amount of quenching varying with the intensity of the radiations 68 on the photoconductor. Thus, for example, if a light image is caused to impinge on the photoconductor 58, a light image will be reproduced on the phosphor layer 56, but the reproduced light image will be a negative image.

What is claimed is:

l. A radiant energy translating device including a source of short wave length radiation, a transparent supporting base, a transparent conductive coating thereon, a phosphor material on said conductive coating and exposed to said source and which emits light substantially uniformly over elemental areas thereof in response to said short Wave length radiation, a photoconductive material in'parallel electrical arrangement with said phosphor material, and means including a series resistor connected to apply a voltage across said parallel phosphor material and photoconductive material tocause a reduction of the light emitted by said phosphor.

2. The invention according to claim l and further including a thin layer of light opaque insulating material shielding Isaid phosphor material from said photoconductive material.

3. A radiant energy Itranslating device comprising a source of short wave length radiation, a luminescent screen in the path of said short wave length radiation and including a transparent foundation member, a light transparent condiuctive layer on a surface thereof, a layer of phosphor material on said light transparent conducting layer in a position to intercept said short wave length radiation and which emits light substantially uniformly over elemental areas thereof in response to said short wave length radiation, and means for applying an electric eld across each of said elemental areas of phosphor for quenching the light emitted from each elemental area according to the strength of the electric iield applied thereacross, said means including photoconductive means responsive to incident radiant energy to vary said electric lield and thus vary said quenching.

4. A radiant energy translating device comprising a source of short wave length radiation, a luminescent screen in the pathof said short wave length radiation and including a transparent foundation member, a light transparent conductive layer on a surface thereof, a mosaic on said conductive layer made np of a multiplicity of spaced apart groups of contiguous fluorescent and photoconductive areas,nsaid fluorescent areas having the ability to emit light in response to said short wave length radiation, superimposed resistive areas partially coveringeach of said. groups, transparent electrical connections between each of said groups, a source of voltage connected across said transparent electrical connections and said conductive layer for subjecting said fluorescent areas to independent electric ields, whereby upon impingement of a radiant image on said photoconductive areas said electric fields will be reduced according to the intensity of radiation and the light emitted from said fluorescent areas will be increased.

5. The invention according to claim 4 and wherein said short wave length radiation comprises ultra-violet light..

6. The invention according to claim 4 and wherein the fluorescent areas of said mosaic comprise dots of zinc sulareas being exposed to said source, superimposed resistive areas partially covering each of said groups, and transparent electrical connections between each of said groups.

8. A radiant energy image translating device comprising a laminated structure including a uorescent layer and a photoconductive layer contiguous thereto, trans- 'parent electrode means on the free surfaces of said layers for applying an electric lield to said laminated structure,

' and a source of ultra-violet light in a position to irradiate absence of a radiant image on said photoconductive layer ,elemental areas of said iiuorescentlayer will have relativelyl'ow electric tield applied thereacross andv will emit `uniform light, but in the presence of a radiant image von said photoconductive layer elemental areas of said lluo` rescent layer Will have correspondingly higher electric fields applied thereacross with correspondingly reduced light emission in the regions of higher electric elds.

9. A radiant energy translating device comprising a source of short wave length radiation, a phosphor material exposed to said source, said material being capable of emitting light in response to said short wave length radiation, and means for applying an electric eld across said phosphor material for quenching the light emitted therefrom according to the strength of said iield, said means including photoconductive means responsive to incident radiant energy to vary said electric eld and thus vary said quenching.

References Cited in the file of this patent UNITED STATES PATENTS 2,650,310 White Aug. 25, 1953 2,739,244 Sheldon Mar. 20, 1956 2,780,731 Miller Feb. 5, 1957 OTHER REFERENCES Orthuber et al.: A Solid State Image Intensier, Jour. of the Optical Society of America, vol. 44, No. 4, April 1954, pages 297-299.

Bramley et al.: Transient Voltage Indicator and Information Display Pane, Review of Scientific Instruments, vol. 24, June 1953, pages 471 and 472.

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