US3348056A

US3348056A - Wavelength converting type radiant energy responsive display device

Info

Publication number: US3348056A
Application number: US368906A
Authority: US
Inventors: Kohashi Tadao
Original assignee: Matsushita Electric Industrial Co Ltd
Current assignee: Panasonic Holdings Corp
Priority date: 1963-05-22
Filing date: 1964-05-20
Publication date: 1967-10-17
Anticipated expiration: 1984-10-17
Also published as: DE1489113B2; DE1489113A1; NL6405716A; GB1025320A; DE1489113C3

Description

0d. 17, 1967 TABAC KOHASH, 3,348,056'

WAVELENGTi CONVERTING TYPE RADIANT ENERGY Filed May 2O' 1964 ESPONSIVE DISPLAY DEVICE 2 Sheets sheet l FIG. l @pagas l 7 ,Zhfe/-Zaye [am in e. cenf [mpeddn ce Ve777 r.l 72de@ Aww/; 2mm' WM f ATTORNEYS Oct. 17, 1967 TADAO KoHAsl-u 3,348,056

- WAVELENGTH CONVERTING TYPE RADIANT ENERGY RESPONSIVE DISPLAY DEVICE Filed May 20. 1964 2 Sheets-Sheet 2 ATTORNEY Y P2M.

United i States Patent O 3,348,056 WAVELENGTH CONVERTING TYPE RADIANT ENERGY RESPONSIVE DISPLAY DEVICE Tadao Kohashi, Yokohama, Japan, assignor to Matsushita Electric Industrial Co., Ltd., Osaka, Japan, a corporation of Japan Filed May 20, 1964, Ser. No. 368,906 Claims priority, application Japan, May 22, 1963, 38/ 27,248 16 Claims. (Cl. Z50-213) ABSTRACT OF THE DISCLOSURE A multilayered radiant energy responsive display device including a light transmitting base, a first electrode,

an electroluminescent layer, an interlayer, a photocon-V ductive layer, a third electrode, an impedance layer, a second electrode, and power supply means. The impedance layer being an energy converting luminescent layer having a radiant energy response to a first spectral distribution and having a luminescence characteristic With a second energy distribution different from the first and at least partly overlapping the spectral photoconductivity distribution of the Vphotoconductive layer whereby, when excited by radiant energy, the luminescent layer produces luminescent light energy to which the photoconductive layer is responsive,

trode including a wire, ribbon or reticular electroconduc-V tive structure and interposedV between said electro-,-

luminescent and impedance layers in electrical contact with said photoconductive layer, electric power supply means for impressing a voltage V1 between said -first 4and third electrodes and another voltage V2 between said iirst and second electrodes, said voltages V1 and V2 being variable or fixed at least in voltage value or polarity where they are fed in DC form and at least in amplitude or phase where they are fed in AC form, and means for electrically controlling the luminescence intensity of vsaid electroluminescent layer through the variation in impedance under the excitation of the radiant energy on said photoconductive layer thereby to convert the radiant energy image into visible form on said electroluminescent layer with or without amplification.

The radiant energy responsive display device has conventionally been called an electrostatic image converteramplifier. For discrimination between the inventive and conventional devices, however, only the latter will conveniently be called an electrostatic image converteramplifier throughout the following description.

The electrostatic image converter-amplifier employs a so-called second electrode and an impedance layer which are both transmissive to radiant energy. With such device, it has been found and made known by the inventor that, by properly selecting or fixing the voltagesrVl and V2, the radiant energy image impinging upon the photoconductive layer can be converted with or without ampliication into a visible image of the positive, negative or Patented Oct. 17, 1967 ICC mixed nature on the electroluminescent layer and that, by controlling the voltages V1 and V2, the contrast ratio, gamma and other operating characteristics of the device can be made variable over a wide range.

Moreover,with the positive image amplification characteristic, which produces an image of the positive nature, the radiant energy displayed can be stored with a bistable luminescence intensity by feeding the luminescent rays from the electroluminescent layer back to the photoconductive layer.

On the other hand, the photoconductive layer employed inthe electrostatic imageV converter-amplifier is vsubject to limitations due to its material nature. For example, the spectral response distribution of the photoconductive layer is held lwithin considerably narrow limits because ofV its properties deriving from the material of which the layer is formed and usually the layer is utterly non-responsive or only slightly responsive to some forms of radiant energy.

To cite an example, photoconductive materials of the type previously used most widely include cadmium sultide and cadmium selenide both activated with copper, chlorine, etc. and solid solutions of cadmium sulfide and cadmium selenide. The spectral photoconductivity distribution of this type of photoconductive material ranges from about 500 ma to 900 mit, only partly covering the visible light range. The range of distribution may be further limited under conditions of preparation for such material. Thus, the photoconductive layer formed of such material is entirely non-responsive to light rays of the wavelength of less than 500 m/L and also to ultraviolet rays. To the radiant energy of X-ray levels the material exhibits some sensitivity, which is very limited when cornpared with the sensitivity to radiant energy in the visible and near infrared regions. In addition, because of the high penetrability, X-ray radiation is transmitted through the photoconductive layer effecting only a limited amount of excitation thereto. This is the reason why the layer exhibits an extremely limited sensitivity to X-ray radiation.

As for the electron radiation, for example, taking the form of a beam of electrons, it is blocked by the impedance layer, which customarily takes the form of a solid layer. Thus, it has been impossible to excite the photoconductive layer by electron the structure ofthe display device.

The present invention is intended to overcome these ditliculties.

According to the present invention, there is provided a radiant energy responsive display device of the typev radiant energy responsive luminescent impedance layer to effectively convert radiation energy into light energy to which the photoconductive layer is most sensitive.

According to the present invention, it will be appreciated, therefore, that the display -device is operable with high sensitivity even to radiant energy 'of the level to which conventional electrostatic image converter-amplifiers have been inoperable or operable only with limited rsensitivity because of the structure of the photoconduc- -tive layer. Moreover, the inventive device is extremely radiation because Vof simple in construction and highly eicient since it includes a radiant energy responsive luminescent impedance layer which apparently serves also as an impedance layer essential to conventional electrostatic image converteramplifiers. Y

The foregoing and other objects and features of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings, which diagrammatically illustrate some embodiments of the invention. In the drawings:

FIG. 1 is a diagrammatic view of one embodiment of the present invention including a luminescent impedance layer in the form of a single or mixed layer, illustratingits structure and the power supply system therefor;

FIG. 2 is a diagrammatic illustration of the embodii' ment as used with radiant energy in the form of a beam of electrons;

FIG. 3 is a cross-sectional diagram of another embodiment of the invention, which includes a luminescent impedance layer in the form of a composite` layer,'illus trating its structure and the power supply system therefor; and Y FIG. 4 illustrates a .modification of the embodiment shown in FIG. 3, which further includes a DC control.

' shown on an enlarge scale. In the following description,

the radiant energy responsive luminescent impedance layer will be referred to briefly as a luminescent impedance layer.

VFIG. l diagrammatically illustrates the structure of a radiant energy responsive display device according to the present invention and .a power supply system therefor which are usable with radiant energy in the form of radiation such as X-rays, 'y-rays or an electron beam.

In FIG. 1, the reference numeral 1 indicates a lighttransmitting base or support plate, for example, formed of transparent glass sheet; and reference numeral 2 indi-l cates a rst electrode which is light-transmitting and formed, for example, of a metal oxide such as tin oxide.

The reference numeral 3 indicates a electrode-luminescent layer which includes a vapor-deposited film such as of ZnSV or a mixed layer formed, for example, of ZnS powder-activated with copper for green luminescence andbound with glass enamel, plastic or the like material. The electroluminescent layer 3 is a solid layer having a thickness of the order of from 5 toV 70,@ and emits light when excited electrically. An opaque interlayer 4 is arranged so as to prevent any unstable operation of the device which may otherwise be caused by the excitation of a photoconductive layer. 5 by the light feedback from the layer 3 or exterior light arriving from the base side of the device. The interlayer 4 is a solid high-resistance layer of the thickness of the order of 1 torlOfL, which includes an evaporated lm of an opaque high resistance material or is formed of a black paint, carbon black or the like material and a binder such as plastic or glass enamel. The photoconductive layer 5 includes a powder of photoconductive substance such as CdS activated with copper or chlorine and bound with plastic, glass enamel or the like binder material into layer form, or an evaporated ilm of such photoconductive substance including no binding agent, or also a sintered film of such substance. The layer 5 has a thickness of the order of from 5 to 100/r, and, containing CdS as an essential element, as described above, exhibits a high sensitivity to the excitation of orange and infrared light energy with decreasing resistance. rIhe reference numeral 6 indicate a grid-like third electrode, which in this embodiment is recticular. The grid-like third electrode 6 includes a continuous metal structure like that of a metal net electrode such as used in television pickup tubes, which includes conducting sections having a width of the order of, say 10 to 30u and a thickness of the order of, say, several microns arranged in about 50 to 100 mesh. Alternatively, the electrode 6 may 'be of a discontinuous metal structure including tungsten or other metal wires of the thickness of the order of, say,

l0 to 30p., weaved into a network of 50 to 100 mesh Vor n be plated with gold or rother metal. The third electrode 6k is arranged in electrical contact with the photoconductiv layer 5. Y p l The reference numeral indicates a luminescent Yirnpedance layer which takes the form of a single or mixed layer. For radiant energy E1 in `the `form of an electron beam, the luminescent impedance layer 100 is formed of a lluorescent material Such as (Zn, Mg)F2 for cathodoluminescence; and, for radiation energy E1, for example, in the form of X-rays or y-rays, the layer 100 is formed of a fluorescent material for'radiation luminescence such as ZnCdSzAg, which emits orange light energy to which said photoconductive layer 5 is responsive withthe highest f sensitivity. Y

One form of such luminescent impedance layer -is' comprised of a mixed layer formed of powdery luminescent or fluorescent material suchV asV described above mixed with abinding agent such as epoxy resin or glass'V a single layer of uorescent material of the type described above formed by vapor depositing -or a single layer" formed by precipitating the fluorescent material preliminarily suspended in a solution of an appropriate cellulose such as nitrocellulose in aY suitable organic solvent such as aluminum acetate and baking the layer of precipitate to vaporize the organic substances included therein while causing the precipitate to cohere together to form a desired single layer, which is particularly well adapted for use with electron beams.

The luminescent impendance layer formed in the manner described above has a high'resistivitypand exhibits a capacitive impendance to AC voltages and whenrexcited with radiant rays or an electron beam emits organge light, to whichrthe photoconductive layer 5 is responsive with high sensitivity. The thickness of the luminescent imped-Y ance layer 100 should be determined in due consideration of its relationship with the impedance of the other layers and of its characteristics relative to the radiant energy, including its transmissiw'ty, dielectric strength and imped`` ance. For example, the Vthickness of the layer 100 is determined on the order of from 20 Vto 200g, depending upon the dimensions of the other layers cited hereinbefore.

The reference numeral 7 indicates a second electrode which is transmissive to radiation energy. For radiation E1 in the form of an electron beam, the electrode 7 preferably includes a thin evaporated ilm of metal such as aluminum. For radiation E1 in the form of X-rays or the like radiant rays of high penetrability, the electrode 7 iSV formed of a similar evaporated film of aluminum or a thin lm of aluminum or the like metal.

The second electrode 7 formed in the manner described above is transmissive to radiant energy and highly reflective to the luminescent light energy as emitted from the layer 100 when the latter is excited with radiation energy so that the light energy is effectively prevented from escaping exteriorly through the electrode 7 and reflected toward the layer 5. Thus, excitation of the photoconductive layer 5 can be effected with high efciency by the luminescent light energy.

For radiations in the form of X-rays, fy-rays or like radiant rays, the second electrode 7 may include an electrically conductive coated layer such as of silver paint.

VThe electrical power supply may be performed, for example, in the manner shown in FIG. 1. In the case where the electroluminescent layer 3 is formed of a powder of electroluminescent material such as ZnS:Cu, Al, which emits green light when excited, and a binding agent such as epoxy resin, an AC power supply source is connected to the device since the layer 3 is luminescent only when excited by an AC voltage. The third electrode 6 includes a conductive strip 17 and an AC power source 8 is connected between the strip 7 and the first electrode 2 by conductor means 9 and 10 so that an AC voltage V1 is applied between

electrodes

2 and 6.

n the other hand, an AC power source 11 is connected between the 'irst and

second electrodes

2 and 7 by way of

conductors

9 and 12 to apply an AC voltage V2 of the same frequency as the voltage V1 between the

electrodes

2 and 7.

Under this condition, assume that V2=0 with the

electrodes

2 and 7 short-circuited through an external circuit and the frequency of voltage V1 is selected on the order of 100 to 5000 cycles per second at an appropriate voltage value. If the device is irradiated with an X-ray energy image E1, the luminescent impedance layer 100 isexcited to emit light which effectively excites the photoconductive layer 5, and simultaneously the radiation E1, which is highly penetrable, pentrates the layer 100 to excite the photoconductive layer 5. Thus, the photoconductive layer is excited by two energy images, i.e. the X- ray energy image and an orange light energy image, which has been formed from the X-radiation through conversion in wavelength, and to which the layer 5 is most sensitive. As the result, the photoconductive layer Y 5 is increased in conductivity in a plane at right angles to the direction of radiation E1, and a corresponding photocurrent is caused through the layer 3 to electrically excite the electroluminescent powder therein thereby to produce in the layer 3 a bright green image of the positive nature, which is amplified 'relative to the X-ray energy image E1 with a sensitivity much higher-than that of anyY conventional electrostatic image converter-amplifier.

Under this condition, if V1 is fixed and a dark state is established with no radiation E1 given, a dark current' 1111 is formed in association with V1, which iiows between the third and

iirst electrodes

6 and 2 through the intermediary of the electrolurninescent layer 3. However, when voltage V2 is applied, a current I2 ilows between the first and second eelctrodes 7 and 2 by way of the luminescent impedance layer 100, the interior of the rcticular structure of the third electrode 6, and the-electroluminescent layer 3.

`Accordingly, if the voltage V1 is fixed and the phase difference of the voltage V2 from V1 is selected so as t'o obtain a differential relation between the currents I1 and I2, the amount of amplitude of the current I3=I1J|I2 iiowing through the layer 3 and hence the dark luminescence thereof are reduced as the amplitude of the voltage V2 increases. Such phase difference is usually selected in the range of l80i90 depending upon the impedance characteristic of the material employed. With radiation E1 given under this condition, a visible image E2 of the positive nature is obtained which has a contrast ratio and a gamma value both increasing with increase in amplitude of V2, rendering the operating characteristics variable in a wide range. Contrariwise, if V1 and V2 are both fixed in amplitude and the phase difference therebetween made variable, the contrast ratio and the gamma value can be controlled over a wide range in a decreasing direction as the phase relationship deviates from that for the above described differential relation between VI1 and I2.

Similarly, where V2 is applied, with V1=0, the current relation 13:12 is obtained and the layer 3 is effectively excited with the current I2. Under this condition, if radiation E1 is given, the photoconductive layer 5 increases in conductivity in-accordance with the distribution pattern of the local strength of E1. It will be noted that, since the

electrodes

2 and 6 are at the same potential, the layer S behaves as an electrostatic shielding layer and the cur- Y rent I2 is bypassed through the third electrode 6 to reduce the current 13:12, which is owing through the layer 3. In this manner, radiation E1 is converted, while being intensified, into a bright visible image E2 of the negative nature relative to radiation E1.

Under this condition with radiation E1, if the phase difference of V1 from V2 is selected to obtain the difierential relation of I1 (ordinarily in the Vrange of i90), the contrast ratio and the gamma value are increased as the amplitude of V1 is increased from zero. On the other hand, as the phase difference is varied to deviate from the differential relation while fixing the amplitude of V1 and V2 at a properly adjusted value, the contrast ratio and the gamma value are reduced accordingly. In the case of V1 has a more or less large amplitude, the deviation of the phase difference causes reduction in contrast ratio and gamma value thereby to obtain a visible image E2 as a mixture of a negative image corresponding to those portions of the image E1 which areV limited in intensity and a positive image corresponding to the high intensity portions of thev image'E1. Where the phase dierence between voltages V1 and V2 gives an additive relationship between currents I1 and I2, an image E2 of the positive nature shows itself which is entirely reverse to the one described above.

Assume that the amplitudes of V1 and V2 are selected so as to obtain a nearly differential relationship between I1 and I2 and to render I3 predominant over I2 when dark with no radiation E1 and over I1 when a radiation E1 of a uniform and high intensity is given.

Under this condition, if an X-ray energy image E1 of a suiciently high intensity is irradiated while maintaining I1 and I2 in a differential relation in whch I1 and I2 are in phase and of the same amplitude, the output image E2 obtained is extremely limited in brightness in areas corresponding to the localized intensity of the X-ray energy image E1 and is of the negative nature in areas corresponding to those regions of the X-ray image having an intensity lower than said localized intensity and of the positive nature in areas corresponding to those regions of the intensity higher than said localized intensity. In this manner, the X-ray energy image is divided into two groups of intensity regions and is converted into an image of a combined negative and positive nature. In other words, the device exhibits a so-called V-shaped operation characteristic. In this case, if the amplitude of V1 is increased or that of V2 decreased, the V-shaped characteristic is shifted in a direction in which the strength of the input X-ray radiation'increases, and similarly, if the amplitude of V1 is decreased or-that of V2 increased, the V-shaped characteristic is shifted in a direction in which the X-ray radiation decreases in strength. Thus, the behavior of the output image E2 can be freely varied simply by controlling the amplitudes of V1 and V2, and precise observation and examination of the X-ray energy image E1 can be performed by a kind of zero method.

On the other hand,if, with the amplitudes of V1 and V2 xed, the phase difference therebetween is caused to deviate from the ditierential relationship, the minimum Y luminescence intensity of the output image E2 increases to reduce the contrast ratio while displacing the V-shaped characteristic. If the phase diiierence between V1 and V2 is varied to obtain an additive relation between I1 and I2, the output image E2 varies continuously into an image of the positive nature.

As apparent from the foregoing description on several modes of operation, the inventive device is highly advantageous in that such operation can be effected continuously from one mode to another by controlling the amplitudes of the voltages V1 and V2 and the phase relationship therebetween. Moreover, the device can be iixed to have a V-shaped characteristic of the positive, negative or intermediate nature simply by properly fixing the voltages V1 and V2.

For the continuous controlling of the device or line adjustment thereof in a limited range, it is preferable to lemploy an AC power supply source including a single signal generator and two electrical signal amplifier sys- Vtems for amplification of its AC signal output'with a variable phase-shifter and amplitude-controlling means arranged in at least one of the amplifier systems. As forY a single power supply if provided with means to open the several electrodes of the device, to insert an impedance element such as a resistor, an impedance or a capacitor or a combination of such elements between the electrodes or between the latter and the power source, or also to fix or make variable the impedance value of such impedance element or combination of impedance elements. Further, la modification in which one power supply system includes an output transformer having vaV-tap changer or slider means on the output coil thereof lmay provide an entirely satisfactory amplitude relationship between the voltages V1 and V2 and also a satisfactory phaseV relationship therebetween at least iu cases where the voltages are in the same or opposite phase.

Having described some forms of luminescent impedance layer 100, it is to be noted that the dielectric strength of the layer 100 often-raises some problem because of the required application of voltage V2. Such problem, however, can be alleviated by admixing a powder of dielectric substance'to the layer 100 which is light-reecting and has a high dielectric strength. For example, the luminescent impedance layer 100 may be lformed by mixing a powder of uorescent material and a powder of highly light-reflecting dielectric substance such as zinc sulfide, zinc oxide, titanium oxide or barium titanate and laminat- 'ing such mixture with a binding agent such as epoxy resin. Apparently, it is necessary to properly select a dielectric strength and an impedance value for the layer 100.

On the other hand, the luminescent material is subject to spectral limitations of the luminescent light energy and to other limitations such as the conversion efficiency of the material. Therefore, the selection of an impedance value resolves itself down to a matter of controlling the thickness of the layer. On the other hand, highly lightreliecting materials provide a considerably wide range for selection of the dielectric constant. For example, barium titanate powder has a dielectric constant of several thousands Vor over, and titanium oxide of theV anatase form of the order of ten. Therefore, one advantage of the luminescent impedance layer 100 is that its impedance value can be selected in a wide range by properly selecting the materials therefor and changing their volume ratio in the mixture. It is to be understood that the present invention also includes within its scope such structures of luminescent impedance layer. Y

Though the foregoing description has been made with respect to the operation on an AC power supply, the voltages V1 and V2 may also be of DC form in cases where the electro-luminescent layer 3 takes the form of an evaporated film such as of ZnS activated with Mn or other element, since such layer 3 is luminescent with a DC power supply. In such cases, various modes of operation as described hereinbefore can be obtained by controlling the voltage values and their polarity.

Also in these cases, it is obvious that the various cornponent layers of the device must be formed to have a proper conductivity to allow a more or less DC current Y flow therethrough.

images, which are highly penetrable, an appropriate con-- ductivity can be obtained without detracting fromtlie` utilizability of the luminescent light energy by admixing a powder of highly light-reecting metal such as silver in a proper volume ratio.

Description will next be made on another embodiment of the present invention, which is shown in FIG. 2 and formed upon the basis of the same principles as the one shown in FIG. 1.

The 'embodiment of FIG. 2 is basically similar to the radiant energy responsive display device of FIG. 1, but isY particularly adapted for use with radiationenergy in the form of a beam of electrons.

For convenience in description, the solid discrjgaortionV of the device is generally indicated at :200 and the power Y, source means for applying voltages V1 and V2 at 300.L

The reference numeral 400 indicates an envelope similar to the picture tube used in a television set. The solid disc' f 200 being arranged in a portion corresponding to rthe uorescent screen infthe latter. An electron gun 401 emits a beam of electrons E1, which is modulated byanV electrical signal S. Reference numeral 402 indicates an electron beam deecting coil.

If, with voltages V1 and V2 applied, an electron beam E1 is directedonto the second electrode 7, which is comprised, for example, of an evaporated, aluminum film and is eletron-beam transmitting, the electron beam penetrating through the second electrode 7 excites the luminescent impedance layer 100, which includes at least aL cathode luminescence material and when excited emits light energy, which excites the photoconductive layer 5. In this manner, it will be understood that the luminescence outputEZ of the electroluminescent layer 3 can be controlled electrically.

For example, if a video signal is used as electrical signal S to control electron beam E1, it can be converted into a visible image image, which appears on the first electrode (2) side of the assembly under control of the dellectingV coil 402.

This embodiment is thus operable in the same manner as conventional television picture tubes and, exhibiting a` Y much higher amplification factor, can operate satis-l y fa-ctorily with an electron beam E1 lower in voltage .and current and with a video signal S weaker than any conventional television picture tube. Infaddition, the output image obtained with the device is brighter because of its higher amplification.

An extremely important advantage of this device is that not only the contrast ratio and the gamma value of the image can be freely controlled by varying voltagesV V1 and V2.

A further embodiment of the present invention shown in FIG. 3 is particularly adapted for use with a radiation energy image E1 such as an ultraviolet image, which is relatively low in penetrability.

A major feature of this embodiment is that its luminescent impedance layer is of composite form.

Description willnow be made assuming that E1 is an ultraviolet image. In FIG. 3, reference numeral 101'indicates a luminescent layer at least containing photoluminescent material such as CdZnS:Ag, which emits orange light when excited with ultraviolet image E1. The layer 101 can be formed in the same manner as described in connection with FIG'. l. When excited, the layer 101 can luminescense `only in its surface region since the ultraviolet image E cannot fully excite the interior of the layer because of its limited penetrability. 1

Therefore, the excitation of photoconductive layer 5 is effected by the luminescense light energy coming fromV the surface region of the layer 101 therethrough and such excitation may not be satisfactorily effective because the light energyY is partly lost by absorption during its 9 passage through the material of the layer 101. To avoid this, the layer 101 itself should be made extremely thin.

On the other hand, the thickness of the layer l101 is subject to limitations from the impedance and dielectric strength requirements to the layer 100 as a luminescent impedance layer.

This situation can be improved by employing an auxiliary luminescence light energy transmitting impedance layer 102 between the luminescent layer 101 and photoconductive layer 5. The impedance layer 102 is formed of a transparent low-loss dielectric substance, such as a polyester lm or a light-transmitting glass enamel. With this construction, the problems of the impedance and dielectric strength can be met with the layer 102, allowing the layer 101 to have any desired thickness. The light energy produced in the layer 101 when excited by ultraviolet image E1 passes through the layer '102 to eiectively excite the photoconductive layer 5,V as will readily be understood. v

By use of the composite luminescent impedance layer 100 including

elementary Vlayers

101 and 102, it will be appreciated that any ultraviolet image E1 can be converted or amplified into visible images E2 of dijerent natures by controlling the voltages V1 and V2, as described hereinbefore in connection with FIG. 1, though such image conversion is impossible with photoconductive layer 5 formed of photoconductive material such as CdS:Cu, Cl.

In this embodiment, the second electrode 7, which is radiant energy transmitting, is comprised of a base or support plate, such as a quartz plate 14, which is transmissive to radiant energy and,.in this instance, to ultraviolet rays, and an electrically conductive lm of tin oxide or the like material coated on said base plate 14.

The third or grid electrode 6,1'nthis embodiment includes an arrangement of spaced parallel metal wires and a conducting strip 17, between which voltage Vlis applied from power supply source 8, as illustrated.

The embodiment of FIG. 3 also includes another form of interlayer 13 lying between the opaque layer 4 and electroluminescent layer 3. The layer 13 is designed to prevent dielectric breakdown between the

electrodes

2 and 6 and also to reliect the luminescence light from layer 3 thereby to enhance the brightness of the visible output image E2. The layer 13 may be formed by evaporating a high dielectric white substance having a high reflection factor and a high dielectric strength, for example, titanium oxide or barium titanate, or by laminating a mixture of a powder of such substance and a binding agent such as glass enamel or plastic.

The thickness of the interlayer 13 is determined so as to give an impedance lower than that of layer'3 with the intention of reducing the voltage loss.

The wire grid electrode 6, serving as a discharge electrode, includes tungsten or other metal wires of the thickness of the order of to 150; arranged at regular intervals of the order of 250 to 700i; and, if required, plated with gold.

In this example, the grid electrode 6 is formed of thin tungsten Wires of the thickness of about 10u and is embedded in the relatively-thick layer 5 midway of its thickness of about 80p. Y

It will be understood that this embodiment is also usable on the same principles with radiation energy El taking the form of X-rays, 'y-rays or like radiant rays of high penetrability. With electron beams, satisfactory results can be obtained by eliminating base plate 14 and making electrode 7 in the form transmissive to electron beams, for example, in the form of a evaporated aluminum film.

FIG. 4 illustrates another embodiment of the present invention which is particularly useful with radiation energy forms of high penetrability such as X-rays and fyrays. In this embodiment, the luminescent impedance layer 100 is also a `composite layer, but of the construction different from that in the embodiment of FIG. 3.

Description will now be made on the embodiment as used with a radiation energy E1 in the form of an X-ray image.

In FIG. 3, the photoconductive layer is excited by luminescent light rays which have passed through the impedance layer 102, which is transmissive to luminescent light energy. In that case, the light rays are often dispersed in the layer 102 to such an extent as to make output image E2 hazy.

This diiiculty is overcome in the embodiment of FIG. 4 by laying directly on thesurface of the photoconductive layer 5 a Vluminescent layer 101 which at least contains a radiation luminescence material as one mentioned hereinbefore. A radiant energy transmitting auxiliary'impedance layer |103 is provided between the second electrode 7 and layer 101, as illustrated, for the purpose of improving the dielectric strength and impedanceV characteristics of the assembly. The auxiliary impedance layer 103 in the illustrated example is formed of an X-ray transmitting dielectric substance such as polyester or other plastic film, glass film or glass enamel.

The second electrode 7 is an X-ray transmitting electrode which is light-reilecting With an aluminum or other metal lilm evaporated or an aluminum or other thin metal foil stuck thereon. Also, utilization may be made of an electrically conducting ilm of a metal oxide such as tin oxide coated on a thin base Iplate of glass.

In addition, an appropriate light reflectivity may be imparted to the layer 103 ,to enhance the utilization factor of luminescent light energy from the layer 101. In other Words, the layer 103 may be made radiation energy transmissive and luminescent light energy reflecting to serve as an auxiliary impedance layer. For example, for use with X-ray images, the layer 103 should leastwise contain a highly light-reilecting substance. One example is a single layer of magnesium oxide or like material coated on the luminescent layer 101. Another example takes the form of a mixed layer including a fine powder of zinc sulfide, titanium oxide, barium titanate or like compound mixed with a binding agent such as epoxy resin orV other plastic or glass enamel. I

Withthese forms of composite layer 103, the luminescent light energy from layer 101 is subjected only to a minimized light-dispersing effect and can be utilized in the photoconductive layer 5 etliciently enough to form a clear and definite output image E2, since it is directly reflected by the layer 103 toward the photoconductive layer 5. These forms of composite layer 103 are obviously highly transmissive to X-rays since they are usually made considerably thin.

In the case where the layer 103.is transmissive to luminescent light energy and the latter is reilected by the second electrode 7, as described hereinbefore, the light energy must proceed over a substantial distance and is subject to a considerably high light-dispersing effect in the layer 103. This sometimes causes an unnegligible detraction from the quality of the output image E2.

If the layer 103 is made opaque or light absorptive in an attempt to eliminate such adverse eiect, it will absorb the luminescent light raysand the utilization factor of the luminescent light energy and hence the sensitivity of the device will be reduced correspondingly. y

It follows, therefore, that to impart a light reectivity to the layer 103 is a very effective way to save this difliculty.

The use of impedance layer 103 is also advantageous when viewed from the standpoint of controlling its impedance value in that an appropriate impedance value can readily be selected for the layer while improving the dielectric strength thereof by properly selecting the thickness of the layer and, if it is a mixed layer, the specific dielectricity and volume ratio of the powder materials forming the layer.

The power supply system shown in FIG. 4 is particularly well adapted to control the operation characteristics of the inventive display device direct-currentwise over a 4 widely extended range While improving'the sensitivity thereof, incase the device includes a photoconductive layer in the .form of a mixed layer including photoconductive powder material such as CdS:Cu, Cl and electrodes, which are connected with a variable current source 16 by way of a polarity changing switch 15 and with an AC power source 300 by way ofdirect-current blocking capacitors C1 and C2 and a conductor 10. Capacitors C1 and C2 should have a capacitance suicient to minimize their AC impedance. Y

As illustrated, the AC power source 300 applies an AC voltage V1 between

conductors

9 and 10 and another AC voltage V1 between

coductors

9 and 12. Under this situation, AC voltage V1 is applied between the-third electrode 3 and light-transmitting electrode 2, and DC voltage VB between the two sets of alternate elements in the third electrode -6 (in the plane of the photoconductive layer 5 extending at right angles to the direction in which the X-ray energy El is irradiated). 4

The AC photoconductive sensitivity of the photoconductive layer, which is formed of a mxiture of photoconductive powder such as CdSzCu, Cl With a .binding agent, as described hereinbefore, is reduced as the .frequency increases because of the nonlinearity of its voltage-current characteristics and inherent AC dependence. This reduction in AC photocoductive sensitivity can be ameliorated -by use of a DC controlling voltage. Y'

On the other hand, the sensitivity, operation characteristics Vcontrast ratio and gamma value of the radiant energy responsive display device shown in FIG. 4 are dependent upon the photoconductive sensitivity in a plane normal to the direction in which E1 is irradiated. Therefore, these characteristics of the device can be made variable by use of DC voltage VB from the variable DC' power source 16 to controllably increase the AC photoconductive sensitivity in a plane normal to the direction of radiation E1. The range of variation in the performance characteristics'will be extended as the voltages V1 and V2 increase in frequency.

In the positive, V-shaped and negative operations of the device as obtained by controlling V1 andKV2 in the same manner as described in connection with FIG. 1, the increase in VB causes displacement of the operation characteristics in a direction in which the irradiation magnitude of the input radiant energy decreases thereby to increase the sensitivity, contrast ratio and gamma value. An irnportant advantage is thus obtained that the range of variation in performance of the device which is obtainable by controlling the amplitude and phase relationships of V1 and V2 (including the case where either V1 or V2 is reduced to zero)can be further extended under the control of DC voltage VB.

On the other hand, use of a photoconductive layer formed of powdery material bound together is very undesirable fon some applications since even after the irradiation of the radiant energy image has been interrupted the speed ofresponse is so limited as to cause a residual image continuing for a period of from several seconds to several minutes.

Utilizing a special phenomenon of this form of photoconductive layer, the residual image can be extinguished rapidly by changing the polarity of the D C. voltage VB, which is being applied between the interlined electrodes 6, thereby to reverse the polarity of the D.C. electric eld in which the photoconductive layer 5 is placed.

The polarity-changing switch 15 is provided to serve the' purpose. With the power supply system shown in'FIG. `4, it will thus be appreciated that not only the performance characteristics of the display device can be made variable over a wider range -but also the residual image can be freely extinguished.

This power supply system can be applied irrespective of the structure ofV the third electrode 6 as long as it includes an arrangement of electrode elements, for example, as shown in FIG. 3, and without regard to the type of radiant energy image El.

Obviously, voltage means 16 is not always required to be variable since the D C. voltage may be fixed if required.

Though the luminescent impedance layer 100 has been shown and described herein as including one'or two layers,

it may take the form of a composite layer including moreA withoutl departing from the than two elementary layersV spirit of the invention. A

` It will be appreciated Yfrom'lthe foregoing that the.

- present invention makes it possible to convert a radiant energy imageV into a visible image vintensified with highY` Y sensitivity, which'has previously been impossible or feas- Y.

ibly only with limited sensitivity.

The radiant energy responsive display-device oftheY`v`r present invention is usable withva wide variety of radiant Yenergy images: including those having wavelengthsshorter than the highest sensitivity wavelength in the` spectrum or conductivity distribution of the photoconductive layer, Y

whether they are visible or invisible` (like ultraviolet rays);

radiation energy images such as X-ray or y-ray radiation' l images; Velectron radiation images or signals such as electron beams; and all other 'radiant energy signals or 1 images elective to excite the luminescent impedancelayer of the device. Particularly, with high penetrability radiation energy images such as X-ray or 'y-ray images, the` inventive device can operate with such a high sensitivity as has previously been unimaginable since the photoconductive layer is excited with two types of image including the radiant energy image passing through the luminescentV impedance layer and a luminescent light energy image formed under the excitation elfect of the radiant energy- Y image.

Though a few embodiments of the present invention Y have been shown and described herein, it is to understood that many changesV and modifications can be made by combining the features and concepts'of the different Y embodiments without departing from the spirit of the in-V vention or from the scope of the appended claims.

What is claimed is:

1. A radiant energy responsive display device of the type including a photoconductive layer, an electroluminescent layer lying on one surface of said photoconductive layer, an impedance layer lying on the other surface of said photoconductivelayer, a first electrode arranged on the outer surface of said electroluminescent layer, Va second electrode arranged on 'the outer surface ofV said impedance layer, a third electrodeinterposed between said electroluminescent and impedance layers lin electricalv l' contact with said photo'conductive layer, electric power supply means for'applying a Iirst voltage between said rst and third electrodes and second voltage between said first and second electrodes, an means for electrically controlling the luminescent intensity of said electrolumil `nescent layer through the variation in impedance under the excitation effect of radiant energy on said photoconductive layer to thereby convert'V a radiant energy image 2 into visible form on said electroluminescent layer, in which said impedance layer is an energy converting luminescent layer having a radiant energy response to` a first spectral distributionV and having a luminescence characteristic with a second energy distribution different i from the rst and at least partly overlapping the Y spectral photoconductivity distribution of the photoconductive layer whereby, when excited with radiant energy,

said luminescent impedance layer produces luminescent light energy to which said photocondnctive layer is responsive. Y

2. A radiant energy responsive display device according to claim 1 wherein an intermediate layer is interposed between said electroluminescent layer and said photoconductive layer.

3. A radiant energy responsive display device according to claim 1 wherein said intermediate layer is an opaque layer.

4. A radiant energy responsive display device according to claim 1 wherein said rst electrode is a light transmitting electrode.

5. A radiant energy responsive display device according to claim 1 wherein said second electrode is a radiant energy transmitting electrode.

6. A radiant energy responsive display device according to claim 1 wherein said third electrode is a grid-like discharge electrode.

7. A radiant energy responsive display device according to claim 1 wherein said radiant energy image is converted to visible form on said electroluminescent layer at a size no less than that of the original image.

8. A radiant energy responsive display device according to claim 1 in which the maximum spectral sensitivity of said photoconductive layer lies within the spectral range of the luminescence of said luminescent impedance layer.

9. A radiant energy responsive display device according to claim 1 in which said luminescent impedance layer has a thickness of 20 to 20D/i.

10. A radiant energy responsive display device according to claim 1 in which said impedance layer comprises a compound of zinc and epoxy resin.

11. A radiant energy responsive display device according to claim 1 in which said impedance layer comprises a compound of zinc and glass enamel. t

12. A radiant energy responsive display device according to claim 1 in which said impedance layer comprises a highly light reflective material.

13. A radiant energy responsive display device according to claim 1 in which said impedance layer comprises a mixture of powder of a luminescent material and a binding material.

14. A radiant energy responsive display device according to claim 1 comprising an auxiliary impedance layer capable of transmitting said luminescent light energy produced by said impedance layer interposed between said impedance layer and said photoconductive layer.

15. A radiant energy responsive display device according to claim 1 comprising an auxiliary impedance layer capable of transmitting said radiant energy interposed between said impedance layer and said second electrode.

16. A radiant energy responsive display device according to claim 1 comprising an auxiliary impedance layer capable of transmitting said radiant energy and capable of reflecting lsaid luminescent light energy produced by said impedance layer interposed between said impedance layer and said second electrode.

References Cited UNITED STATES PATENTS 2,929,950 3/ 1960 Hanlet 313-108 2,999,941 9/ 1961 Klasens et al Z50-213 3,101,408 8/1963 Taylor 250-83.3 X 3,217,168 11/ 1965 Kohashi 250--213 WALTER STOLWEIN, Primary Examiner.

RALPH G. NILSON, Examiner.

I. D. WALL, Assistant Examiner.

Claims

1. A RADIANT ENERGY RESPONSIVE DISPLAY DEVICE OF THE TYPE INCLUDING A PHOTOCONDUCTIVE LAYER, AN ELECTROLUMINESCENT LAYER LYING ON ONE SURFACE OF SAID PHOTOCONDUCTIVE LAYER, AN IMPEDANCE LAYER LYING ON THE OTHER SURFACE OF SAID PHOTOCONDUCTIVE LAYER, A FIRST ELECTRODE ARRANGED ON THE OUTER SURFACE OF SAID ELECTROLUMINESCENT LAYER, A SECOND ELECTRODE ARRANGED ON THE OUTER SURFACE OF SAID IMPEDANCE LAYER, A THIRD ELECRODE INTERPOSED BETWEEN SAID ELECTROLUMINESCENT AND IMPEDANCE LAYERS IN ELECTRICAL CONTACT WITH SAID PHOTOCONDUCTIVE LAYER, ELECTRIC POWER SUPPLY MEANS FOR APPLYING A FIRST VOLTAGE BETWEEN SAID FIRST AND THIRD ELECTRODES AND SECOND VOLTAGE BETWEEN SAID FIRST AND SECOND ELECTRODES, AN MEANS FOR ELECTRICALLY CONTROLLING THE LUMINESCENT INTENSITY OF SAID ELECTROLUMINESCENT LAYER THROUGH THE VARIATION IN IMPEDANCE UNDER THE EXCITATION EFFECT OF RADIANT ENERGY ON SAID PHOTOCON-