US3112404A - Photosensitive radiant-energy transducers - Google Patents

Description

Nov. 26, 1963 w. o. REED 3,112,404

PHOTOSENSITIVE} RADIANT-ENERGY TRANSDUCERS Filed June 1'7, 1953 l3 l6 l3 Photofignductive Element Photoconductlve Element Electroluminescent Electroluminescent Element Element Voltage Voltage Fl G, 1 Source Source FIG. 4

Electroluminescent Element 22 l3 l4 l5 l6 *5 j 1 O. .I I 5 4 Y o j :5 3 .9 j 2 j A; E I (a I I E t I00 zoo 300 400 v, S Voltage across Electroluminescent conducflw/ |9 Layer Element 2 Voltage Source a: 400 Q U 3300 2oo 2 I00 cc 0 l 2 3 4 5 Relative Intensity of Incident Radiation WILLIAM O. REED Fl G. 3 INVENTOR.

H'IS ATTORNEY.

United States Patent 3,112,404 PHOTGSENSITWE RADIANT-ENERGY TRANSDUQERS William 0. Reed, Chicago, Ill, assignor to The Rauland Corporation, a corporation of Illinois Filed June 17, 1953, Ser. No. 362,195 2 Claims. (Q1. 250-213) This invention relates to radiant-energy transducers, and more particularly, to such transducers comprising at least one electroluminescent element. For purposes of this specification, electroluminescence may be defined as the characteristic of emitting visible or invisible light radiations in response to the application of a suitable electric field.

Known radiant-energy transducers use the phenomenon of photoemission or cathodoluminescence to efiect the reproduction in observable form of radiations representing an image. For example, image converters which depend upon the phenomenon of photoemission to translate light energy of one wavelength into light energy at a diflerent wavelength, preferably in the visible light spectrum, are well known. In general, devices of this type comprise photoemissive and fluorescent elements arranged within an evacuated glass envelope, and while not exceptionally bulky or cumbersome, are not as compact as might be desired. Moreover, in the fabrication of such image converters, extreme care must be taken to avoid contamination of the fluorescent screen While activating the photoemissive surface, and other costly and time-consuming operations are involved in the manufacturing process.

In the field of projection television, one of the major limitations has always resided in the loss of picture brightness accompanying image magnificaiton by optical means. Electronic image intensification through the use of specially constructed image converters, while obtainable, has not been commercially feasible because of the inordinately high cost of projection television systems employing such devices as compared with that of the structurally less complex and more compact arrangements for direct-view image reproduction.

It is a primary object of this invention to provide an improved radiant-energy transducer comprising at least one electroluminescent element.

It is a further object of the invention to provide a new and improved radiant-energy transducer for translating light radiations from one wavelength to a different wavelength.

It is another object of this invention to provide a novel radiant-energy transducer for intensifying or amplifying visible light radiations.

A further object of this invention is to provide a new and improved image intensifier well adapted for use in projection television systems to increase the brightness of the reproduced image.

Yet another object of the invention is to provide a new and improved radiant-energy transducer of simple and compact construction, which is well adapted to economical fabrication on a mass production basis.

A new and improved radiant-energy transducer constructed in accordance with the invention comprises a pair of electrodes with interposed contiguous layers of radiant-energy-sensitive and electroluminescent materials respectively, in registration with the electrodes to constitute a compact laminar structure. The radiant-energysensitive material has an impedance characteristic per unit area which is variable in response to incident radiations, while the electroluminescent element emits light radiations in response to an applied electrical potential. One of the electrodes is at least substantially transparent to incident radiations, and one of the electrodes is at least ice substantially transparent to the light radiations originating at the electroluminescent layer. Means are also provided for establishing an electrical potential difference between the electrodes to cause the electroluminescent element to emit light in response to the variations in the voltage applied to the electroluminescent element resulting from variations in the electrical impedance of the radiant-energy-sensitive element; the invention contemplates that the applied potential comprises a combination of alternating and unidirectional voltages. The transducer of the invention may respond either to electromagnetic or particle radiation.

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood, however, by reference to the following description taken in connection with the accompanying drawings, in the several figures of which like reference numerals indicate like elements, and in which:

FIGURE 1 is a cross-sectional view, partly schematic, of a preferred embodiment of the present invention,

FIGURE 2 and 3 are graphical representations of certain operating characteristics of the embodiment of FIG- URE 1,

FIGURE 4 is a cross-sectional view, partly schematic, of another embodiment of the present invention, and

FIGURE 5 is a cross-sectional view, partly schematic, of a third embodiment of the invention.

A preferred embodiment of the invention, as illus trated in FIGURE 1, comprises a layer 14 of radiantenergy-sensitive or photosensitive material afiixed to a first electrode 13, and a layer 15 of electroluminescent material afilxed to a second electrode 16 and disposed between the latter electrode and the side of photosensitive layer 14 opposite electrode 13. For certain applications it may be advantageous to provide a suitable binder, such as sodium or potassium silicate, between layers 14 and 15. From another viewpoint, electrodes 13 and 16 oom prise the plates of a condenser the dielectric of which is composed of juxtaposed photosensitive and electroluminescent layers 14 and 15. A source 2% of substantially constant alternating voltage is connected to the electrodes 13 and 16 by means of wire conductors 19 or the like.

For purposes of illustration there are also depicted an image it and an optical lens system 11 The image 10 may be any object which is capable of emitting radiations or reflecting radiations representative of it. The optical lens system may be of any suitable construction, and, for convenience, is here schematically represented as a single biconcave lens 11. Specifically, image 10 may constitute the image formed at the fluorescent screen of a projection-type cathode-ray tube image-reproducer of a television receiver, while lens system 11 may comprise a Schmidt optical system or other image-magnifying lens system associated with the image reproducer.

The two electrodes 13 and 16 are composed of a material which is electrically conductive and, in the preferred embodiment, electrode 13 is at least substantially transparent to the incident light radiations from image 10, while electrode 16 is at least substantially transparent to the light radiations emitted by the electroluminescent layer 15. For example, electrodes 13 and 16 may each be composed of a plate of insulating glass upon the inner surface of which is provided an electrically conducting coating of tin oXide or the like; Corning E-C or electrical conducting glass has been found satisfactory. The conductive coating may be of the order of .00002 inch in thickness with a resulting transparency of about with respect to the incident or the emitted light radiations.

The radiant-energy-sensitive element or photosensitive layer 14 is preferably formed of photoconductive mate- .9 rial, as for example lead sulphide or selenium in a transducer of the image intensifier type intended for use at wavelengths within the visible portion of the spectrum, or thallous sulphide, lead telluride, or lead selenide in an image converter for translating infra-red light radiations to visible images. A typical composition of the electroluminescent material may be approximately 80% zinc sulphide and Zinc selenide with copper as an activator, although other electroluminescent materials such as silicon carbide may be employed. Of course, the wavelength-response characteristic of the transducer is determined largely by the compositions of the photosensitive and electroluminescent layers; consequently the choice of materials is dependent on the application for which the transducer is intended. For convenience, the invention is hereinafter explained by reference to its image intensifier or light amplifier embodiments, although it is to be clearly understood that such embodiments may be modified to constitute image converters by merely altering the composition of the photosensitive material.

While the thicknesses of the photosensitive and electroluminescent layers is partly dependent on the compositions employed and on the desired operating characteristics, the photoconductive layer is generally thinner than the electroluminescent layer; in one embodiment, the photoconductive layer may be about microns thick, while the thickness of the electroluminescent layer may be of the order of 100 microns, although thicknesses of much greater magnitude may be employed.

The operating characteristics of the transducer are materially afiected by the frequency of the alternating voltage applied between electrodes 13 and 16 from source 20. In general, for a given alternating voltage, light output is increased with an increase in the frequency. Thus, although the transducer is operative with 60-cycle alternating voltages derived directly from the public utility power lines, it is preferred to employ higher frequencies of the order of 2000 cps. or higher, in order to achieve increased brightness; to this end, voltage source 2i) may comprise a voltageand frequencystabilized audio-frequency oscillator. Moreover, with presently known materials, useful electroluminescence is only achieved with applied alternating voltages exceeding a predetermined threshold voltage which is a function of the material; with the copper-activated zinc sulphide-zinc selenide described, this threshold voltage is about 200 volts R.M.S. If desired, a direct-voltage bias from any suitable source, as for example a battery or other direct-voltage power supply such as the rectified voltage supply of a projection television receiver (not shown), may be superimposed in either polarity on the alternating voltage applied between electrodes 13 and 16.

The fabrication of the radiant-energy amplifier of the preferred embodiment may be accomplished in the following manner. The photosensitive or photoconductive element may be composed of lead sulphide which may be produced from natural sources, such as galena crystals, or prepared synthetically. Commercial grades of galena of chemical purity suificient for use in this device are readily available. The galena crystals are crushed into a fine powder and pressed through a suitable screen to provide minute crystals of uniform size. The pulverized crystals may be sublimated to the desired thickness (about 0.001 inch) on the surface of a section of electrically conducting glass to form the photoconductive layer 14 upon one side of electrode 13. During the process of sublimation it has been found advantageous to introduce moist oxygen in order to aid materially in photosensitizing the lead sulphide, although the reasons for the reaction of the moist oxygen with the lead sulphide to produce more sensitivity in the photoconductive characteristic of this chemical compound are not fully understood.

The electroluminescent element, comprised of a suitable composition of zinc sulphide and zinc selenide with copper as an activator as previously discussed, may be suspended in a solution of ethylene dichloride and polyethyl methacrylate; if desired, a small amount of barium titanate or other high dielectric constant material may be added for the purpose of increasing the dark admittance of the electroluminescent layer. The mixture is allowed to dry and the residue is crushed to a fine powder. The powder is pressed between two heated steel platens with accurately ground surfaces to form a film approximately microns thick. This film is then squeegeed against a conducting glass surface covered with a viscous grade of silicone oil. After the application of this sheet of electroluminescent material upon the surface of the conductive glass, the excess oil is removed by careful scraping so that there is intimate contact between the film and the glass. The two glass sheets are then mounted together in such a manner that the coatings which have been placed on their surfaces are in registration and in intimate contact with each other.

Much or" the theory involved in the function and nature of electroluminescence is not completely understood; however, it has been experimentally determined that if an alternating potential or a recurrent pulse signal of amplitude above a predetermined threshold is impressed upon an electroluminescent material, the material emits light radiation in proportion to the R.M.S. value of the alternating voltage. In short, the electroluminescent layer emits light radiation per unit area with an intensity directly, although not necessarily linearly, proportional to the magnitude of the applied energizing signal. For the zinc sulphide-zinc selenide material described, the characteristic of brightness in foot lamberts emitted by the electroluminescent layer as a function of the R.M.S. value of the impressed alternating potential is shown in FIGURE 2. It has been experimentally determined that the brightness of films containing definite amounts of electroluminescent phosphor depends critically on the electric field strength, but only very slightly on the amount of phosphors, provided there is at least about 6 milligrams of phosphor per square centimeter. Experiments have indicated that within a temperature range from 100 C. to +50 C. the brightness response is substantially independent of temperature variations.

FIGURE 3 shows the interdependence of the voltage components applied respectively to the photoconductive and electroluminescent layers as a function of the intensity in foot lamberts of the incident light. Curve 1 of FIGURE 3 represents a constant applied alternating voltage between electrodes 13 and 16 of 400 volts R.M.S. Curve 2 of FIGURE 3 illustrates the variation of that portion of the applied voltage which is impressed across the electroluminescent layer as the intensity of incident light is increased. Curve 3 represents the alternating voltage component applied across the photosensitive layer as a function of the intensity of the incident light. As shown by these characteristics, as the relative intensity of the incident light increases there is a corresponding decrease in the potential applied across the photosensitive layer. There is also a correlative increase in the potential applied across the electroluminescent element. For any condition of incident light intensity, the sum of the voltage components across the respective layers 14 and 15 (curves 2 and 3) corresponds to the constant applied voltage of curve 1.

It is known that the dielectric constant of a photoconductive material varies in proportion to the intensity of incident light. More specifically as the incident light increases in intensity, the specific-inductive-capacity or dielectric constant of the photoconductive layer also increases. Furthermore, with an increase in incident light the resistance of the photoconduetive layer decreases to permit a greater flow of conductive current. Consequently, the two layers constitute a capacitive voltage divider, with the voltage division ratio varying as a function of the incident light intensity. As the intensity of the incident light increases, the dielectric constant of the photoconductive material, and hence the voltage impressed across the electroluminescent element, also increases. When the energizing signal across the electroluminescent layer exceeds a certain threshold value, an emission of visible light radiation from the electroluminescent material ensues. In this Way the incident radiation, representative of image 10, which forms a charged area upon the photosensitive layer 14, causes an image to be reproduced on the electroluminescent layer 15. The variable response characteristics of the photosensitive and electroluminescent elements permit a reproduction of the image in gradations of brightness corresponding to the intensity of the incident radiations, and hence, corresponding to the half-tones or shade values of the image.

The radiant-energy transducer of FIGURE 1 is considerably more compact than hitherto known image intensifiers and image converters and may he constructed and operated without being enclosed in an evacuated envelope; thus many of the difliculties encountered in the manufacture of previously known devices, such as the precautions against contamination of the fluorescent screen during activation of the photoemissive cathode, are eliminated. Moreover, the transducer of FIGURE 1 is readily adaptable to use in projection television systems to provide increased brightness of the reproduced image, thus overcoming one of the most severe limitations heretofore encountered in such systems.

A further embodiment of this invention may be considered in conjunction with FIGURE 1 in which the voltage source 20 comprises a direct current source, such as an A.C. rectifier or battery, of about 400 volts D.C. In this embodiment image consists of a source of rapidly varying illumination, such as the output of a motion picture projector, the viewing screen of a television picture tube, or a pulsed light source. In this case the transducer operates in a manner similar to that discussed in the previous embodiment except that the change in impedance characteristic of the photosensitive layer causes the production of a pulsed signal which although derived from a constant direct current source, has an alternating potential component. The electroluminescent layer has of its very nature a leakage resistance which permits a decay in voltage across the photosensitive layer so that an effective constant potential cannot remain across the photoconductive layer. In this manner a constant source of D.C. potential is effectively substituted for the alternating potential source which has proved to be necessary in other embodiments of this invention.

An additional embodiment of this invention is shown in FIGURE 4. The laminar structure of FIGURE 4 and the general theory of operation of this structure are largely the same as discussed in connection with the embodiment of FIGURE 1. However, in this figure the incident light from object 10 is projected onto photoconductive layer 14 through electrode 16 and electroluminescent layer 15. For such operation, electrode 16 must be at least substantially transparent both to the incident light radiation from object 10 and to the light radiations originating at electroluminescent element electrode 13 need not be transparent, and if desired, may be formed as a highly polished metal plate or metal-coated conductive glass plate to constitute a reflector immediately behind photoconductive layer 14 so that a large portion of the light emitted from the electroluminescent layer may be utilized in conjunction with the incident radiation to vary the efiective reactance and conductivity of the photosensitive element. This results in a regeneration of light energy which can prove to be very effective in the amplification of weak incident light radiation.

It may prove desirable in some applications to provide a means for controlling the amount of regeneration and for this purpose a coating of finely divided carbon (not 6 shown) may be interposed between the photoconductive and electroluminescent layers. Such a carbon layer may be formed to provide longitudinal conduction between the two layers while being substantially non-conducting along its surface dimension. Alternatively, undesirable regeneration may be inhibited by a judicious choice of the photoconductive and electroluminescent materials such that the wavelength response characteristics of the photoconductive and electroluminescent layers do not overlap.

Another embodiment, shown in FIGURE 5, may be used as a detector of atomic radiation. This embodiment comprises the same transducer disclosed in conjunction with FIGURE 1, with the additional feature of an added layer 22 of a substance which is responsive to atomic radiation. Element 22 may comprise a layer of chemical compound responsive to the radiation of alpha, beta, or gamma radiation, such as naphthalene or anthnacene, enclosed in a glass structure 23 affixed to electrode 13. Upon bombardment by atomic articles or rays, the radiation-sensitive layer 22 emits radiations which in turn aifect the dielectric constant of the photoconductive layer in the manner described in connection with the embodiments of FIGURES 1 and 4. As previously explained, such changes in the dielectric constant of the photoconductive layer result in corresponding changes in the voltage impressed across the electroluminescent layer with a resulting emission of visible light radiation which may be observed through electrode 16.

Additional embodiments within the scope of the present invention include cascade structures in which additional photoconductive and electroluminescent layers are arranged so that the light output from one stage may be directed to a second stage to provide further image intensification.

The present invention provides a compact and simple structure for amplifying light radiations which is particularly useful in the construction of an efiicient and commercially practicable projection television system. It also provides an image converter for the translation of light radiations of one wavelength into those of another by the use of a compact and simple structure which is well adapted to economical fabrication on a mass production basis. The invention is also adaptable to other types of radiant-energy transducers, such as atomic radiation detectors and the like.

While particular embodiments of the invention have been shown and described, modifications may be made and it is intended in the appended claims to cover all such modifications as fall Within the true spirit and scope of the invention.

1 claim:

1. A radiant-energy transducer comprising: a first electrode; a photoconductive element in registration with said electrode and having an electrical impedance per unit area which is variable in response to incident radiations; an electroluminescent element, responsive to an applied electrical potential exceeding a predetermined threshold magnitude 'for emitting light radiations, in registration with said photoconductive element; a second electrode in juxtaposition with said electroluminescent elernent opposite said photoconductive element; said first electrode being at least substantially transparent to said incident radiations and said second electrode being at least substantially transparent to said light radiations; means for projecting incident radiations through said first electrode upon said photoconducti-ve element to cause variations in its electrical impedance per unit area; and means, including a source of unidirectional potential superimposed upon an alternating potential with said unidirectional and alternating potentials individually of less magnitude than said threshold potential magnitude but together having a magnitude exceeding said threshold potential magnitude, for establishing an electrical potential diiference between said electrodes to cause said electroluminescent element to emit light in response to the variations in the voltage 7 applied to said electroluminescent element resulting from said variations in said electrical impedance of said photoconductive element.

2. A radiant-energy, transducer comprising: a first electrode; a pho-toconductive element in registration with said electrode and having an electrical impedance per unit area which is variable in response to incident radiation; an electroluminescent element, responsive to an applied electrical potential exceeding a predetermined threshold magnitude for emitting light radiation, in registration with said photocondu'ctive element; a second electrode in juxtaposition with said electroluminescent element opposite said photoconductive element; said first electrode being at least substantially transparent to said incident radiation and said second electrode being at least substantially transparent to said light radiation; means for projecting incident radiation through said first electrode upon said photoconductive element to cause variations in its electrical impedance per unit area; and means, including a source of unidirectional potential superimposed upon an alternating potential, for establishing an electrical potential difference between said electrodes to cause said electroluminescent element to emit light in response to the variations in the voltage applied to said electroluminescent element resulting from said variation in said electrical impedance of said photoconductive element.

References Cited in the file of this patent UNITED STATES PATENTS 2,120,916 Bitner June 14, 1938 2,645,721 Williams July 14, 1953 2,650,310 White Aug. 25, 1953 OTHER REFERENCES Thornton: A.C.D.C. Elec-troluminescence; Physical Review; vol. 113; No. 5; Mar. 1, 1959; pp. 1187-1191.

Claims (1)

1. A RADIANT-ENERGY TRANSDUCER COMPRISING: A FIRST ELECTRODE; A PHOTOCONDUCTIVE ELEMENT IN REGISTRATION WITH SAID ELECTRODE AND HAVING AN ELECTRICAL IMPEDANCE PER UNIT AREA WHICH IS VARIABLE IN RESPONSE TO INCIDENT RADIATIONS; AN ELECTROLUMINESCENT ELEMENT, RESPONSIVE TO AN APPLIED ELECTRICAL POTENTIAL EXCEEDING A PREDETERMINED THRESHOLD MAGNITUDE FOR EMITTING LIGHT RADIATIONS, IN REGISTRATION WITH SAID PHOTOCONDUCTIVE ELEMENT; A SECOND ELECTRODE IN JUXTAPOSITION WITH SAID ELECTROLUMINESCENT ELEMENT OPPOSITE SAID PHOTOCONDUCTIVE ELEMENT; SAID FIRST ELECTRODE BEING AT LEAST SUBSTANTIALLY TRANSPARENT TO SAID INCIDENT RADIATIONS AND SAID SECOND ELECTRODE BEING AT LEAST SUBSTANTIALLY TRANSPARENT TO SAID LIGHT RADIATIONS; MEANS FOR PROJECTING INCIDENT RADIATIONS THROUGH SAID FIRST ELECTRODE UPON SAID PHOTOCONDUCTIVE ELEMENT TO CAUSE VARIATIONS IN ITS ELECTRICAL IMPEDANCE PER UNIT AREA; AND MEANS, INCLUDING A SOURCE OF UNIDIRECTIONAL POTENTIAL SUPERIMPOSED UPON AN ALTERNATING POTENTIAL WITH SAID UNIDIRECTIONAL AND ALTERNATING POTENTIALS INDIVIDUALLY OF LESS MAGNITUDE THAN SAID THRESHOLD POTENTIAL MAGNITUDE BUT TOGETHER HAVING A MAGNITUDE EXCEEDING SAID THRESHOLD POTENTIAL MAGNITUDE, FOR ESTABLISHING AN ELECTRICAL POTENTIAL DIFFERENCE BETWEEN SAID ELECTRODES TO CAUSE SAID ELECTROLUMINESCENT ELEMENT TO EMIT LIGHT IN RESPONSE TO THE VARIATIONS IN THE VOLTAGE APPLIED TO SAID ELECTROLUMINESCENT ELEMENT RESULTING FROM SAID VARIATIONS IN SAID ELECTRICAL IMPEDANCE OF SAID PHOTOCONDUCTIVE ELEMENT.

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