US3535574A - Image pick-up tube with a photosensitive transmission secondary electron multiplication layer - Google Patents

Description

IMAGE PICK--UP SECONDARY M. Filed Feb. 12, 1968 5,535,574 "FIVE TRANSMISSION. 'UIGATION LAYER 2 Sheets-Sheet 1 FIG; I

INVENTOR H/mu 0 MHE D H ATTORNEY Oct. 20, 1970 wimuo MAEQA IMAGE PICK-UP TUB WITH 32 PHOTOSENSITIVE TRANSMISSION SECONDARY ELECTRON MULTIPLICATION LAYER 2 Sheets-Sheet 2 Filed Feb. 12, 1968 INVENTOR HRRLLD IIHEDH ATTORNEYJ United States Patent Ofice Patented Oct. 20, 1970 3,535,574 IMAGE PICK-UP TUBE WITH A PHOTOSENSITIV E TRANSMISSION SECONDARY ELECTRON MUL- TIPLICATION LAYER Haruo Maeda, Tokyo, Japan, assignor to Matsushita Electric Industrial C0., Ltd., Osaka, Japan, a corporation of Japan Filed Feb. 12, 1968, Ser. No. 704,882 Claims priority, application Japan, Feb. 24, 1967, 42/ 12,202; May 29, 1967, 42/334,767; May 30, 1967, 42/35,044; July 25, 1967, 42/48,283 Int. Cl. H013 31/48, 43/02; H011 15/00 US. Cl. 313-65 Claims ABSTRACT OF THE DISCLOSURE An image pick-up tube comprising a target, wherein the transmission secondary electron multipling porous layer has photoconductivity and the transmission secondary electron multiplication is controlled by the impedance change of said photoconductive porous layer corresponding to the incident light of an image.

This invention relates to a novel image pick-up device which utilizes the transmission secondary electron multiplication effect of a porous photoconductive material. This invention resembles a known vidicon in that a photoconductive material is used as a photosensitive surface, but it is quite different in its fundamental operation.

In a vidicon type image pick-up tube, a photoconductive evaporated film layer provided as a photosensitive surface exhibits a resistance distribution corresponding to an intensity distribution of the light image projected thereon and a television image signal is obtained by discharging a charge from a NESA film through a load resistance, said charge being accumulated during a scanning period in the distributed capacitance considered to exist between the scanning surface of the photoconductive film and the NESA conductive layer of the substrate when the surface of said photoconductive layer is scanned by a low velocity electron beam. Thus, in a conventional vidicon, the combination of a sensitive photoconductive film and a low velocity scanning electron beam usually below several hundred volts is the basis of its operation.

The image pick-up tube of this invention also employs a photoconductive layer as a photosensitive layer, but a high speed electron beam of more than several kilovolts to several tens kilovolts is used as a scanning beam. Moreover, said scanning high velocity beam is not used for discharging a charge accumulated on a photoconductive sensitive layer whose quantity corresponds to the image as in a conventional vidicon, but the high velocity beam passes through a low density porous photoconductive film which is a photosensitive target characteristic of this invention and produces secondary electrons to provide current multiplication when passing. Said electrons are collected with a collector and thus a television image signal is obtained.

The objects of the invention generally set forth, together with ancillary advantages, are attained by the construction and arrangement shown by way of illustration in the accompanying drawings, in which:

FIG. 1 is a diagram illustrating the structure and the principle of operation of an image pick-up tube according to an embodiment of this invention;

FIG. 2 is a magnified composition diagram of a target shown in FIG. 1;

FIGS. 3a and 3b are magnified composition diagram of a TSEM target for use in a pick-up tube of this invention;

FIG. 4 is a sectional diagram showing the state wherein electrons are multiplied at the photoconductive TSEM layer;

FIG. 5 is a cross-sectional view of TSEM photoconductive target wherein two kinds of materials compose a mixed layer intertwined with the porous layer; and

FIG. 6 is a cross-sectional view of a porous TSEM target according to another embodiment of this invention.

This invention is now described in detail with reference to FIG. 1. The reference numeral 1 indicates a photosensitive surface of the target made of a low density photoconductive film characteristic of this invention, 2 designates an electron gun projecting an electron beam of more than several kilovolts onto the target 1 with a high resolution and 3 shows a face for introducing an image from a face in front of the tube into the target 1 the inner surfaces of which are coated with a transparent conductive treated surface or a NESA film 3. Reference numeral 4 designates a lens system for electromagnetically focusing the electron beam, 5 designates a high energy focused electron beam emitted from the electron gun 2, 6 indicates a glass tube and 7 indicates an optical lens system for focusing a picture to be picked-up into an image on the target 1. During ordinary performance, the target 1 is grounded, the electron gun 2 is provided with a high negative potential and a high energy electron beam is made to collide with the target 1 by use of the electron beam 5. A suitable positive potential is applied to the NESA film 3 with respect to the target 1 through a resistor R. Now, the mechanism of the sensitive photoelectric conversion of the target 1 as a pick-up tube will be explained. FIG. 2 shows a magnified composition diagram of the target 1. The target 1 consists of a metal film 8 and a deposited low density insulating layer 9 of quite fine photoconductive particles as shown in the figure. Generally, in the case of a conventional transmission secondary electron multiplication device, when a high energy electron from electron gun 2 is injected, as shown in the drawings, from the side of the metal film into the target formed by depositing insulators like KCl or other alkali halides, etc. on the metal film support member in a way to make a porous low density insulating layer, the high energy electron transmits through the metal film, enters the porous low-density insulating layer, undergoes multiple reflection therein and produces secondary electrons multiplied in geometrical progression. As a result, a large number of secondary electrons are emitted from the back surface of the low density insulating layer as shown by arrows 12. Such a phenomenon is known as transmission secondary electron multiplication (hereafter denoted as TSEM).

This invention intends to provide a novel pick-up tube wherein a material producing said TSEM is used as a photoconductive material and the secondary electron multiplication is controlled by external light. TSEM is based on the multiplication of secondary electrons in geometrical progression produced when a primary electron is injected on a low density porous insulating layer as described hereinabove, but it has no photoelectric sensitivity and the photoelectric conversion effect thereof has not been utilized. According to this invention, the control effect due to the external light is added to the TSEM effect of a photoconductive body at a porous layer by forming a low density porous photoconductive layer of a high resistance photoconductive material having a very small dark current like CdS, CdSe, Sb S etc. instead of said low density porous insulating material. According to the conventional TSEM effect of an ordinary porous insulator, a large number of positive charges are left on the surface of the insulator inside the porous space'of the low density insulating layer of FIG. 2. In the generating process of the secondary electron within the low density insulating layer a progressive generation of the secondary electron is taking place in the porous insulating layer as said positive charges appear on the insulator surface producing an accelerating electric field and further in turn contribute to the generation of the subsequent secondary electrons emission. When the secondary electrons 12 generated in this way are emitted in a right hand direction from the surface of the porous low density insulating layer, positive charges corresponding to the quantity of the emitted secondary electrons are left on the surface of the layer and this time these work to inhibit the emission of secondary electrons from the surface. Accordingly, when a suitable positive potential is applied to the collector electrode 3 so as to capture the emitted secondary electrons, the surface potential of the low density insulating layer becomes equal to the potential of the collector electrode and the secondary electrons are continuously emitted from the surface until equilibrium is reached if the insulating property of the layer is good.

The quantity of the finally emitted secondary electrons 12 depends upon accelerating voltage, current density of the primary electron beam 5 incident on the metal film of the target, the material, structure and thickness of the low density insulating layer 9, the distance between the collector electrode 3 and the secondary electron emission surface, the potential of the target electrode 3 and the like and the gain of the TSEM target becomes several tens to several hundreds. The aim of this invention is to provide a photoconductive TSEM target, wherein the gain of the secondary electron emission can be controlled by the external light 13, by using a photoconductive material having quite a high dark resistance instead of the ordinary insulator materials performing TSEM operation as described hereinabove and to utilize said TSEM target as an image pick-up device.

Namely, the porous photoconductive layer 9 is composed of a photoconductive body material with a higher resistivity, and the SnO film (UESA film) 3 formed on the inner surface of the window glass plate forms a collector electrode which captures the transmitted secondary electrons emitted from the surface of the low density photoconductive porous layer 9 when a suitable positive potential is applied to the film 3.

The object 10 to be picked up is projected on the surface of the low density photoconductive layer 9 as shown at 11 by the lens system 7 through the window glass 6 and the transparent conductive layer 3.

Said low density photoconductive layer has a thickness of 40,u, but because of its low density the incident light penetrates inside and exhibits photoconductivity corresponding to the quantity of the incident light.

When the high velocity electron beam 5 is made to impinge on the low density photoconductive layer through the metal film 8, multiplied secondary electrons are generated at the part where the beam hits due to the TSEM effect and turn into a collector current at the collector 3.

As has been described hereinabove, the gain of the photoconductive TSEM lowers according to the electrical resistance of the low density photoconductive layer 9 because the influence of the positive charge left by the secondary electron emission and thus the output current of the secondary electron in this case becomes inversely proportional to the light intensity of the image projected onto the surface of the low density photoconductive layer. Accordingly, if this light image 11 is focused on the surface of the low density photoconductive layer 9, the conductivity of the target depends on the brightness of the image 11. The primary electron beam 5 is scanned by the television scanning system 4 as shown in FIG. 1 and a suitable positive potential is applied to the collector electrode 3 composed of NESA film. The multiplied transmission secondary electron current corresponding to the brightness of the image 11 flows through the load resistance R provided at the collector and an image signal can be taken out by way of a capacitor C.

As has become apparent from the foregoing descrip tion, the device according to this invention comprises a low density porous photoconductive layer exhibiting conductivity corresponding to the incident light image and means for scanning said photoconductive layer by a high energy electron beam, and in this device the effect of transmission secondary electron multiplication is controlled by the incident light. According to this device, not only a quite high resolution can be obtained but also a large output can be derived easily by use of the transmission secondary multiplication effect in the low density layer of the photoconductive material.

Further, a target having a porous low density spongelike structure can be fabricated easily, e.g., by evaporation in low pressure (below several mm. Hg) inert gas.

Now, another form of photoconductive TSEM target for use in a pick-up tube of this invention will be described.

For example, the target 1 consists of a fine metal mesh 81 and a porous sponge-like deposited layer 9 formed of fine particles of a photoconductive material as shown in FIGS. 3a and 312. Since the photoconductive TSEM target previously shown in FIG. 2 is formed in a way that a metal film like aluminum film is used as a support member and that a porous low density layer is formed thereupon, a high velocity electron beam of at least more than several kv. is required to penetrate through the metal film which works as a support member.

In the embodiment shown in FIGS. 3a and 3b, a metal layer having holes like a fine metal mesh is used as the support layer in place of the metal film in order to make it unnecessary to give the primary electron an energy high enough to transmit through the conventional metal film or the support layer. When the electron beam 5 is made to impinge on the low density photoconductive layer 9 through the fine metal mesh 81, multiplied secondary electrons are generated at the part where the electron beam is injected on account of the TSEM effect and they turn to a collector current of the opposing collector.

Thus, the image pick-up tube of this invention is similar to conventional ones in that a photoconductive film is used as a sensitive layer, but diiferent in the point that a scanning electron beam of several tens to several hundred volts is used as a primary electron beam for generating transmission secondary electrons at the photoconductive TSEM target. The structure of the target includes two typical examples shown in FIGS. 3a and 3b. Though the targets shown in FIGS. 3a and 3b are similar to each other, the grating holes of the support mesh 81 of FIG. 3a are not packed with a low density porous photoconductive material Whereas these of the mesh of FIG. 3b are packed with photoconductive material. The setting of the porous low density sponge-like photoconductive material to the grating of the metal mesh is done by setting collodion film, which can be burned out in oxygen atmosphere, like nitrocellulose, etc. to one side of the mesh, depositing a photoconductive material from either side in inert gas atmosphere like argon of several mm. Hg and burning collodion film in oxygen. When the photoconductive material is deposited on the mesh from the side not comprising the collodion film, the structure of the mesh becomes as shown in FIG. 3a and when the evaporation is done from the side comprising the film, the structure as shown in FIG. 3b is provided.

FIG. 4 is a sectional diagram showing the state wherein electrons are multiplied at the photoconductive TSEM layer and turned into emitted secondary electrons 12 when the primary electron beam 5 impinges on the mesh target from the left.

In a photoconductive transmission secondary electron multiplication target of such a structure, a resolution of several to several tens of lines per mm. is possible when a suitably fine mesh is used and the target can be used not only for simple transmission secondary electron multiplication (TSEM), but also for a high resolution image pick-up tube utilizing a photoconductive secondary electron target.

When an output sensitivity of such a pick-up tube using a TSEM target is to be increased, the multiplication effect of the transmission electrons at the porous low density photoconductive layer becomes an important problem. When photoelectric sensitivity is not taken into account, KCl, other alkali halides or other metal oxide insulators are usually used as the porous layer material for the ordinary transmission secondary electron multiplication both because the secondary electron emission rate of these materials, 5, is approximately equal to -6 and remarkably high and because they have a good insulating property. On the other hand, the TSEM target is formed of various photoconductive materials like CdS and it has an intrinsic resistance or a dark resistance as well as its photoconductive property in contrast to said insulator and moreover the secondary electron emission rate is not so high. Therefore, the gain of TSEM is not sufficient if the TSEM layer is formed only of photoconductive materials. In such a case, it is necessary to increase the secondary electron multiplication effect by some means and at the same time to provide a porous TSEM layer having a high photoconductive sensitivity.

For such a purpose, the sponge-like porous layer is formed by using alkali halides like KCl or other metal oxides in combination with photoconductive materials to enhance the dark current as well as photoconductivity and the transmission secondary electron multiplication factor. These two kinds of materials compose a mixed layer intertwined with the porous layer as shown in FIG. 5. FIG. 5 shows a cross-section of such a TSEM photoconductive target, wherein 8 indicates a support film supporting the porous layer which consists of metal, semiconductor, one kind of insulator or the combination thereof, the black particles 92 indicate particles of photoconductive materials for forming the porous layer, 93 designates particles of alkali halides like KCl or other metal oxides or the mixture thereof, 5 designates a primary electron beam having a high acceleration voltage sufiicient to transmit through the support film layer and 12 shows the transmission secondary electrons. Such a porous low density sponge-like layer formed of the mixture of photoconductive materials and insulators like alkali halides can be fabricated by simultaneously depositing said two kinds of materials in vacuum of less than several mm. Hg or inert gas like argon. When the rate of TSEM is to be enhanced, the rate of combination of the photoconductive material and the insulator, the deposition of speed of the two, the method of composing a porous layer, the method of heat treatment after deposition etc. must be suitably selected.

. FIG. 6 shows a cross-section of a porous TSEM target according to another embodiment of this invention, which is formed into sandwiched laminated layers consisting of photoconductive particles 92 and insulating particles 93 by depositing photoconductive materials and insulators alternatively in low pressure inert gases.

In such a target, the porous low density layer is formed by mixing photoconductive materials with materials having a high secondary electron emission rate and using layer distribution. In such a device, the transmission sec ondary electron multiplication gain increases remarkably and an image pick-up tube of this invention which has a high photoelectric sensitivity can be obtained.

Further, in such a case where a photoconductive material is mixed with a material having a high secondary electron emission rate and a layer distribution thereof is employed, a support member consisting of a metal mesh as described hereinabove can be used as the support layer instead of the metal film.

What is claimed is:

1. An image pick-up tube having at one end a window for receiving light images and at the other end an electron gun for providing a scanning beam of high speed electrons, a transparent collector on the inner face of said window, a target spaced from said collector, said target consisting of a metal support member provided with a porous photosensitive layer of material exhibiting transmission secondary electron multiplication when struck by haid high speed electron scanning beam from said electron gun, said collector facing the porous photosensitive layer to receive therefrom the transmitted m-ultiplied secondary electrons from said porous layer.

2. A pick-up tube according to claim 1, further comprising a target having a metal support member having holes like a fine metal mesh and a porous photoconductive layer exhibiting conductivity corresponding to the incident image.

3. A pick-up tube according to claim 2, wherein a part of said porous photoconductive layer is packed into the holes of said metal support member.

4. A pick-up tube according to claim 1, further comprising a photoconductive layer including a photoconductive material and at least a kind of material having a high secondary electron emission rate like alkali halide.

5. A pick-up tube according to claim 2, further comprising a photoconductive layer including a photoconductive material and at least one kind of material having a high electron emission rate like alkali halide.

References Cited UNITED STATES PATENTS 2,579,772 12/1951 Wilder 117-210 2,683,832 7/1954 Edwards et al. 313- 2,910,602 10/1959 Lubszynski et al. 313-65 3,128,406 4/1964 Goetze et al 313-65 3,148,297 9/1964 Schneeberger et al. 313-65 3,213,315 10/1965 Lempert 313-65 JAMES W. LAWRENCE, Primary Examiner V. LAFRANCHI, Assistant Examiner US. Cl. X.R.

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