DE1240549B - Method of operating an image pick-up tube - Google Patents

Description

GERMAN 007Ϊ ^ PATENT OFFICE

EDITORIAL

German KL: 21 al - 32/35

Number: 1240 549

File number: W 35752 VIII a / 21 al

J 240 549 filing date: December 3, 1963

Open date: May 18, 1967

The invention relates to a method for operating an image pickup tube in which the image information the incident radiation is detected on a storage electrode and then by means of an electron beam is read.

A well-known image pickup tube is the Image-Orthicon. This has a photo electrode that consists of consists of a thin sheet of glass. A photocathode is arranged on one side of this glass film, from which the photoelectrons released during exposure to the storage electrode by means of electron optics be directed. As a result, there is a charge distribution on the storage electrode, the corresponds to the image information. On the other side of the storage electrode is an electron gun arranged, the lower the scanning side of the storage electrode with an electron beam Scans the speed and leaves electrons at each elementary point of the storage electrode, the number of which corresponds to the charge distribution. The remaining electrons of the scanning beam return to a collecting electrode and provide the output of the image orthicon.

In this known pick-up tube, the photoelectrons striking the storage electrode are released a secondary emission, and the secondary electrons are discharged from a mesh electrode, the is located immediately in front of the storage electrode. This mesh electrode is at a positive potential, related to the cathode of the scanning beam generator. When the resistivity of the storage electrode is chosen high enough in the direction of the image plane to allow usable storage times, so the resistance perpendicular to the plane of the picture is also correspondingly high, and therefore it is possible that more electrons leave the receiving side of the storage electrode than through the storage electrode wander through from the scanning side to the receiving side. So can the recording side charge to an equilibrium potential that is slightly more positive than that of the mesh electrode until as many electrons leave the screen as land on it. Under these circumstances then can no further image information can be recorded. The basic problem with the Image-orthicon consists in the fact that the charge balance of the storage electrode is perpendicular within one scanning period must be carried out to the image plane, while at the same time this electrode has a sufficiently specific Must have resistance in the transverse direction in order to limit a discharge of charge in this direction.

Procedure for operating an image pickup tube Applicant:

Westinghouse Electric Corporation,
East Pittsburgh, Pa. (V. St. A.)

Representative:

Dipl.-Ing. G. Weinhausen, patent attorney,
Munich 22, Widenmayerstr. 46

Named as inventor:

Gerhard W. Goetze, Monroeville, Pa .;

Alvin H. Boerio, Turtle Creek, Pa. (V. St. A.)

Claimed priority:
V. St. v. America December 3, 1962
(241641)

Another known image pickup tube uses a storage electrode which is an induced one Has conductivity when bombarded by electrons. This tube also has a high sensitivity low exposure, similar to the Image Orthicon. The storage electrode here consists of a thin one conductive layer, e.g. B. made of aluminum, which is arranged at some distance from a photocathode. This layer, which is permeable to electrons, has a layer of high specific resistance, z. B. a semiconductor or insulator, coated on the side facing away from the photocathode.

This high resistivity material has the property that it becomes conductive when it is is bombarded with electrons of high energy. A typical such material is e.g. B. arsenic trisulfide. To keep the free surface of the insulating coating at a fixed equilibrium potential, that deviates from the voltage on the conductive base layer, a scanning electron beam may be lower Speed can be used. The electrons emanating from the photocathode become strong accelerates, penetrate the screen and induce conductivity at the point of impact, i.e. H. between the free surface scanned by the scanning beam and the base layer. This will make that The potential on the surface is adjusted to the potential of the base layer, and it arises her a charge distribution that corresponds to the brightness distribution on the photocathode. The initial

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signal can be sent from the storage electrode to a collecting electrode in the same way as with the Image-Orthicon returning slow electrons are derived. But it can also be a signal from the conductive layer of the storage electrode can be derived by scanning this electrode occurring charging current is used for this purpose by means of a simple capacitive coupling. this corresponds to the reading process of the well-known vidicon tube.

In the case of the storage electrode with induced conductivity, the primary electrons increase the conductivity of the storage layer because they generate secondary pairs of electrons and holes. This increase in conductivity can be in the amount of 1,000 or more. To make this possible, the material used should have a narrow energy band gap, high carrier mobility and a short transition time. In addition, the storage layer should be as thin as possible. However, in order to avoid too large a dark current, the number of carriers present at the non-excited locations must be small. The specific resistance of the material must therefore have at least a value of 10 ¹² ohm · cm. Such values are only found for substances with band gaps of 2.0 electron volts or more. The use of very thin layers is limited by their low output voltage, as this is inversely proportional to the layer thickness. In the case of very thin storage layers, there are still very high capacitance values, which are responsible for a strong aftereffect. This is noticeable in the fact that recording and scanning take too long.

Furthermore, because of the requirement that the material must have a band gap of 2.0 electron volts or more, only substances can be used whose charge carriers have the low mobility of about 10 ^-4 cm ² / volt * sec. exhibit. This also results in a strong aftereffect. Several seconds may be required to scan and erase a charge image on such a storage electrode. This makes electrodes with conductivity induced by electron bombardment unsuitable for numerous applications.

There is also known an image pickup tube having a storage electrode which consists of a conductive layer and a porous insulating layer in contact therewith and having a secondary electron yield greater than 1 . The density of the porous insulating layer is very low compared to the density of the insulating material in compact form. When charged particles with a suitable energy strike the conductive layer, they penetrate the same and cause secondary emission in the porous insulating layer. There is an electric field on the insulating layer, through which the emitted electrons are guided to the free surface of the insulating layer and charge it. The free surface is scanned with an electron beam that tries to bring it back to an equilibrium potential. It has been shown that the yield of such a storage electrode with a porous insulating layer is considerably better than that of one with a compact insulating layer.

However, the after-effects described above also affect the image recording

tubes with porous storage electrode noticeable. So far, you have used such storage electrodes severely disabled.

The object of the invention is to create a method for operating an image pickup tube in this way with porous storage electrode that the described after-effects avoided will.

The inventive method for operating an image pickup tube with a storage electrode, which consists of a conductive layer and a porous insulating layer in contact therewith with a secondary electron yield greater than 1 , charged particles impinging on the electrode with such energy that they penetrate the conductive layer and Cause secondary emission in the insulating layer, the free surface of the insulating layer is scanned with an electron beam, which tries to bring it to an equilibrium potential, and an electric field is applied to the insulating layer, which has a positive potential on the free surface of the insulating layer opposite the conductive layer generated, is characterized in that the field strength on the porous insulating layer is chosen so that the secondary electrons emitted into the pores of the insulating layer are dissipated, but at the same time no conductivity in the solid parts of the insulating layer ever induced by electron bombardment.

If the operating conditions according to the invention are observed, the local charge on the free surface of the insulating layer solely by the secondary electrons emitted into the pores of the same evoked. If a higher field strength is selected, the sensitivity still increases, but as a result of the Maltese effect, which is associated with the conductivity induced in the solid parts of the insulating layer is linked, then occurs a very strong aftereffect, which is avoided according to the invention.

The invention is explained below with reference to the drawings. In here is

F i g. 1 shows a schematic representation of an image pick-up tube according to the invention,

F i g. 2 shows a sectional view of the storage electrode according to FIG. 1 on an enlarged scale,

F i g. 3 is a sectional view of a modified storage electrode;

4 shows a graphic representation of the image intensification factor as a function of the electric field strength on the insulating layer,

F i g. 5 is a graph of the image enhancement factor as a function of the potential of FIG Exit surface of the storage electrode for a constant brightness signal and

F i g. 6 shows a graph of the integrated signal as a function of the potential of the exit surface of the storage electrode.

F i g. 1 shows an image pickup tube according to the invention with a piston 10 and a translucent end plate 12. On the inner surface of the end plate there is a photocathode 14 which, for visible light, can be used. B. consists of cesium antimonide. At the other end of the piston 10 there is an electron beam generator 20 for forming a scanning electron beam which is directed onto a storage electrode 30 (shown enlarged in FIG. 2). The electrode 30 is located between the electron gun 20 and the photocathode 14.

Between the electrode 30 and the photocathode 14 electrodes 16 and 18 are arranged, the one Form electron lens in order to accelerate the photo electrons emanating from the photocathode 14 and focus on electrode 30. Located between electrode 30 and electron gun 20 a mesh electrode 40 made of conductive material, e.g. B. Nickel, which is at a distance of about 3 mm from the surface of the electrode 30. The electrode 30 consists of a carrier ring 32 made of metal, e.g. B. an alloy of nickel, iron and cobalt, over the opening is a conductive Layer 34, e.g. B. aluminum, extends. On the side facing the electron gun 20 According to the invention, conductive layer 34 is a porous layer 36 made of an insulating or semiconducting one Material applied which has the property of secondary electrons when bombarded with electrons generate, wherein the electrons can migrate through the cavities of the porous layer 36.

The porous layer 36 can for example consist of an alkali or alkaline earth compound, such as Potassium Chloride, Magnesium Chloride, or Magnesium Oxide. The mesh electrode 40 serves as a collector for the secondary electrons emanating from the free surface of the porous layer 36. The mesh electrode 40 also helps maintain a uniform electric field between the Mesh electrode 40 and the storage electrode 30. Furthermore, a conductive coating 44 is on the inner wall of the piston 10 is provided in the space between the electron gun 20 and the electrode 30. The coating 44 is at the potential of the mesh electrode 40 to create a suitable electrostatic field to create.

The electron gun 20 can in a known manner from a cathode 22, a control grid 24 and an acceleration anode 26 exist. The electrodes 22, 24 and 26 generate together with coating 44, a small diameter electron beam directed onto electrode 30. A deflection device 50, shown in the form of a deflection coil, causes the electron beam the surface of the electrode 30 scans in a known manner. To concentrate the from the electron beam emanating from the electron beam 20 onto the electrode 30 as well as from the Electrons emanating from photocathode 14 onto electrode 30 are also used by a focusing coil 52.

The storage electrode 30 is produced as follows, for example: the aluminum layer 34 is formed by placing aluminum in a vacuum on a membrane made of organic material, such as Cellulose nitrate, is evaporated. The thickness of the aluminum layer 34 should be one electrode diameter of about 25 mm will be about 1000 angstroms. The cellulose nitrate is then through Heating removed leaving the aluminum layer 34 on the ring 32. This manufacturing method is known and is described, for example, in U.S. Patent 2,905,844. The conductive foil 34 forms the carrier. Another carrier layer is described in US Pat. No. 2,898,499. Even this type can be used. The only requirement in each case is that the carrier layer for impinging Electrons is permeable and has sufficient conductivity to the electrons flowing away to be able to replace.

The conductive layer 34 is then placed with the carrier ring 32 in a recipient, in which there is Argon or other inert gas is at a gas pressure of about 1 torr. Located in the recipient a boat (e.g. made of tantalum) with a resistance heater. In the Shuttle a certain amount of the coating material, e.g. B. given 16 mg of potassium chloride. That The boat is then placed about 75 mm below the aluminum layer 34 and heated. That Coating material is heated just to the melting point and then held at that temperature. This temperature is considerably lower than the melting point under atmospheric pressure. The vapor pressure of the material at the melting point is sufficient to allow evaporation with sufficient speed to cause. The material is completely evaporated and it is found that the density of the the aluminum layer of the vapor-deposited material is only about 1 to 5% of the density in the compact state amounts to.

The resulting layer 36 has a thickness of about 20 microns.

The values of the potentials on the individual electrodes are shown in FIG. 1 specified. The photocathode 14 is at a potential of about -8000 volts with respect to the conductive layer 34 around to accelerate the electrons emanating from the photocathode 14. The distribution corresponds the respective brightness of the image of the object 51 designed on the photocathode 14. The conductive layer 34 can have a voltage of approximately +5 volts against the radiation cathode 22. The latter is grounded so that the free surface of the porous layer 36 by the scanning beam on a Equilibrium potential is held, which corresponds to the earth potential. The electrodes 44 and 40 are located at a voltage of about +250 volts to earth. The retarding field between the storage electrode 30 and the electrode 40 decelerates the electron beam, so that this the electrode 30 with reaches a potential which is below the first zero crossing of the secondary emission curve of the porous Layer 36 lies. As a result, electrons are left on the free surface in such a way that this surface seeks to assume an equilibrium potential which is essentially equal to the potential of the Cathode 22 is.

The electrons emanating from the photocathode 14 and distributed according to the brightness of the image are focused on the electrode 30. As a result of their high energy of about 8000 electron volts penetrate they penetrate the conductive layer 34 and penetrate the layer 36. The acceleration voltage should be like this be set that as possible all primary electrons from the photocathode 14 the storage electrode SO penetrate almost completely, but not leak on the other side.

It has been shown, for example, that in the case described above, a conductive layer 34 made of aluminum about 1000 angstroms thick and a porous insulator 36 about 20 microns the acceleration voltage should be around 8000 volts. The primary electrons lose here when passing through the conductive layer 34 about 25% of its initial energy, and the the remaining 75%, i.e. about 6000 electron volts, are destroyed in layer 36. The primary electrons

generate a certain number of secondary electrons of low energy in the layer 36, which is several orders of magnitude higher than the number of incident primary electrons. For example, the number of secondary electrons generated is about 200 times that of the primary electrons when the ionization energy is about 30 electron volts. Is the storage electrode 30 has been polarized prior to incidence of the photoelectrons, by applying a positive voltage of about 5 volts to the to conductive layer 34 and the free surface of the layer was brought 36 by electron scanning at ground potential, the potential is determined by the generated secondary electrons of Free surface changed locally because electron conduction across the layer 36 through the vacuum between the solid parts of the porous layer 36 to the positive conductive layer 34 and at the same time secondary emission occurs at the free surface of the layer 36. The emerging secondary electrons are picked up by the mesh electrode 40. This local change in potential can be used in a known manner to generate an output signal. In Fig. 1, the output signal is tapped capacitively, for example, at the lead to the conductive layer 34.

For a more detailed explanation of the mode of operation of the storage electrode 30, the course of the free surface potential F _{s of} the layer 36 is considered as a function of time for a given current density /. If at time T = 0 the surface potential V _{s is} at the value of the cathode potential, ie earth, then when the electrode 30 is bombarded with photoelectrons, secondary emission occurs on the surface of the layer 36 and electron conduction through this layer occurs. The surface potential V _s consequently tends towards the potential F _{r of} the conductive layer 34 because of the electric field applied to the layer 36. While the effect of the electron conduction predominates in the layer 36 and the secondary emission on the surface facing away from the impact surface only contributes a small fraction of the total charge change, the situation is reversed when the surface potential V _{s reaches} the layer potential F _r . This is clear from FIG. 5 emerges. The electron conduction within the storage layer disappears when both surfaces of the porous layer 36 are at the same potential. The external secondary emission continues, however, whereby the potential V _{s is raised} above the potential F _{r of} the conductive layer, so that the polarity of the electric field in the layer 36 is reversed. If V _{s is a} few volts more positive than F ₇ -, electron conduction occurs again, but this time in the opposite direction and with a smaller increase. It is found that when using a layer 36 which exhibits field-enhanced secondary emission, the secondary emission gain is greater than the now negative gain due to electron conduction in the layer 36 until the potential of the surface is adjusted to a certain value V _e. The potential V _e is determined by the voltage at which the number of secondary electrons leaving the surface is equal to the number of free electrons picked up by the conductive layer 34. This results in the in FIG. 5 solid curve which intersects the axis at the value V _e. F i g. 6 shows that about the

Gain G integrated signal as a function of the surface potential V _s .

This shows that the following minimum conditions must be observed in order to be able to operate the storage electrode 30 in the manner described. The electrode 30 must have external secondary emission and at the same time free electron conduction within the porous layer 36. The capacity of the storage electrode 30 should be small enough to enable a discharge without delay. Because of the porous structure, the layer 36 is relatively thick and therefore has a low capacity. In spite of the fact that the surface density is low enough, acceptable acceleration voltages can still be used. Layer 36 must also have a very high electrical resistivity, and precautions must be taken to avoid "runaway" surface potential. This effect is illustrated below with reference to FIGS. 3 explained. The arrangement described meets these requirements. It has the high gain of 200 to 300 within the storage electrode at acceleration voltages of around 8 kilovolts for the primary electrons. Furthermore, the discharge of the storage electrode is possible with a very short delay, so that an image disappears within less than Vao seconds. The electrode also has an extremely long storage time, which is more than 60 minutes, since the specific resistance of the layer 36 is more than 10 ¹⁸ Ohm -Cm. An image can be stored on electrode 30 for several hours with the scanning beam turned off. The electrode 30 is free of after-effects, ie it shows no afterimage as a result of trapped electrons, as occurs in known storage electrodes with conductivity induced by electron bombardment. This is due to the fact that the electric field at the layer 36 is very small, so that the conductive layer 34 can dissipate the electrons occurring in the cavities of the layer 36. Conductivity triggered by electron bombardment as a result of a carrier flow through the solid parts of the layer 36 does not contribute to the conduction produced according to the invention, since the low field strength is unable to convey the carrier through the solid substance. The influence of the field strength on the gain is shown in FIG. 4 shown. The storage electrode 30 also has the good resolution of more than 20 line pairs per millimeter. The known deficiencies of the storage electrodes with conductivity triggered by electron bombardment as a result of the required high field strength do not occur here even with a long operating time. If potassium chloride is used as the basic substance of layer 36, operating temperatures of up to 325 ° C are possible without the risk of destroying the electrode.

F i g. 3 shows a modified electrode arrangement in which an auxiliary grid 41 is located between the Storage electrode 30 and the known mesh electrode 40 is located. In this embodiment lies the electrode 40 at a voltage of about +450 volts opposite the cathode 22 and has a Distance of about 3 mm from the storage electrode 30. The auxiliary grid 41 is at a potential of about +50 volts to earth and is about 1.5 mm from electrode 30.

It has the following task: As mentioned above, the

Claims (4)

free surface of the storage electrode 30 due to secondary emission and electron conduction positive. When the free surface reaches the potential Vt of the conductive layer 34, the charging of the surface continues in the positive direction because the S external secondary emission outweighs the free electron conduction current. With some substances it is possible that the free surface becomes so positively charged with respect to the cathode of the scanning beam that the energy of the electrons of the scanning beam rises above the zero crossing of the secondary emission potential of the material of the layer 36. A well-known feature of secondary emission is that when the primary energy is low, the number of secondary electrons ejected from a surface is less than the number of primary electrons, i.e. That is, the secondary emission factor is less than 1, and the surface tries to be negatively charged. From a certain primary energy onwards, more secondary electrons are generated than primary electrons strike. In this case, the surface is positively charged. The potential at which the number of secondary electrons is equal to that of primary electrons is referred to as the potential of the first zero crossing. If the free surface of the porous layer 36 assumes a higher potential than this potential of the first zero crossing, then the surface is not only charged more positively by electron bombardment from the photocathode, but the scanning beam also tries to further positively charge the surface. As a result, the voltage on the free surface of the layer 36 continues to rise until the field strength that occurs breaks through the insulating or semiconductor layer 36 and thus destroys it. By using the auxiliary grid 41 between the mesh electrode 40 and the storage electrode 30 and applying a positive potential below the potential of the first zero crossing, according to the invention, a "rise" of the surface potential and thus a destruction of the porous layer 36 can be avoided. Since the potential of the surface cannot exceed that of the auxiliary grid 41, a breakdown is impossible, even with a high intensity of the input signal or a long integration time. The operating mode of the storage electrode 30 according to the invention is not adversely affected if the operating voltages and spacings between the electrodes are selected in such a way that the occurrence of moiré effects is avoided in a known manner. The relationships are illustrated again on the basis of FIG. 4 explained. There, the gain of the storage electrode 30 is plotted on the porous layer 36 as a function of the field strength. If there is no field strength on the layer, the gain is due solely to the secondary emission on the free area of the layer 36. If the conductive layer 34 is positively biased against the free surface, the gain due to the conduction of free electrons through the cavities of the layer 36 increases up to approximately the value 200. Beyond this point the profit increase is considerably less. This additional gain comes from electrons in the conduction band as a result of the well-known conductivity caused by electron bombardment that takes place inside the solid. In this area, the field strength at the porous layer 36 is so high that the electrons can penetrate into the solid parts and cause solid-state conduction there. The conduction current in the range of low field strength results in a response time of about Vao seconds or less, while at high field strengths due to the induced conductivity the response time is V2 seconds and more. Instead of the photocathode 14, a controlled scanning beam system can be provided, so that a scanning transducer results. The storage electrode operating according to the invention can also be used in a storage tube as described, for example, in US Pat. No. 3,002124. In this case, the porous storage layer is operated in such a way that the charge due to electron conduction in the porous layer is increased by reflected secondary electrons. The electron guns for the write beam and for the scanning beam are both located on the side of the free surface of the storage electrode, which is provided with one or more holes. Instead of electrons, light or other radiations can also be directed directly onto a porous storage layer operating according to the invention in order to generate electrons by photoemission. The photoemission is either already a property of the material used or can be caused by adding suitable substances such as cesium antl · monide. Patent claims: 1. A method for operating an image pickup tube with a storage electrode which consists of a conductive layer and a porous insulating layer in contact therewith with a secondary electron yield greater than 1, charged particles impinging on the conductive layer with such energy that they penetrate the same and secondary emission cause in the insulating layer, while the free surface of the insulating layer is scanned with an electron beam, which tries to bring it to an equilibrium potential, and an electric field is applied to the porous layer, which has a positive potential with respect to the conductive layer on the free surface of the insulating layer generated, characterized in that the field strength on the porous insulating layer (36) is selected so that the secondary electrons emitted into the pores of the insulating layer are dissipated, but in the solid parts of the porous insulating layer no conductivity by electron bombardment is induced. 2. The method according to claim 1, characterized in that a part of the secondary electrons generated by the charged particles in the porous layer (36) emerges at the free surface of the layer (36) and causes a corresponding positive charge, while another portion of the secondary electrons is captured by the conductive layer (34) and thus amplifies the positive charge distribution generated. 3. The method according to claim 1 or 2, characterized in that the field strength prevailing on the porous insulating layer (36) is less than 10 kV / cm, preferably less than 2.5 kV / cm. 4. The method according to any one of the preceding claims, characterized in that a 709 580/183