US2735935A - Image-translating device - Google Patents

Description

Feb. 21, 1956 e. s. P. FREEMAN 2,735,935

IMAGE-TRANSLATING DEVICES Filed Feb. 3, 1950 GEORGE STANLEY PEROIVAL FREEMAN INVENTOR.

HTS ATTORNEY IlVIAGE-TRANSLATING DEVICE George Stanley Percival Freeman, Chiswick, London, England, assignor to Cinema-Television Limited, Lon-v don, England, a British company Application February 3, 1950, Serial No. 142,111

Claims priority, application Great Britain February 7, 1949 9 Claims. (Cl. 250-27) This invention relates to image-translating devices and more particularly to television pickup tubes or the like.

Conventional pickup tubes of the types known as the orthicon and the transparent signal plate iconoscope employ a target electrode comprising a light-transparent insulating sheet having a photosensitive mosaic disposed on one surface and a conductive film deposited on the other surface. The conductive film serves as a capacitive electrode and is generally formed of silver, aluminum, or the like which is sufliciently thin to be substantially transparent to light. An optical image to be analysed by the device is projected through the metal film and the insulating sheet onto the photosensitive mosaic elements; this process involves a loss of sensitivity due to absorption and reflection of the light from the image at the metal film. In order to be robust enough to withstand the conditions imposed by glass Working on the envelope during sealing of the tube and subsequent evacuation, the metal film must be of such thickness that about of the incident light is absorbed or reflected; in many applications, the absorption figure is as high as It is an important object of the present invention to provide an improved image-translating device which avoids one or more of the disadvantages of the prior art.

It is a further object of the invention to provide an improved television pickup tube, similar to the orthicon and the transparent signal plate. iconoscope, which does not require a metal film on the target electrode through which light from the image to be transmitted must pass.

Yet another object of the invention is to provide an improved method of deriving image signals from a target electrode comprising a thin insulating sheet and a photosensitive mosaic disposed on one surface of the insulating sheet and having a charge distribution representing an optical image.

In accordance with the present invention, a new and improvedimage-translating device comprises a target electrode including a thin sheet of insulating material on one surface of which a photosensitive mosaic is disposed. Means are provided for projecting an optical image on the mosaic to produce a charge pattern representing the optical image. An electron source is provided for projecting a flood of electrons on the surface of the insulating sheet opposite the mosaic at a velocity above the cross-over velocity of the insulating material to maintain. a uniform. positive charge over that surface. The device further comprises an electrode system for projecting a scanning electron beam toward the mosaic and a load impedance coupledvto. the electrode system for developing an output signal determined by the charge distribution on the mosaic.

The invention also provides a new and improved method for deriving image signals from a target electrode comprising a thin insulating sheet and a photosensitive mosaic disposed on one surface of the insulating sheet nited States Patent Q 2,735,935 Patented Feb. 21, 1956 and having a charge distribution representing an optical image. The improved method comprises the steps of flooding the surface of the target electrode opposite the mosaic with electrons of a velocity above the cross-over velocity of the insulating material to maintain a uniform positive charge over that surface, scanning the mosaic with an electron beam to provide a modulated return beam representing the charge distribution on the mosaic, and collecting the modulated return beam to derive the image signals.

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 drawing, in which the single figure is a schematic representation of an image-converting device constructed in accordance with the present invention.

In the drawing, a television pickup tube 10 constructed in accordance with the invention comprises a target electrode 11 including a thin transparent sheet 12 of insulating material such as glass, mica, or the like, and a photosensitive mosaic 13 disposed on one surface of the insulating sheet 12. An optical system, schematically represented for convenience in the drawing as a single lens 14, is provided for projecting the optical image onto the photosensitive mosaic 13 through the transparent insulating sheet 12.

An electron source for projecting a flood of electrons on the surface of insulating sheet 12 opposite mosaic 13 comprises an electron gun including a cathode 15, a grid 16, a first anode 17, and second anode 18 which may be in the form of a conductive coating on the inner wall of envelope 10.

On the opposite side of the target electrode 11, there is provided an electrode system for projecting a scanning electron beam towards the mosaic, that system comprising an electron gun including a cathode 19, a grid 20, a first anode 21, and a second anode 22 which may also be formed as a conductive coating on the inner wall of envelope 10. Focus coils 23 and scanning coils 24 are provided for controlling the scanning electron beam in a manner well known in the art and for this purpose are connected to a conventional scanning system (ot shown).

An output electrode 25, which may be in the form of a conductive ring, is disposed between the scanning electron gun and mosaic 13 for collecting the modulated re turn beam from target electrode 11 to derive an electrical signal representing the charge distribution on mosaic 13.

The several electrodes may be supported within envelope 10 in any desired manner, as well known in the art. The photosensitive mosaic 13 may be provided on one surface of insulating sheet 12 in any well known manner, as by evaporating a thin layer of silver onto the insulating sheet, baking the silver-coated insulating sheet to form silver globules on the surface, oxidizing the silver globules, caesiating to form a complex photo surface, and evaporating a small amount of silver onto the activated surface to increase the sensitivity. The envelope may then be evacuated and gettered in a conventional manner; external connections are provided for the various conductive electrodes.

Cathode 15' of the flooding electron gun is connected to ground, and grid 16 is connected to a variable tap 26 on a potentiometer 27 connected in parallel with a suitable source of unidirectional bias voltage such as a bat tery 28, the positive terminal of which is connected to ground. First anode 17 is connected to a variable tap 29 on a potentiometer which is connected in parallel with a suitable source of unidirectional operating potential such as a battery 31, the negative terminal of which is connected to ground. If desired, variable taps 26 and 29 may be ganged for unicontrol operation. Second anode 18 is directly connected to the positive terminal of battery 31.

Cathode 19 of the scanning electron gun is directly connected to ground, and grid 20 is connected to ground through a negative bias voltage source, here shown as a battery 32. In order to provide suitable operating potentials for first anode 21, second anode 22, and output electrode 25, resistors 33, 34, and are seriesconnected across a suitable source of unidirectional operating potential, here shown as a battery 36 the negative terminal of which is grounded and the positive terminal of which is bypassed to ground by means of a condenser 37; it may be desirable to bypass each of the positive electrodes to ground individually (not shown). First anode 21 is connected to a point 38 intermediate resistors 34 and 35, and second anode 22 is connected to a point 39 intermediate resistors 33 and 34. Output electrode 25 is connected to the positive terminal of battery 36 through a load resistor 40.

In operation, the surface of target electrode 11 opposite mosaic 13 is irradiated with flooding electrons of a velocity above the cross-over velocity of the insulatng material 12, so that the secondary emission ratio is greater than unity. Consequently, the irradiated surface of the target is stabilized at a. potential within a few volts of that of second anode 18 of the flooding system.

When any element of the photosensitive mosaic 13 loses electrons under the action of light, a positive charge appears at that element, and an equal induced charge instantaneously appears on the irradiated surface opposite that element. If the intensity of the flooding electron stream is great enough, this charge is instantly removed from the irradiated surface and that surface is restored to equilibrium potential. Thus, in effect, the irradiaton by flooding electrons causes the surface of the target opposite the mosaic to appear to be coated with an electrical conductor, although no metal film or other light absorbing or reflecting material is used. The virtual conductor resulting from the action of the flooding beam provides the increased capacity desirable for improved storage properties of the device, and the light efiiciency is from 20 to 40% greater than that of presently known orthicon or transparent signal. plate iconoscope type image-converting devices.

As a further advantage of the invention, it is possible to achieve greater utilization of the photo-electrons, and to control more readily the generation of spurious or tilt signals than in tubes of the conventional iconoscope type. For this purpose, conventional tubes are provided with a dielectric sheet 12 having aslght electrical leakage in order to maintain the final average potential of the mosaic surface at some value intermediate that of the final accelerating anode 22 of the scanning system and that of the target electrode surface opposite the mosaic. In conventional structures, as a practical matter, it is difficult to achieve the optimum leakage resistance, and there is often an undesirable loss of signal and picture resolution due to excessive leakage to the conductive film normally used as the capacitive electrode. This undesirable condition may be somewhat abated by using a conductive film of very high resistance, but a practical disadvantage inherent with such an arrangement resides in the difficulty of providing a high-resistance fihn of suflicient strength to withstand the glass-working and evacuating operations.

In accordance with the present invention, the apparent resistance of the target electrode surface opposite the photo-sensitive mosaic may be controlled by adjusting the intensity of the flooding electron stream. For this purpose, potentiometers '27 and 30 are provided for controlling the operating potentials applied to grid 16 and first anode 17 respectively of the flooding electron source. The intensity of the flooding electron stream may be controlled by varying the potential of grid 16, for example, and a control of the potential of first anode 17 is desirable in order to maintain the proper focussing conditions for the flooding stream. Variable taps 26 and 29 of potentiometer 27 and 39 may be ganged for unicontrol operation to provide optimum beam focus for all conditions of beam intensity. Thus, the invention provides a convenient means for obtaining and critically adjusting the leakage resstance of the target electrode to suit the particular application of the analysing tube.

While it has been convenient to show an electron gun as the source of flooding electrons, the irradiating electron stream may also be provided by means of a photosensitive cathode, by field emission, or by any other suitable means known to the art.

In illustrating the invention in the accompanying drawing a device utilizing a high-velocity scannng beam has been shown and described. Thus, the scanning system comprises cathode 19, grid 20, first and second anodes 21 and 22, and output electrode 25 constituting an electrode system for projecting a high-velocity scanning electron beam toward photo-sensitive mosaic 13. This scanning beam is focussed by means of a coil 23 disposed about the neck of the scanning gun, and electromagnetic scanning coils schematically indicated at 24 are provided to impart the desired scanning motion to the electron beam. Scanning electrons impinging upon photosensitive mosaic 13 liberate secondary electrons in proportion to the local charge at the point of impingement, and these secondary electrons constitute a modulated return beam which is collected by output electrode 25 to develop image signals across output resistor 40.

The invention is also applicable to image-translating devices of the orthicon or low-velocity scanning type (not shown) in which a low-velocity scanning electron beam is projected toward the photosensitive mosaic. With such a device, substantially no secondary electrons are liberated byimpingement of. the scanning beam, but the scanning beam itself is reversed and modulated in accordance with the charge distribution on the mosaic. With such a device, the modulated return beam may be directed to an electron multiplier arrangement having a final output electrode between .the :electron gun and the target electrode to provide thedesired output signal.

Thus, the present invention provides a new and improved image-translating device which affords increased light sensitivitycver that obtainable with a conventional orthicon or transparent signal plate iconiscope. Furthermore, the invention provides a convenient and practical arrangement for controlling the apparent leakage resistance of the insulating portion of the target electrode to afford bettercontrol of the generation of spurious or tilt signals and to provide greater utilization of the photo-electrons emitted by the photosensitive mosaic than are characteristic ofprior-art devices.

While particular embodiments of thepresent invention have been shown. and described, it is apparent that various changes and modifications may be made, and it is therefore contemplated in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

I claim:

1. An image-translating device comprising: a target electrode consisting essential of a thin sheetof insulating material and a photosensitivemosaic disposed on one surface of said insulatingsheet; means for projecting an optical image on said mosaic; an electron source facing the surface of said sheet opposite said mosaic; an electrode system for projecting a scanning electron beam towards said mosaic; and a load impedance coupled to said electrode system for developing an outputsignal dedetermined by thecharge distribution on said mosaic.

2. An image-translating device comprising: a target electrode consisting essentially of a thin sheet of transparent insulating material and a photosensitive mosaic disposed on one surface of said insulating sheet; means facing the surface of said sheet opposite said mosaic for projecting an optical image through said insulating sheet onto said mosaic; an electron source facing said opposite surface; an electrode system for projecting a scanning electron beam towards said mosaic; and a load impedance coupled to said electrode system for developing an output signal determined by the charge distribution on said mosaic.

3. The method of deriving image signals from a target electrode consisting essentially of a thin insulating sheet and a photosensitive mosaic disposed on one surface of said sheet and having a charge distribution representing an optical image, which method comprises: flooding the surface of said target electrode opposite said mosaic with electrons of a velocity above the first cross-over velocity of said insulating material to maintain a uniform positive charge over said opposite surface; scanning said mosaic with an electron beam to produce a modulated return beam representing the charge distribution of said mosaic; and utilizing said modulated return beam to derive said image signals.

4. An image-translating device comprising: a target electrode consisting essentially of a thin sheet of insulating material and a photosensitive mosaic disposed on one surface of said insulating sheet; means for projecting an optical image on said mosaic; an electron source facing the surface of said sheet opposite said mosaic for projecting a flood of electrons on said opposite surface; an electron gun for projecting a scanning beam toward said mosaic; an output electrode disposed between said electron gun and said mosaic for collecting the modulated return beam from said mosaic; and a load impedance coupled to said output electrode to develop an output signal representing said optical image.

5. An image-translating device comprising: a target electrode consisting essentially of a thin sheet of insulating material and a photosensitive mosaic disposed on one surface of said insulating sheet; means for projecting an optical image on said mosaic; an electron source facing the surface of said sheet opposite said mosaic for projecting a flood of electrons on said opposite surface; means for varying the intensity of said electron flood; an electrode system for projecting a scanning electron beam towards said mosaic; and a load impedance coupled to said electrode system for developing an output signal determined by the charge distribution on said mosaic.

6. An image-translating device comprising: a target electrode consisting essentially of a thin sheet of insulating material and a photosensitive mosaic disposed on one surface of said insulating sheet; means for projecting an optical image on said mosaic; an electron source facing the surface of said sheet opposite said mosaic for projecting a flood of electrons on said opposite surface; an electron gun for projecting a scanning electron beam towards said mosaic; and means including an output electrode spaced from said mosaic on the same side thereof as said electron gun for collecting the modulated return beam from said mosaic.

7. The method of deriving image signals from a target electrode consisting essentially of a thin insulating sheet and a photosensitive mosaic disposed on one surface of said sheet and having a charge distribution representing an optical image, which method comprises: flooding the surface of said target electrode opposite said mosaic with electrons of a velocity above the first crossover velocity of said insulating material to maintain a uniform positive charge over said opposite surface; controlling the apparent resistance of said surface by altering the intensity of said flooding electrons; scanning said mosaic with an electron beam to produce a modulated return beam representing said charge distribution; and utilizing said modulated return beam to derive said image signals.

8. The method of deriving image signals from a target electrode consisting essentially of a thin insulating sheet and a photosensitive mosaic disposed on one surface of said sheet and having a charge distribution representing an optical image, which method comprises: flooding the surface of said target electrode opposite said mosaic with electrons of a velocity above the first crossover velocity of said insulating material to maintain a uniform positive charge over said opposite surface; scanning said mosaic with a high-velocity electron beam to produce a modulated return beam comprising secondary electrons emitted by said target electrode in response to impingement of said scanning beam and representing said charge distribution; and collecting said modulated return beam to derive said image signals.

9. The method of deriving image signals from a target electrode consisting essentially of a thin insulating sheet and a photosensitive mosaic disposed on one surface of said sheet and having a charge distribution representing an optical image, which method comprises: flooding the surface of said target electrode opposite said mosaic with electrons of a velocity above the first crossover velocity of said insulating material to maintain a uniform positive charge over said opposite surface; scanning said mosaic with a low-velocity electron beam to provide a modulated return beam comprising said scanning electrons and representing said charge distribution; and utilizing said modulated return beam to derive said image signals.

References Cited in the file of this patent UNITED STATES PATENTS 2,147,760 Vance Feb. 21, 1939 2,251,573 Morton Aug. 5, 1941 2,269,588 Iams Jan. 13, 1942 2,494,670 Rajchman Jan. 17, 1950 2,540,632 Rose Feb. 6, 1951 2,549,072 Epstein Apr. 17, 1951

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