Title:
System for television transmission
United States Patent 2247138


Abstract:
This invention relates to television, and especially to a system for television transmission, especially by the aid of a cathode ray scanning device. Efficient television systems depend upon advantageous use of amplifier stages of the audion type. At best the signaling impulses obtained at...



Inventors:
Sukumlyn, Thomas W.
Application Number:
US23275538A
Publication Date:
06/24/1941
Filing Date:
10/01/1938
Assignee:
Sukumlyn, Thomas W.
Primary Class:
Other Classes:
313/2.1, 313/329, 313/374, 313/379, 315/11, 427/68
International Classes:
H01J31/40
View Patent Images:



Description:

This invention relates to television, and especially to a system for television transmission, especially by the aid of a cathode ray scanning device.

Efficient television systems depend upon advantageous use of amplifier stages of the audion type. At best the signaling impulses obtained at the transmitter end by conversion of light impulses into electrical impulses, are minute; and effective high amplication is essential to render such signals perceptible at the receiver.

It is one of the objects of this invention to make it possible to increase the intensity of the signaling impulses without the necessity of supplementing the amplifier stages.

In one previously known application of a cathode ray tube for television transmission, the ray (comprising a stream of electrons) is caused to scan a mosaic of photoelectric material. This mosaic in such prior systems comprises a series of isolated photoelectrically active areas, supported on an insulation plate; this plate in turn is supported on a metal member, having with the active elements, capacities that are in parallel relation with each other. The picture to be transmitted is focused on the photoelectric mosaic; accordingly the charge on each photoelectrically active element depends upon its illumination, and therefore varies as its Illumination varies. The capacity between each active element and the conducting plate support determines the potential difference across this capacity due to the charge. Now just as soon as a cathode ray impinges upon a group of such active elements in the course of the scanning operation, the photoelectric charge on these elements can be discharged by the ray. The resultant discharge current flow is passed through a large resistance so as to set up a corresponding potential difference across the resistance. This potential difference Is then utilized to affect an amplifier system in a well-known manner.

In such old systems as outlined above, advantage is taken of the fact that the active elements of the photoelectric mosaic are continuously effective to determine the quantity of electricity to be discharged by the operation of the scanning cathode ray; and this effectiveness occurs at any group of active elements of the mosaic between successive discharge by the ray. For this reason, such systems have been termed "storage" systems.

It has been demonstrated that the intensity of the television impulse is dependent upon the capacity between the active elements that are being discharged, and the metal plate support that carries the mosaic as well as upon the capacity of the anode that collects the electrons. In general it may be stated that the smaller this capacity is, the greater the signal impulse that can be passed to the amplifier system. When the capacity of this collector is made small as by making its area small, much fewer electrons need be received thereon to cause it to have a potential sufficient to provide a large signaling impulse.

It is another object of this invention to make it possible to increase greatly the signaling impulses in such storage systems, especially by providing that the discharge take place through a smaller capacity. This smaller capacity is formed, not by the supporting plate for the mosaic, but by a supplemental small anode or collector that is spaced from the source of those electrons that are to be received and collected thereon.

By providing such an anode, the intensity of the signal is further increased by an amplifier action. The discharge and charge of the mosaic elements do not serve directly as a signal current, but they cause the potential of these elements to change, and thus the mosaic serves as a control electrode controlling a much larger current in the form of secondary electrons from a supplemental emitter to the anode. Thus the charge and discharge current to the mosaic elements is really a grid current, and the mosaic elements perform the function of a grid as in the usual amplifying audion.

This invention possesses many other advantages, and has other objects which may be made more apparent from a consideration of one embodiment of the invention. For this purpose there is shown a form in the drawing accompanying and forming part of the present specification. This form will now be described in detail, illustrating the general principles of the invention; but it is to be understood that this de45 tailed description is not to be taken in a limiting sense, since the scope of this invention is best defined by the appended claims.

Referring to the drawing: Figure 1 is a diagram illustrating a system incorporating the invention; Fig. 2 is an enlarged fragmentary view of a Photoelectrically active mosaic utilized in connection with the invention; Fig. 3 is an enlarged sectional view taken along the plane 3--3 of Fig. 2, and Figs. 4, 5 and 6 are graphs illustrating the phases of operation of the system.

The television transmission system includes an evacuated tubular envelope I in which substantially all of the elements of the system may be enclosed and sealed. One portion of the tube is a cathode ray device; this corresponds in Fig. 1 to the left hand portion of the tube 1. The right hand portion of the tube I is a photoelectric cell system.

In transmitting electrical impulses corresponding in intensity to the illumination obtained by projecting an image on a surface, it is essential to make use of photoelectrically active elements such as those produced by the distillation of alkali metals on a supporting surface. The impulses may be secured by the aid of a scanning cathode ray for causing the flow of an electric current, corresponding to the illumination on successive elemental areas of the photoelectrically active surface.

In one form of television system use is made of a photoelectrically active mosaic. Upon this mosaic the image of the scene to be transmitted by television is cast. The present system utilizes such a mosaic. It has a supporting plate 3. The construction of this mosaic can be best explained in connection with Figs. 2 and 3.

The supporting plate 3 is shown as having perforations 4. Upon this supporting plate is spread the mosaic 2 which includes mutually insulated elemental photoelectric areas. The resultant effect is that there are numerous small spots such as indicated at 5 on Fig. 2, which are electrically insulated from each other -and from the supporting plate 3, but each of which forms a photoelectric .cathode capable of emitting electrons when illuminated. The intensity of emission is dependent, as in the usual photoelectric cell, upon the intensity of illumination falling upon these elements.

Various ways may be utilized to form the mosaic. One manner suggested by prior investigators is to produce minute metallic globules on a layer of insulation, as for example by reducing particles of silver oxide that had been dusted over the insulation layer. The reduced silver globules form the individual minute metal areas.

These areas may be sensitized after the mosaic has been mounted in the tube I and the tube evacuated, as required for such cathode ray devices. The sensitization may be accomplished by oxidizing the metal surface and exposing it to caesium vapor, and then heat treating.

No matter how produced, the mosaic 2 includes a very large number of elemental areas closely spaced on the insulation 6, which in turn is supported on the perforated screen or plate 3. The mosaic 2 and its supporting plate 3 are shown as appropriately supported within the evacuated tube I.

In order to focus an image on to the photoelectrically active mosaic, use is made of any suitable lens system. Such a lens system 7 is diagrammatically illustrated in Fig. 1 and is shown as located exterior of the tube 1. Illumination from the scene passes through the lens system 7 and part of it passes through the perforations 4 of the supporting plate 3. Preferably the perforations comprise about one-half of the to'tal over-all area of this support 3. The illumination is reflected backward on to the mosaic 2 by the aid of a metallic mirror film 8 disposed adjacent and opposite the mosaic 2.

Since for the purpose of this invention the image cast upon the mosaic 2 is formed by light passing through the perforations 4, a portion of the light is lost, but in view of the tremendous advantage obtained in signal intensity, this reduction in illumination is tolerable.

The elemental active areas of the mosaic 2 emit electrons corresponding to the Intensity of the illumination produced on these areas from the scene to be transmitted. The emission of o1 electrons causes an increase in the potential of these areas. This increase .in potential with respect to the metallic film 8 is made use of to determine the intensity of another stream of electrons, in the manner of control electrodes 1. used for aniplifiers of the thermionic type. How this is accomplished will now be described.

Thus the film 8 is made of appropriately thin continuous metal so that it may serve as a secondary emitter of slow electrons when bombarded by a cathode ray; but it is thick enough to prevent the passage of any high velocity primary rays that are used for the bombardment. The emission of electrons toward the right as viewed in Fig. 1, from elemental areas of the secondary emitter 8 is shown as controlled by a scanning cathode ray 9, which serves as .the bombarding ray. This ray is caused toe scan film 8, as for example by the aid of the usual scanning coils 10 and 11. This is all accomplished in a well understood manner. The cathode ray 0 is produced by the aid of a heated -cathode 12 and directing anode 13 by the aid of which an "electron gun" is produced.

The electrons in the ray 9 are quite well concentrated along this ray.

The anode 13 is maintained at a sufficiently high potential with respect to the cathode 12 as by the aid of the battery 14 connected between these two electrodes. Furthermore, the secondary emitter 8 is shown as maintained at a positive potential with respect to the cathode 12 as by the aid of .the battery 15. Thus the path of the electrons utilized in the gun includes the ray 9, and a return through the battery 15.

The right hand portion of the evacuated tube I utilizes the emissive right hand surface of the secondary emitter 8 as well as the mosaic 2 with its support 3 to form a structure analogous to a three electrode electronic emission amplifier of the audion type, the mosaic 2 serving as a control grid and its potential being varied by the photoelectric effect. The emitter 8 corresponds to a cathode, and a plate 17 serves as the anode. The secondary emitter 8 is maintained at a potential normally positive with respect to the mosaic 2 and the plate 3, as by the aid of the battery 16. In other words, if the mosaic 2 is not illuminated, the intensity of illumination of any particular elementary active portion of the mosaic, produces a corresponding emission of electrons from the elementary portion, causing the potential to increase with respect to the secondary emitter 8. Most of the electrons emitted from the mosaic 2 find their path to the emitter 8. Accordingly the potential ,difference between any elemental area of the emitter 8 and the corresponding opposite photoelectrically active elements of the mosaic 2 is varied in accordance with the intensity of illumination of the elemental area .of the mosaic.

Now at the ingstan-t that the cathode ray 9 reaches any elemental portion of the emitter 8, a secondary emission of slow electrons takes place 'from that elemental area toward the right. Although some of these electrons might find their way back to the emitter 8, some of them can proceed through the apertures 4 of the plate 3 in to the region to the right of the mosaic 2, and the balance is utilized to replenish electrons on the mosaic 2. They are accelerated to some extent by the potential of the corresponding portion of the mosaic 2, toward the anode 17. This anode I7 is spaced some distance to the right of the emitter 8. It is maintained at a potential 1C positive with respect to the plate 3 and is also positive with respect to the emitter 8, as by the aid of battery 18. The intensity of electron flow along the path 1'9 between the emitter 8 and the anode 17 is dependent upon the potential 'difference between the mosaic 2 andithe emitter 8; and this in turn, as heretofore explained, is dependent upon the intensity of illumination of that portion of the mosaic 2 which is opposite the region on the emitter 8 which 2C is acted upon by the cathode ray 9. Accordingly the intensity of the current impulses flowing through the path connecting the anode 17 and emitter 8 is dependent upon the intensity of illumination of the corresponding elementary portions of the mosaic.

In order to affect the input circuit of an amplifier system 20 by this current impulse, the path between anode 17 and emitter 8 includes a resistance 21 of appropriate size across which the input circuit 2.2, 23 of the amplifier system may be connected.

By virtue of the fact that the anode 17 forms a very small capacity with respect to the other elements of the system, a comparatively small volume of electron flow to the anode 17 is sufficient to alter the potential of the anode 17 to a signal strength. The potential of anode 17 is the potential applied to the amplifier system 20, as by having the input side of this system coupled across the resistance 21 that is in the circuit of anode 17.

The manner in which the potential differences along the tube are controlled is illustrated to best advantage in Figs. 4, 5 and 6. Thus in Fig. 4 the potential of cathode 12 is taken as the datum or zero potential 24. The potential of the directing anode 13 is indicated by the point 25. This increase in potential between points 24 and 25 is caused by the battery 14. The potential of the emitter 8 is indicated at the point 26 and is higher than that of anode 13 by virtue of the battery 15. Assuming that there is no illumination on the mosaic 2, and that the cathode ray 9 is ineffective, the plate 3 has a potential below that of the emitter 8. This lower potential is represented by the point 27, and is due to the battery 16 interposed between these two elements. From the point 27 corresponding to the supporting plate 3 there is a comparatively steep rise of potential to the point 28 corresponding to the anode 17, and due to the battery 18.

Now when the mosaic is illuminated, the potential of the photoelectrically active elements' thereof at any particular point corresponding to the position of the cathode ray 9, and the potentials of other elements of the system may be represented by the graph of Fig. 5. In this graph the potential of emitter 8 is represented as before by the point 26. However, due to the emission of electrons from the photoelectrically active elements under consideration, these elements have a potential represented by the point 29 which is considerably higher than before. The point 27 again represents the potential of the plate 3. Accordingly conditions are more favorable for the acceleration of electrons through the plate 3 to anode 17. In Fig. 6, the electric field through a path for the electrons from emitter 8, is indicated. Here the point 30 represents the potential at the corresponding aperture 4 through which the electrons from emitter 8 are accelerated. This potential 30 at Sthe aperture is affected by the potential 29 on the adjacent portion of the photoelectric control mosaic 2. Since this electric field is such as to accelerate the electrons through path 19 to the anode 17, there is a flow of secondary electrons, the intensity of which'is dependent upon the illumination of the corresponding part of the mosaic 2.

The ray 9 thus serves to discharge a current by the aid of the electron stream 19 to the anI ode 17. This current being due to the collection of negative electrons emitted from the emitter 8, once these electrons reach the region of the mosaic 2, they are accelerated toward the collecting anode 17. This anode serves, as is apparent, as a plate electrode of an amplifier including an emitter 8 and a control electrode or grid 2.

It is of importance to ensure, under any circumstances, that the capacity of plate 17 be as low as possible; the smaller this is, the greater the advantages obtained by this system. It can be shown that the output of the storage type of pickup is NC, C2 times the output of a non-storage pickup (assuming equal photoelectric sensitivities and perfect efficiencies), where N is the number of elements making up the picture, Ci is the capacity of the photoelectric cell and associated circuits in the case of the nonstorage type of pickup, and C2 is the capacity of the circuit of anode 17, connecting the pickup and amplifier in the case of the storage type. Now since in the usual storage type of pickup, the capacity C2 is necessarily very large, being of the order of hundreds of times larger than the capacity Ci usually found in the non-storage type of pickup, the gain in output resulting from the storage principle is in large measure annulled. Therefore if we reduce the capacity in a pickup of the storage type to a value as low as that of the usual photoelectric cell and its associated circuit, the gain in output over the gain possible with the non-storage type of pickup becomes equal to the number of picture elements; this is up in the thousands of times. The capacity of the electrode 17 in the present invention, together with the capacity of the circuit associated with the amplifier system 20 can be easily made as small as this.

What is claimed is: 1. In a television transmission system, means forming a mosaic of photoelectrically active elements, a cathode ray scanning device, a secondary electron emitter interposed in the path between the mosaic and the cathode ray, an apertured plate supporting the mosaic, said secondary electron emitter having a reflecting surface opposed to the mosaic, means for casting an optical image on the said reflecting surface, and through the apertured plate, means whereby the potential difference between an elementary portion of the surface of the secondary emitter and the opposite elements of the mosaic is dependent upon the illumination on said elements, and an anode spaced from the mosaic and on that side of the mosaic opposite from the secondary emitter, for collecting the secondarily emitted electrons.

2. In a television transmission system, means forming a mosaic of photoelectrically active elements, a perforated plate support for said mosaic, a reflecting member opposite the mosaic, means for casting an optical Image on the mosaic by the passage of light through the plate onto the reflecting member, means whereby successive elemental areas of said reflecting member are caused to emit electrons in accordance with the illumination of successive elemental portions of the mosaic, an anode for collecting said electrons, and an amplifier system having an input circuit connected to said anode.

3. In a television transmission system having a mosaic of photoelectrically active elements, as well as means for casting an optical image on said elements to cause emission of electrons from said elements in accordance with the intensities of illumination on said elements, and also an anode to receive electrons for affecting a transmission circuit, characterized by the fact that the mosaic: elements have their active surfaces tarned away from the direct illumination and receive their image by reflection, and by the provision of means to cause secondary electrons to pass to the anode and in accordance with the optical image.

4. In a television transmission system having a mosaic of photoelectrically active elements, as well as means for casting an optical image on said elements to cause emission of electrons from said elements in accordance with the intensities of illumination on said elements, and also an anode to receive electrons for affecting a transmission circuit, characterized by the fact that the mosaic elements have their active surfaces turned away from the direct illumination and receive their image by reflection, said reflection being provided by a secondary electron emitter having a reflecting surface, which receives primary photoelectrons from the elements, and by the provision of means to cause said reflecting surface to emit secondary electrons corresponding in succession to elements of the optical image, and received by the anode.

THOMAS W. SUKUMLYN.