US2256300A - Device applicable mainly to television - Google Patents

Description

Sept. 16, 1941. s. VAN MIERLO DEVICE APPLICABLE MAINLY TO'TELEVISION Filed Oct. 15, 1937 2 Sheets-Sheet l m ml w 5. mm Mxzmo ATTORNEY p 16, 1941- s. VAN MIERLO DEVICE APPLICABLE MAINLY T0 TELEVISION Filed Oct. 15, 1937 2 Sheets-Sheet 2 ATTORNEY Patented Sept. 16, 1941 DEVICE APPLICABLE MAINLY TO TELEVISION Stanislas Van Mierlo, Paris, France, assignor to International Standard Electric Corporation,

I New York, N. Y;

Application October .15, 1937, Serial No. 169,185 In France November 6, 1936 1 Claim. (Cl. 178-72) The present invention relates to devices particularly applicable to television systems and more particularly to television cameras for use in such systems.

One of the objects of the invention consists in the provision .of television cameras in which great variations maybe obtained in the potential of the screen of suchdevices independent of the number of scanning lines.

Another object of the invention consists in the provision of devices such as television cameras requiring only low energy in order to control the electrons. produced by an independent electron source for example an electron gun.

One feature of the invention consists in the provision of devices such as television cameras combined with an arrangement such as an electron multiplier for obtaining a high output power while, reducing the noise due in particular to thethermic agitation.

Another feature of the invention consists in the provision of screens adapted to be employed in such devices and comprising layers of photoemissive or photo-conducting substances.

Fig. 1 shows an embodiment of a tube particularly adapted to be used as electron camera in a television system;

Fig. 2 shows another embodiment of an electron camera;

7 Fig. 3 shows an example of a photo-emissive screen such as is employed in the devices of Figs. 1 and-2;

Fig. 4 shows in schematic circuit form an element of a photo-conductor screen which may be employed in the arrangement of Figs. 1 and 2;

Figs. 5 and 6 show views in partial section and in plan of a screen formed of elements similar to those represented in Fig. 4, and

Fig. 7 shows a modification of the screen of Figs. 5 and 6.

cally in the drawings. These two portions of the glass envelope are closed by suitable end caps 6 and I through which the connecting wires are passed. In the central portion of the envelope is arranged a screen 8 on which is projected a luminous image of a picture or field of View through .a rlens system 9, outside the tube or forming a part of the tube and through an internal grid In. The electron gun 4, the grid ID, the screen 8 and the electron multiplier 5 are fed by batteries ll, l2, [3, I4 and IT as shown or by a common source and suitable potentiometers. A resistance I5 is inserted in the external circuit of the multiplier 5 in series with its collecting electrode and serves as a load resistance. A grid may in certain cases be provided in front of the first electrode l 6 of the multiplier.

The image to be transmitted is projected on the screen 8 which may for example be composed of a support member of wire-gauze covered with an insulating substance and provided on one of its surfaces with discrete photo-emissive particles. Each of these photo-emissive particles assumes a potential dependent on the illumination at this point. sweeps or scans this photo-emissive surface by means of an electron beam which may be retarded by the grid l0 which is suitably polarised. The electrons released by the photo-emissive particles are absorbed for example by the grid l0. Owing to this release, each photo-emissive particle takes a charge which depends at each moment on the illumination and its insulation resistance with respect to the wire-gauze of the screen 8. Consequently the potential at each moment depends upon the illumination of the particular particle. The electrons proceeding from the gun 4 are retarded by the grid l0 and the latter can thus be considered as a source of electrons all the points of which become successively active during scanning by the beam of the electron gun 4. The screen 8 may thus be considered as the grid of a, triode system constituted by the grid l0, screen 8, and anodic electrode l6 of the multiplier 5. This screen 8 thus controls the number of electrons passing to each point in r a similar manner to that in which the grid of-a triode controls the plate current. The current produced by the multiplier and collected on the terminals of the resistance I5 will thus at any moment be in proportion to the quantity of electrons collected by the electron multiplier, and thus upon the illumination of a, given point of the screen. The low power available to the photo The electron gun 4 U gun is maintained constant the level of the signals does not diminish when the number of lines increases.

Fig. 2 shows an arrangement adapted to be used in place of the device shown in Fig. 1. In Fig. 2 a cathode ray tube of the usual type I! having a fluorescent screen is arranged in front of an evacuated glass envelope 18 containing a screen system 8--l0 and an electron multiplier arranged as shown in the drawings. Between the 'ends of tube I1 and envelope [8 is a lens or other suitable optical system I9 and another system of optical concentration 9 is arranged on the other side of the screens 8 and [0 outside the envelope 18. These two screens 8 and I0 must be placed as near as possible to each other a1" though for the convenience of the drawing they are shown as separated by a substantial distance.

The screen 8 consists as before of wire-gauze covered with insulating material and provided on the side of its surface where the luminous image to be transmitted is projected, that is to say on the same side as the optical system 9 with photo-emissive particles. The screen l0 consists of a continuous photo-emissive layer 20 deposited on a transparent supporting plate 2|. The emissive layer of this screen [0 is scanned by a luminous ray coming for example from the fluorescent screen of the cathode ray tube l1 and the electrons emitted from it explore the surface of the screen 8 the photo-emissive elements of which are brought to a potential corresponding to the illumination of the image projected on the said screen 8 at each point. The electrons coming from H) and passing through the screen 8 are then collected by the electron multiplier as previously indicated. The two functions, namely, exploration of the image and production of signal current are thus separate.

Various structures may be conceived for the screen 8 shown in Figs. 1 and 2. This screen may consist either as stated of wire-gauze or of a conducting plate covered with an insulating substance and covered on one of its surfaces with photo-emissive particles, or again of a conducting plate covered with photo-conductive particles. Figs. 3 to 7 show examples of suitable structures which may be employed.

Referring to Fig. 3, a conducting plate I is shown with a full edge 2. This plate is perforated with openings 3 regularly arranged. The narrow portions of the plate are partially covered with photo-emissive surfaces as shown at 4. The openings 3 have been represented in square form, but they could obviously be of any desired shape, circular, oval or polygonal, or even of irregular shape.

The conducting plate I may consist of a metal plate having a thickness of the order of the linear dimensions of the openings 3. In a practical embodiment the said plate may have the form of a square with its sides 12 cm. long, perforated with 400 rows each of 400 holes, and each opening being in the form of square the length of its sides being 0.2 mm., and the intervals between holes being about 0.1 mm.

In the case in which the metal employed for the supporting plate is, for example, of copper, this plate may be covered with a lacquer or ink by means of a photographically prepared plate so that the surfaces to be out are not covered. Then the plate so treated may be dipped in a bath of acid or a composition which chemically or electro-chemically attacks the metal not covered with lacquer until the plate is perforated at the parts not covered by the lacquer. Once the perforation is thus obtained, the protecting lacquer is dissolved by any suitable means and the plate is covered with a suitable insulating substance. Another photographic plate is then printed on the plate so produced and by em-' ploying a salt or an, amalgam of a metal such as silver, a certain number of small insulating surfaces are obtained. After reduction and washing the plate is introduced in the vacuum tube and heated so as to volatilise the remaining impurities, the silver surfaces are oxidised and the photo-sensitive layer is obtained by any suitable means.

In this method of producing the plate some difiiculty may be experienced in covering the perforated metal with an insulator which is capable of resisting relatively high temperatures. According to another method, however, it is possible to take a plate of aluminium and to cover it with oxide by means of anodic oxidation. It is also possible in the case of a plate of aluminium to dispense with the step of covering the plate with lacquer or ink before causing it to be attacked with acid, and to obtain the same result by means of local cold hammering, for example, by pressing the sheet of aluminium between two engraved plates: the parts compressed are then cold hammered and only resist slightly to the attack of the acid. After oxidation and before applying the conducting particles it is possible to impregnate the layer of aluminium with a mineral oil, for example, in order to prevent the salt or conducting material from penetrating the pores of the aluminium. This oil will then be evaporated.

It is obvious that instead of using the silver salt for the second printing of the sheet of metal, it is possible to apply the silver particles by pulverisation or in any other manner. It is also clear that instead of copper or aluminium the use of other metals may be conceived, the oxide of which has a suitably high specific resistance, such as iron, nickel, etc.

As previously mentioned the invention provides in accordance with certain of its characteristics, for the use in place of photo-emissive screens of screens comprising photo-conducting substances.

Such screens may consist of a set of elementary electrical circuits similar to that shown in Fig. 4 and comprising a resistance l, whose value depends on th illumination applied thereto, a source of current H and a fixed resistance 2. The conductor indicated at 3 completely surrounds the circuit described to which it is connected, and consequently takes a potential depending on the light received, with respect to a point A of the circuit which is connected through a source of current such as a battery l2 to an electron emitting electrode 4.

A screen composed of a set of such elementary circuits will control at each of its points the electrons proceeding from an external source of electrons, the electrode 4 for example, which sweeps the surface of the screen. Those of these electrons passing between th elements of the screen will be received by the electron multiplier as shown in Figs. 1 and 2.

Various methods of construction of a screen employing such elementary circuits may be conceived. For example, two of the conducting plates similar to those prepared in the embodiment of a screen previously described may be used. On each of these plates, however, the insulating material is removed and the two plates are then superposed as shown in Fig. 5.

In this drawing, the plan of which is shown in Fig, 6, I and 2 represent the conducting plates,

3 the holes in these plates, and 4 the insulating substance interposed between these plates. A layer of photo-conducting substance 5 (for example selenium or mercurous iodide) is then applied to the external surfaces of the plate I by any suitable means, such for example as printing. On the external face of the plate 2 is applied in the same manner a resisting layer 6. A fine conducting layer I is arranged so as to envelope the whole. this layer which may, for example, be produced by a vaporisation of metal in vacuum, is transparent at least at the parts where it covers the photo-conducting substanc 5. If between the two conducting plates a difference of potential is applied by means of a suitable source such as a battery II and another battery I2 is provided for a connection to an electron emitting electrode, the unit thus composed forms a group of circuits similar to that shown in Fig. 4.

The battery I2 may either be directly connected to an electrode which may be considered as a source of electrons when it is scanned by an electron beam (as in Fig. 1), or connected to an intermediate screen permitting the substitution for the electronic scanning of luminous scanning (as shown in Fig. 2). This intermediate screen which is shown in the drawings may, for exam- .ple, consist of 'a transparent support 8 of mica or other suitable insulating substance covered with a transparent layer 9 of a metal such as silver, itself rendered photo-emissive as shown at I 0. The source I2 establishes a difference of potential between the metallic layer 9 and the plate I, so that in the absenc of all illumination, the enveloping conductor 1 is negative with respect to the silver layer 9. The scanning by the luminous ray whose direction is indicated by the arrowE, produces an electron emission in the layer I0. vWhen no luminous image is projected on the other surface of the screen, the electrons emitted by the layer I will not pass through the holes 3 in the screen. When the luminous image of the object to be transmitted is projected through the photo-conducting layer 5, its resistance varies, th potential of the metallisation I with respect to the layer 9 may become positive at certain places and the electrons emitted by the layer III will then be able to pass through the corresponding openings 3 and reach the electron multiplier.

The metal layer I may be continuous provided it is sufiiciently resistant, the efiect of a variation of resistance of an element of the photo-conducting layer only having a local influence on the second plate 2 is dispensed with. This layer must then be of such a resistance (particularlyat the places 'I) that a variation of resistanceof the photo-conducting layer 5 results in a modification of the potential of the plate I in the the point under consideration.

As the utilisation of a photo-conducting sub,- stance permits relatively high variations of potential of the screen to be obtained, it is also possible to conceive in the devices shown in Figs.il and 2 of the elimination of the electron multiplier; a simple collecting anode will then be sufficient, the current discharged being of sufiicient intensity for the noise due to the thermic agitation in the coupling resistances then to be negligible. It is consequently possible in this case to employ an external electron multiplier or even an ordinary thermionic amplifier.

It is clear that the photo-conducting substances employed may be chosen so as to be more or less sensitive to certain wave lengths, which for example would make it possible to conceive .of the use of such devices in infrared ray systems.

Although the devices described have been conceived in their application to two dimensional scanning, they may also be employed in the case in which the scanning is only effected in one direction.

What is claimed is:

A television camera comprising an evacuated vessel having disposed therein an electron multiplier and a screen consisting of a metallic grid coated with insulating material and provided on one face with a mosaic of photo-emissive particles, a source of electrons disposed on the opposite side of said screen to said electron multiplier, means for projecting a luminous image on the photo-emissive surface of said screen and means for causing an electron beam from said electron source to scan said screen, said electron multiplier being positioned to receive electrons which pass through said screen from said electron source, and a grid electrode negatively biased with respect to said electron source interposed between said source andsaid screen for retarding the speed of the electrons as they approach said screen, thereby permitting the photo-emissive particles on said screen to act as a control grid to control the electrons passing through said screen from said electron source.

STANISLAS VAN MIERLO.

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