US2250721A - Image storage tube - Google Patents

Description

1941- R. M6L ER arm.

IMAGE STORAGE TUBE Filed Feb. 4; 1959 Patented July. 29, 194i IMAGE STORAGE TUBE Rolf Miiller, Klein Maclmow, and Werner Hartlslimnn, Zelilcndorf,

Zehlendorl, Germany Application near Berlin, Germany, asgnors to-Fernseli Aktlengescllschaft, Berlin'- February 4, 1939, Serial No. 254,592 Germany February 8,1938

. 3 Claims. (01. 250-150) Our invention relates to storage pick-up tubes for television purp ses, and particularly to such tubes utilizing a mosaic of insulated photoelectric elements upon'which a charge image is stored in accordance with an optical image, and in which this mosaic is scanned point by point. Under scansion a picture signal is generated and the charges on the-mosaic, elements under scansion neutralized, whereupon recharging takes place.

In known tubes, scansion of the mosaic is effected by impact of a high-velocity electron beam which causes secondary electrons to be emitted and the charges on the mosaic elements to become neutralized. The potential which the elements will acquire after neutralization depends upon the secondary emission produced by the scanning beam and adjusts itself to an equilibrium potential which is only several volts below that of the electrode to which the secondary electrons are accelerated. Consequently only a 9 weak accelerating field exists in front of a mosaic during the charge and it becomes extremely diflicult to achieve saturation of the photoelectrons under the influence of the incident optical image. Another consequence thereof is that not all secondary electrons generated at the mosaic by the scanning beam are drawn away, but a substantial portion thereof is drawn to neighboring mosaic elements and prematurely discharge the latter. This largely impairs the efiiciency of the tube as well 'as the sharpness of the image produced. These defects exist also in known tubes with double-sided mosaics. In that case, a strong accelerating field can be used for the photoelectrons. n the side impacted by the scanning beam, however, a strong accelcrating field still cannot be used because the equilibrium potential is determined by the secondary emission.

It is the object of this invention to overcome the aforesaid disadvantages wholly or in part; to provide a mosaic and to provide strong accelerating fields for both photoelectrons and scanning electrons. Other objects and features of our invention will become clear from the following description.

Referring to the drawing, Fig. 1 schematically illustrates a cross section through a storage tube according to our invention, reduced to the simplest terms for sake of simplicity; Fig. 2 schematically shows a cross section through a mosaic screen embodying our invention; and Fig. 3 schematically shows a cross section through a modified type of mosaic screen according to our invention.

' place. This invention,

screen simple in construction,

Broadly considered, our invention provides the use of a strong accelerating field in frontof the mosaic and scansion with low-velocity electrons which liberate no or only a few secondary electrons. The low-velocity electrons used for scansion are generated photoelectrically. While it is known in the art to produce trons by photoelectric means, the disadvantage of this method when hitherto used has been that the mosaic needed to be double-sided and that a diiiusion of the scanning electrons would take therefore. provides that the photoelectrically produced scanning electrons impact the mosaic on the same side from which the picture photoelectrons are liberated, and that the fields traversed by the scanning electrons are separated from the field which accelerates. the picture photoelectrons. By doing so the impacting velocity of the scanning electrons is determined by the potential difference between the potential of the mosaic elements in discharged condition and the potential produced by the charges caused by exposure to light. In order to prevent diffusion or spreading of the lowvelocity scanning electrons, the fields traversed by the scanning electrons are preferably separated for each individual picture element, so that the scanning electrons again impact only the corresponding mosaic elements. For this purpose, all mosaic elements can be screened off against each other.

In operation,-an optical image is projected upon the photomosaic. Photoelectrons are accelerated by means of a strong field and the mosaic elements acquire a positive charge,the magnitude of which is proportional to the intensity of light incident thereupon. Under scansion, low-velocity photoelectrons are emitted in the immediate proximity of the mosaic elements under scansion, whereby a portion of these low-velocity scanning electrons will .be drawn toward the mosaic element by virtue of the potential difference betwen the mosaic element and the emitter of scanning electrons. This portion of scanning electrons will neutralize the charge on the mosaic elements, while the remainder of the scanning electrons will be used as the picture signal. The invention will now be described in detail in connection with the drawing.

Fig. 1 shows a vacuum receptacle l housing an electron gun consisting of a cathode 8, an

anode 6- and two pairs of deflecting plates I.

' Reference numeral 2 designates the mosaic screen upon which an optical image is proiected by means of lens 3. A is a wide open wire mesh scanning elec-' screen 4 servesas an accelerating electrode or picture signal collector. Two electron multipliers 5 and 5a, each comprising a plurality of secondary-emissive grids and a solid plate collector, may be provided. A voltage source 9 serves to heat the cathode 8. A voltage divided I! in cooperation with a voltage source i8 for supplies anode 6, accelerating screen 4 and electron multipliers 5 and 5a with operating voltages. A signal output resistor l9, across which the signal can be taken off by way of a condenser 20, is connected to the anodes of the electron multipliers 5 and 5a.

Fig. 3 shows a modification of the mosaic screen which includes a transparent insulating layer I I, a fluorescent screen l4, a wire mesh screen l2 and the mosaic electrodes l3 which, in this case, are button-like in shape, possessing a photosensitive front surface and side surfaces insensitive to light and secondary emission.

Fig. 2 shows a screen comprising two layers H of an insulating material, such as mica, between which a translucent metal layer I6 is disposed. One of the layers II is provided with a fluorescent screen l4, while the other layer II is in close contact with a metal wire mesh screen I2. In order to secure a close contact it may be preferable to provide the layers H and the metal layer l6 with a slight curvature and to stretch the metal screen l2 across the same. In the openings of the screen i 2 photoelectric mosaic elements l3 are deposited upon the last-mentioned of the layers ll. Portions l5 of the screen l2 are made to be photoemissive.

Fig. 3 shows a modification of the mosaic screen which includes a transparent insulating layer II, a fluorescent screen l4, a wire mesh screen I! and mosaic electrodes l3 which. in this case, are button-like in shape, possessing a photosensitive front surface and side surfaces insensitive to light and secondary emission.

In operation, an optical image is focused upon the right side of a mosaic screen 2, as shown in Fig. l, and photoelectrons emitted by mosaic elements i3 are accelerated toward grid 4, which is held approximately 100 volts positive with respect to the assumed potential of the photomosaic. The accelerating field developed by the potential applied to the electrode 4 reaches into the openings of the screen I2 and exerts In case some photoelectrons should be intercepted by the screen l2, the latter rapidly accumulates a negative charge thereby, hence preventing any further collection of photoelectrons from the mosaic elements l3 and enhancing the focusing above referred to. The cathode ray beam produced by cathode 8 which is deflected by applying sawtooth voltages to deflecting plates 1, as is well-known in the art, is scanned across fluorescent screen l4 and produces light upon impact therewith. Due to the bombardment by high velocity electrons the fluorescent screen l4 will assume a potential which is positive to the cathode 8. In accordance with standard practice, the portion of the inner wall of the envelop I surrounding the discharge space between the cathode 8 and the fluorescent screen l4 may, if desired, be provided with a conductive coating (not shown) connected to the anode 6 in order to collect the secondary electrons emitted by the fluorescent screen l4 and anode 6,

and to prevent wall charges. By virtue of electro-static induction the screen I 2, separated from the fluorescent screen by a layer of insulating material, will assume a potential negative with respect to that of the screen i4.

these photoelectrons will mosaic elements I3, as indicated by the dotted lines in Figs. 2 and 3. This portion of the lowvelocity scanning electrons will neutralize the charges on mosaic elements i3 until the positive charges on these elements are neutralized to charge of the mosaic elements exceed about 5 volts, whereby the imvelocity of the scanning electrons proportions l5 remains below this value, secondary emission is practically avoided. The storage eflect, however, is nearly completely each other. Therefore, the eiiiciency is only slightly below because complete neutralization of the charges is not advisable, it being preferable to discharge only to a residual charge of, for instance, 10%. Further discharge would not considerably amplify the picture signal. It would, however, increase the noise level of the tube because the main portion of the photocurrent produced at portions l5 would then be collected.

It is preferable to avoid too high a photosensitivity on the side of mesh the insulating layer because the noise level of current is then reduced. For this purpose a mechanical mask can be used during evaporation of the silver, or during oxidation, respectively, which can be removed later on.

It may be diflicult to sensitize the portions I5 uniformly over the entire mosaic area. In order to cause uniform photoemission by the scanning light spot, network H can be made of a material which does not oxidize very readily (nickel, molybdenum, platinum). The work function of the surface will then be considerably higher than that of a silver oxide-caesium layer. Light of a short wave length, particularly ultra violet light, is then used for generation of scanning electrons, The luminescent substance is then so chosen that it has its maximum efficiency, or at least emits suflicient light, in the spectral range following the shortest wave length of the light used in the optical range.

instance M mm., in order to avoid light dispersion. If a mosaic plate of an area of about 9x11 cm. is used, elements l3 must have a size of about 0.2x0.2 mm. for 441 lines. The meshsize of network 12 is determined thereby.

Instead of scanning fluorescent screen ll with a cathode ray beam, a fluorescent screen 01 a separate cathode ray tube can also be optically reproduced in the plane of network l2. A pattern of uniform brilliancy is recorded on the screen of this tube in the known manner so that photoelectrons are emitted from portions l5 point by point. Fluorescent screen I can then be omitted.

We claim:

1. A television signal generating device comprising a plurality of photoelectric sources, means for developing a spot of light of substantially constant brightness, means for efiecting movements of said spot across said source in accordance with a scanning pattern to cause said sources to emit electrons in successive order, photoelectric mosaic means adapted to have an optical image focused thereon for collecting portions of the electrons emitted from each of said sources in accordance with variations in the illumination over said image, and means for collecting the remainder of said emitted electrons to produce a signal representing said image.

2; A television signal generating device comprising a plurality of photoelectric sources, means for developing a spot of light of subprising a stantially constant brightness, means for scanning said sources by said light spot in accordance with a scanning pattern to cause said sources to emit electrons in successive order, photoelectric mosaic means adapted to have an optical image focused thereon for collecting portions of the electrons emitted from each of said sources in accordance with variations in the illumination over said image, and means for collecting the remainder of said emitted electrons to produce a signal representing said image.

3. A television signal generating device comtranslucent insulating member, a coating of fluorescent material on one side of said member, means facing said one' side for generating an electron beam, means for deflecting said beam across said fluorescent coating in accordance with a scanning pattern, an apertured metallic screen having photosensitive areas adjacent the other side .of said member, photoelectric mosaic elements disposed on said other side of said member within the apertures of said screen and adapted to have an optical image focused thereon from a point facing said other side, and means for collecting electrons emitted from said photosensitive areas and said mosaic elements to produce a signal representative of said image.

ROLF M6LLER. WERNER HARTMANN.

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