US2153646A - Television scanner - Google Patents

Description

F. SCHRTER TELEVIS ON SCANNER Filed July 25, 1935 April l 1, 1939.

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Patented Apr. 1l, 1939 UNITED sra'ras PATENT ori-ics TELEVISION SCANNER tion of application July 23, 1935, serial No. 32,768

Germany July 14, 1934 2 Claims.

In the so-called'iconoscope, the television picture to be transmitted is projected on a surface, consisting' of a mosaic of minute photoelectric cells. Each of these small cells represents a min- 5- iatiue condenser which stores the energy of the photoelectric' electron emission over a period of time; approximating the duration of the scanning of the picture, for instance 3&5" second. The charges thereby accumulated, are, for the purpose of modulating the transmitter, scanned by means of a cathode ray, passing in a parallel line movement over the mosaic surface, and moving in synchronism with that in the Brauns tube of thesuiiicient length of time, the luminous sensitivity,

essentially present in the iconoscope but only of short duration therein.

The invention makes use of the so-called electron microscope, i. e., the possibility of highly enlarged reproduction on a. luminous screen of an electron source contained in high` vacuum, by

means of electrostatic or magnetic lenses. In the present case, the electron source is represented by a photoelectric cathode I as shown in the drawing, on which the picture to be transmitted or a portion thereof, preferably an entire picture line 2, is projected in suitable reduction by means of an optical means 3. In the subsequent description, only the last mentioned case will be further considered, i. e., only a single line acts on the photo-cathode. In decomposing a lm, the line alternation is obtained by the continuous progression oi the film, in scanning a two-dimensional picture iield which is statlonaryin space, this is done by interposing an optical means supplying the second component of the decomposition, for instance a prismatic reflector wheel. At the place where the photo-cathode I is impinged by the light of the picture line projection, there occurs, along the cathode, a photoelectric emission, varying as regards density, in accordance with the distribution of the brilliancy, the linear course of said emission being reproduced in eularged size on a separate scanning devicel by means of the electron microscope (electrodes 4, 5, 6). The scanning device consists of a line of individual microscopic electrodes 8, insulated from (Cl. TIS-7.2)

each other, and arranged within a highly evacuated tube, the said electrodes being impinged and charged by the accelerated photoelectrons. These individual electrodes 8 therefore` receive different charge in accordance with the course of the in- 5 tensity in the picture line. The capacity thereof, relative to a common positive collecting electrode 9, can be utilized for storing, analogous to the iconoscope, so that in the case herein referred to, the accumulation of the luminous effect takes 1o place throughout the period of transmission of a picture line resulting in a large gain of luminous sensitivity, in comparison with the hitherto applied principle of scanning the television picture from point to point. The increase of the energy l5 of the photoelectrons caused in the electron microscope due to the electrical acceleration is utilized in the Widest degree, in accordance with the invention, since it is possible in this manner to charge the individual electrodes 8 to a high nega- 20 tive potential relative to the opposite electrode 9.

In order to exploit the varying accumulated charges along the contact line l, for the modulation of the television transmitter, an independent constant cathode ray having a sufficiently 25 small cross section of incidence, is used. This cathode ray, indicated by the dotted line I0, is derived from an independent source (glow cathode I I) and is moved along 'I in synchronism and equal phase with the line recording ray of the 30 image receiving tube. Since it is hereby aimed at neutralizing the more or less high negative charges of the individual condensers 8, it is necessary to liberate from the incident ray I0, a. corresponding positive charge. 'I'his will advanta- 35 geously be accomplished by means of secondary electron emission, released by impinging or primary electrons on the electrodes 8 of the contact line, or preferably on separate contact surfaces I2 individually connected to the said electrodes. 40 The secondary emission can be enhanced bycovering the surfaces l2 with coatings favoring electron emission or forming said surfaces of suitable metal; furthermore by providing suitable velocities of particles in the scanning cathode ray, an'd 4,5 placing opposite the repulsing faces I2 a collecting electrode I3, having a relatively high positive potential, and whose iield makes possible the withdrawal by way of suction of the secondary electrons. On the contrary, in the electron mi- 50 croscopic reproduction of the photoelectron source I, a detrimental secondary emission of this contact series towards the electrodes 8 can be prevented by means of-a highly negative co1- lecting electrode I4 placed opposite the electrodes 5g 8 and acting like a suppressor grid of a pentode. The high secondary emission at the scanning surfaces? I2 has the effect that the latter emit during the time unit, a greater number of electrons than was supplied thereto by the ray I0. In other words, positive charge will be liberated for compensating the negative charge of the electrodes 8, the compensating effect to be utilized for modulating the transmitter varies, as desired, in its intensity in accordance with the distribution of brilliancy along the picture line.

By interrupting, at scanning, the cathode ray I by known means (auxiliary electrode near the cathode II) in the rhythm of a high frequency whose duration of the cycle be a Whole number fraction of the duration of the traversing of the individual surface elements I2 (in order to avoid disturbing interference between the period of interruption and the screen of the contact line), there will be readilyobtained the carrier oscillation desired for further intensifying the picture signals.

An advantage of the described arrangement resides in that the photoelectrically effective surface I is not continuously impinged by rapid electrons and thus disactivated as in the case of the iconoscope. Positive ions arrive in excess at the highly negative intermediate electrode 5 protecting the photo-cathode I against rapid attack. If, as pointed out, only a single picture line is reproduced on i it is easy to find thereon a zone of suiciently uniform photoelectric sensitivity. Furthermore, it would be possible to displace the picture line on the surface i for instance in vmaking it displaceable or rotatable from the outside, in usin'g for example a rotating' surface, or it is provided with a very fine screen of sharp edges on which an especially high and uniform photoelectron emission appears.

A further advantage of the arrangement according to the invention resides in the fact that it is not necessary to pass separate, insulating lead-ins from the always numerous individual elements 8 of the contact line 1, through the wall of, the tube, and which would otherwise render the manufacture thereof extremely difficult. If the photo-cathode I is to be utilized for the reproduction of larger picture portions, the linear contact series 'I must be substituted by a corresponding two dimensional screen having a very large number of -individual electrodes. In this case, the common positive opposing layer 9 is formed as a fine net, the scanning ray I 0 passing through the meshes thereof, While the individual electrodes 8 are formed-as separate conductors, passed through an insulating carrier layer (for instance mica) and ending on the rear side into points standing freely in the meshes of the net 9. When the cathode ray I0 impinges onthese points, the desired secondary emission occurs thereon by means of which the individual charges of the electrodes 8 are momentarily destrayed. The same steps may also be taken, as

is obvious, where 1 represents a single 'contactl line, as above referred to.

'I'he compensating current impulses produced at the discharge of the single elements 8--9 of the contact line or of the contact screen, act across a coupling resistance I5, on the input tube I8 of the image amplier I'I. By means of a suitable circuit care is to be taken that the coupling resistance I5 is passed only by the desired current impulses of the picture point scanning and which are defined by the frequency thereof, while the sum of the lpositive charging currents, released in 9 by the photoelectrons, and whose period is given by the picture line transmission period, is rendered ineffective with respect to the modulation, by the introduction of 'a frequency independent shunt, or in an equivalent manner.

The surfaces 8 impinged by the photoelectrons may themselves be utilized for the scanning by means of the cathode ray IB, the latter then arriving from the front side, and to this end a common auxiliary electrode will be placed opposite the said surfaces, which during the in cidence of the photoelectrons is maintained predominantly negative, while during the incidence of the scanning cathode ray it is maintained highly positive for the purpose of withdrawing by way of suction, the secondary electrons. In

this case it appears however necessary to limit the illumination to a fraction for instance 10% of the transmission period of each line, in order to permit scanning by I 0 during the remaining period of time.

Having thus described the invention, what is claimed and desired to secure by Letters Patent is the following:-

1. In a television transmitter, an evacuated envelope havingV at least one elongated section containing a photoelectric cathode, means responsive to the optical image of the object to be transmitted, reflecting means embracing at least a part of the elongated section of said envelope for directing the image on to said photoelectric means and developing a current image thereof, a double sided photoelectric mosaic containing a plurality Iof discrete photoelectric elements on the side thereof facing said photoelectric cathode, means for focusing the current image on to said photoelectric mosaic whereby a charge image is formed on said mosaic corresponding to the current image, means f or developing a cathode ray beam, means for deiiecting the said beam in at least one coordinate, and means for directing said beam to said double sided mosaic on the side thereof remote from said photoelectric means whereby said beam neutralizes the charge image formed on the photoelectric section of the mosaic.

2. Apparatus in accordance with claim 1 wherein said photoelectric mosaic containing said discrete photoelectric elements comprises a single row of said elements corresponding to one linear section of the optical image to be transmitted.

FRITZ SGHRTER.

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