US2342521A - Photoelectric signal amplifier - Google Patents

Description

Feb. 22, 1944.

K. THGM PHOTQELECILRIC SIGNAL AMPLIFIER Y -Filfad July 16, 1940 XXX /NVE/V TOR y K. THOM ATTORNEY Patented Feb. .22, 1944 STATES ersr OFFlCE PHOTOELEC'I'RIC SIGNAL AMPLIFIER Kurt Thtirn, Berlin I-ichtertelde, Germany; vested in the Alien Property Custodian Application July 16, 1940, Serial No. 345.82%

Germany July 20, 1939 Claims.

values giving a natural and true representation of the original image or scene. It is a further object to improve the type of transmitter known as light spot scanner which is particularly useful for the two way transmission of images of persons or objects. A transmitter of this type contains usually a plurality of photocells having a sensitivity versus wave length distribution corresponding to the effective light emission of the movable spot of light which is produced, for example by a' cathode ray tube, a vapour lamp or an. incandescent lamp. If for example incaudescent lamps are used it is preferable to employ a photocell having the highest sensitivity in the red part of the spectrum. The image on the receiving side however makes then a chalky, flat and lifeless impression because halt-tones are not sufilciently reproduced. It has been tried to overcome this drawback by employing simultaneously different types of photocells having sensitivities in the red and in the blue part of the spectrum. This however does not produce the desired effect because the source of light which is used for the scanning process radiates either more red ormore blue light so that a good efficiency. for both photoelectric cells cannot be obtained. A further drawback or this arrangement is that the life and the primary sensitivity of the different photocells varies considerably.

It is an object of the present invention to overcome these and other difiiculties by means of a special amplifier arrangement in connection with a number of photocells having the same properties with regard to sensitivity and spectral range. According to the invention the transmission is effected by means of a number of photocells having the same properties and by connecting each photocell to an amplifier having an amnlitude characteristic and a band Width which is different from that of the remaining amplifiers. By regulating separately the different amplifiers the character of the image and the quality of reproduction with respect to the gradation can be varied within wide limits and brought to an optimal value.

Other aspects of my invention will he appareat or will be specifically pointed out in the description forming a part of this specification, but if do not limit myself to the embodiment of the invention herein described, as various forms may be adopted within the scope of the claims.

Referring to the drawing Fig. 1 shows an embodiment of the invention with two photoelectric cells,

Fig. 2 shows the characteristic curvesof the amplifiers arranged according to Fig. 1, while Figs. 3 and t show other circuit arrangements in accordance with the invention.

The embodiment of Fig. 1 contains two photoelectric cells I and 2 connected each to a pream- 4 pllfier 3 and t. The output circuits of the preamplifiers are connected to a further common amplifier 5. The photoelectric cells I and 2 are of the same type and have substantially the same sensitivity in the same ranges of the spectrum. In the present case cells using secondary emission amplification are employed. The preamplifiers 3 and l are different from one another on account of their amplification factor as a function of amplitude and frequency band width. Fig. 2 represents the properties of these amplifiers in a diagram showing the amplitude a of the signal as a function of the brightness b of the original image. The light signal is amplified linearly in the photoelectric cells l and 2. Curve 6 shows the amplification obtained by preamplifier 3 in which the bright lights are amplified less than the small values of brightness. Curve 1 shows the linear curve of preamplifier 4 producing an undistorted amplification so that the signals are proportional to the original brightness. The combination of the two amplifier outputs results in a signal producing. a distribution of brightness which appears natural and true to the eye. The use. of two separate amplifiers makes it possible to transmit the amplitudes of small brightness values with a smaller band width than the amplitudes of large brightness values. This effect cannot be obtained by an amplifier having a characteristic correspond ing to the combined characteristic of amplifier 3 and d. The use of amplifiers having difierent band widths allows a considerable lowering of the shot effect for smaller values of brightness. This is important because in consequence of the high amplification of small amplitudes according to curve the shot noise would be amplified to an undesired degree. The band width of this amplifier can be made smaller because the range of smaller brightness contains only small differences in brightness so that no high frequencies are present which would be necessary for a good reproduction of the image.

It is therefore a feature of the invention that photoelectric cells 6 has a smaller band width than the linear preamplifier 4.

In the embodiment of Figs. 3 and 4 the two photocells I and 2 contain secondary emission electrodes and have themselves different amplitying properties. Two different ways of achieving this object are represented. According to Fig. 3 the potential dividers II and I! of the secondary emission photocells are so dimensioned that the currents flowing through the potential dividers differ from one another. The current in the ar rangement ll of the photocell I is for instance. 0.5 mA. and that in the arrangement I2 of photoelectric cell the potential dividers When a bright image point is transmitted corresponding to a high amplitude the voltage of photoelectric cell I is reduced so that the amplifica- 2 is 10 mA. The resistance values of 18 II and I2 are different.

tion factor also goes down. The output resistance 20 8 is made larger than the output resistance 9 of photoelectric cell 2, in order to obtain the same total amplitude in both circuits. The decrease of band width is obta ned by the larger anode resistance 8 of photoelectric cell I and this has a favourable influence upon the shot noise. The potential of the potential divider l2 of photoelectr c cell 2 does not break down so that the amplification factor remains constant. The photoelectric cell 2 will producing an image on the receiving side having maximal definition and sharp contrasts while the circuit of photoelectric cell I produces signals corresponding to the lower brightness values havtherefore deliver impulses 80 ing a relatively small contrast. Experiments have 85 shown that in this case the eye has the impression of a correct distribution of brightness values. The preamplifiers I4 and I5 of this arrangement may both have a linear characteristic curve.

Fig. 4 shows a simplified arrangement. Both photoelectric cells I and 2 are connected to the same potential divider I3. In order to regulate the amplification of photoelectric cell I, one or more of the leads connecting the grids of the secondary emission amplifier with the potential divider I3 contain adjustable resistances ill allowing an adjustment of the potentials between single electrodes of the secondary emission amplifier and therefore of th total characteristic of this amplifier, so that the eifect described in connection with Fig. 3 is obtained. Alsojin this case the output resistance 8 of photoelectric cell I is made larger than the output resistance 9 of photoelectric cell 2 in order to obtain the same total amplitude in both circuits.

The arrangement can be modified in various directions. The arrangement containing two can be employed in connection with two way television transmitting booths. Each individual cell hitherto used in the booth may be replaced by an arrangement as represented in Figs. 1, 3 or 4. It lies also within the scope of the invention to place the photoelectric cells I and 2 on difierent sides of the scanning 6 light ray.

The invention is not limited to the described embodiments. It can be used in connection with other types of television transmitting devices, for

example, in connection with the amplifiers of dissector tubes in order to influence the gradation of the image.

What I claim is:

1. A television signal amplifier arrangement including a plurality of photoelectric cells with v a second amplifier connected to the collecting photoelectric cells having substantially equal properties and construction, and a voltage divider for each photoelectric cell, said voltage dividers having diflerent resistance values.

-2. A television signal amplifier arrangement including a plurality of photoelectric cells with secondary emission multiplier electrodes, said photoelectric cells having substantially the same properties and construction, a common voltage divider for all photoelectric cells, a plurality of adjustable resistances in the leads connecting the tapping points of the voltage divider with a corresponding plurality of secondary emission elec trodes of one of said cells while the corresponding electrodes of the other of said cells are connected directly to said tapp points.

3. In a television system, a photoelectric electron multiplier having a plurality of multiplying stages, a second photoelectric electron multiplier substantially like said first multiplier, a voltage divider comprising a resistance having voltage taps corresponding to said multiplying stages, means to connect the taps of said voltage divider to the stages of said first multiplier, means to .connect the same taps of said voltage divider to the corresponding stages of said second multiplier comprising a variable resistance connected in at least one of said connections, and a common circuit associated with said multipliers forthe current resulting from concurrently energizing both said multipliers from the same succession of elemental areas.

4. In a television system, a photoelectric electron multiplier having a plurality of multiplying electrodes and a collecting electrode, a second photoelectric electron multiplier substantially identical with said first-mentioned multiplier, an amplifier connected to the collecting electrode of said first multiplier, said amplifier being adapted to amplify currents corresponding to large light values on said first multiplier to a lesser degree than currents corresponding to small light values,

electrode of said second multiplier, said second amplifier being adapted to amplify currents corresponding to both large and small light values linearly, and a third amplifierconnected to both said first and second amplifiers for concurrently amplifying currents received from both said amplifiers.

5. In a television system, a photoelectric electron multiplier having a plurality of multiplying electrodes and 9. collecting electrode, a second photoelectric electron multiplier substantially identical with said first-mentioned multiplier, an amplifier connected to the collecting electrode of said first multiplier, said amplifier being adapted to amplify currents corresponding to large light values on said first multiplier to a lesser degree than currents corresponding to small light values, a second amplifier connected to the collecting electrode of said second multiplier, said second amplifier being adapted to amplify currents cor responding to both large and small light values 0 linearly, said first amplifier being designed to transmit a narrower band of frequencies than said second amplifier, and a third amplifier connected to both said first and second amplifiers for concurrently amplifying currents received from both said amplifiers.

6. In a television system, a photoelectric electron multiplier having a plurality of multiplying electrodes, a voltage divider for said multiplier having voltage taps connected to said multiplysecondary" emission multiplier electrode aid ing electrodes, the current in said voltage diof the order of mA., means to impress the same voltageon both said voltage divideraand means to impress the signaling current from both said multipliers on a common circuit.

7. In a television system, a photoelectric electron multiplier having a plurality of multiplying electrodes, a voltage divider for said multiplier having voltage tam connected to said multiplying electrodes, the current in said voltage divider being of the order of 0.5 mA., a, second photoelectric electron multiplier having the same number of multiplying stages as said first multiplier, a voltage divider for said second multiplier having corresponding voltage taps connected to corresponding multiplying electrodes, the current in said second voltage divider being of the order of 10 mA., means to impress the same voltage on both said voltage dividers. linear amplifiers indi vidual to said multipliers connected to said multipliers to amplify signaling current therefrom, and means to impress the signaling current from said amplifiers on a common circuit.

8. In a television system, a photoelectric electron multiplier having a plurality of multiplying electrodes, a voltage divider for said multiplier having voltage taps connected to said multiplying electrodes, the current in said voltage divider being of the order of 0.5 mA., a second photoelectric electron multiplier having the same number oi multiplying stages as said first multiplier, a voltage divider for said second multiplier having corresponding voltage taps connected to cor-- responding multiplying electrodes, the current in said second voltage divider-being of the order of 10 mA., means to impress the same voltage on both said voltage dividers, a linear amplifier having a large input coupling resistance connected to the output of said first multiplier, a second linear amplifier having a smaller input coupling resistance connected to the output of said second multiplier, said coupling resistances being so proportioned that the maximumamplitudes oi signaling currents in both circuits is the same and the band width of signaling current is less in said first amplifier than in said second amplifier, and means to impress the signaling currents from both said amplifiers on a common circuit.

9. In a television system, a first photoelectric electron multiplier having a photoelectric cathode, a plurality of secondary electron emissive electrodes, a collecting electrode and a retarding electrode, a second photoelectric electron multiplier substantially identical with said first multiplier, a voltage divider having voltage taps supplying progressively increasing voltages from negative to positive, connections from the most negative tap to both said photoelectric cathodes, connections of negligible resistance from succeeding positive taps to corresponding secondary electron emissive electrodes of both multipliers, a connection including a large resistance from the most positive tap to said collecting electrode of said first multiplier, a connection including a smaller resistance from said most positive tap to said collecting electrode of said second multiplier, V a connection including a variable resistance from the next to the most positive tap to the retarding electrode of said first multiplier, a connection of negligible resistance fromsaid same nextto the most positive tap to the retarding electrode of said second multiplier, a variable resistance included in the connection to the secondary electron emissive electrode of said first multiplier which is adjacent to said collecting electrode, linear amplifiers coupled to said large and small resistances respectively, and means to impress signaling current from both said amplifiers on a common circuit;

10. In a television system, a first circuit comprising a photoelectric electron multiplier having a plurality of multiplying electrodes and a collecting electrode and an amplifier connected to the collecting electrode of said multiplier, said circuit being adapted to amplify currents corresponding to large light values on said multiplier to a lesserdegree than currents corresponding to small light values, a second circuit comprising a second photoelectric electron multiplier also having a plurality of multiplying electrodes and a collecting electrode and a second amplifier connected to the collecting electrode of said second multiplier, said second circuit being adapted to amplify currents corresponding to both large and small light values linearly, said first amplifier being designed to transmit a narrower band of frequencies than said second amplifier, and a common circuit additively associated with both said first and second amplifiers to receive current resulting from concurrently energizing both said multipliers from the same succession oi elemental areas.

KURT mom.

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