434,876

PATENT SPECIFICATION

Application Date: Feb. 5, 1934. No. 3776/34.

Complete Specification Left: Dec. 4, 1934.

Complete Specification Accepted: Sept. 5, 1935.

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PROVISIONAL SPECIFICATION

Improvements in and relating to Television and the like Systems

I, ALAN DOWER BLUMLEIN, a British Subject, of 7, Courtfield Gardens, Ealing, London, W.13, do hereby declare the nature of this invention to be as follows:-

The present invention relates to television and the like systems and more particularly to systems employing for transmitting purposes a cathode ray tube provided with a "mosaic" screen.

In one form of tube of this kind, an image to be transmitted is projected on to a "mosaic" of small photo-electric elements insulated from one another and from a conducting back-plate upon which they are carried. The elements are usually deposited upon an insulating layer provided on the back-plate which will be referred to as the signal plate.

Within the tube are provided, in addition to the mosaic screen, a source of electrons, means for directing the electrons in a narrow beam upon the screen, an anode for collecting electrons emitted by the screen and deflecting means for causing the electron beam to scan the screen.

The deflecting means may be in the known form of coils or electrostatic plates which when supplied with electrical oscillations of saw-tooth wave form cause the ray to scan the screen in parallel strips, each scanning stroke being followed by a return stroke.

When the image is cast upon the screen, each element of the "mosaic" emits electrons in proportion to the brightness of its illumination and these electrons pass to the anode. When the cathode ray passes over an element during the scanning operation, the element is charged negatively with respect to the anode and when the ray leaves the element the potential of the element rises due to the emission of photo-electrons to the anode. The charge given to each element by the cathode ray when it passes over the element is therefore dependent upon the amount of photo-electric discharge that has taken place since the last scanning of the element and consequently upon the brightness of illumination of the element.

If a resistance be arranged in series between the signal plate and the anode, there are developed across the resistance potential pulses which are representative of the brightness of illumination of the successively scanned elements of the "mosaic".

So long as the average illumination of the "mosaic" screens remains constant, the average current flowing to the signal plate is zero. When the cathode ray scans an element which has been brightly illuminated, the potential of the signal plate is made negative because of the arrival thereupon of charging electrons which are greater in number than the sum of the secondary electrons emitted owing to bombardment by the electrons of the cathode ray beam and the total photo-electric electrons emitted.

When the cathode ray strikes an element which has not been illuminated since the last scan, no charge is given up by the ray, the number of secondary electrons emitted being equal to the number of electrons arriving; but at this instant the potential of the signal plate will be positive owing to the steady photo-electric discharge occurring from all the elements. Thus the signal generated across the resistance will represent true differences in light and shade between successively scanned elements but will not give any indication of the average illumination.

In order that the average illumination may be transmitted, the D.C. component of the signal may be established at some convenient point, for example in the manner set forth in co-pending Application No. 11204/33. According to the method described in this prior application, the signal is caused at intervals to assume maximum or minimum values corresponding to some fixed absolute light intensity and the D.C. component is inserted at any desired point with reference to these recurring maxima or minima.

These recurrent maxima or minima may be established in the case of a cathode ray system such as that above described by cutting off the cathode ray at the end of each scanning stroke so that there is no bombardment of the "mosaic" screen during the return stroke, that is to say whilst the ray is being returned to one side of the screen after scanning one strip of the image. Thus, during the return stroke, no charge is delivered to the "mosaic." The signal generated at this time on the signal plate will not necessarily be zero, but will depart from zero by an amount depending upon the total steady photo-electric current arising from electrons leaving the "mosaic." The signal, however, corresponds to that generated, during a scanning stroke, when scanning an unilluminated element and therefore corresponds to full black. Consequently by shutting off the cathode ray beam during each return stroke, recurrent minima (or maxima, depending upon the sense chose) of value corresponding to full black are obtained and these minima may be used in the manner described in the co-pending application above referred to, to re-establish the average picture intensity.

In the above discussion it has been assumed that the average brightness of the image remained constant. If, however, the average brightness should change suddenly, the operation of the tube, during the first scanning cycle after the change, will be quite different from that described above. A sudden change of image brightness will cause a sudden change in the photo-electric current from the "mosaic" and this change will cause a momentary additional current in the signal plate circuit. The additional current will be opposite in sense to that associated with the change when considering changes form element to element with the average brightness constant. For example if the image be assumed to be quite dark, then the whole "mosaic" will remain fully charged and no signals will be transmitted during scanning, the conditions during the forward (or scanning) stroke being the same as those during the return stroke when the ray is cut off.

If the "mosaic" now be suddenly illuminated, there will flow from the signal plate a photo-electric current tending to make the signal plate positive by virtue of the resistance in series with it. The signal so generated would normally be associated with a black portion of the screen and therefore this new signal, generated by a sudden increase in average brightness, is in the reverse direction to that normally associated with brighter illumination. In effect the signal represents "blacker than black."

Immediately after the change of illumination, the cathode ray beam will not deliver large charges to the "mosaic" because the increased light will not have been active for long enough to discharge the "mosaic" appreciably. There will not therefore be any marked change of signal during return strokes, when the ray is cut off, and the device for re-establishing D.C. will not operate immediately to change the D.C. component of the signal in such a way as to present the change in brightness that has occurred. At the end of one scanning cycle (when the image has been completely scanned once since the change) normal operating conditions will have re-established themselves: during scanning strokes the signal plate will be driven towards the negative condition by the charge delivered by the cathode ray and during return strokes the signal plate will tend towards positive owing to the steady photo-electric current flowing.

Similar, but rather more dangerous conditions arise when the illumination of the "mosaic" is suddenly reduced. There may first be considered the works case which occurs if the image which is being transmitted has a uniform intensity equal to the brightest white transmitted (referred to as full white) and if this image be replaced suddenly by complete blackness. Before the change all the photo-electric elements are fully discharged by photo-electric current in the intervals between successive scannings by the ray and the ray delivers the maximum charge as it passes over the elements. At the end of each scanning line and during the return stroke this charging action ceases and a large positive pulse is generated on the signal plate owing to the un-neutralised photo-electric discharge of the elements. Th signal generated then consists of small negative values followed by large positive pules constituting the minima which are used for inserting the D.C. component. When this component is inserted the signals consist of substantially steady strong signals representing full white interspersed with returns of short duration to a level corresponding to fully black, corresponding to the return strokes.

When the illumination is suddenly reduced to zero, the photo-electric current component is complete stopped. The elements of the first few strips scanned by the ray after the change are still fully discharged by the previous illumination and in consequence there are delivered to these elements by the ray charges which as there is no photo-electric emission, make the signal plate highly negative. Also during the return strokes there is no photo-electric current to drive the signal plate positive so that no "black" signal, such as was transmitted before the change, is generated. The result for the first instant after the change of illumination is that instead of the full "white" signals falling to "black" level, they rise to a value corresponding to nearly twice full "white" and the "black" signals between strips will rise to a level corresponding nearly to full "white."

Thus the initial effect of reducing maximum brightness of image to zero is to generate a signal corresponding to almost twice full "white," that is to say the transient signals obtained are the reverse of what they should be.

The effect of these transient signals may be either to overload the apparatus, such as amplifiers, radio frequency transmitter or receivers or, if the apparatus is free for overload, to produce a very bright white flash at the receiver at just the moment at which the received picture is intended to become black after being white. Further, the false transient signal is very likely to cause incorrect operation of any device serving to reestablish D.C. either at the transmitter or receiver. Even if no means are provided in the system for re-establishing D.C., the transients produced will probably cause a serious upset of the apparatus.

It is an object of the present invention to provide means whereby the disturbing effect of transients such as above described may be reduced or eliminated.

According to the present invention, a method of transmitting an image of an object comprises forming an image of the object upon a mosaic screen of photo-electric elements, scanning the screen to generate in an output circuit picture currents representative of the light and shade of the object, generating currents representative of the general brightness of the object and combining these latter currents with the picture currents in such a way as to reduce the amplitude of reverse picture currents produced by transient changes in general brightness of the object.

In carrying the invention into effect I may proceed as follows:-

A mosaic screen is mounted within a bulbous portion of a closed glass envelope having a cylindrical neck in which are mounted in electron emitting cathode and suitable electron directing electrodes. The anode is constituted by a silvering of the inside of the envelope in the neighbourhood of the junction between the bulbous and cylindrical portions thereof. The screen is arranged at an angle of about 60° to the axis of the cylindrical portion and therefore to the mean direction of the cathode ray beam. The deflection of the ray over the screen may be effected by means of suitable electromagnets arranged outside the cylindrical part of the tube and fed with currents of saw-tooth wave-from. Outside the envelope is arranged an optical system adapted to project an image of an object such as a photographic film or an actual scene upon the screen. The optical axis of the optical system is arranged to be normal to the screen. An arrangement of this kind is described for example by V. K. Zworykin in an article entitled "Television with Cathode-Ray Tubes" appearing in the Journal of The Institution of Electrical Engineers, Vol. 73, and commencing on page 437.

The signal plate of the mosaic screen may be connected to the grid of an amplifying valve and through a suitable resistance and bias battery or other source of E.M.F. to the cathode of the valve. The cathode of the valve is connected to the anode of the cathode ray tube which is maintained at a suitable positive potential relative to the cathode of the cathode ray tube.

In order to neutralise the transient effects above described, an auxiliary photo-electric cell may have its anode connected to the grid of the amplifying valve and its cathode connected through a suitable bias battery or other source of E.M.F. to the cathode of the valve.

The auxiliary photo-electric cell is arranged to receive light from the whole of the object of which the image is formed upon the mosaic screen. The sensitivity of this cell is arranged to be such that, with sudden changes of illumination of the object, the transient voltages developed across the resistance by the signal plate and by the auxiliary cell are substantially equal and opposite.

It may be noted that with the arrangement described the pulse generated and impressed upon the grid of the amplifying valve during return strokes of the cathode ray, when the ray is cut off, will correspond to full black and will therefore be suitable for use in the re-insertion of the D.C. component when required. This is because the positive voltage produced upon the grid by the signal plate owing to the photo-electric emission from all the illuminated elements of the "mosaic" will be equal and opposite to the voltage produced by the auxiliary cell whatever may be the brightness of illumination of the object.

The relative sensitivities of the photo-cell constituted by the "mosaic" screen and signal plate and the auxiliary cell may be adjusted in any known or suitable manner, for example by connecting either the signal plate or the anode of the auxiliary cell or both to a variable tapping point on the resistance or by adjusting the relative amounts of light falling upon the two devices. The correct adjustment may be determined by causing the brightness of illumination of the object to vary rapidly and so adjusting the relative sensitivities that the signals obtained correctly represent the changes in illumination.

For very rapid alternation of illumination, such as an illumination which changes at a frequency much greater than the frame frequency, no appreciable output signal should be produced. No transient having a duration less than the time of one complete scanning cycle can be accurately transmitted, although a very intense transient such as an electric spark discharge may be transmitted owing to it causing a rapid discharge of the mosaic elements and being therefore transmitted in the form of a flash of lower intensity and longer duration.

When transmitting pictures from film, the successive frames being projected one at a time upon the "mosaic," it is possible to move the film forwards during the scanning of the "mosaic." If this is done with an uncompensated system, however, the sudden change in illumination of the mosaic owing to the interposition of a shutter or even, if no shutter is used, owing to the black strip between frames, produces an undesired transient in the middle of the picture signals. With the aid of the compensating arrangement of the present invention, however, the film may be moved forward during the scanning cycle without the generation of a harmful transient and more time is therefore available for moving the film than the interval between the transmission of two successive frames and these intervals may be made as short as practicable.

The auxiliary cell may be housed at one end of a rectangular box having its opposite end open and having its interior blackened. The open end of the box is so arranged that the cell receives light from the same area as that which illuminates the "mosaic." It is not necessary to focus an image of the object upon the cell, in fact this is usually undesirable because the cathode of the auxiliary cell may not be of uniform sensitivity over its surface. When the object is a picture film, light from a source additional to that used for illuminating the "mosaic" may be passed through the film on to the auxiliary cell.

As an alternative to the arrangements described above, the auxiliary photo-electric cell may be mounted in the envelope of the cathode ray tube.

The output from the "mosaic" cell and the auxiliary cell may be amplified separately and combined in opposition at a later stage. It is desirable in such an arrangement either that the time constants of the coupling or couplings in the amplifiers be made greater than 1/25th of a second (where 25 pictures are transmitted per second) or that the time constants of the two amplifiers should be made substantially equal to one another.

Although the invention has been described in some detail with reference to a particular form of cathode ray tube, it is also applicable to other forms of "mosaic" type tubes which tend, on a change of illumination of the object, to generate a transient having a sense opposite to that normally corresponding to that change of illumination.

Further the invention is not limited to system in which the D.C. component is retained or re-inserted before transmission of the signal. It can also be applied to systems in which the D.C. component is suppressed or at least not utilised.

 

Dated this 5th day of February, 1934.

REDDIE & GROSE,

Agents for the Applicants,

6, Bream’s Buildings, London, E.C.4.

COMPLETE SPECIFICATION

Improvements in and relating to Television and the like Systems

I, ALAN DOWER BLUMLEIN, a British Subject, of 7, Courtfield Gardens, Ealing, London, W.13, do hereby declare the nature of this invention and in what manner the same is to be performed, to be particularly described and ascertained in and by the following statement:

The present invention relates to television and the like systems and more particularly to systems employing for transmitting purposes a cathode ray tube provided with a "mosaic" screen.

In one form of tube of this kind, an image to be transmitted is projected on to a "mosaic" of small photo-electric elements insulated from one another and from a conducting back-plate upon which they are carried. The elements are usually deposited upon an insulating layer provided on the back-plate which will be referred to as the signal plate.

Within the tube are provided, in addition to the mosaic screen, a source of electrons, means for directing the electrons in a narrow beam upon the screen, an anode for collecting electrons emitted by the screen and deflecting means for causing the electron beam to scan the screen.

The deflecting means may be in the known form of coils or electrostatic plates which when supplied with electrical oscillations of saw-tooth wave form cause the ray to scan the screen in parallel strips, each scanning stroke being followed by a return stroke.

When the image is cast upon the screen, each element of the "mosaic" emits electrons, and accordingly rises in potential, in proportion to the brightness of its illumination and these electrons pass to the anode. What can be regarded as an electrostatic image is thus formed on the "mosaic" by the electric charges on the elements thereof. When the cathode ray passes over an element during the scanning operation, the charge thereon is neutralised thus bringing the element to a datum potential which is negative with respect to the anode potential. When the ray leaves the element the potential of the element again rises due to the emission of photo-electrons to the anode. The neutralising charge given to each element by the cathode ray when it passes over the element is therefore dependent upon the amount of photo-electric discharge that has taken place since the last scanning of the element and consequently upon the brightness of illumination of the element.

The operation of the device described above may, in practice, be more complex than that given but the simple explanation set forth is believed to be sufficient for the understanding of the present invention. The present invention can be carried into effect without exact knowledge of the physical method of operation of the device.

If a resistance be arranged in series between the signal plate and the anode of the device above described, there are developed across the resistance potential pulses which are representative of the brightness of illumination of the successively scanned element of the "mosaic."

So long as the average illumination of the "mosaic" screen remains constant, the average current flowing to the signal plate is zero. When the cathode ray scans an element which has been brightly illuminated, the potential of the signal plate is made negative because of the arrival thereupon of charging electrons which are greater in number than the sum of the secondary electrons emitted owing to bombardment by the electrons of the cathode ray beam and the total photo-electric electrons emitted.

When the cathode ray strikes an element which has not bee illuminated since the last scan, no charge is given up by the ray, the number of secondary electrons emitted being equal to the number of electrons arriving; but at this instant the potential of the signal plate will be positive owing to the steady photo-electric discharge occurring from all the elements. Thus the signal generated a cross the resistance will represent true differences in light and shade between successively scanned elements but will not give any indication of the average illumination.

In order that the average illumination may be transmitted, the D.C. component of the signal may be established at some convenient point, for example in the manner set forth in the Specification of Patent No. 422,906. According to the method described in this prior Specification, the signal is caused at intervals to assume maximum or minimum values differing from picture "black" by a fixed amount but lying outside the range of picture signals and the D.C. component is inserted at any desired point with reference to these recurring maxima or minima.

These recurrent maxima or minima may be established in the case of a cathode ray system such as that above described by cutting off the cathode ray at the end of each scanning stroke so that there is no bombardment of the "mosaic" screen during the return stroke, that is to say whilst the ray is being returned to one side of the screen after scanning one strip of the image. Thus, during the return stroke, no charge is delivered to the "mosaic." The signal generated at this time on the signal plate will not necessarily be zero, but will depart from zero by an amount depending upon the total steady photo-electric current arising from electrons leaving the "mosaic." The signal, however, corresponds to that generated, during a scanning stoke, when scanning an unilluminated element and therefore corresponds to full black. Consequently by shutting off the cathode ray beam during each return stroke, recurrent signals of value corresponding to full black are obtained. A pulse in the "blacker than black" sense may be superimposed upon each of the black signals and the recurrent minima so obtained may be used in the manner described in the prior Specification above referred to, to re-establish the average picture intensity.

The pulses in the "blacker than black" sense may also be used to control the generation of the saw-tooth scanning oscillations and, by applying them to a modulator electrode in the cathode ray tube, they also serve to cut off the ray during return strokes.

In the above discussion it has been assumed that the average brightness of the image remained constant. If, however, the average brightness should change suddenly, the operation of the tube, during the first scanning cycle after the change, will be quite different from that described above. A sudden change of image brightness will cause a sudden change in the photo-electric current from the "mosaic" and this change will cause a momentary additional current in the signal plate circuit. The additional current will be opposite in sense to the associated with the change when considering changes for element to element with the average brightness constant. For example if the image be assumed to be quite dark, then the whole "mosaic" will remain fully charged and no signals will be transmitted during scanning, the conditions during the forward (or scanning) stroke being the same as those during the return stroke when the ray is cut off.

If the "mosaic" now be suddenly illuminated, there will flow from the signal plate a photo-electric current tending to make the signal plate positive by virtue of the resistance in series with it. The signal so generated would normally be associated with a black portion of the screen and therefore this new signal, generated by a sudden increase in average brightness, is in the reverse direction to that normally associated with brighter illumination. In effect the signal represents "blacker than black".

Immediately after the change of illumination, the cathode ray beam will not deliver large charges to the "mosaic" because the increased light will not have been active for long enough to discharge the "mosaic" appreciably. There will not therefore be any marked change of signal during return strokes, when the ray is cut off, and the device for re-establishing D.C. will not operate immediately to change the D.C. component of the signal in such a way as to represent the change in brightness that has occurred. At the end of one scanning cycle (when the image has been completed scanned once since the change) normal operating conditions will have re-established themselves: during scanning strokes the signal plate will be driven towards the negative condition by the charge delivered by the cathode ray and during return strokes the signal plate will tend towards positive owing to the steady photo-electric current flowing.

Similar, but rather more dangerous conditions arise when the illumination of the "mosaic" is suddenly reduced. There may first be considered the worst case which occurs if the image which is being transmitted has a uniform intensity equal to the brightest white transmitted (referred to as full white) and if this image be replaced suddenly by complete blackness. Before the change all the photo-electric elements are fully discharged by photo-electric current in the intervals between successive scannings by the ray and the ray delivers the maximum charge as it passes over the elements. At the end of each scanning line and during the return stroke this charging action ceases and a large positive pulse is generated on the signal plate owing to the un-neutralised photo-electric discharge of the elements. The signal generated then consists of small negative values followed by large positive pulses constituting the minima which are used for inserting the D.C. component. When this component is inserted the signals consist of substantially steady strong signals representing full white interspersed with returns of short duration to a level corresponding to full black, corresponding to the return strokes.

When the illumination is suddenly reduced to zero, the photo-electric current component is completely stopped. The elements of the first few strips scanned by the ray after the change are still fully discharged by the previous illumination and in consequence there are delivered to these elements by the ray charges which, as there is no photo-electric emission, make the signal plate highly negative. Also during the return strokes there is no photo-electric current to drive the signal plate positive so that no "black" signal, such as was transmitted before the change, is generated. The result for the first instant after the change of illumination is that instead of the full "white" signals falling to "black" level, they rise to a value corresponding to nearly twice full "white" and the "black" signals between strips will rise to a level corresponding nearly to full "white".

Thus the initial effect of reducing maximum brightness of image to zero is to generate a signal corresponding to almost twice full "white", that is to say the transient signals obtained are the reverse of what they should be.

The effect of these transient signals may be either to overload the apparatus, such as amplifiers, radio frequency transmitter or receivers or, if the apparatus is free from overload, to produce a very bright white flash at the receiver at just the moment at which the received picture in intended to become black after being white. Further, the false transient signal is very likely to cause incorrect operation of any device serving to re-establish D.C. ether at the transmitter or receiver. Even if no means are provided in the system for re-establishing D.C., the transients produced will probably cause a serious upset of the apparatus.

It is an object of the present invention to provide means whereby the disturbing effect of transients such as above described may be reduced or eliminated.

The present invention provides a method of transmitting an image of an object comprising the steps of projecting said image upon a photo-sensitive screen to produce electron emission therefrom and to form an electrostatic image of said object, scanning the electrostatic image to generate, in an output circuit, picture currents representative of the light and shade of the object generating, with the aid of an auxiliary light sensitive device, currents representative of the general brightness of the object and combining these latter currents with the picture currents in such a way as to reduce the amplitude of reverse picture currents produced by transient changes in general brightness of the object, the auxiliary light sensitive device being exposed to light from said object, other than light reflected from said screen.

The present invention also provides a method of television transmission in which an image of an object to be transmitted is projected upon a photo-sensitive screen to produce electron emission therefrom, thereby producing an electrostatic image of said object, and in which, during said projection, the electric charges constituting said electrostatic image are periodically neutralised by scanning, the present invention being characterised in that the amplitude of undesired impulses produced in the picture signal circuit by sudden changes in the average intensity of the light falling upon said screen is reduced by developing and superimposing upon said undesired impulses, corrective impulses of opposite sense to said undesired impulses.

The present invention also provides apparatus adapted for use in carrying out the methods above set forth.

The invention will be described with reference to the accompanying diagrammatic drawing.

Referring to the drawing, a mosaic screen 1 is mounted within a bulbous portion of a closed glass envelope 2 having a cylindrical neck 3 in which are mounted an electron emitting cathode 4 and suitable electron directing electrodes 5. The anode is constituted by a silvering 6 of the inside of the envelope 2 in the neighbourhood of the junction between the bulbous and cylindrical portions thereof. The screen 1 is arranged at an angle of about 60° to the axis of the cylindrical portion and therefore to the mean direction of the cathode ray beam. The deflection of the ray over the screen may be effected by means of suitable electromagnets (not shown) arranged outside the cylindrical part of the tube and fed with currents of saw-tooth wave-form. Outside the envelope is arranged an optical system 7 adapted to project an image of an object 8 upon the screen 1. The optical axis of the optical system is arranged to be normal to the screen. An arrangement of this kind is described for example by V. K. Zworykin in an article entitled "Television with Cathode-Ray Tubes" appearing in the Journal of the Institution of Electrical Engineers, Vol. 73, and commencing on page 437.

The signal plate of the mosaic screen 1 may be connected to the grid of an amplifying valve 9 and through a suitable resistance 10 and bias battery 11 or other source of E.M.F. to earth and to the cathode of the valve 9. The cathode of the valve 9 is connected to the anode of the cathode ray tube which is maintained at a suitable positive potential relative to the cathode of the cathode ray tube by means of a battery or other source 12.

In order to neutralise the transient effects above described, an auxiliary photo-electric cell 13 is arranged with its anode 14 connected to a tapping point on the resistance 10 and its cathode connected through a suitable bias battery 15 or other source of E.M.F. to earth.

The auxiliary photo-electric cell 13 is arranged in a box 16 which is closed excepting for an aperture 17 through which the cell 13 receives light from the whole of the object 8. The interior of the box may be blackened. The sensitivity of this cell 13 and the adjustment of the tapping point on the resistance 10 (which adjustment determines the relative effective sensitivities of the photo-cell 13 and the mosaic screen) are arranged to be such that, with sudden changes of illumination of the object 8, the transient voltages developed across the resistance 10 by the signal plate of the mosaic screen 1 and by the auxiliary cell 13 are substantially equal and opposite.

It may be noted that with the arrangement described the pulse generated and impressed upon the grid of the amplifying valve 9 during return strokes of the cathode rya, when the ray is cut off, will correspond to full black and will therefore be suitable for use in the re-insertion of the D.C. component when required. This is because the positive voltage produced upon the grid of the valve 9 by the signal plate owing to the photo-electric emission from all the illuminated elements of the "mosaic" will be equal and opposite to the voltage produced by the auxiliary cell 13 whatever may be the brightness of illumination of the object.

The relative effective sensitivities of the photo-cell constituted by the "mosaic" screen and signal plate and the auxiliary cell may be adjusted in any other know or suitable manner, for example by adjusting the relative amounts of light falling upon the two devices with the aid of suitable adjustable diaphragm stops. The correct adjustment may be determined by causing the brightness of illumination of the object to vary rapidly and so adjusting the relative sensitivities that the signals obtained correctly represent the changes in illumination.

For very rapid alternation of illumination, such as an illumination which changes at a frequency much greater than the frame frequency, no appreciable output signal should be produced. No transient having a duration less than the time of one complete scanning cycle can be accurately transmitted, although a very intense transient such as an electric spark discharge may be transmitted owing to it causing a rapid discharge of the mosaic elements and being therefore transmitted in the form of a flash of lower intensity and longer duration.

When transmitting pictures from motion picture film, the successive frames being projected one at a time upon the "mosaic", it is possible to move the film forwards during the scanning of the "mosaic". If this is done with an uncompensated system, however, the sudden change in illumination of the mosaic owing to the interposition of a shutter or even, if no shutter is used, owing the black strip between frames, produces an undesired transient in the middle of the picture signals. With the aid of the compensating arrangement of the present invention, however the film may be moved forward during the scanning cycle without the generation of a harmful transient and more time is therefore available for moving the film than the interval between the transmission of two successive frames and these intervals may be made as short as practicable.

It is not necessary to focus an image of the object 8 upon the auxiliary cell 13, in fact this is usually undesirable because the cathode of the auxiliary cell may not be of uniform sensitivity over its surface. When the object is a picture film, light from a source additional to that used for illuminating the "mosaic" may be passed through the film on to the auxiliary cell.

As an alternative to the arrangements described above, the auxiliary photo-electric cell may be mounted in the envelope of the cathode ray tube.

The output from the "mosaic" cell and the auxiliary cell may be amplified separately and combined in opposition at a later stage. It is desirable in such an arrangement either that the time constants of the coupling or couplings in the amplifiers which determine the lower cut-off frequency be made greater than 1/25th of a second (where 25 pictures are transmitted per second) or that the time constants of the two amplifiers should be made substantially equal to one another.

Although the invention has been described in some detail with reference to a particular form of cathode ray tube, it is also applicable to other forms of "mosaic" type tubes which tend, on a change of illumination of the object, to generate a transient having a sense opposite to that normally corresponding to that change of illumination.

Further the invention is not limited to systems in which the D.C. component is retained or re-inserted before transmission of the signal. It can also be applied to systems in which the D.C. component is suppressed or at least not utilised.

Patent Specification No. 431,258 and 431,207 which relate to inventions similar to that forming the subject of the present Application were not published at the date of the present Application although they bear an earlier dated.

In Patent Specification No. 431,258 there are described and claimed a method of and apparatus for television or like transmission in which undesired impulses produced in a picture signal circuit by sudden changes in average illumination of a mosaic screen structure, upon which the images to be transmitted are projected, are cancelled out or substantially reduced in amplitude with the aid of corrective impulses of opposite sense to the undesired impulses.

In Patent Specification No. 431,207 there are described and claimed a method of and apparatus for television transmission in which an image is projected upon a screen structure and is removed before the screen structure is scanned. According to a feature of this specification the undesirable effect of the disturbing impulse produced when each image is projected upon the screen structure is substantially removed with the aid of an electrical impulse of opposite polarity to the disturbing impulse.

Neither of these specifications make specific mention of correcting for undesired impulses developed in consequence of changes in average brightness of the image in cases where scanning takes place during the project of an image on the screen structure. Further, both of the specifications above referred to make specific mention of the use, for developing corrective impulses, or an auxiliary photo-sensitive device exposed to light reflected from the active surface of the screen structure and neither specification contains the proposal to expose the auxiliary photo-sensitive device to light from the object other than that reflected from the screen structure.

Having now particularly described and ascertained the nature of may said invention and in what manner the same is to be performed, I declare that what I claim is:-

  1. A method of transmitting an image of an object comprising the steps of projecting said image upon a photo-sensitive screen to produce electron emission therefrom and to form an electrostatic image of said object, scanning the electrostatic image to generate, in an output circuit, picture currents representative of the light and shade of the object, generating, with the aid of an auxiliary photo-sensitive device exposed to light from said object, currents representative of the general brightness of the object and combining these latter currents with the picture currents in such a way as to reduce the amplitude of reverse picture currents produced by transient changes in general brightness of the object, the auxiliary light sensitive device being exposed to light from said object, other than light reflected from said screen.
  2. A method of transmitting an image of an object comprising the steps of forming an image of the object upon the photo-electric surface of a screen, scanning the screen to generate, in an output circuit, picture currents representative of the light and shade of the object, generating, with the aid of an auxiliary light sensitive device, currents representative of the general brightness of the object and combining these latter currents. With the picture currents in such a way as to reduce the amplitude of reverse picture current produced by transient changes in general brightness of the object, the auxiliary light sensitive device being exposed to light from said object, other than light reflected from said screen.
  3. A method of television transmission in which an image of an object to be transmitted is projected upon a photo-sensitive screen to produce electron emission therefrom, thereby producing an electrostatic image of said object, and in which, during said projection, the electric charges constituting said electrostatic image are periodically neutralise by scanning, characterised in that the amplitude of undesired impulses produced in the picture signal circuit by sudden changes in the average intensity of the light falling upon said screen is reduced by developing and superimposing upon said undesired impulses, corrective impulses of opposite sense to said undesired impulses.
  4. A method of television transmission in which an image to be transmitted is projected upon a mosaic photo-electric screen and in which, during said projection, the potentials of the mosaic elements are periodically brought to a fixed value by scanning, characterised in the amplitude of undesired impulses produced in the picture signal circuit by sudden changes in the average intensity of the light falling upon said screen is reduced by developing and superimposing upon said undesired impulses, corrective impulses of opposite sense to said undesired impulses.
  5. Apparatus for transmitting an image of an object, said apparatus comprising means for forming an image of the object upon a photo-sensitive screen, the electrons emitted by said screen causing an electrostatic image of said object to be formed, scanning means for periodically neutralising the electric charges constituting said electrostatic image, an output circuit for receiving picture signals generated during the scanning process, an auxiliary light sensitive device arranged to receive light, other than light reflected from said screen, simultaneously from the whole of the object and means for applying corrective signals generated in said auxiliary device to said output circuit in opposition to said picture signals.
  6. Apparatus according to claim 5, in which means are provided for adjusting the relative amplitudes of said picture signals and said corrective signals in said output circuit.
  7. Apparatus according to claim 6, wherein means are provided for adjusting the relative illumination of said auxiliary device and said screen.
  8. Apparatus according to any of claims 5 to 7, in which the object to be transmitted is a film and in which the film is moved forward during the scanning of the electrostatic image.
  9. Apparatus according to any of claims 5 to 8, wherein the object to be transmitted is a film and wherein light is passed through the film on to the auxiliary light-sensitive device from a source separate from that used to form an image of the film upon said screen.
  10. Apparatus according to any of claims 5 to 9, wherein the auxiliary light sensitive device is arranged with said screen in a single evacuated envelope.
  11. Apparatus according to any of claims 5 to 10, wherein said screen is arranged within an evacuated envelope containing means for generating a cathode ray beam and means for seeping the beam over the electrostatic image for scanning purposes.
  12. Apparatus according to any of claims 5 to 11, wherein said screen is composed of mutually insulated photo-electrically sensitive elements, the electric charges produced in these elements by photo-emission constituting said electrostatic image.
  13. A method of or means for transmitting images of objects, substantially as described.

Dated this 4th day of December, 1934.

REDDIE & GROSE,

Agents for the Applicants,

6, Bream’s Buildings, London, E.C.4.

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Leamington Spa: Printed for His Majesty’s Stationery Office, by the Courier Press. - 1935.