446,661

PATENT SPECIFICATION

Application Date: Aug. 3, 1934. No. 22724/34.

Complete Specification Left: Aug. 2, 1935.

Complete Specification Accepted: May 4, 1936.

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PROVISIONAL SPECIFICATION

Improvements in or relating to Television Transmitting Systems

We, ALAN DOWER BLUMLEIN, a British Subject, of 7, Courtfield Gardens, Ealing, London, W.13, and JAMES DWYER McGEE, a British Subject, of 9, King’s Avenue, Ealing, London, W.5, do hereby declare the nature of this invention to be as follows:-

The present invention relates to television transmitting systems.

A television transmitting system is known in which an optical image of the object to be transmitted is projected upon a mosaic screen of photo-electrically active elements and the photo-electric surface of the screen is scanned by a cathode ray. The mosaic screen consists of a multiplicity of elements which are insulated from each other and from a common signal plate, each element forming, with the signal plate, a small condenser. In between successive scans, each elemental condenser acquires a charge determined by the number of photo-electrons emitted since the last scan, that is to say, a charge determined by the intensity of the light falling upon the element and at each scan each elemental condenser is discharged. Picture signals are derived, in an external circuit associated with the common signal plate, from the electric impulses capacity-fed to the signal plate. Such a system consists of the following essential features:

  1. A mosaic screen having a large number of small photo-electric elements, each element having capacity to a collection electrode or signal plate.
  2. Means for projecting an optical image of the object to be transmitted on to these elements.
  3. Means for applying polarising potentials to these elements, thus causing the capacities to be charged (or discharged) in accordance with the brightness of the images.
  4. A switching means such as a cathode ray or other electrical "switch" which periodically discharges (or charges) the condensers, bringing the photo-electric elements to a definite potential. The discharging (or charging) currents so produced developing the required picture signals.

Two methods of forming the photo-electric elements are known. Firstly the elements may be formed of small particles of metal insulated from a plate and lying on one side of the plate. In this case, the light is thrown on to the same side of the plate as that with which the cathode ray or electrical switch co-operates and such a plate may be called a "single sided mosaic screen." Secondly, the elements may be formed on a grid or open work plate form which they are insulated, or may be formed of rivets projecting through a plate from which they are insulated. The elements thus appear at both sides of the plate and it may be arranged that the light image is thrown on one side and the cathode ray or other "switch" operates on the other side. Used in this manner, the plate and elements may be spoken of as a "double sided mosaic screen."

A television transmitting system employing a cathode ray tube having a single sided mosaic screen is described by Zworykin in the Journal of the Institute of Electrical Engineers of October 1933, page 437. In this system a single sided mosaic screen is employed and the optical image and the cathode ray operate on the same side of the mosaic screen. The cathode ray serves to stabilise the potential of the mosaic a few volts negative with respect to the anode. This voltage serves to polarise the photo-electric elements. With this device there are a number of difficulties which are caused by the dependence of the photo-electric polarising voltage on the effect of the ray, the scattering of secondary electrons over the mosaic, the irregular action of the switching means at the end of scanning strokes, the failure of the ray to discharge (or charge) the elements sufficiently fully, and the fluctuations of the signals caused by sudden changes of illumination.

It is an object to the present invention to effect improvements in the polarising means and photo-electric elements, in the storing condensers associated with the elements, in the switching means used to charge (or discharge) the condensers and periodically stabilise the potentials of the elements, and in the circuits with which the device is employed. Many of the improvements described may be used separately; for example the improved switching means may be employed with a mosaic and circuit as suggested by Zworykin, or the improved mosaic and circuits may be used with a switching means dependent on a secondary electron equilibrium.

According to the present invention, a discharge device, such as a cathode ray tube, adapted for television transmission, comprises a mosaic screen of small elements having capacity to a common signal plate, wherein the direct capacity between adjacent groups of elements each constituting a "picture dot" is less than the capacity of each such group to the signal plate, and wherein the total capacity of all elements within the scanned area to the signal plate is so small that the time constant defined by this total capacity multiplied by the effective resistance of the cathode ray or other switching means is not greater than the period occupied by one component scan of the whole area.

By "picture dot" is meant an area representing one elementary dot on the transmitted picture. A picture dot is generally taken as a round or rectangular area equal in area to a square, the side of which is equal to the distance between adjacent scanning lines. Where scanning is effected with the aid of a cathode ray the area of the beam is made of the same order as that of one picture dot.

The signal plate may be composed of small rivets inserted in and insulated from a common metallic supporting plate, the surface of the rivets on one side being made photo-electrically sensitive.

According to a further feature of the invention, a discharging device such as a cathode ray tube, adapted for television transmission, comprises a mosaic screen of small elements and a mesh-work or multi-apertured late mounted adjacent one another in parallel planes, each element of the mosaic plate or a large proportion of these elements having a corresponding aperture in the mesh work or multi-apertured plate opposite to it.

Henceforth the mesh work or multi-apertured plate will be called a "grid."

The elements and the apertures in the grid may have different pitches and/or configurations as for example a triangular spaced set of mosaic elements co-operating with a rectangularly spaced set of apertures in the grid.

According to a further feature of the present invention there is provided a method of television transmission in which a mosaic screen, which may be either single or double-sided, is scanned with the aid of an electrical switching means such as a cathode ray, it being arranged that said switching means serve to stabilise the potential of the elements to approximately the potential of the cathode supplying the electrons which constitute the switching means.

Where a cathode ray is used as switching means the ray serves to stabilise the potential of the mosaic screen approximately to the potential of the cathode supplying the electrons of the ray and in this case there may be interposed in the track of the cathode ray, and in front of the mosaic screen, a conducting mesh-work or grid which is held at a potential positive with respect to the cathode supplying the cathode ray.

The potential of the grid relative to the mosaic surface is made so positive as to produce a strong electrostatic field normal to the mosaic surface, thus preventing any appreciable travel of electrons in a direction normal to the surface of the mosaic screen. Furthermore, the grid may provide the polarising potential for the photo-electric emission from the elements.

According to a further feature of the present invention a method of television or like transmission comprises the steps of projecting an optical image of the object to be transmitted upon a mosaic screen composed of a plurality of photo-electrically active elements and scanning said screen with a beam of electrons, each element acquiring

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Preferably the potential of the signal plate is maintained slightly negative with respect to the cathode providing the electrons which comprise the scanning means, and the photo-sensitive elements may be polarised through a circuit which does not include the impedance element or elements across which the signals to be amplified are produced.

The photo-electric currents from the photo-electric anode may, if desired, be so mixed with the currents from the signal plate either before or after amplification, that sudden change of illumination does not produce any sudden change in amplitude of the signals given out from the television transmitter, due to the sudden changes in photo-electric current.

According to a further feature of the invention, in the case where it is desired to stabilise the mosaic screen at approximately the potential of the cathode providing the electrons which constitute the switching means, an order of switching on various potentials is maintained such that the mosaic elements never attain a potential greatly differing from the required stabilised cathode potential.

Furthermore the signal plate may be maintained at such a potential that failure of the insulation between a mosaic element and the signal plate will not cause a bright "white" signal to be transmitted but will tend to produce a "black" signal.

The present invention may be carried out in the following way.

Television transmitting apparatus comprises a cathode ray tube having an envelope in the form of a cylindrical portion flaring out into a bulbous portion. Within the envelop are arranged in the order mentioned, and starting from the closed end of the cylindrical portion, a cathode, on or more electrodes adapted to take part in focusing the ray, an anode, a grid, a mosaic screen and a photo-electric anode. The three last-mentioned electrodes are disposed within the bulbous portion of the envelope and the remaining electrodes within the cylindrical portion. The cathode and anode may be of any suitable kind and between the anode and grid there are disposed either electrostatic or electromagnetic means for defecting the ray so as to scan the mosaic screen upon which the ray is focused.

The mosaic screen is in the form of a metal "signal" plate, disposed normally to the mean direction of the cathode ray, carrying elements in the form of rivets passing right through the plate. The rivets are insulated from the plate and the ends of the rivets on the face of the plate opposite the face scanned by the cathode ray are coated with photo-electric material, the other ends of the rivets being uncoated.

The grid is in the form of a plane, fine wire mesh disposed parallel to, and about 1 mm. from the mosaic screen, the cathode ray passing through this electrode on its way to the mosaic screen. The apertures in this grid should either register exactly with each mosaic element, so that each mosaic element has an aperture directly opposite, or else the apertures of the grid should have no correspondence at all with the mosaic element. In order to achieve this latter arrangement, it is convenient to arrange that the pitch of the apertures in the grid is entirely different from the pitch of the mosaic elements, or that the arrangement of the apertures in the grid and the mosaic elements is on a definite basis. For example, the mosaic elements may be arranged in triangular spacing, whereas the grid apertures may be arranged in a rectangular system as would be possible by using wire gauze. Alternatively, the mosaic may be formed of a very large number of small particles in random arrangement co-operating with a comparatively widely spaced set of apertures in the grid. It is desirable, however, that the pitch of the apertures in the grid be finer than the distance between adjacent picture dots.

The photo-electric anode is in the form of a wire bent into the shape of a rectangle or circle and is disposed at some distance from the mosaic screen in a plate parallel thereto. Alternatively the photo-electric anode may consist of a metal coating on the glass wall.

Means are also provided, preferably outside the tube, for focusing an image of the object to be transmitted upon the photo-electrically active surfaces of the elements of the mosaic screen, light from the object passing through the central hole in the photo-electric anode.

The cathode of the tube is earthed, the anode is maintained at about 1000 volts positive with respect to the cathode and the grid is maintained at between 100 volts and 1500 volts positive with respect to the cathode.

The signal plate, which is insulated from the photo-electric elements of the mosaic screen is connected through a "signal" resistance of 2000 ohms to the negative terminal of a four-volt source of current, the positive terminal of this source being earthed.

The photo-electric elements of the mosaic screen are polarised by a battery of sufficient voltage to saturate the elements, thus the negative end of a battery of 200 volts is connected through a resistance of one megohm to the lead to the signal plate at a point lying between the signal resistance and the plate, and the positive end of this battery is connected, through a second resistance of one megohm, to the photo-electric anode. A decoupling condenser is also connected in shunt with the 200 volt battery and two one-megohm resistances between the photo-electric anode and the signal plate. The two one-megohm resistances are inserted to prevent the battery capacity being introduced in shunt with the signal resistance. The 200 volt battery must be well insulated from earth.

In order to operate the device, the various members are switched on in the following order: cathode and deflecting means, anode and focusing electrodes and finally the photo-electric anode.

During operating an image of the object to be transmitted is projected on to the face of the mosaic screen which is coated with photo-electric material and the opposite face is scanned by the cathode ray.

Between the cathode and anode, the cathode ray is accelerated to a velocity equivalent to 1000 volts and assuming for the moment that the signal plate is at cathode potential, the ray is brought substantially to rest just as it reaches the mosaic screen.

In between successive scans however, each element acquires a positive potential proportional to the intensity of the light falling upon the element. Therefore when the ray is deflected on to a positively charged element, the latter attracts the electrons of the ray. At each scan of an element therefore, the element is charged negatively by the scanning ray until it reaches approximately cathode potential this potential may differ slightly from the potential of the cathode due to the temperature velocity of the emitted electrons and contact potentials, etc. At this stage the velocity of the ray is reduced to zero, so that an element cannot be charged negatively. The ray thus operates as a switch, reducing the potential of each element in turn almost instantaneously to zero. Furthermore, provided the elements are not charged too highly, the velocity of the ray on reaching the elements is at all times zero or very small, so that practically no secondary electrons are emitted from the uncoated faces of the elements. These faces are preferably surface so as to be poor emitters of secondary electrons.

The sudden changes of potential of the elements are capacity fed to the insulated signal plate and give rise to picture signals in the signal resistance connected to the signal plate. These signals are amplified and transmitted in any know or suitable manner.

The photo-electrons emitted by the elements are collected by the photo-electric anode and flow by way of the 200 volt polarising battery or decoupling condensers back to the signal plate, not passing through the signal resistance, so that large transient variations of intensity of the object do not give rise to pulses of large amplitude in the signal resistance, although the currents flowing through the signal resistance, being unidirectional, are representative of zero and low frequency changes of intensity of the object. Alternatively, the currents from the photo-electric anode may be amplified separately and mixed with the currents arriving from the signal pate at a later stage, it being so arranged that sudden changes of illumination produce no signal other than that produced when the switching means discharges (or charges) the elements which have been affected by the illumination.

The purpose of the grid is two-fold. In the first place it is extremely improbable that the ray would keep in good focus if its velocity approached zero over a large part of its flight; the grid serves to screen the ray, over the grater part of its flight, from the zero potential of the mosaic screen, and thus operates to preserve the focus of the ray. Secondly, the grid provides a strong potential gradient normal to the signal plate. When the ray approaches an element at cathode potential, it is turned back without striking the element. The strong potential gradient normal to the signal plate causes the electrons which have been stopped, to be accelerated back to the grid without any change of these electrons being pulled into elements of an adjacent dot which are at a positive potential. This strong field gradient thus prevents "fogging" of the signals.

If a single-sided mosaic be employed instead of the double-sided mosaic described, the gird which is positive with reference to the stabilised potential of the mosaic elements, will operate as a photo-anode and is capable of providing sufficient field at the mosaic surface to saturate the photo-electric emission. In such a case, the optical image must be thrown on to the mosaic screen through the mesh-work of the grid so that the wires forming the mesh-work must be of extremely small diameter.

The mosaic screen may be constructed in the following manner:-

A metal plate, about 0.15 mm. thick, is pierced with about 160,000 holes. These holes may be made by etching the plate (preferably from both sides) so that the holes are countersunk. Alternatively, the holes may be made mechanically. The holes may conveniently be 0.2 mm. in diameter and for a picture of 200 lines, four of such holes co-operate in forming what is referred to above as a picture dot. The plate is insulated all over by a thin layer of insulating material, with a thickness of at least 0.02 mm. and the holes in the plate are then filled with metal rivets. These can be formed in position by plating through from one side of the plate to the other. The surfaces of these rivet elements are coated with silver on one side (for example by plating). The silver surfaces are then activated with caesium to form photo-electric elements.

With such a construction, the direction capacity between adjacent elements is much smaller than the capacity of an element to the signal plate. Taking these dimensions and assuming an electric constant of 3 for the insulating material, the capacity per picture dot (four elements) is approximately 0.5 micro-microfarad. For a picture consisting of 200 lines, each of 200 dots, making 40,000 dots to the complete picture, the total capacity of all elements to the signal plate will be 20000 micro-microfarads.

In order that moving pictures may be satisfactorily represented, it is necessary that the cathode ray shall be capable of discharging these substantially during a complete scanning cycle. By a complete cycle is meant a cycle during which the ray occupies all possible positions and such a cycle would include more than one "frame period" of a scanning cycle employing interlaced scanning. If for example the complete scanning cycle occupies a 25th of a second, it is necessary that the time constant of discharge of this capacity shall be less than a 25th of a second, i.e. the ray resistance should be such that the product of the ray resistance and 20000 micro-microfarads will be less than 1/25th second, that is to say, the ray resistance should be less than two megohms and preferably should not exceed one megohm.

This requirement of the product of the ray resistance and mosaic capacity can be looked at from another point of view. In the example quoted above, 40000 picture dots were scanned in a 25th of a second, so that the ray rested on each picture dot for one millionth of a second. During this period, it is necessary that the capacity of one picture dot, viz. 0.5 micro-microfarads, shall be substantially discharged. Hence the produce of the ray resistance and the dot capacity must be less than one millionth of a second, that is, the dot time. For the example taken, this again leads to a requirement of a ray resistance less than two megohms and preferably one megohm. If a ray resistance of this value cannot be obtained, it will be necessary to reduce the capacity of the elements to the signal plate, as for instance, by increasing the thickness of the insulation.

By ray resistance is meant the effective resistance of the cathode ray or other electrical switch looked upon as a switching resistance. If the elements of the mosaic depart from their stabilised voltage by a voltage V, and such voltage departure produces a re-stabilising current equal to I, then the ray resistance is given by the ration EQUAT. HERE. The moment the ray hits the mosaic element, the voltage V is rapidly reduced, but for any value V of the mosaic element, a certain resultant number of electrons corresponding to the current I will be attracted by the elements. Suppose in the above example the mosaic elements were all short circuited to the signal plate, and the signal plate were held 10 volts more positive than the normal stabilised voltage of the mosaic elements (approximately cathode), then a certain charging current would be obtained from the cathode ray. If this charging current were 10 microamps, then the average ray resistance over the range of 0 to 10 volts would be EQUAT. HERE which is one megohm.

It must be arranged that the light intensity falling on the mosaic elements is never so great that a very large positive potential is developed on the elements during the period between successive scannings of the beam. If such a large potential is developed, the electrons hitting the mosaic elements will liberate secondary electrons, which may be more numerous than the primary electrons, and thus prevent the ray stabilising the mosaic elements down to approximately cathode potential.

The limiting voltage to which these elements should be allowed to rise is probably in the order of 5 to 20 volts positive with respect to the cathode ray cathode. Similarly, it is important that during the process of switching on the tube, the elements do not attain a high positive potential. Suppose for example that before the cathode ray is switched on the photo-electric anode is polarised and light falls on the mosaic, the elements will become positively charged to an abnormal extent, and not only will the ray when switched on be unable to stabilise these to cathode potential, but there is also a risk in respect of breaking down the insulation of the elements to the signal plate. To prevent this, a suitable order of switching on the various potentials may be observed. For example, the cathode of the cathode ray gun and the current to the scanning means (deflecting coils or deflecting plates) may be switched on first. After that, the potentials are applied to the anode of the cathode ray gun, then the grid, and finally the photo-electric anode.

As explained above, the signal plate is connected to the negative terminal of the small source of e.m.f. which holes its potential slightly negative relative to the cathode. This ensures that any leakage in the insulation of the mosaic elements shall tend to held these elements negative relative to the cathode, rather than allow them to drift to a dangerously high positive voltage. Similarly, this arrangement has the advantage that if the insulation of any element to signal plate breaks down, this element is held negative and becomes inoperative. This will cause a slightly darker spot on the received picture, whereas had the signal plate been positive, a very bright spot might have been produced.

When a mosaic surface is stabilised to approximately cathode potential, it is advantageous to arrange that the scanning ray scans right up to or slightly beyond the edges of the area of the active mosaic, since any mosaic surface which is not scanned, may rise in potential to a dangerously high value. In order that the cathode ray, when it scans off the edge of the mosaic, shall not cause unwanted charges to be set up, it is convenient to place a screen round the edges of the mosaic. In the preferred construction described, the double-sided mosaic may with advantage be fixed into, or slightly behind, a square aperture in a conducting mask which divides the tube into two portions. The cathode ray is adjusted to scan just beyond the edges of the area of the mosaic screen on to the mask, which is preferably maintained at a slightly negative voltage with respect to the cathode. Alternatively, this mask may be an uninsulated part of the signal plate, or may be connected to the signal plate. It is advantageous to make the grid mesh-work extend beyond the edges of the mosaic, and beyond the limiting scan of the ray. The ray may be arranged to scan beyond the edges of the mosaic by starting the tube up with large deflecting fields so that a very large area is scanned. Then by observation of the received picture, it will be possible to adjust the deflecting amplitudes so that the scanning ray passes just beyond the edge of the active mosaic. In this connection it may be necessary to provide not only controls of the amplitude of scan, but also controls (direct current bias) for the mean positive of the scan.

In any case the mosaic screen must stand the limiting voltage to which it can rise, or else that the grid potential must be run sufficiently low to prevent the mosaic ever assuming a dangerously high voltage. In the latter case, the mask described above would not be necessary.

It is to be noted that the presence of the grid materially increases the capacity of the mosaic screen to earth. If the mosaic and grid are each 10 cm. square and are separated by 1 mm. as described above, the effective capacity of the signal plate to earth is of the order of 100 micro-microfarads. In order to reduce the abnormally high noise to signal ratio which would be produced by working this device into a single ordinary valve, the device may be worked into several valves in parallel, it being arranged that the input capacity of the valves equals the effective capacity of the device.

As another alternative, the grid may be more widely spaced from the mosaic screen, thus reducing the increase in capacity. In any case, however, the loss due to the added capacity is more than compensated by the gain in efficiency due to saturating the photo-electric elements.

The grid must not be placed near enough, and its potential must not be high enough, to extract electrons from the mosaic screen. Also the tube must be highly evacuated in order to prevent trouble arising for the presence of ions.

If preferred, the photo-electric anode need not be connected through its battery to the signal plate, but may be connected through resistance, the voltage across which may be amplified. The amplified signals so obtained can then be mixed with the amplified signals from the signal plate so as to neutralise any sudden change of illumination.

The device described above employs a double ended mosaic, stabilised to cathode potential. The advantages of photo-electric saturation and/or the neutralising of the effects of sudden changes of illumination, may be obtained with such a mosaic even if the mosaic elements are not stabilised to cathode potential, but are stabilised to some potential approximating to the anode potential of the cathode ray gun in the manner described by Zworykin in the article referred to above.

Similarly, the stabilisation of the mosaic elements to cathode potential may be employed on a single-sided mosaic similar to that described by Zworykin. In this case, the tube would be as described above, except that the mosaic screen would be inclined to the cathode gun to allow the optical image to be thrown on the same side as that on which the cathode ray falls. No separate photo-electric anode would be provided but the grid in front of the mosaic would act as the photo-electric anode, since its potential is positive with respect to the stabilised (cathode) potential of the mosaic.

In this arrangement it may be necessary to limit yet more severely the maximum potential to which the mosaic elements may rise, since the photo-active surface may emit secondary electrons at quite low voltage bombardment. Also it may be necessary to employ a comparatively thick mica plate in order to keep the capacity of the mosaic to the signal plate sufficiently low.

It is not essential that the elements should be switched to the cathode, or equilibrium, potential by means of a moving cathode ray. Thus, for example a wide stationary beam of electrons may be accelerated towards the mosaic screen so as, in the absence of an obturator, to flood the whole of the screen. Close to the screen are disposed two gratings of parallel wires, the wires of one grating being perpendicular to those of the second. The potentials of the wires of the gratings are then varied in such manner that electrons pass through the holes formed by the wires of the two gratings in turn, thus scanning the mosaic screen in a manner similar to the normal. For example, it would be arranged that all such wires are normally held negative to prevent any electrons arriving at the mosaic surface. Each wire of one set would in turn be made positive, the whole set being so controlled during the time of one line. Each member of the other set would in turn be made positive, the whole cycle occupying the time of one frame.

When the "aperture" constituted by a wire of one grating and a wire of the other grain is positive, electrons can pass through the "aperture" and since the positive of this electron passing aperture is continually changing the mosaic screen in, in effect, scanned.

Dated this 3rd day of August, 1934.

REDDIE & GROSE,

Agents for the Applicants,

6, Bream’s Buildings, London, E.C.4.

COMPLETE SPECIFICATION

Improvements in or relating to Television Transmitting Systems

We, ALAN DOWER BLUMLEIN, a British Subject, of 7, Courtfield Gardens, Ealing, London, W.13, and JAMES DWYER McGEE, a British Subject, of 9, King’s Avenue, Ealing, London, W.5, 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 transmitting systems.

A television transmitting system is known in which an optical image of the object to be transmitted is projected upon a mosaic screen of photo-electrically active elements and the photo-electric surface of the screen is scanned by a cathode ray. The mosaic screen consists of a multiplicity of elements which are insulated from each other and from a common signal plate, each element forming, with the signal plate, a small condenser. Between successive scans, each elemental condenser acquires a charge determined by the number of photo-electrons emitted since the last scan, that is to say, a charge determined by the intensity of the light falling upon the element, and at each scan each elemental condenser is discharged. Picture signals are derived, in an external circuit associated with the common signal plate, from the electric impulses capacity-fed to the signal plate. Such a system consists of the following essential features:

  1. A mosaic screen having a large number of small photo-electric elements, each element having capacity to a signal plate.
  2. Means for projecting an optical image of the object to be transmitted on to these elements.
  3. Means for collecting photo-electrons emitted by these elements, thus causing the capacities to be charged (or discharged) in accordance with the brightness of the images.
  4. Switching means such as a cathode ray or other electrical "switch" which periodically discharges (or charges) the condensers, bringing the photo-electric elements to a definite potential, the discharging (or charging) currents so produced developing the required picture signals.

Two arrangements of the photo-electric elements are known. Firstly the elements may be formed of small particles of metal insulated from a plate and lying on one side of the plate. In this case, the light is thrown on to the same side of the plate as that with which the cathode ray or electrical switch co-operates and such a plate may be called a "single sided mosaic screen." Secondly, the elements may be formed on a grid or open work plate from which they are insulated, or may be formed of rivets projecting through a plate from which they are insulated. The elements thus appear at both sides of the plate and it may be arranged that the light image is thrown on one side and the cathode ray or other "switch" operates on the other side. Used in this manner, the plate and elements may be spoken of as a "double sided mosaic screen."

A television transmitting system employing a cathode ray tube having a single sided mosaic screen is described by Zworykin in the Journal of the Institution of Electrical Engineers of October 1933, page 437. In this system the optical image and the cathode ray operate on the same side of the mosaic screen. The cathode ray serves periodically to bring the potential of the mosaic elements to a few volts negative with respect to the potential of an anode. This potential difference causes the photo-electrons emitted by the mosaic elements to flow to the anode. With this device there are a number of difficulties which are caused by the fact that the collection of the photo-electrons depends on the potential to which the mosaic elements are brought when scanned by the ray, and also by the fluctuations of the signals caused by sudden changes of illumination, by the scattering of secondary electrons over the mosaic, by the irregular action of the ray at the end of scanning strokes, and by the failure of the ray to discharge (or charge) the elements sufficiently fully.

Objects of the present invention are to effect improvements in the methods of operating apparatus which includes a mosaic screen either of the single-sided kind or of the double-sided kind and improvements in the arrangement and construction of such apparatus.

Television transmitting apparatus has been proposed, as described in Patent Specification No. 442,666, which comprises an optical system for projecting an image of an object upon a photo-electrically active screen to cause emission of photo-electrons therefrom, a mosaic screen comprising mutually insulated elements or being in the form of a sheet having a high resistance in directions parallel to its surface, the mosaic screen being spaced apart from the photo-electrically active screen and being arranged with the mosaic screen within an evacuated envelope, focusing means for causing said photo-electrons to form an electron image upon the mosaic screen, means for developing a beam of electrons, means for causing this beam to scan the mosaic screen, and a signal plate capacitively associated with the mosaic screen. With this apparatus, also, difficulties may arise from the fluctuations of the signals caused by sudden changes of illumination, and by the scattering of secondary electrons over the mosaic screen in consequence of the scanning operation, and it is a further object of the present invention to overcome these difficulties.

The present invention in one aspect comprises a method of television transmission in which an electrostatic image of an object to be transmitted is formed upon a mosaic screen, and the screen is scanned by electrical switching means which include a cathode supplying a beam of electrons, the switching means serving periodically to bring the potential of the elements of the mosaic screen to approximately the potential of the cathode. Where the mosaic screen is photo-electrically active, the electrostatic image is formed as a consequence of the emission of photo-electrons due to the projection of an optical image on the screen.

The present invention in a further aspect comprises television transmitting apparatus including a double-sided mosaic screen of photo-electrically active elements insulated from one another and from a common signal plate, means for projecting an optical image of an object to be transmitted on to said screen to cause emission of photo-electrons from said elements, an anode adapted to receive said photo-electrons, electrical switching means for scanning said screen be a beam of electrons, and an impedance electrically connected with said signal plate and across which the picture signal voltages to be transmitted are developed, such apparatus including also a circuit arrangement between said anode and said signal plate adapted to prevent impulsive changes in average brightness of said image from producing corresponding impulsive changes in the voltage across said impedance.

The invention in another aspect comprises television transmitting apparatus including a double-sided mosaic screen, means for forming an electrostatic image on said screen by causing emission of photo or secondary electrons from the mosaic elements thereof, an anode for receiving said electrons, means for scanning said mosaic screen to bring the potentials of said elements thereof, an anode for receiving said electrons, means for scanning said mosaic screen to bring the potentials of said elements to a datum value, a signal plate capacitively associated with said mosaic screen, and an impedance which is connected to said signal plate and across which picture signal voltages are developed when the mosaic screen is scanned, such apparatus including also a circuit which is associated with said anode and which comprise means for mixing with the signal voltage from said impedance, before of after amplification thereof, a voltage derived from current constituted by said electrons or from the alternating component of this current.

The invention in a further aspect comprises television transmitting apparatus including a double-sided mosaic screen, means for forming an electrostatic image on said screen by causing emission of photo or secondary electrons from the mosaic elements thereof, an anode for receiving said electrons, means for scanning said mosaic screen to bring the potentials of said mosaic elements to a datum value, a signal plate capacitively associated with said mosaic screen, and an impedance which is connected to said signal plate and across which picture signal voltages are developed when the mosaic screen is scanned, such apparatus including also a circuit whereby the current due to said electrons, or at lest the alternating component of this current, is fed to a point between said signal plate and said impedance.

Where the mosaic screen comprises mutually insulated metal elements, the direct capacity between adjacent groups of these elements, each group constituting a "picture dot," is less than the capacity of each group to the signal plate, and it is preferably arranged that the total capacity of all the elements within the scanned area to the signal plate is such that the time constant, defined by this total capacity multiplied by the effective resistance of the cathode ray of the switching means, is not greater than the period to be occupied by one complete scan of the whole area.

By "picture dot" is meant an area representing one elementary dot on the transmitted picture. A picture dot is generally taken as a round or rectangular area equal in area to a square, the side of which is equal to the distance between adjacent scanning lines. Where scanning is effected with the aid of a cathode ray the area of the beam is made of the same order as that of one picture dot.

The mosaic screen may be composed of small rivets inserted in and insulated from a common metallic supporting plate which serves as the signal plate, and the surface of the rivets on one side may be made photo-electrically sensitive.

Apparatus according to the present invention preferably includes a multi-apertured plate disposed adjacent to the mosaic screen on the side thereof that is adapted to be scanned by the beam of electrons, and means for maintaining this plate at a potential positive with respect to the cathode which serves to supply the beam of electrons.

Henceforth the multi-apertured plate will be called a "grid." The mosaic elements and the apertures in the grid may have different pitches and/or configurations, as for example a triangularly spaced set of mosaic elements co-operating with a rectangularly spaced set of apertures in the grid.

The potential of the grid relative to the mosaic surface is made so positive as to produce a strong electrostatic field normal to the mosaic surface, thus preventing any appreciable travel of electrons in a direction parallel to the surface of the mosaic screen. Furthermore, where the grid is associated with a single-sided mosaic screen, the grid may serve to collect the photo-electric emission from the elements.

The invention will be further described with reference to the examples of parts of television transmission apparatus, shown in the accompanying diagrammatic drawings, in which similar parts in the different Figures have the same reference numerals.

Fig. 1 shows a cathode ray tube having a mosaic screen of the double-sided type, with certain of the associated electrical circuits,

Fig. 2 is a section, to a greatly enlarged scale, or a portion of the mosaic screen in Fig. 1 and showing also the arrangement of an additional grid adjacent to the screen,

Figs. 3 and 4 show respectively two modifications of a part of the arrangement shown in Fig. 1,

Fig. 5 shows a cathode ray tube of the single-sided type, with cerate of the associated electrical circuits, and

Fig. 6 is a section, to a greatly enlarged scale, of a portion of the mosaic screen shown in Fig. 5 and a grid adjacent thereto.

The apparatus shown in Figs. 1 and 2 comprises a cathode ray tube having an envelope in the form of a cylindrical portion 1 flaring out into a bulbous portion 2. Within the envelope are arranged in the order mentioned, and starting from the closed end of the cylindrical portion 1, a cathode 3 (the heating means for which are not shown), one or more electrodes, such as 4, which are adapted to take part in focusing the ray, an anode 5, a grid 6, a double-sided mosaic screen 7 and an anode 8 which serves to collect photo-electrons. The cathode 3 and anode 5 may be of any suitable kind and between the anode 5 and grid 6 there are disposed either electrostatic or electro-magnetic means for deflecting the ray so as to scan the mosaic screen 7 upon which the ray is focused. In the present example these means have the form of deflecting coils 9 and 10.

The mosaic screen 7 is in the form of a metal "signal" plate 11 (Fig. 2), provided with regularly arranged perforations and disposed normally to the mean direction of the cathode ray. The signal plate 11 carries elements in the form of rivets 12 passing right through the perforations in the plate. The rivets are insulated from the plate, for example by coating the plate, before the insertion of the rivets, with a layer 13 of insulating material, and the ends of the rivets on the face of the plate opposite the face scanned by the cathode ray are coated with photo-electric material 14, such as caesium, the other ends of the rivets being uncoated.

The grid 6 is in the form of a platen, fine wire mesh disposed parallel to, and about 1 mm. from the mosaic screen, the cathode ray passing through this electrode on its way to the mosaic screen. The apertures in this grid should either register exactly with each mosaic element, so that each mosaic element has an aperture directly opposite, as shown in Fig. 2, or else the apertures of the grid should have no correspondence at all with the mosaic elements. In order to achieve this latter arrangement, it is convenient to arrange that the pitch of the apertures in the grid is entirely different from the pitch of the mosaic elements, or that the arrangement of the apertures in the grid and the mosaic elements is on a definite basis. For example, the mosaic elements may be arranged in triangular spacing, whereas the grid apertures may be arranged in a rectangular system as would be possible by using wire gauze. Alternatively, the mosaic may be formed of a very large number of small particles in random arrangement co-operating with a comparatively widely spaced set of apertures in the grid. It is desirable however, that the pitch of the apertures in the grid be finer than the distance between adjacent picture dots.

The photo-electron anode 8 may consist of a metal coating on the glass wall, or it may be in the form of a wire or sheet metal electrode bent into rectangular or annular form. It is disposed at some distance from the mosaic screen in a plate parallel thereto.

A lens system 15 is provided, preferably outside the tube, as shown, for focusing an image of the object to be transmitted upon the photo-electrically active surfaces 14 of the elements 12 of the mosaic screen, light from the object passing through the central hole in the photo-electron anode 8.

The cathode of the tube is earthed by a conductor 16, the anode 5 is maintained at about 1000 volts positive with respect to the cathode by a source of potential difference 17 and the grid 6 is maintained at between 100 volts and 500 volts positive with respect to the cathode by a source of potential difference 18.

The signal plate 11 of mosaic screen 7 is connected by a conductor 26 to one terminal of a signal resistance 19 of 2000 ohms the other terminal of which is connected to the negative terminal of a four-volt source of current 20, the positive terminal of this source being earthed.

In order to ensure that the photo-electrons emitted by the mosaic elements 12 are all collected by the photo-electron anode 8, the negative end of a battery 21, of 200 volts, is connected to the terminal of the signal resistance 19 which is connected by the lead 26 to the signal plate 11 of mosaic screen 7, and the positive end of this battery is connected to the photo-electron anode 8. A decoupling condenser 24 may be connected in shunt with the battery 21, if this battery has appreciable impedance. The battery 21 must be well insulated from earth, and its capacity to earth must be kept low.

In order to operate the device, the various members are switched on in the following order: cathode 3 and deflecting means 9 and 10, anode 5 and focusing electrode 4, and finally the photo-electron anode 8.

During operation an image of the object to be transmitted is projected by the lens system 15 on to the face of the mosaic screen 7 and the back of the screen is scanned by the cathode ray.

Between the cathode 3 and anode 5, the cathode ray is accelerated to a velocity equivalent to 1000 volts, and, assuming for the moment that the parts of the mosaic screen 7 are at cathode potential, the ray is brought substantially to rest just as it reaches the mosaic screen.

Between successive scans, however, each element 12, owing to the emission of photo-electrons, acquires a positive potential proportional to the intensity of the light falling upon the element. Therefore when the ray is deflected on to a positively charged element, the latter attracts the electrons of the ray. At each scan of an element therefore, the element is charged negatively by the scanning ray until it reaches approximately cathode potential. This potential may differ slightly from the potential of the cathode due to the temperature velocity of the emitted electrons and contact potentials etc. At this stage the velocity of the ray is reduced to zero, so that an element cannot be charged negatively. The ray thus operates as a switch, reducing the potential of each element in turn almost instantaneously to zero. Furthermore, provided the elements are not charged too highly, the velocity of the ray on reaching the elements is at all times zero or very small, so that practically no secondary electrons are emitted from the uncoated faces of the elements.

The sudden changes of potential of the elements 12 are capacity fed to the insulated signal plate 11 and give rise to picture signals in the signal resistance 19. These signals are tapped at 25 and amplified and transmitted in any known or suitable manner.

This device may be regarded as operating in the following manner. When an optical image is formed on the mosaic screen, a photo-electric current, which is representative of the instantaneous average intensity of illumination of the screen, flows from the mosaic elements 12 to the photo-electron anode 8 and through the circuit 24 or 21 and 26 to the signal plate 11 of mosaic screen 7. This may be regarded as the current charging the condensers formed by the mosaic elements and the signal plate, and it contains no picture signal component. Thus any impulsive components of the photo-electric current are sent back from the photo-electron anode to the signal plate, so that sudden changes of light, which are much quicker than the picture period, produce no impulsive potential changes across the signal resistance 19, and the objectionable results of such transient changes are thereby eliminated.

When the cathode ray strikes a mosaic element which has acquired a positive charge owing to photo-emission, it lowers the potential of this element to substantially cathode potential, that is to say, it discharges the condenser formed by the element and the signal plate, and the signal thus capacity fed to the signal plate causes a flow of electrons to earth through the signal resistance, the number of the electrons being proportional to the charge previously acquired by the element. Thus the picture signal currents, due to successive discharge of the mosaic elements, are unidirectional and are accordingly representative of the absolute intensity of illumination of the individual mosaic elements. In this way, although transient variations of intensity of illumination do not adversely influence the transmitted signals, low-frequency changes of intensity of illumination are adequately transmitted.

The purpose of the grid 6 is two-fold. In the first place it serves to screen the ray, over the greater part of its flight, from the zero potential of the mosaic screen, and thus operates to preserve the focus of the ray. Secondly, the grid provides a strong potential gradient normal to the signal plate. When the ray approaches an element at cathode potential, it is turned back without striking the element. The strong potential gradient normal to the signal plate causes the electrons which have been stopped, to be accelerated back to the grid without any chance of their being pulled into elements of an adjacent dot which are at a positive potential. This strong field gradient thus prevents "fogging" of the signals.

It may in certain cases be desirable to provide an additional grid maintained at cathode potential and disposed between the grid 6 and the screen 7, for the purpose of suppressing any secondary emission that may arise from the mosaic elements 12. This additional grid, which is shown by dotted lines at 27 in Fig. 2, may be formed by a continuous mesh of electrically conducting material deposited on the insulating layer 13 of the signal plate 11. The introduction of this additional grid 27 forms the subject of co-pending patent application No. 27120/34 (Serial No. 446,664), and we make no claim for it as a feature of the present invention.

The mosaic screen may be constructed in the following manner. A metal plate, about 0.14 mm. thick, is pierced with about 160,000 holes. These holes may be made be etching the plate (preferably from both sides) so that the holes are countersunk. Alternatively, the holes may be made mechanically. The holes may conveniently be 0.2 mm. in diameter and for a picture of 200 lines, four of such holes co-operate in forming what is referred to above as a picture dot. The plate is insulated all over by a thin layer of a suitable insulting material, such as glass, having a thickness of at least 0.02 mm. and the holes in the plate are then filled with metal rivets. These can be formed in position by plating through from one side of the plate to the other. The surfaces of these rivet elements are coated with silver on one side (for example by plating). The silver surfaces are then oxidised and activated with caesium to form photo-electric elements.

With such a construction, the direct capacity between adjacent elements is much smaller than the capacity of an element to the signal plate. Taking these dimensions and assuming a dielectric constant of 3 for the insulating material, the capacity per picture dot (four elements) is approximately 0.5 micro-microfarad. For a picture consisting of 200 lines, each of 200 dots, making 40,000 dots to the complete picture, the total capacity of all elements to the signal plate is 20,000 micro-microfarads.

In order that moving pictures may be satisfactorily represented, it is necessary that the cathode ray shall be capable of discharging this total capacity substantially during a complete scanning cycle. By a complete cycle is meant a cycle during which the ray occupies all possible positions and such a cycle would include more than one "frame period" of a scanning cycle employing interlaced scanning. If for example the complete scanning cycle occupies a 25th of a second, it is necessary that that time constant of discharge of this capacity shall be less than a 25th of a second, i.e. the ray resistance should be such that the product of the ray resistance and 20000 micro-microfarads will be less than 1/25 second, that is to say, the ray resistance should be less than two megohms and preferably should not exceed one megohm.

This requirement of the produce of the ray resistance and mosaic capacity can be looked at for another point of view. In the example given above, 40,000 picture dots were scanned in a 25th of second, so that the ray rested on each picture dot for one millionth of a second. During this period it is necessary that the capacity of one picture dot viz. 0.5 micro-microfarads, shall be substantially discharged. Hence the product of the ray resistance and the dot capacity must be less than one millionth of a second, that is, the dot time. For the example taken, this again leads to a requirement of a ray resistance less than two megohms and preferably one megohm.

By ray resistance is meant the effective resistance of the cathode ray or other electrical switch looked upon as a switching resistance. If the elements of the mosaic depart from their datum voltage by a voltage V, and such voltage departure produces a re-establishing current to equal to I, then the ray resistance is given by the ratio EQUAT. HERE. The moment that the ray hits the mosaic element, the voltage V is rapidly reduced, but any value V of the mosaic element, a certain resultant number of electrons corresponding to the current I will be attracted by the elements. Suppose in the above example the mosaic elements were all short circuited to the signal plate, and the signal plate were held 10 volts more positive than the normal datum voltage of the mosaic elements (approximately cathode voltage), then a certain charging current would be obtained from the cathode ray. If this charging current were 10 microamps, then the average ray resistance over the range of 0 to 10 volts would be EQUAT. HERE which is one megohm.

The light intensity falling on the mosaic elements must never be so great that no over large positive potential is developed on the elements during the period between successive scannings of the beam. If such a large potential is developed, the electrons hitting the mosaic elements will liberate secondary electrons, which may be more numerous than the primary electrons, and thus prevent the ray from restoring the mosaic elements to approximately cathode potential.

In the example shown in Fig. 1, it is desirable to limit the rise in voltage of the mosaic elements to between 5 and 20 volts positive with respect to the cathode ray cathode. Similarly, it is important that, during the process of switching on the tube, the elements should not attain a high positive potential. Suppose for example that before the cathode ray is switched on the photo-electron anode is energised and light falls on the mosaic, the elements will become positively charged to an abnormal extent, and not only will the ray when switched on be unable to restore them to cathode potential, but there is also a risk of breaking down the insulation of the elements to the signal plate. To prevent this, a suitable order of switching on the various potentials should be observed. For example, the cathode 3 and the current to the deflecting coils 9 and 10 may be switched on first. After that, the potentials area applied to the anode 5, then the grid 6 and finally the photo-electron anode 8.

The introduction of the additional grid 27, which, as already mentioned, forms the subject of co-pending patent application no. 27120/34 (Serial No. 446,664), will materially reduce the number of any secondary electrons which are liberated by the elements 12 and pass to the grid 6. The additional grid is preferably joined to the signal plate by a connection to lead 26 which will maintain the electrode 27 slightly negative with respect to the cathode by virtue of the battery 20. By this connection to the signal plate the additional grid does not cause an additional capacity load across the signal resistance 19.

The provision of the battery 20, which holds the signal plate 11 slightly negative relative to the cathode, ensures that any leakage in the insulation of the mosaic elements shall tend to hold these elements negative relative to the cathode, rather than allow them to drift to a dangerously high positive voltage. Similarly, this arrangement has the advantage that, if the insulation of any element to the signal plate breaks down, this element is held negative and becomes inoperative. This will cause a slightly darker spot on the received picture, whereas had the signal plate been positive, a very bright spot might have been produced,.

The presence of the grid 6 material increased the capacity of the mosaic screen to earth. If the mosaic and grid are each 20 cm. square and are separated by 1 mm. as described above, the effective capacity of the signal plate to earth is of the order of 400 micro-microfarads. In order to reduce the abnormally high noise-to-signal ratio which would be produced by working this device into a signal ordinary valve, the drive may be worked into several valves in parallel, it being arranged that the input capacity of the valves equals the effective capacity of the device.

As an alternative, the grid may be more widely spaced from the mosaic screen, thus reducing the increase in capacity.

The grid must not be placed near enough, and its potential must not be high enough, to extract electrons from the mosaic screen in the absence of primary bombardment. Also the tube must be high evacuated in order to prevent trouble arising from the presence of ions. There will be no risk of extracting electrons from the screen if the grid is 1 mm. away from it, and if its voltage is not more than about 500 volts above that of the screen.

In a modified arrangement, not shown in the drawings, but which is similar to that shown in Figs. 1 and 2, except that the grids 6 and 27 are omitted, the second anode 5 being earthed, and the cathode 3 being maintained at a high negative potential relative to earth, the scanning periodically restores the mosaic elements to about the potential of the second anode 5. With this modified arrangement also, although transient variations of intensity of illumination do not adversely influence the transmitted signals, low-frequency changes of intensity of illumination are adequately transmitted.

In the modification shown in Fig. 3, the negative terminal of the battery 21, which maintains the photo-electron anode 8 at a suitable positive potential with respect to the signal plate and the cathode, is connected to earth instead of to the signal plate. The positive terminal of this battery is connected to the anode 8 through a resistance 23, while the condenser 24, as in Fig. 1, is connected between the anode 8 and the signal plate. With this modified arrangement, the alternating component of the current flowing to the photo-electron anode 8 is fed through the condenser 24 to a point in the lead 26 which connects the signal plate to the signal resistance. However, the direct component of this current in this case passes to earth through the resistance 23 and the battery 21.

In the modification shown in Fig. 4, the condenser 24 of Fig. 3 is omitted, and the arrangement is such that both the total current due to the photo-electrons and the picture signal currents produced by scanning pass through the signal resistance 19. A voltage proportional to the total photo-electron current is developed across the resistance 23. The changes in potential difference across this resistance are fed to an amplifier 31, while the picture signals together with the superimposed variations due to the total photo-electric current are fed to an amplifier 28. The outputs from the amplifiers 31 and 28 are fed to a mixing device 29 so arranged that the variations of the picture signal due to variations in the total photo-electric current passing through the signal resistance 19, are neutralised by the output from the amplifier 31. Thus the signals developed in the mixer output leads 30 are free from effects due to transient changes in intensity of illumination of the object being transmitted.

In applying the method of periodically restoring the potential of the mosaic screen to cathode potential according to the present invention, to the kind of apparatus described in patent specification No. 442,666 hereinbefore referred to, the potentials applied to the various electrodes are such that photo-electrons liberated by the photo-electrically active screen strike the mosaic screen with such a velocity that the number of secondary electrons emitted by the mosaic screen and collected by a suitable anode exceeds the number of photo-electrons received by that screen. The elements of the mosaic screen therefore become more positive to an extent proportional to the intensity of the light falling on the corresponding parts of the photo-electrically active screen. The signal plate, which is capacitively associated with the mosaic screen, is maintained at approximately the potential of the cathode of the electron gun which is disposed on the opposite side of mosaic screen to the photo-electrically active screen, and a grid maintained at a suitable positive potential with respect to the cathode is placed adjacent to the mosaic screen and between this screen and the cathode. Thus during scanning, the cathode ray strikes the mosaic screen with such a low velocity that substantially no secondary electrons are emitted and the mosaic elements are therefore restored to cathode potential. If it is desired to prevent fluctuations of the signals due to transient changes in the mean intensity of illumination of the photo-electrically active screen, the anode, which is provided to collected the secondary electrons emitted by the mosaic screen as a result of its bombardment by the photo-electrons, may be connected, through a battery which serves to maintain it as a suitable positive potential, to the signal plate, similarly to the anode 8 in Fig. 1, so that impulsive changes in the secondary emission current do not cause transient variations in the potential difference across the signal impedance in circuit with the signal plate.

The devices shown in Figs. 1 to 4 employ a double sided mosaic screen, the mosaic elements of which are periodically restored to cathode potential. The advantage of neutralising the effects of transient changes of illumination, may be obtained with such a mosaic, even if the mosaic elements are periodically restored to some potential approximating to the potential of the anode of the device emitting the beam of electrons, in the manner described by Zworykin in the article referred to above.

Similarly the improved arrangement whereby the mosaic elements are periodically resorted to cathode potential may be employed with a single-sided mosaic screen, as shown in Figs. 5 and 6. In this case the tube is similar to that shown in Fig. 1, except that the mosaic screen 7 is inclined to the electron gun 3, 4, 5 to allow the optical image to be thrown in the same side as that on which the cathode ray falls, and the lens system 14 is placed on the opposite side of the screen. No separate photo-electron anode is provided; the grid 6 acts as the photo-electron anode, since its potential is positive with respect to the maximum potential acquired by the mosaic elements. The wires of the grid 6 must be of extremely small diameter, since the image is projected through the grid. The mosaic screen 7 includes a mica sheet 13a (Fig. 6). On the side of this sheet facing the electron gun is formed a mosaic of separate silver elements 12a which are oxidised and photo-sensitized with caesium. On the opposite side of the mica sheet is a continuous silver signal plate 11a. In this arrangement it is desirable to limit to a low value the maximum potential to which the mosaic elements may rise, since the photo-active surface may emit secondary electrons at quite low voltage bombardment. Also it is desirable to make the mica sheet 13a comparatively thick in order to keep the capacity of the mosaic elements to the signal plate sufficiently low.

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

  1. A method of television transmission in which an electrostatic image of an object to be transmitted is formed upon a mosaic screen which has a common signal plate capacitively associated with its elements, and said screen is scanned by electrical switching means which include a cathode supplying a beam of electrons, characterised in that said switching means serve periodically to bring the potential of the elements of the mosaic screen to approximately the potential of said cathode.
  2. A method as claimed in claim 1 wherein an optical image is formed on said mosaic screen which is made photo-electrically active, so that the emission of photo-electrons from said mosaic screen in proportion to the intensity of illumination of the individual mosaic elements thereof forming said electrostatic image.
  3. In the method of television transmission according to claim 1 or claim 2, the step for initially maintaining the elements of the mosaic screen at approximately the potential of the cathode, which consists in switching on the potentials to the various elements thereof in such an order that the elements of the mosaic screen never attain a potential substantially higher than their normal maximum operating potential.
  4. Television transmitting apparatus comprising a double-sided mosaic screen of photo-electrically active elements insulated from one another and from a common signal plate, means for projecting an optical image of an object to be transmitted on to said screen to cause emission of photo-electrons from said elements, in anode adapted to receive said photo-electrons, electrical switching means for scanning said screen by a beam of electrons, and an impedance electrically connected with said signal plate and across which the picture signal voltage to be transmitted is developed, characterised by a circuit arrangement between said anode and said signal plate adapted to prevent impulsive changes in average brightness of said image from producing corresponding impulsive changes in the voltage across said impedance.
  5. Television transmitting apparatus comprising a double-sided mosaic screen, means for forming an electrostatic image on said screen by causing emission of photo or secondary electrons from the mosaic elements thereof, an anode for receiving said electrons, means for scanning said mosaic screen to bring the potentials of said elements to a datum value, a signal plate capacitively associated with said mosaic screen, and an impedance which is connected to said signal plate and across which picture signal voltages are developed when the mosaic screen is scanned, characterised by the provision of a circuit which is associated with said anode and which includes means for mixing with these gain voltages from said impedance, before or after amplification thereof, a voltage derived for current constituted by said electrons or from the alternating component of this current.
  6. Television transmitting apparatus comprising a double-sided mosaic screen, means for forming an electrostatic image on said screen by causing emission of photo or secondary electrons from the mosaic elements thereof, an anode for receiving said electrons, means for scanning said mosaic screen to bring the potentials of said mosaic elements to a datum value, a signal plate capacitively associated with said mosaic screen, and an impedance which is connected to said signal plate and across which picture signal voltages are develop when the mosaic screen is scanned, characterised by the provision of a circuit whereby the current due to said electrons, or at least the alternating component of this current, is fed to a point between said signal plate and said impedance.
  7. Apparatus for carrying out the method of claim 1, 2 or 3 characterised in that the direct capacity between adjacent groups of photo-electric elements of the mosaic screen, each group constituting a "picture dot," is less than the capacity of each group to the signal plate, and that the total capacity of all the elements within the scanned area to the signal plate is such that the time constant, defined by this total capacity multiplied by the effective resistance of the cathode ray of the switching means, is not greater than the period to be occupied by one complete scan of the whole area.
  8. Apparatus as claimed in claim 7 characterised in that a multi-apertured plate is disposed adjacent to the mosaic screen on the side thereof that is adapted to be scanned by the beam of electrons, and means are provided for maintaining this plate at a potential positive with respect to the cathode which serves to supply the beam of electrons, for the purpose hereinbefore set forth.
  9. Apparatus as claimed in claim 7 or 8 characterised in the provision of means adapted to maintain the signal plate at a potential which is slightly negative with respect to the potential of the cathode which serves to supply the beam of electrons.
  10. Television transmitting apparatus substantially as herein described with reference to Figs. 1, 3, 4 and 5 and 6 of the accompanying drawings.

Dated this 1st day of August, 1935.

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. – 1936.