US2277009A - Television image projection tube - Google Patents

Description

M. VON ARDENNE TELEVISION IMAGE PROJECTION TUBE Filed June 6, 1940 Match 17, 1942.

G I I I I I V INVENTOR MAIVFRED VON ARDEN/V5 ATTORNEY v Patented Mar. 17, 1942' OFFICE TELEVISION IMAGE PROJECTION TUBE Manfred von Ardenne, Berlin-Lichterfelde, Germany Application June 6, 1940, Serial No. 339,052 In Germany December 6, 1938 3 Claims.

This invention relates to television image projection systems wherein optical means are provided for projecting a television image upon a screen. Television receiving tubes for use in such a system are shown and described in applications Serial #292,017, filed August 26, 1939, and in Serial #306,610, filed November 29, 1939, as well as in other'applications filed by me.

In experiments carried out with arrangements such as shown and described in the application Serial #292,017 and more especially with those of the application Serial #306,610, polarization charges were encountered at the separating surface between the crystal layer and auxiliary layer. These charges appear gradually and, in accordance with the proportion of the conductivity of the auxiliary layer and of the crystal, they caused either a slow disappearance of an originally intense luminescence at a constant storage potential, or else a longer sustaining of a luminescence following the extinction of the storage potential. These charges, therefore, cause in part either profound falsiflcations of the tone values of the image having constant brightness value, or in images where the brightness varies they cause trailing disturbances such as are produced, for instance, by fluorescent screens having the property of after slow. The elimination of these disturbances is an important problem when employing crystal screens with auxiliary layers having a low secondary emissivity.

Th present invention may best be understood by referring to the drawing wherein like reference characters represent like parts, and wherem:

Figure 1 shows one form of the present invention.

Figure 2 shows an enlarged view of one form of the crystal plate structure or target electrode.

In Figure l of the drawing is shown a cathode ray tube 8 in which is positioned a gun structure It for producing a narrow focused beam of electrons. The intensity of the beam of electrons is determined by the potential of the control electrode l2. Als positioned within the tube is a target electrode which comprises a support plate 22, a layer of crystals 20, and an auxiliary layer or film of transparent material 26 which has a predetermined (preferably low) secondary electron emissive characteristic. Positioned in front of the target electrode is an electron accelerating screen 52 for producing rapid acceleration of the electrons which constitute the beam in the vicinity of the target electrode. The cathode ray beam H which is generated by the gun structure I0, is caused to be deflected over the target electrode including the crystal plate by means of the horizontal and vertical deflecting means l6 and I8, respectively.

A source oi. light, 36, is provided, and the light which originates from the source is directed toward the receiving tube by means of the reflector 34. The light is passed through a lens 38 and a polarizing screen 46 before being projected upon the crystal plate 20. Positioned on the other side of the receiving tube is a still further polarizing screen 48 and lens system 40, in order to focus the produced optical image upon a viewing screen 44 in an enlarged manner.

As described in the above mentioned application, Serial #292,017, the amount of light which is permitted to pass through the crystal plate is determined by the charge which the various elements of the crystal plate have assumed in accordance with the intensity of the scanning cathode ray beam, and the light rays so modulated are finally projected upon the screen 44 where the enlarged and intensified television image is reproduced.

One form of the crystal plate or target electrode is shown in Figure 2, and as stated above, this includes the support 22 upon which ispositioned a layer of crystals 20 which may be prepared and arranged in the manner suggested in application Serial #307,573, filed December 5, 1939. The layer of crystals 20 may be composed of Segnette salts or the crystal layer may be formed of sulphide of zinc or zinc blende. The auxiliary layer 26, which is preferably light transparent, and which has a predetermined secondary electron emissive characteristic, is attached to the layer of crystals 20 by any appropriate means, such as, for example, by means of a layer of cement or adhesive material 24. When cement is used it should Preferably be clear or transparent and should be a substance that is stable in vacuum, such as, for example, waterglass. In an alternative form the layer of cement may be omitted and the auxiliary layer may be placed on the layer of crystals while in a liquid state and later permitted to become hardened. It is necessary that very intimate contact be maintained between the auxiliary layer 26 and the crystal layer 20, as will be explained later. The thickness of the auxiliary layer 26, as well as its conductivity and dielectric constant, are chosen in accordance with the corresponding characteristics of the particular crystals used in the layer 20, as will be more fully explained. As suggested in application Serial No. 306,610, the layer 26 must possess high insulating qualities and may consist of glass or a vitreous layer. Alternatively, mica platelets or scales of similarly clear transparent substances may be used.

In order to eliminate the disturbances caused by polarization charges various ways are open. The most obvious measure resides in preventing the appearance of true charges at the separating surface between the auxiliary layer 26 and the crystal layer 20. A calculation carried through in this respect shows that true charges cannot occur if the proportion between the conductivity and the dielectric constant of he auxiliary layer 28 is the same as that between the conductivity of the .crystal and its dielectric constant (in the direction of the field). Polarization charges hence ditions the conductivity within limits (for inh stance, the conductivity'of various types of glass, i. e., the special Schott glass having a high conductivity), the control and choice of the conductivity of the auxiliary layer to conform to the above requirement involves no principal difllculties.

' It is obvious that care must beataken that vacuum interspaces between the crystal and auxiliary layer caused by insufilcient contacting do not exist. Very intimate contact between the 1 two layers may be attained either by employing a cement 28 previously referred to (see Application Serial #306,610) but which would have to gomply with the aforesaid condition, or else by applying the auxiliary layer in a heated liquid state, to1 the crystal layer and causing a Joining of both. There could also be used as auxiliary layer a layer of cementing substance proper which fulfills the aforesaid requirement and which has at the same time the property of low secondary emissivityjand optical clarity combined.

Another entirely diiferent way of eliminating the charge disturbances consists of dimensioning the layer thicknesses and conductivities in such a manner that the polarization charges can pass ofi at such rapidity that the trailing disturbance referred to above will be suppressed in images where the light content is variable or where motion is present. This signifies that the polarization charges must be passed off within about second, At this procedure there exists next the danger; that the useful or desired charges will be weakened to a critical point by the leakage during the time period extending from the beginsumciently' thin layer fulfilling the requirement of suitable conductivity it can be readily achieved that during the period of extinction the polarization charge together with the useful chargewill be passed through the auxiliary layer without detriment to the sharpness of the contours of the image on account of lateral leakage of the useful charges. This is explained by the fact that the conductivity of an auxiliary layer as so dimensioned differs by several order values in the various-directions thereby accomplishing the de:

sired result.

The last mentioned procedure increases in eftelevision standard with scanning according to the line interlace method. The time period be tween the storing and extinction of the individual image elements would in this case be of the order of 1/60 second so that the conditions of the conthe thinner the auxiliary layer can be chosen with respect to the penetration depth of the scanning cathode'ray beam of electrons. At volt velocities of the order of 10,000 volts and with the layer materials to be considered the depth of penetration is of the order of some 10- mm. If the storing device can be operated with a plate potential of 10,000 volts, i. e., if auxiliary layers having such a. small thickness can be employed, thelast mentioned method affords an especially suitable and safe solution of the present problem since in this method an absolutely accurate matching of the conductivities is not required but.

solely a proper dimensioning of the conductivities in respect to order values.

I claim:

l. A cathode ray tube for a television receiving system having means therein adapted for generating a beam of electrons, a target area, means for deflecting the beam of electrons to cause the same to scan the target area, said target area comprising transparent support base,

alayer of crystals of substantially uniform thickness positioned on the support base, and a translayer being substantially equal to the correspond ing characteristics of the crystal layer.

2. A cathode ray tube for a television receiving system having means therein adapted for generating a beam of electrons, a target area, means for deflecting the beam of electrons to cause the same to scan the target area, said target area comprising a transparent support base, a layer of crystals of substantially uniform thickness positioned on the support base, and a translucent film or auxiliary layer of material positioned upon the crystal layer, said .auxiliary layer having a predetermined secondary electron emissive charductivity, which have the polarization charges leak off within second still do not produce a critical weakening of the useful charge in the substantially shorter period of storing.

A special case' of the above discussed second -measure resides'in that at the given properties of the crystal plate 20 the auxiliary layer 26 is com- .product of the conductivity of the crystal layer acteristi-c, the product of the conductivity of the auxiliary layer times the dielectric constant of the crystal layer' being substantially equal to the in a direction normal to its surface times the ch electric constant of the auxiliary layer.

posed of a material having a much better conductivity (conductivity superior to that of the crystal) but it should be produced with such a 3. A target electrode for a television receiving tube comprising a transparent support base, a layer ofcrystals of substantially uniform thickness positioned on said support base, and an auxiliary film of translucent material positioned upon the crystal layer, the ratio of the conduc-' tivity of the auxiliary film to" the dielectric con- 'stant ofthe auxiliary film being substantially equal to the ratio of conductivity of the crystal layerin a direction through the layer to the dielectric constant of th "crystal layer,

" ED VON "ARDENNE.

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