Image display element

Abstract: In the image display element, where a cathode (2), and a control electrode (3-7) for controlling the electron beams (102) radiated from the cathode are provided within the vacuum cell, a phosphor layer (20) which emits the light by the application of the electron beams, and a metal back layer (101) which is positioned on the phosphor layer (20), is made of aluminum and brings the light emission of the phosphor to the front face (21) by the mirror face effect are provided on the internal face (21) of the face plate for constituting the vacuum cell, the present invention is characterized in that the carbon layer (104) for reducing the generating of the rearward dispersed electrons (103) is provided on the metal back layer (101), and the thickness of the aluminum is adjusted to reduce the rerush of the rearward dispersed electrons (103) as much as possible.

Description: The invention relates to a flat image display comprising a vacuum envelope having a face plate, a back cell and a plurality of vacuum cells having a potential gradient, a plurality of cathodes disposed within said vacuum envelope for radiating electron beams toward said face plate, control electrodes for controlling the electron beams radiated from said cathodes, a phosphor layer on said face plate and positioned to be irradiated by the electron beams and for emitting light when irradiated by the electron beams, and an aluminum back layer on the face of said phosphor layer towards said cathode for directing light emitted by said phosphor layer to the face of said face plate. Conventionally, although Braun tubes are mainly used as display elements for color television image displays, it is impossible to make the conventional Braun tube shorter, because the depth thereof is much longer as compared with the size of the picture face. A flat image display having the same light emitting principle as that of the Braun tube is known from USA patents Nos. 4,451,846 and 4,449,148. Such a flat image display uses electron beams from a plurality of linear thermal cathodes, controlled by a plurality of control electrodes, and colliding against a fluorescent screen. It can display letters, images and so on. Such a flat image display is constructed as shown in Fig. 1. In Fig. 1, a back electrode 51 is adapted to direct into the front face direction electron beams 72 emitted from a plurality of linear thermal cathodes 52a through 52d. An electron beam fetching electrode 53 fetches the electrons of the linear thermal cathode 52a through 52d. Through holes 62 are provided in the electrode 53 to let the electron beams 72 pass through them. A signal electrode 54 which is provided to apply video signals and is composed of a plurality of control electrodes 64. The control electrodes 64 have through holes 63 therein to let the electron beams 72 pass through it. A first focusing electrode 55 and a second focusing electrode 56 are provided to focus the electron beams 72 in the horizontal and vertical directions. Through holes 65 and 66 are provided in the electrodes 55 and 56 to let the electron beams 72 to pass through them. A horizontal deflection electrode 68 deflects the electron beams in the right, left directions of the picture face, and is composed of one set of comb type electrodes 57a and 57b. The electrodes of the comb type electrodes 57a and 57b constitute a slot 67 to let the electron beams 72 pass through. A vertical deflection electrode 71 is provided to deflect the electron beams 72 in the vertical direction of the picture face, and is composed of a set of comb type electrodes 58a and 58b. The comb type electrodes 58a and 58b constitute a slot 70 between the electrodes to let the electrode beams 72 pass through it. A face plate (surface glass cell) 60 has a screen 73 composed of a three color phosphor layers of red, green, blue, a black stripe layer provided between them, and a metal back layer provided behind them on the inner face thereof. A metallic plate 61 made of a back cell, and the face plate 60 constitute a vacuum cell. But in such a conventional display apparatus as described hereinabove, the rearward dispersed electrons generated by the electrons applied upon the metal back layer of the face plate 60 as the interior of the display element have a potential gradient, instead of equipotential like in conventional Braun tubes, and are applied again upon the face plate, thus resulting in lowering the contrast ratio. Fig. 2 A, B are views showing the internal construction of a conventional Braun tube and of a conventional flat image display. In the drawings, the portions which are not necessary for illustration are omitted. In the case of the Braun tube (Fig. 2A) electron beams 82 transmitted from an electron gun 81 are applied upon the metal back 84 positioned on a face 83. Approximately 80 % of the electron beams pass through the metal back 84 and become incident to the fluorescent screen applied upon the face 83 so as to emit the light. But approximately 20 % of the electron beams 82 are reflected on the metal back 84 and become rearward dispersed electrons 85 which are absorbed by a funnel 86 and a shadow mask 87. This is because the interior of the funnel 86 is equipotential. Although approximately 20 % of the electron beams 82 transmitted from a cathode within the electrode 88 become rearward dispersed electrons 85 as in the conventional Braun tube, in the case of the flat image display element of the conventional embodiment of Fig. 2B, a high voltage of approximately 10KV is applied upon the above described metal back 84 with respect to approximately 300V of the electrode 88. Therefore, the element has an electrode gradient therein. The rearward dispersed electrons 85 are applied again upon the metal back 84 of the face 83, and the fluorescent screen, except for the place where the electron beams 82 become incident primarily, emits the light, thus reducing the contrast ratio considerably. It is to be noted that the metal back 84 is composed of aluminum layer. GB-A-2 120 840 discloses a cathode ray tube of the BRAUN type in which the electron beams are accelerated through a large potential difference immediately before impact with a fluorescent screen. This acceleration is provided in a large area channel plate electron multiplier which is disposed close to the face plate of the cathode ray tube. However, within the electron multiplier there occurs impact of electrons so that dispersed secondary electron beams are caused. Accordingly, no high accuracy in deflection can be obtained as is necessary in a flat thin type display device of the type as described in the present application. It is an object of the present invention to avoid the problems inherent in the prior art of flat image display devices, i.e. to prevent the reduction in contrast by rearward dispersed electrons in order to display distinct images of good contrast and brightness. According to the invention, in a flat image display as defined in the preamble of claim 1, a layer of material having an atomic number which is smaller than that of aluminum is provided on top of said aluminum back layer and facing said cathodes for reducing the generation of rearward dispersed electrons at the time of irradiation of said phospor layer by the electron beams, and the aluminum back layer having a thickness in relation to its supply voltage such that the transmission factor of the incident electrons becomes larger than 77 % and the transmission factor of the reflected and dispersed electrons becomes less than 16 %. The layer of material having an atomic number which is smaller than that of aluminum, preferably consists of a carbon layer. Preferable embodiments defining the thickness of the aluminum back layer in relation to the supply voltage of the back layer are defined in the dependent claims. By the above described construction, a flat image display is provided where the generation of the rearward dispersed electrons is reduced by approximately half, the disturbing light emission of the fluorescent screen, except for the location where the electron beams become primarily incident, is also reduced by half, and the contrast ratio is improved twice. These and other objects and features of the present invention will become apparent from the following description taken in conjunction with the preferred embodiments thereof with reference to the accompanying drawings, in which; Fig. 1 is an exploded perspective view showing the basic construction of a conventional image display; Fig. 2 A, B are views showing the inner construction of a conventional Braun tube and of such a conventional image display element; Fig. 3 is a side sectional view of an image display element in one embodiment of the present invention; Fig. 4 is a structural model showing one example of a carbon layer forming method; Fig. 5 is a characteristic graph showing the generation factor (rearward dispersion coefficient) η of the rearward dispersed electrons with respect to the atomic numeral Z of a target to which the electron beams become incident; Fig. 6 is a structural model view showing another example of a carbon layer forming method; and Fig. 7 is a graph showing the relationship of electron energy against energy transmission factor with the thickness of the metal back as parameters. Before the description of the present invention proceeds, it is to be noted that like parts are designated by like reference numerals throughout the accompanying drawings. In Fig. 3, reference numeral 1 is a back electrode equivalent to the back electrode 51 of Fig. 1, reference numeral 2 is a linear cathode equivalent to a linear cathode 52 of Fig. 1, reference numerals 3 through 7 are electrode blocks equivalent to a beam fetching electrode 53, a signal electrode 54, a focusing electrode 55, horizontal . vertical deflecting electrodes 57, 58 of Fig. 1, reference numeral 8 is a screen plate equivalent to a screen 73 of Fig. 1, the screen plate being composed of a glass plate 21, a phosphor 20 to be positioned on it, a metal back (aluminum layer) 101 provided on the phosphor 20, a carbon layer 104 provided on the metal back 101. Reference numeral 102 shows electron beams to be generated from the linear cathode 2, reference numeral 103 is a rearward dispersed electrons (secondary electrons). As can be seen from Fig. 5, the rearward dispersed electrons 103 are 18% of the electron beams 102 incident in a case of aluminum (atomic number 13) normally used even in the metal back 101. The rearward dispersed electrons 103 become 9% of the electron beams 102 incident in the case of carbon (atomic number 6). If a carbon layer 104 is formed on the metal back 101, the generation of the rearward dispersion electrons 103 can be prevented by half, and the contrast ratio is improved twice. Fig. 4 is a structural model of a carbon layer forming method. In the drawing, assume that phosphor 20 and a metal back 101 are already formed on the internal face of a glass plate 21. Carbon liquid 11 with powdered carbon being dissolved in a solvent such as water, alcohol or the like is put into a sprayer 12, is sprayed onto the metal back 101 of the glass plate 21 so as to provide the carbon layer 104. Thereafter, it is burned at approximately 450°C and the face plate is completed as a whole. The thickness of the carbon layer 104 can be adjusted by the spraying time or the spraying amount of the sprayer 12. When the carbon layer 104 is too thick, the passing ratio of the electron beams is lowered, thus reducing the brilliance. Fig. 6 is a structural model of another carbon layer forming method. Assume that phosphor 20 and a metal back 101 are already formed on the internal face of a glass plate 21. A sufficient amount of carbon powder 13 is prepared and a glass plate 21 is placed above it with the metal back 101 being directed downwards. A high voltage of a high tension generator 14 connected with the metal back 101 is applied, and a carbon layer 104 is formed on the metal back 101 by the electric evaporation. Thereafter, it is burned at approximately 450°C and the face plate is be completed as a whole. When the carbon layer is formed by electric evaporation, a more uniform carbon layer may be obtained than by the spraying in the first method. When the high-tension voltage to be applied upon the metal back on the face is comparatively low (in a case 15KV or lower), the face where uneven brilliance is not caused may be formed. Although the carbon is used in the present embodiment, the equal effect may be obtained if the normal temperature solid material which is smaller at the atomic number than aluminum is used. A method of setting the thickness of the metal back 101 will be described with reference to the drawings. Fig. 7 is a graph showing the relation of the electron incident energy to the energy transmission factor when the thickness of the metal back 101 is provided as a parameter. Assume that the metal back 101 is 1000Å in thickness with an electric potential of 10KV being applied upon it in Fig. 3. In this case, the electron beams 102 generated from the linear cathode 2 (potential OV) are accelerated by the potential gradient with respect to the metal back 101, and are applied upon the metal back with the incident energy of 10keV. When the target is aluminum, 18% of the incident electrons are dispersed as rearward dispersed electrons 103, and the energy of the rearward dispersed electrons 103 becomes approximately 6keV (approximately 60 % of the incident energies). The secondary electrons dispersed rearwards rush into the metal back again with an energy of approximately 6keV by the above described potential gradient. When the thickness of the metal back 101 is 1000Å, the energy transmission factor of the incident electrons (10keV) is 92%, the energy transmission factor of the rearward dispersion electrons (6keV) is 64%. While the brilliance is extremely high, the transmission factor of the rearward dispersed electrons is also high, and the contrast is deteriorated. Assume that the thickness of the metal back is made 2000Å, the energy transmission factors of the incident electrons and of the rearward dispersed electrons are respectively 77%, 16%. When the thickness of the metal back 101 is changed from 1000Å to 2000Å, the energy transmission factor (which is proportional to brilliance) of the incident electrons changes from 92% to 77% and the brilliance is also lowered somewhat. But the energy transmission factor (proportional to halation) of the rearward dispersed electrons is reduced as extremely low from 64% to 16%. Therefore, the brilliance is satisfactory and the contrast is also extremely good. But when the thickness of the metal back 101 is increased extremely, the brilliance is also extremely lowered, so that a proper thickness is demanded. By the experiment, it has been found out that a good compromise is in the brilliance and the contrast is obtained if the energy transmission factor of the rearward dispersed electrons is 30% or lower. On the basis of it, the thickness is proper to be 2000Å or more and 3500Å or lower when the voltage of the metal back is 10KV. In the case of 9kV, it is proper to be 1500Å or more and 3000A or lower. In the case of 8KV, it is proper to be 1500Å or more and 2000Å or lower. As described hereinabove, the halation may be considerably reduced within some brilliance reduction by the adjustment of the thickness.

Claims: A flat image display circuit including a flat image display and circuitry having a voltage supply; the flat image display comprising a vacuum envelope having a face plate (8), a back cell (1) and a plurality of vacuum cells having a potential gradient; a plurality of cathodes (2) disposed within said vacuum envelope for radiating electron beams (102) toward said face plate (8); control electrodes (3 ... 7) for controlling the electron beams (102) radiated from said cathodes (2); a phosphor layer (20) on said face plate (8) and positioned to be irradiated by the electron beams (102) and for emitting light when irradiated by the electron beams; and an aluminum back layer (101) on the face of said phosphor layer (20) towards said cathode (2) for directing light emitted by said phosphor layer (20) to the face of said face plate (8); characterized by said image display comprising a uniform layer (104) of material having an atomic number which is smaller than that of aluminum on top of said aluminum back layer (101) and facing said cathodes (2) for reducing the generation of rearward dispersed electrons at the time of irradiation of said phosphor layer (20) by the electron beams (102), this uniform layer being deposited by an electric evaporation process (Fig. 6); said aluminum back layer (101) having a thickness of 1500Å to 3500Å for limiting the transmission factor of rearward dispersed electrons (103) generated at the time of irradiation of the phosphor layer (20) with the electron beams (101) to no more than 30%; and said voltage supply applying a voltage of 8KV to 10KV to said back layer (101). An image display element in accordance with claim 1, where the layer (104) of material having an atomic number which is smaller than that of aluminum, consists of a carbon layer. An image display element in accordance with claim 1 or 2, wherein for a supply voltage of the aluminum back layer (101) of 10KV the thickness is from 2000Å to 3500Å. An image display element in accordance with claim 1 or 2, wherein for a supply voltage of the aluminum back layer (101) of 9KV the thickness is from 1500Å to 3000Å. An image display element in accordance with claim 1 or 2, wherein for a supply voltage of the aluminum back layer (101) of 8KV the thickness is from 1500Å to 2000Å.

Inventors: AIKAWA NOBORU, HOSOKI ATSUNORI, OTAKE KEIICHI, AIKAWA, NOBORU, HOSOKI, ATSUNORI, OTAKE, KEIICHI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1991
Main classification: H01J29
IPC codes: H01J29