WO1998011731A1

WO1998011731A1 - Flat-panel type picture display device

Info

Publication number: WO1998011731A1
Application number: PCT/IB1997/000891
Authority: WO
Inventors: Jeroen Van Velzen
Original assignee: Philips Electronics NV; Philips Norden AB
Current assignee: Koninklijke Philips NV; Philips Norden AB
Priority date: 1996-09-10
Filing date: 1997-07-17
Publication date: 1998-03-19
Anticipated expiration: 1999-03-10

Abstract

Picture display device comprising electron sources and electron transport ducts. The picture display device comprises detectors with measuring elements. The transport duct has measuring apertures through which test currents can be extracted from the transport duct and directed towards the measuring element. The test currents are measured by means of a difference measurement between two measuring electrodes, with a test current being applied to one of the measuring electrodes. This greatly improves the signal-to-noise ratio.

Description

Flat-panel type picture display device

The invention relates to a picture display device having a vacuum envelope provided with a transparent face plate and a luminescent screen, which display device comprises a plurality of electron sources, a plurality of electron transport ducts cooperating with the sources, selection means for extracting each electron current from its transport duct via exit apertures and for directing said current to pixels of the luminescent screen, and detector means tor measuring a test current transported through the transport duct

A display device of this type is known from European Patent Application EP-A 0 663 134 Electron sources are herein understood to mean sources for generating electrons, and electron transport ducts are understood to mean ducts for transporting electrons in the form of electron currents Said Patent Application describes a picture displav device of the flat-panel type Such picture display devices relate to constructions having a transparent face plate and. spaced apart at a small distance, a rear plate, which plates are connected together, for example, by means of side walls and in which pixels in the form of a phosphor pattern are provided on the inner side of the face plate When (video information- controlled) electrons impinge upon the luminescent screen, a visual image is formed which is v isible through the front side of the face plate The face plate may be flat or, if desired, curved

The display device described in European Patent Application EP-A 0 663 134 comprises a plurality of juxtaposed sources for emitting electrons, electron transport ducts cooperating with the sources for transporting electron currents through the transport duct and selectively energizable electrodes for extracting each electron current from its transport duct via exit apertures at predetermined extraction locations, and further means for guiding extracted electron currents towards the luminescent screen for producing an image consisting of pixels The known picture display device further comprises a detector for each transport duct so as to measure a test current passed through the transport duct If the electron currents flowing through the transport ducts exhibit non-uniformities during operation of the picture display device, the displayed image gives a striped impression. By passing a test current through a transport duct and measuring this current by means of a detector, it is possible to distinguish the differences m the ducts and, if desired, to apply corrections in the number of electrons introduced from a source into a transport duct during operation.

Further examples of display devices having electron transport ducts are known from US 5,442.253, US 5,347,199 and US 5,497,046.

However, it has been found that the uniformity of the displayed image can be further improved in practice.

It is an object of the invention to provide a picture display device of the type described in the opening paragraph, with which an improved uniformity of the displayed image can be obtained.

To this end, the picture display device according to the invention is characterized m that the detector means comprise a measuring unit having at least two measuring electrodes, and means for measuring the voltage at a measuring electrode and/or the current applied to a measuring electrode, while, in operation, a first measuring electrode of the measuring means receives the test current transported through a transport duct and a second measuring electrode does not receive the test current, and the detector means comprise means for determining a difference of voltage and/or current measurement.

In the known state of the art, the detector means comprise a measuring electrode which measures the number of electrons arriving in the upper part of the transport duct The known state of the art mentions a few causes of the occurrence of non-uniformity of the displayed image. All of these causes relate to variations in driving a wire cathode, the geometry of the wire cathode and the position of the wire cathode with respect to the transport ducts The invention is also based on the recognition that non-uniformity still occurs, even if there are no variations in the above-mentioned properties. In addition to the test current, a measuring electrode also measures a noise signal. A display device of the type described in the opening paragraph comprises, inter alia, selection means. Alternating electric voltages are applied to these selection means. These alternating voltages disturb the voltage at the measuring electrode. Moreover, applied voltages such as the high voltage, may produce an interference signal. By performing a difference measurement, in which the difference in voltage or current between a first measuring electrode, to which the test current is applied, and a second measuring electrode is determined, it is possible to considerably improve the signal-to-noise ratio by suppressing the noise signal

The detector means preferably comprise a measuring unit for each transport duct.

For example, if there is only a slight extent of non-uniformity, it is possible to provide a relatively small number of transport ducts with a measuring unit For example, if the picture display device is provided with a plurality of juxtaposed transport ducts, every n"¹ transport duct (n = 2, 3, 4 etc.) may be provided with a measuring unit The number of measuring elements and hence the complexity of the measurement is relatively simple in such an implementation The possibilities of correcting non-uniformity are, however, limited, and each transport duct is preferably provided with a measuring element and the associated extraction means and further means.

The picture display device preferably comprises shielding electrodes for electromagnetically shielding the measuring electrodes

Electromagnetic shielding reduces the disturbing influence, which may notably be caused by capacitive coupling, on the detector means by alternating voltages which are applied to other electrodes during operation

The invention also relates to a selection structure for a display picture tube, the selection structure having a number of apertured plates, the selection structure having entrance apertures and entrance electrodes and exit apertures and selection means for selectively directing electron current from the entrance apertures to exit apertures, characterized in that the selection structure comprises at least one measuring unit integrated in the apertured plates, the measuring unit comprising a measuring aperture and at least two measuring electrodes, means for directing a test current entering the measuring aperture to one of the measuring electrodes and leads for measuring current and/or voltages on both of at least two measuring electrodes

Preferably the selection structure also comprises shielding electrodes for electromagnetically shielding the measuring electrodes. The selection structure in itself can be made and sold separately from the rest of the display unit, and although the invention, is very suitable for display device in which currents are extracted from electron ducts, for the inventive idea in its most general form the source of the electron currents entering the selection structure is of secondary importance The electron entering the entrance apertures of the selection structure could for instance be directly (i.e. not via a transport duct) injected into the entrance apertures e.g. from a point-like electron source (such as a field-emitting tip) or a source in a line form (such as a line cathode or a plasma channel)

These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.

In the drawings:

Fig. 1A is a diagrammatic perspective elevational view, partly broken away, of a display device according to the invention,

Fig. IB is a cross-section through a display device of Fig. 1A,

Fig. 2 A is a diagrammatic perspective elevational view, partly broken away, of a display device provided with a preselection and a fine selection,

Fig. 2B is a cross-section of a display device of Fig. 2A, Fig. 3 is a cross-section of a detail of a further embodiment of a picture display device according to the invention,

Fig. 4 shows a measuring circuit, and

Fig. 5 shows a further measuring circuit.

The Figures are diagrammatic and generally not to scale. Figs. 1A and IB show a flat-panel type display device 1 having a display panel ( window or face plate) 3 and a rear wall 4 opposite said panel. A luminescent screen 7 having a repetitive pattern (lines or dots) of. for example, triplets of red (R), green (G) and blue (B) luminescing phosphor elements is provided on the inner surface of window 3. The luminescent screen may alternatively be a monochrome screen. To be able to apply the required high voltage, the luminescent screen 7 is either provided on a transparent, electrically conducting layer, for example a layer of ITO (indium tin oxide) or may have an electrically conducting layer (for example, an aluminum layer). In a preferred embodiment, the (doi-shaped) phosphor elements of a triplet are located at the vertices of a substantially isosceles or equilateral triangle.

An electron source arrangement 5, for example a line cathode which, by means of control electrodes, provides a large number of (for example, 600) electron emitters or a corresponding number of separate emitters is arranged proximate to a bottom poπion 2 interconnecting display panel 3 and rear wall 4 Each of these emitters is to supply a relatively small current so that many types of cathodes (cold or thermionic cathodes) are usable as emitters For example, the picture display device may be provided with a separate cathode for each transport duct Each emitter may be driven separately or, if they are combined to one line cathode, driven jointly The electron source arrangement 5 is arranged opposite entrance apertures 6 of a row of transport ducts extending substantially parallel to the screen In this embodiment, each entrance aperture is provided with a first electrode Gl and a second electrode G2 In this embodiment, Gl is a control electrode which can be driven separately for each duct and G2 is an electrode which is common for some or all ducts Together, they constitute the line cathode and the electrodes Gl and G2 constitute a tπode By heating the line cathode and applying a potential difference between cathode 5 and electrode G2, electrons are emitted in the relevant transport duct The potential difference between cathode 5 and electrode Gl is controllable, so that the intensity of the electron current introduced into the transport duct can be controlled In this embodiment, the transport ducts comprise ducts 11 , 11 ', 11 " defined by the rear wall 4 and partitions 12, 12' . 12' , which ducts are further referred to as ' transport ducts 1 1 , 1 1 ' , 1 1 " " for the sake of simplicity At least one of the walls, preferably the wall opposite the exit apertures 8, 8^' . 8", is made of a material which is suitable for realizing electron transport by means of secondary emission The electrical resistance of the wall material has a suitable high resistance in the longitudinal direction of the ducts, and in the area where electron transport is to take place it has a secondarv emission coefficient of more than 1 The electrical resistance of the wall material has such a vaiue that little current, preferably less than 10 mA, flows as a total current in the walls at a field strength in the transport ducts of the order of several tens to several hundred volts per cm required for electron transport through the transport ducts During operation, a voltage generating the field strength required for transport is present across a transport duct (in the longitudinal direction of the duct) By applying a voltage of several tens to several hundred volts between the wire cathodes 5 (or electron sources of other types) and electrodes G 1 , G2 arranged at the entrance apertures of the transport ducts in which the value of the last-mentioned voltages is dependent on circumstances such as the size of the apertures 6 and the distance between the cathode and the apertures as well as the desired current, electrons are accelerated towards the transport ducts and introduced into these transport ducts In the transport ducts, the electrons impinge upon a wall, while secondary electrons are generated which in their turn are further transported by the electric field applied in the transport duct The secondary electrons impinge upon the wall and generate further secondary electrons. The result is that an electron current flows in the transport duct, with the incoming current being equal to the outgoing current. For a detailed description of the operation of such transport ducts, reference is made to said European Patent Applications EP-A 0 663 134 and EP-A 0 400 750. An electron current can be guided from the transport duct via exit apertures 8, 8' , 8" in a perforated plate 10. By means of electrodes 9, 9', 9" , an electric field attracting the electrons towards the exit aperture 8 can be created locally, i.e. in the ambience of an exit aperture 8. This is denoted in the Figure by means of a pulse symbol. Extracted electrons can be subsequently accelerated towards the screen 7 by means of an acceleration voltage applied in operation between the plate 10 and the luminescent screen 7. In this embodiment, horizontal partitions 112, 112', 112" are placed between the display panel 3 and the perforated plate 10. Instead of the partitions shown in the Figure, it is alternatively possible to use a second perforated plate. In the known state of the art, each transport duct is provided with a measuring element which measures the number of electrons arriving in the upper part of the relevant transport duct. Fig. IB shows a very simple embodiment of a picture display device according to the invention. Measuring aperture 21 is provided with an electrode 22. This provides the possibility of extracting electrons from the transport duct 11. The extracted electrons are directed towards a measuring electrode 23 A. This may be effected by applying, for example a potential difference between the measuring electrode 23 A and the electrode 9. A circuit for measuring the current and/or the voltage at measuring electrode 23A is coupled to this measuring electrode 23 A. The adjacent compartment comprises a measuring electrode 23B. A circuit for measuring the current received by the measuring electrode 23B is coupled to this electrode. The measurements on electrodes 23 A and 23B are compared with each other in a comparison means. This comparison yields a much better measurement of the current applied because interference signals are approximately equally large for both electrodes 23A and 23B, while only electrode 23A supplies a signal as a result of the test current. The signal-to-noise ratio is thereby improved considerably.

In this embodiment, the perforated plate 10 is provided with the exit apertures 8, 8' , 8" as well as the measuring aperture 21. Such an embodiment is preferred to an embodiment in which measuring apertures 21 are implemented in a separate element. The number of required elements is reduced by making use of one and the same perforated plate 10. Moreover, the position of the measuring apertures 21 with respect to the exit apertures 8, 8' , 8" can be accurately determined and is not dependent on the accuracy with which an extra element is positioned with respect to the perforated plate 10. A variation of the position of the measuring apertures 21 with respect to the exit apertures 8, 8', 8" leads to variations of the conformity between the measured currents and the currents directed towards the display screen.

Fig. IB shows that, viewed from the entrance aperture of the transport duct, the measuring aperture is the first aperture. This is a preferred embodiment. The preferred positions for the measuring aperture are either the first aperture or the last aperture. With respect to a position of the measuring apertures between the exit apertures, the disturbing influences of the signals applied in operation to electrodes 9, 9', 9" and other selection electrodes are reduced at these positions. The measuring aperture is preferably the first aperture. The electron current emitted m a transport duct is dependent on the potentials applied to the source 5 and the electrodes Gl and G2. By positioning the measuring aperture, and hence the detector, relatively close to source 5, Gl and G2, it is possible to establish relatively short electrical connections between the detector and the source. This reduces the risk of disturbing influences by other signals. In Fig. IB, the detector is provided with shielding electrodes 28 and 29. The shielding electrodes operate as an electromagnetic

Faraday cage and reduce the capacitive coupling between the detector and the electrodes 9, 9' , 9" and further selection electrodes. Within the context of the invention, the first and last apertures are understood to mean the first and last apertures through which electrons can be extracted from the transport ducts, i.e. the first and last "active" apertures Blind apertures may be present between the entrance aperture and the measuring aperture, or further blind apertures may be present beyond the last "active" aperture

Figs 2A and 2B show a picture display device according to the invention in which stepped selection is used Stepped selection is herein understood to mean that the selection from the transport ducts 11 , 11 ' , 11 " to the luminescent screen 7 is realized in at least two steps, l e a first (coarse) step of selecting, for example the pixels and a second, fine step of selecting, for example the color pixels An active color selection system 100 comprising an (active) preselection plate 10a, a spacer plate 10b and an (active) (fme- )selectιon plate 10c is arranged in the space between the transport ducts and the luminescent screen which is provided on the inner wall of the display panel 3 Plates 10a, 10b and 10c are perforated plates, i.e. they are provided with a pattern of apertures Structure 100 is separated from the luminescent screen 7 by a spacer plate 101 , for example, an apertured insulating plate.

Fig. 2B is a diagrammatic cross-section of a part of the display device of Fig 2A in greater detail, particularly the active color selection plate structure 100. This plate structure 100 comprises a preselection plate 10a with exit apertures 8, 8' and a fme-selection plate 10c with groups of apertures R, G, B. In this embodiment, the apertures R, G, B are positioned in a triangle, but for the sake of clarity all three of them are shown diagrammatically in the cross-section in Fig 2B. A phosphor element R' , G\ B' corresponds to each aperture R, G, B. In this case, three fine selection apertures R, G, B are associated with each exit aperture 8, 8' Other numbers are alternatively possible, for example, 6 fine- selection apertures for each preselection aperture, etc. A spacer plate 10b is arranged between the preselection plate 10a and the fine-selection plate 10c. This spacer plate includes communication leads 30, 30' having a shape adapted to the shape of the phosphor color pixels (for example, circular or triangular triplets). The apermres R, G, B are provided with electrodes 13, 13' , 13" with which the electron current can be extracted from spaces 30

The electron transport ducts 11 , 1 1 ', 11 " are formed between the structure 100 and the rear wall 4 To be able to extract the electrons from the transport ducts through the exit apertures 8, 8' , 8", electrodes 9, 9', 9" are arranged on the screen-sided surface of the plate 10a in this embodiment

In addition to exit apertures 8, 8' , 8", plate 10a also has a measuring aperture 21 which is provided with an electrode 22 for extracting the electrons from the transport duct 1 1. Moreover, plate 10b has a further measuring aperture 27 and an electrode 28 for extracting electrons from space 30" Measuring element 23 in the form of a measuring electrode is arranged behind aperture 27 Shielding electrodes 102 and 103 are arranged in the icinity of the measuring element 23 for electromagnetically shielding the measuring element from the signals applied, in operation, to electrodes 9 and 13

A blind duct 8'" is present between the measuring aperture 21 and the most proximate active exit aperture 8 This duct increases the distance between the electrodes 9 and 13 and the measuring element 29 The disturbing influence by the signals applied to the electrodes 9 and 13 on the signal to be measured is thereby reduced. Measuring electrode 23B is arranged proximate to measuring electrode 23A

Fig 3 is a detailed cross-section of a further embodiment of a picture display device according to the invention In this embodiment, the picture display device comprises a stack of plates 51, 52, 53, 54, 55, 56, 57 and 58 Plate 51 corresponds to plate 10a, plates 52 and 58 are spacer plates and plates 53 to 57 are selection plates. The selection plates comprise apertures, for example aperture 61 in plate 53 having one entrance aperture 63 and two or more exit apertures 64, 65, with the associated selection electrodes 66, 67 By applying suitable voltages to the electrodes, an electron current can be extracted from the transport duct 11 via the exit aperture 8' and can subsequently be further passed on to one of the phosphor elements R, G. B. In this embodiment, an arrow indicates a possible trajectory of an electron current. The selection is accompanied by the application of alternating electric voltages to the different electrodes around selection apertures Particularly as a result of capacitive coupling, these alternating voltages may induce a "false" signal on the measuring element Such a "false" signal reduces the accuracy with which the test current can be measured. Even if all selection voltages are switched off during measurement, there is still a disturbing effect of residual voltages on the selection electrodes Moreover, disturbances occur as a result of interference or noise of power supply voltages. A blind duct 111 is present between measuring aperture 21 and the most proximate exit aperture 8' This increases the distance between the electrodes around the selection apertures and the detector, more particularly the measuring element of the detector This reduces interferences on the measuring signal as a result of capacitive coupling Plate 56 is provided with measuring electrodes 71A and 71B The test current is directed to measuring electrode 71A Plates 54, 56 and 58 are further provided with shielding electrodes 72, 73 and 74 Unlike the other electrodes, these shielding electrodes are not provided with drive voltages for controlling electron currents but are used for shielding the measuring electrode 71 The impedance for high frequencies between the electrodes 72, 73 and 74 is preferably low, which may be realized for example by interconnecting these electrodes The assembly of shielding electrodes is preferably capacitively coupled to a point at a fixed interference- free potential, for example earth Interference is now decoupled in a very effective wa\ because the electromagnetic "Faraday cage" is formed by which the shielding electrodes shield capacitive interference signals It is to be noted that the favorable effect of providing the detector means improves the measuring sensitivity of the detector, irrespective of the way in which the measuring current is generated, I e whether electron currents are extracted via a measuring aperture in the transport duct

An alternative arrangement of the wire cathode is also shown in this embodiment Wire cathode 5 is arranged more or less adjacent to the detector, with plate 51 comprising the electrodes G l and G2 In this embodiment, the perforated plates comprise both the exit apertures and the means for directing the electron currents towards the phosphor elements, as well as the different components of the measuring unit The measuring unit is thus integrated in the perforated plates It is not necessary to manufacture extra components, which is a great advantage The measuring electrode measures the number of electrons landing on the electrode. This measurement can be utilized for analyzing the operation of the apparatus, and this analysis can be used in turn to apply feedback signals to the wire cathode or to the electrodes Gl and/or G2. For example, if it appeared that relatively few electrons (with respect to an expected value or measured values of other transport ducts) would reach the measuring electrode, which is an indication that relatively few electrons are emitted in the transport duct, then either the temperature of the wire cad ode, or the voltage at G2 or at Gl might be increased, or any combinations of these measures might be used so that more electrons are emitted in the transport duct. The effect of these measures can then be checked again by measuring the number of electrons at the measuring element. The uniformity of the displayed image can thereby be improved.

Fig. 4 shows an arrangement for measuring a difference voltage and/or a current between the electrodes 23A and 23B, or 71A and 71B. A voltage V_ιn is applied across resistors R, and R₂ and the capacitances C. If necessary, switch S, and S_; may be added. The voltages denoted by V_a and V_b can be compared in electrodes 80. V_out is equal to V_j-V,,, in which V_a is the measuring signal plus the interference signal at electrode 71 A, or 23A, and V_b is the interference signal at electrode 71B, or 23B. The difference measurement yields the signal plus the difference between the interference signals. This interference signal difference is much smaller than the interference signals themselves so that the signal-to-noise ratio is greatly improved. Due to the presence of the shielding electrodes, as in the preferred embodiments, the difference in interference signals can be reduced, which further improves the signal-to-noise ratio.

Fig. 5 shows an alternative arrangement. Measuring electrodes 71A. 23A are connected to a power supply V_m via coil 51 A of transformer 51. Electrodes 7 IB, 23B are connected to power supply V,,. via coil 51B which is wound in a sense opposite to coil 51 A. The voltage difference is measured and amplified by means of coil 51D at core 51C.

It will be evident that many variations are possible within the scope of the invention. In the embodiments described, the test current is measured, for example, by measuring electrodes. The current applied to the measuring electrode can be measured by means of a current or voltage measurement. For the sake of simplicity of construction, this is a preferred way of measuring the test currents. Electron transport ducts formed as channels are shown in the embodiments. Within the scope of the invention, transport ducts are understood to mean any means by which, in operation, an electron current is passed from one or more starting points to the exit apertures by means of secondary emission. The means for directing the currents, which are shown in the Figures, are relatively complex means. Within the scope of the invention, these means for directing the current are understood to be any means which, in operation, direct a test current from the measuring aperture to the measuring element. In the Figures shown, each measuring unit has its own measuring electrode. In a preferred embodiment, a plurality of measuring units comprises a common measuring electrode. The number of measuring electrodes is thereby reduced.

Claims

CLAIMS:

1. A picture display device having a vacuum envelope provided with a transparent face plate and a luminescent screen, which display device comprises a plurality of electron sources, a plurality of electron transport ducts cooperating with the sources, - selection means for extracting each electron current from its transport duct via exit apertures and for directing said current to pixels of the luminescent screen, and detector means for measuring a test current transported through the transport duct, characterized in that the detector means comprise a measuring unit having at least two measuring electrodes, and means for measuring the voltage at a measuring electrode and/or the current applied to a measuring electrode, while, in operation, a first measuring electrode receives the test current transported through a transport duct and a second measuring electrode does not receive the test current, and the detector means comprise means for determining a difference of voltage and/or current measurement.

2. A picture display device as claimed in Claim 1 , characterized in that the detector means comprise a measuring unit for each transport duct.

3. A picture display device as claimed in Claim 1 or 2, characterized in that the picture display device comprises shielding electrodes for electromagnetically shielding the measuring electrodes.

4. A selection structure for a display picture tube, the selection structure having a number of apertured plates, the selection structure having entrance apertures and entrance electrodes and exit apertures and selection means for selectively directing electron current from the entrance apertures to exit apertures, characterized in that the selection structure comprises at least one measuring unit integrated in the apertured plates, the measuring unit comprising a measuring aperture and at least two measuring electrodes, means for directing a test current entering the measuring aperture to one of the measuring electrodes and leads for measuring current and/or voltages on both of at least two measuring electrodes.

5. A selection structure as claimed in claim 4, characterized in that the selection structure also comprises shielding electrodes for electromagnetically shielding the measuring electrodes.