WO1997022135A1

WO1997022135A1 - Flat-panel type display device

Info

Publication number: WO1997022135A1
Application number: PCT/IB1996/001307
Authority: WO
Inventors: Nicolaas Lambert; Jeroen Van Velzen
Original assignee: Philips Electronics N.V.; Philips Norden Ab
Priority date: 1995-12-11
Filing date: 1996-11-27
Publication date: 1997-06-19
Also published as: EP0808511A1; JPH11500862A

Abstract

A display device comprising electron sources and electron-transport ducts. The display device includes detectors with measuring elements. The transport duct has measuring apertures through which trial currents can be extracted from the transport duct and guided to the measuring element. Such a construction enables an improved uniformity of the picture displayed to be achieved.

Description

Flat-panel type display device.

The invention relates to a display device having a vacuum envelope which is provided with a transparent front wall with a luminescent screen, which display device comprises a number of electron sources, - a number of electron-transport ducts which cooperate with said sources, selection means for withdrawing each electron current from its transport duct via exit apertures and guiding it to pixels of the luminescent screen, and detector means for measuring a trial current transported through the transport duct. Such a display device is known from European Patent Application EP-A 0

663 134. The expression "electron sources" is to be understood to mean herein sources for generating electrons, and the expression "electron-transport ducts" is to be understood to mean herein ducts for transporting electrons in the form of electron currents. Said Patent Application describes a flat-panel type display device. Such display devices relate to constructions having a transparent face plate and a rear plate, which is arranged at a small distance from said face plate, said face plate and rear plate being interconnected, for example, by side walls, and pixels in the form of a phosphor pattern being provided on the inner surface of the face plate. When (video-information controlled) electrons are incident on the luminescent screen, an image is formed which is visible via 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 which cooperate with said sources and which serve to transport electron currents through the transport duct, and selectively energizable electrodes for withdrawing each electron current from its transport duct at predetermined extraction locations via exit apertures, and further means for guiding extracted electron currents to the luminescent screen for producing a picture composed of pixels. The known display device further comprises a detector for each transport duct to measure a trial current which is sent through the transport duct. If, during operation of the display device, non-uniformities occur in the electron currents flowing through the transport ducts, the picture displayed exhibits smears. By sending a trial current through a transport duct and measuring said current by means of a detector, the differences between the ducts can be distinguished and, if necessary, corrections regarding the number of electrons which, in operation, are introduced into a transport duct from a source can be carried out.

It has been found, however, that the uniformity of the picture displayed can be further improved in practice.

It is an object of the invention to provide a display device of the type mentioned in the opening paragraph, which enables an improved uniformity of the picture displayed to be achieved.

To this end, a display device in accordance wim the invention is characterized in that the detector means comprise a measuring unit having an extraction means for extracting electrons from the transport duct through a measuring aperture in the transport duct, and - directing means for directing the extracted electron current from the measuring aperture towards a measuring element.

The state-of-the-art detector means comprise measuring elements which measure the number of electrons which arrive at the top of the transport duct. The above document in accordance with the prior art mentions a number of causes of non-uniformity in the picture displayed. These causes all relate to drive-mode variations of a wire cathode, in the geometry of the wire cathode and in the position of the wire cathode relative to the transport ducts. The invention is also based on the recognition that non-uniformity will always occur, even if variations in the above properties do not occur. In operation, the electron currents which are directed towards the phosphor screen are extracted from a transport duct. This extraction process may cause variations in the brightness of the picture displayed. By arranging the measuring element behind an extraction means, the term "behind" meaning in this respect "viewed in the direction of the electron current", the measuring element is sensitive to differences in the effectiveness with which the electron current is extracted from the transport duct. In the prior art, the measuring elements measure the number of electrons which arrive at the top of the transport ducts, i.e. if no electron currents at all are extracted. Consequently, the known measuring elements are incapable of determining differences in extraction efficiency. As the measuring elements in the display device in accordance with the invention are capable of measuring such differences, it is possible to improve the uniformity of the picture displayed. Preferably, the detector means comprise a measuring unit for each transport duct.

For example, if only a small degree of non-uniformity occurs, it is possible to provide a relatively small number of transport ducts with a measuring unit. For example, if the display device comprises a number of juxtaposed transport ducts, every n"¹ (n = 2, 3, 4, etc.) transport duct can be provided with a measuring unit. The number of measuring elements and hence the complexity of the measuring process is relatively low in such an embodiment. However, the possibilities of correcting non-uniformity are limited and, preferably, each transport duct is provided with a measuring element and the associated extraction means and further means.

Preferably, the exit apertures in a transport duct and the measuring aperture in said transport duct are similar in form.

By giving the exit apertures in a transport duct and the measuring aperture in said transport duct a similar form, it is precluded that undesirable differences in measurements occur due to differences in the form of the apertures, i.e. differences between the measuring aperture and the exit apertures.

Preferably, the display device comprises a perforated plate in which both me exit apertures and the measuring apertures are formed.

By forming the exit apertures and the measuring apertures in one and the same perforated plate, both the construction of the display device and the process of giving exit apertures and measuring apertures a similar form are simplified. In addition, a possible error caused by variations in the position of the measuring apertures relative to the exit apertures is precluded.

In an embodiment, the measuring aperture is one of the exit apertures, and the directing means comprise a drivable coupling channel with the means for guiding electron currents extracted through the relevant exit apertures to the luminescent screen.

In this embodiment, the electron current, which is extracted from a transport duct through an exit aperture, is drawn off by the directing means via the drivable coupling channel, and the directing means guide said electron current to the measuring element.

This embodiment has the advantage that a measuring element measures an electron current, which is extracted from the transport duct and, if the coupling is not activated, which actually contributes to the picture displayed. This enables the measurement to be improved. The above embodiment has the disadvantage, however, that the drive of the means for directing electron currents can disturb the measurements by capacitive interference.

For this reason, preferably the measuring aperture is not an exit aperture and there is no coupling duct between the means for guiding the electron current and the directing means.

As a result, capacitive crosstalk between the means for guiding the electron currents to the luminescent screen and the measuring element is reduced. By virtue thereof, the accuracy with which measurements are carried out is improved, which has a positive effect on the uniformity of the picture displayed.

If the display device comprises further, perforated elements on which and/or in which the means for guiding the electron currents are secured, these perforated elements also include the directing means and the measuring element.

Relative to a construction in which the directing means and/or the measuring element is/are secured on or in other elements, such a construction leads to a reduction of the number of elements to be used. This leads to lower costs and precludes variations which could occur as a result of a correct or incorrect positioning of the various elements relative to each other. In the preferred embodiment, the perforated elements comprise parts of the means for guiding electron currents to the luminescent screen as well as parts of the directing means and/or of the measuring element.

Preferably, the display device comprises shielding electrodes for electromagnetically shielding the measuring element.

An electromagnetic shield reduces the disturbing influence on the measuring element, which may occur, in particular, as a result of capacitive coupling of different voltages which, in operation, are applied to other electrodes.

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

In the drawings:

Fig. IA is a schematic, perspective view, partly broken away, of a display device in accordance with the invention,

Fig. IB is a cross-sectional view of a display device shown in Fig. IA, Fig. 2A is a schematic, perspective view, partly broken away, of a display device which is provided with a pre-selection and a fine selection.

Fig. 2B is a cross-sectional view of a display device as shown in Fig. 2 A,

Fig. 2C is a cross-sectional view of a detail of a further embodiment of a display device in accordance with the invention,

Fig. 3 is a sectional view of a detail of a further embodiment of a display device in accordance with the invention.

The Figures are schematic and, in general, not drawn to scale.

Figs. IA and IB show a flat-panel type display device 1 having a display panel (window or face plate) 3 and a rear wall 4 which is located opposite said display panel. On the inner surface of window 3 there is provided a luminescent screen 7 having a repetitive pattern (lines or dots), for example, of triads of phosphor elements luminescing in red (R), green (G) and blue (B), respectively. The luminescent screen may alternatively be a monochrome screen. The luminescent screen 7 is either provided on a transparent, electroconductive layer, for example a layer of ITO (indium tin oxide), or provided with an electroconductive layer (for example, an aluminium layer) to enable the necessary high voltage to be applied. In accordance with a preferred embodiment, the (dot-shaped) phosphor elements of a triad are situated at the vertices of a substantially isosceles or equilateral triangle. An electron-source arrangement 5, for example a line cathode which provides a large number (for example 600) of electron emitters by means of control electrodes, or a corresponding number of separate emitters, is arranged near a bottom portion 2 which connects the display panel 3 and the rear wall 4 with each other. Each individual emitter only has to supply a relatively small current, so that many types (either cold or thermionic) of cathodes can be used as emitters. For example, for each transport duct, the display device can be provided with a separate cathode. Each of me emitters can be arranged separately or, if the emitters are combined to form one line cathode, they can be arranged jointly. The electron-source arrangement 5 is arranged opposite entrance apertures 6 of a row of transport ducts which extend substantially parallel to the screen. In this example, each entrance aperture is provided with a first electrode GI and a second electrode G2. In this example, GI is a control electrode which can be driven for each individual duct, and G2 is an electrode which is common to several or all ducts. The line cathode and the electrodes GI and G2 togemer form a triode. Electrons are emitted in the relevant transport duct by heating the line cathode and applying a potential difference between cathode 5 and electrode G2. The potential difference between cathode 5 and electrode GI can be adjusted, so that the intensity of the electron current introduced into the transport duct can be adjusted. In this example, the transport ducts comprise ducts 11, 11', 11 ", for simplicity hereinafter also referred to as "the transport ducts 11, 11', 11", which are defined by the rear wall 4 and intermediate walls 12, 12', 12". At least one of said walls, preferably the wall opposite the exit apertures 8, 8', 8", is made of a material which is suitable for electron transport by means of secondary emission. The electric resistance of the wall material has a suitable high value in the longitudinal direction of the ducts, and a secondary emission coefficient above 1 across the area where electron transport should take place. The electric resistance of the wall material has such a value that, at a field intensity in the transport ducts of the order of several tens to several hundred volts per centimeter, which is necessary for the electron transport through the transport ducts, a small total amount of current, preferably less than 10 mA, will flow in the walls. In operation, a voltage is applied to a transport duct (in the longitudinal direction of the duct), which generates the field intensity necessary for transport. By applying a voltage of the order of several tens to several hundred volts (the value of the voltage being governed by factors such as the size of the apertures 6, the distance between the cathode and the apertures and the desired current) between the wire cathodes 5 (or different electron sources) and the electrodes GI, G2 provided near the entrance apertures 6 of the transport ducts, electrons are accelerated towards the transport ducts and introduced into said ducts. In the transport ducts, the electrons impinge on a wall, leading to the generation of secondary electrons which, in turn, are transported by the electric field applied in the transport duct. The secondary electrons are incident on the wall and generate further secondary electrons. As a result, an electron current flows in the transport duct, 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 Application EP-A 0 663 134 and European Patent Application EP-A 0 400 750. An electron current can be withdrawn 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 which draws the electrons towards the exit aperture 8 can be created locally, that is in the vicinity of an exit aperture 8. In the Figure, this is shown by means of a pulse sign. Extracted electrons can subsequently be accelerated towards the screen 7 by means of an accelerating voltage which, in operation, is applied between plate 10 and the luminescent screen 7. In this example, horizontal partitions 112, 112', 112" are arranged between the display panel 3 and the perforated plate 10. It is alternatively possible to use a second perforated plate instead of the partitions shown herein. In the prior art, each transport duct is provided with a measurmg element which measures the number of electrons which arrive at the top of the relevant transport duct. Fig. IB shows a very simple embodiment of a display device m accordance with the invention. Measurmg aperture 21 is provided with an electrode 22. This enables electrons to be extracted from the transport duct 11. The extracted electrons are guided to a measurmg element 23. This can be achieved by applying a potential difference between the measuring element 23 and the electrode 9. The measuring element may be, for example, an electrode to which a circuit for measuring the current received by the measuπng element is coupled. This method of measuring a current has the advantage, relative to measuring the current which arrives at the end of a transport duct, as in the known display device, that an extracted electron current is measured. Extraction of electron currents from the transport duct may cause variations intensity on the screen. By measurmg an extracted electron current for a number of transport ducts or, preferably, for each transport duct, it is possible to measure these variations and, if desired or necessary, said extracted electron current can be corrected for these variations. Preferably, the measuring aperture and the exit apertures are substantially similar in form. The expression "substantially similar in form" is to be understood to mean that the shape of the measurmg aperture does not deviate substantially from the average shape of the exit apertures m the relevant transport duct. A shape which does not deviate substantially from the shape of the exit apertures precludes that, due to a difference in shape, the efficiency with which the electrons are extracted from a transport duct is different for the measurmg aperture and the exit apertures. Said differences lead to inaccuracies in the measurements.

In this example, the perforated plate 10 is provided with exit apertures 8, 8', 8" and the measuπng aperture 21. Such an embodiment is preferred to an embodiment in which measuring apertures 21 are formed in a separate element. The number of necessary elements is reduced by using one and the same perforated plate 10. Further, the position of the measuring apertures 21 relative to the exit apertures 8, 8', 8" can be accurately determined and does not depend on the accuracy with which an additional element is positioned relative to the perforated plate 10. A variation in the position of the measuring apertures 21 relative to the exit apertures 8, 8', 8" leads to variations in the correspondence between the measured currents and the currents which are guided to the display screen.

Fig. IB shows that the measuring aperture, viewed from the entrance aperture of the transport duct, 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. When the measuring apertures are m these positions instead of between the exit apertures, the disturbing influences of the signals which are supplied, in operation, to electrodes 9, 9', 9" and other selection electrodes are reduced. Preferably, the measuring aperture is the first aperture. The electron current which is emitted in a transport duct is governed by the potentials applied to the source 5 and the electrodes GI and G2. By arranging the measuring aperture and hence the detector relatively close to the source 5, GI and G2, relatively short electric connections can be made between the detector and the source. By virtue thereof, the risk of disturbing influences of other signals is reduced. In Fig. IB it is shown that the measuring aperture 21 and the next following exit aperture 8"' are situated at a distance from each other which is larger than the distance between the exit apertures in a transport duct. In this example, a blind aperture 27, that is an aperture which cannot be energized, is situated between the measuring aperture 21 and the exit aperture 8", which blind aperture is not provided with an electrode and hence electrons cannot be extracted through said aperture. If mis aperture is provided with an electrode, in operation, this electrode is not energized in such a way that electrons are extracted from the transport duct via said apertures. The distance between the measuring aperture and the nearest exit aperture causes capacitive coupling between the detector and the electrodes 9, 9', 9" and further selection electrodes to be reduced. In Fig. IB, the detector is provided with shielding electrodes 28 and 29. Said shielding electrodes act as an electromagnetic Faraday cage and reduce capacitive coupling between the detector and the electrodes 9, 9', 9" and further selection electrodes. Within tlie scope of the invention, the expressions "first aperture" and "last aperture" are to be understood to mean the first aperture and the last aperture, respectively, through which electrons can be withdrawn from the transport duct. Consequently, the first "active" aperture and the last "active" aperture, respectively. Blind apertures (similar to aperture 27) may be situated between the entrance aperture and the measuring aperture, or further blind apertures may be situated beyond the last "active" aperture.

Figs. 2A and 2B show a display device in accordance with the invention, in which multistage selection is used. The expression "multistage selection" is to be understood to mean herein that the selection from the transport ducts 11, 11', 11 " towards the luminescent screen 7 is carried out in at least two steps. In a first (coarse) step, for example, the pixels are selected and in a second, fine step, for example, the color elements are selected. In this case, an active color-selection system 100, which comprises an (active) pre-selection plate 10a, a spacer plate 10b and an (active) (fine) selection plate 10c, is arranged in the space between the transport ducts and the luminescent screen, which is provided on the inner surface of the display panel 3. Plates 10a, 10b and 10c are perforated plates, i.e. they are provided with a pattern of apertures. A structure 100 is separated from the luminescent screen 7 by a spacer plate 101 , such as an apertured, electrically insulating plate. Fig. 2B is a schematic, sectional view of a part of the display device of

Fig. 2 A in greater detail, in particular the active color-selection plate structure 100. This plate structure 100 comprises a pre-selection plate 10a with exit apertures 8, 8', and a fine- selection plate 10c with groups of holes R, G, B. In this example, the holes R, G, B are positioned so as to foπn a triangle, but, in the sectional view of Fig. 2B, all three are represented schematically for the sake of clarity. 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 also possible, for example 6 fine-selection apertures etc. for each pre-selection aperture. A spacer plate 10b is arranged between the pre-selection plate 10a and the fme-selection plate 10c. In said spacer plate, communication ducts 30, 30' are formed whose shape is chosen to be such that it corresponds with the shape of the phosphor color elements (for example circular or triangular triads). The apertures 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, 11', 11 " are formed between the structure 100 and the rear wall 4. In order to be able to withdraw the electrons from the transport ducts via me exit apertures 8, 8', 8", electrodes 9, 9', 9" are provided, in this example, on the surface of the plate 10a facing the screen.

In addition to the exit apertures 8, 8', 8", plate 10a comprises a measuring aperture 21 which is provided with an electrode 22 to extract electrons from the transport duct 11. Besides, plate 10b has a further measuring aperture 27 and an electrode 28 to extract electrons from space 30". Measuring element 23, which is in the form of a measuring electrode, is arranged behind aperture 27. Shielding electrodes 102 and 103 are arranged in the vicinity of the measuring element 23 to electromagnetically shield the measuring element from the signals which, in operation, are supplied to electrodes 9 and 13. A blind duct 8"' is situated between the measuring aperture 21 and the nearest active exit aperture 8. As a result, the distance between the electrodes 9, 13 and the measuring element 29 is increased. This results in a reduction of the disturbing effect which the signals which are supplied to the electrodes 9 and 13 have on the signal to be measured. Fig. 2C shows a detail of a display device, in which a coupling duct is formed between space 30' and 30" via passages provided with electrodes 42 and 43. This enables the electron current actually supplied to a number of phosphor elements to be drawn off for the measuring process. This results in an improved accuracy of the measurement and, in this respect, such a construction is better than the constructions shown in Fig. 2B. However, the interferences which may occur as a result of capacitive coupling are generally greater in the construction shown in Fig. 2C than in the construction shown in Fig. 2B.

Fig. 3 is a sectional view of a detail of a further embodiment of a display device in accordance wim the invention. In this example, the 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. Selection plates have apertures, for example aperture 61 in plate 53, which apertures have one input 63 and two or more ouφuts 64, 65 and 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 entrance aperture 8' and, subsequently, guided to one of the phosphor elements R, G, B. In this example, a possible trajectory for an electron current is indicated by means of an arrow. Selection is accompanied by the application of varying voltages to the various electrodes around selection holes. Said varying voltages can induce a "false" signal on the measuring element, particularly as a result of capacitive coupling. Such a "false" signal reduces the accuracy with which the trial current can be measured. Even if all selection voltages are switched off during me measuring process, residual voltages still cause a disturbing effect on the selection electrodes. In addition, disturbances occur as a result of supply voltages. A blind duct 111 is situated between measuring aperture 21 and the nearest exit aperture 8'. This leads to a larger interspace between the electrodes around the selection holes and the detector, more particularly the measuring element of the detector. By virtue thereof, disturbances to the measuring signal as a result of capacitive coupling are reduced. Plate 56 is provided with a measuring electrode 71. Plates 54, 56 and 58 are provided wim shielding electrodes 72, 73 and 74. Unlike the other electrodes, these shielding electrodes are not provided with drive voltages for driving electron currents, but serve to shield the measuring electrode 71. Preferably, the impedance to high frequencies between the electrodes 72, 73 and 74 is small, which may be attained by interconnecting said electrodes. The assembly of shielding electrodes is preferably coupled capacitively to an earth point. Decoupling of disturbances now takes place very effectively as the electromagnetic "Faraday cage", which is formed by the shielding electrodes, provides a shield against capacitive disturbances. It is noted that the favorable effect obtained by the provision of detector means results in an improved measuring sensitivity of the detector, irrespective of the way in which the measuring current is generated, i.e. whether extraction takes place via a measuring aperture in the transport duct or not.

This example also shows an alternative arrangement of the wire cathode. In this case, the wire cathode 5 is situated more or less next to the detector, and plate 51 comprises the electrodes GI and G2.

In this example, the perforated plates comprise the exit apertures and the means for guiding the electron currents to the phosphor elements as well as the various parts of the measuring unit. Thus, the measuring unit is integrated in the perforated plates. The manufacture of additional parts is unnecessary, which is a big advantage. The measuring electrode measures the number of electrons incident on the electrode. This measurement can be used to analyze the operation of the apparatus, which analysis can be employed to send feedback signals to the wire cathode or to the electrodes GI and/or G2. If, for example, it turned out that relatively few electrons (relative to an expected value or measured value of other transport ducts) reach the measuring electrode, which means that relatively few electrons are emitted in the transport duct, either the temperature of the wire cathode can be raised or the voltage on G2 or on GI can be increased, or any combination of these measures, so that more electrons are emitted in the transport duct. The effect of these measures can then be checked by measuring the number of electrons on the measuring element. By virtue thereof, the uniformity of the picture displayed can be improved.

It will be obvious that, within the scope of the invention, many variations are possible. In the above examples, the trial current is measured, for example, by measuring electrodes. The current impinging on the measuring electrode can be measured by means of a current or voltage measurement. Owing to the simplicity of the construction, this is a preferred way of measuring trial currents. However, it is not the only way. For example, it is alternatively possible to make the electron current impinge on a trial-phosphor area or several trial-phosphor areas and measure the luminescence of said trial area(s). Although this is a more complicated construction, as it requires a photo-sensitive element, such a construction can determine possible differences in luminescence efficiency and is less sensitive to capacitive disturbances. In the examples, electron-transport ducts in the form of channels are shown. Within the scope of the invention, "transport ducts" are to be understood to include any means by which, in operation, an electron current is passed, by means of secondary emission, from one or more starting points to the exit apertures. The directing means shown in the Figures are relatively complex directing means. Within the scope of the invention, "directing means" are to be understood to include all means which, in operation, guide a trial current from the measuring aperture to the measuring element. In the Figures shown, each measuring unit has a measuring electrode. In a preferred embodiment, a number of measuring units have a common measuring electrode. This results in a reduction of the number of measuring electrodes.

Claims

CLAIMS:

1. A display device having a vacuum envelope which is provided with a transparent front wall with a luminescent screen, which display device comprises a number of electron sources, a number of electron-transport ducts which cooperate with said sources, - selection means for withdrawing each electron current from its transport duct via exit apertures and guiding it to pixels of the luminescent screen, and detector means for measuring a trial current transported through a transport duct, characterized in that the detector means comprise a measuring unit having an extraction means for extracting electrons from the transport duct through a measuring aperture in the transport duct, and directing means for directing me extracted electron current from the measuring aperture towards a measuring element.

2. A display device as claimed in Claim 1 , characterized in that a measuring unit is coupled to each transport duct.

3. A display device as claimed in Claim 1 or 2, characterized in that the measuring aperture of a transport duct and the exit apertures are substantially similar in form.

4. A display device as claimed in Claim 1 , 2 or 3, characterized in that the display device comprises a perforated plate in which the exit apertures and the measuring apertures are formed.

5. A display device as claimed in Claim 1, 2, 3 or 4, characterized in that the measuring aperture is one of the exit apertures, and the directing means comprise a drivable coupling channel with the means for guiding electron currents extracted through the relevant exit apertures to the luminescent screen.

6. A display device as claimed in Claim 1 , 2, 3 or 4, characterized in that the measuring aperture is not an exit aperture.

7. A display device as claimed in any one of the preceding Claims, characterized in that me display device comprises further perforated elements on which and/or in which the means for guiding the electron currents are secured, and in that these 14 perforated elements also include the directing means and the measuring element.

8. A display device as claimed in Claim 1, 2, 3 or 4, characterized in that the display device comprises shielding electrodes for electromagnetically shielding the measuring element.

9. A display device as claimed in Claim 6, characterized in that the distance between the measuring aperture and the nearest exit aperture is larger than the distance between exit apertures.

10. A display device as claimed in Claim 8, characterized in that a blind aperture is situated between the measuring aperture and the nearest exit aperture.

11. A display device as claimed in any one of the preceding Claims, characterized in that the measuring aperture is the first or last aperture in a transport duct, viewed from an entrance aperture of the relevant transport duct.

12. A display device as claimed in Claim 10, characterized in that the measuring aperture is the first aperture.

13. A display device having a vacuum envelope which is provided with a transparent front wall with a luminescent screen, which display device comprises a number of electron sources, a number of electron-transport ducts which cooperate with said sources, selection means for withdrawing each electron current from its transport duct via exit apertures and guiding it to pixels of the luminescent screen, and detector means for measuring a trial current, characterized in that the detector means comprise a measuring unit with a measuring element and means for directing the trial current to a measurmg element, the display device comprising shielding electrodes for electromagnetically shielding the measuring element.