GB2258077A - Display device. - Google Patents

Abstract

A flat panel display device comprises a generally planar phosphor screen vacuum tube 2 have one of a column or row array of electrodes 10 for sequentially illuminating column or row segments phosphor 17 of the screen 2 by driving electrons 13 thereonto. The device also has an LCD screen 4 extending across the front of the phosphor screen 2 in a substantially parallel plane. The LCD screen 4 has the other of an array of row or column LCD elements formed and arranged orthogonally to said phosphor screen segments 17, which has a generally opaque condition but is sequentially selectively switchable to a generally clear condition for exposing parts of said phosphor screen 2 column or row segments 17 respectively. In use of the device 1 there is displayed an image 25 made up of pixel elements each of which is defined by overlaying electrodes 10 and LCD elements. <IMAGE>

Description

DISPLAY DEVICE The present invention relates to a flat panel display device suitable for use in displaying a video image i.e.

an electronically generated visual image.

Conventional display devices are primarily cathode ray tubes (CRT) which are relatively bulky to the substantial depth required to accommodate the electron beam gun and deflectors, or liquid crystal devices (LCD) which have only limited performance characteristics.

It is an object of the present invention to avoid or minimise one or more of the above disadvantages.

The present invention provides a flat panel display device comprising a generally planar phosphor screen vacuum tube having one of a column or row array of electrodes for sequentially illuminating column or row segments of the screen by driving electrons thereonto, and an LCD screen extending across the front of the phosphor screen in a substantially parallel plane and having one of an array of row or column LCD elements formed and arranged orthogonally to said phosphor screen segments, for screening said phosphor screen in a generally opaque condition and sequentially selectively switchable to a generally clear condition for exposing respective parts of said phosphor screen column or row segments, respectively, whereby in use of the device there is displayed an image made up of pixel elements each of which is defined by overlaying electrode and LCD elements.

Preferably the device has a columnar array of electrodes and a row array of LCD elements. With such an arrangement there is obtained a relatively wide horizontal viewing angle e.g. up to 1400 with a smaller vertical viewing angle e.g. up to 700 which is compatible with the preferred characteristics of video image display devices.

The flat panel display devices of the invention have various advantages over previously known displays. The use of the phosphor screen vacuum tube provides a much brighter image than conventional LCD displays, with the further advantage of a much larger grey scale or brightness range. In addition significantly higher contrast is possible and bright light readability is improved since both phosphor emitted light and reflection from the phosphor coating are restricted by the selective switching of the LCD elements to their clear state. Moreover by using small width or pitch electrodes very high resolutions are readily achievable.It will also be appreciated that the disadvantages of relatively slow LCD element switching found in conventional LCD displays are substantially reduced since in the display device of the present invention the individual LCD elements are only required to switch at the screen rate instead of at the line rate (e.g. 1/625 of the frequency of the line rate in a 625 line display). This considerably extends the low temperature operating range of the device.

Other advantages that may be mentioned include minimum side wall thickness allowing video wall arrays of display devices with minimal intersection zones; the use of substantially precisely square or rectangular screen construction without curved corners thereby minimising image distortion; and the use of a regular outside wall structure facilitates installation of the display device in various apparatus.

A further particularly significant advantage is the considerable design flexibility afforded by the mode of construction employed in relation to for example the number and size of electrode and/or LCD elements used, and the possibility of software control for facilitating use of different line standards and interlace or non-interlace screen operating modes.

The electrode construction moreover is substantially more rugged, less bulky and easier, and hence more economical, to manufacture than conventional CRT devices. In particular it will be appreciated that the new display device does not rely on an active effect caused by the intersection of column and row addressing and therefore allows a very simple form of panel construction to be utilised.

It will be understood that especially in the case of larger screen sizes, if it is desired e.g. in order to reduce power consumption, the,row or column electrode elements may be segmented so that only part of each row or column phosphor screen segment corresponding to a part behind a generally clear LCD element at any given time is illuminated i.e. non-useful illumination of those parts which are screened by opaque LCD elements is reduced. It is also possible, if desired, to segment the LCD elements, though this is generally less preferred, due to increase in complexity of the control means required and the limited benefits of such segmentation.

Various forms of phosphor screen (or indeed any generally similar screen which can be illuminated by electrons impingeing thereonto) may be used.

Conveniently, the screen segments are in the form of strips such as those used in the well-known Trinitron (Trade Mark) design of the Sony Corporation.

Advantageously there is used a black matrix guard band structure between the phosphor strips in generally known manner to ensure good purity.

Various suitable LCD materials are known in the art which can be switched between a generally "clear" condition with a light transmission of e.g. 35%, and an "opaque" condition with a light transmission of e.g.

0.3%. A major advantage of the use of LCD elements each of which corresponds to a substantial number of "pixel" elements of the displayed image, is the substantial reduction in the control means required as compared with conventional LCD displays which require a drive transistor for every single pixel, which in turn reduces the available area of the individual LCD elements.

In general the electrode elements comprise a cathode for emitting electrons, and at least one anode for accelerating the electrons towards the phosphor screen, the arrangement being such that a flood of electrons from the full length of the cathode strikes the whole of an area of the screen generally corresponding to that of the cathode, when the cathode is activated. Thus the device of the present invention avoids the need for all the complex beam deflection arrangements used in conventional CRT display devices, as well as the substantial depth of such devices required to allow the electron beam to be manipulated sufficiently.

Advantageously each electrode element also includes at least one electron gate means for controlling the number of electrons passing to the screen at any given time and hence the brightness of the light generated at the respective screen segment.

The display device also includes connector means for connecting the individual electrode elements and LCD elements to suitable control and power supply means formed and arranged for sequentially driving the electrode elements so as to emit electrons and impinge them onto the phosphor screen, and for sequentially switching the LCD elements between their opaque and clear conditions. As noted above the control means may be formed and arranged for sequential driving and switching in various different modes, advantageously with the facility for switching between or selecting from two or more different modes. Thus for example in the case of a column-electrode element and row - LCD element system the control means may be formed and arranged for either interlaced or non-interlaced mode operation; or for pre-setting to operation in either one of these modes; or for user-selection for operation in different ones of these modes as may be required at different times.

It will be appreciated that various different LCD materials are available with different operating characteristics e.g. different times for switching to a clear condition and different times for switching to an opaque condition. In general though LCD switching times are relatively slow in relation to line scan rates, especially for display modes using larger numbers of scan lines, and thus it may be preferred for there to be a greater or less degree of overlap in switching of successive LCD segments i.e. the next segment is switched to clear before the previous one has been switched to opaque.

Further control means that may be used with display devices of the present invention include those used in conventional display devices such as signal synchronisation means, pulse drive means, video signal processing means e.g. gamma correction means, video modulator means etc.

Further preferred features and advantages of the present invention will appear from the following detailed description given by way of example of a preferred embodiment illustrated with reference to the accompanying drawings in which: Fig. 1 is a schematic sectional view of a display device of the invention provided with control and power supply means; Fig. 2 is a schematic sectional view through the display device showing the electrode and LCD elements and the phosphor screen therebetween; Fig. 3 is a detail front perspective view on an enlarged scale of part of the front of the display device showing how a displayed image is made up; Fig. 4 is a block circuit diagram of the control means of the device of Figs. 1 to 3; Fig. 5 is a block diagram of the power supply interface for both low and high voltage supplies; and Fig. 6 is a block circuit diagram of one example of an optional user input circuit.

Fig. 1 shows a display device 1 of the present invention which comprises a planar vacuum tube module 2 having at its front face 3 an LCD screen module 4, and at its rear 5 an electronics control module 6, power supply means 7, and connector means 8 for interfacing with image generating apparatus such as a TV tuner, video camera, computer etc. (not shown).

As shown in Fig. 2 the vacuum module 2 comprises a glass envelope 9 having mounted therein an array of vertically extending electrode means 10 each of which comprises a planar strip cathode 11, a first anode 12 for accelerating a blanket electron flow 13 from the cathode 11 towards a respective phosphor strip coating 14 at the rear side 15 of the front panel 9a of the glass envelope 9. A pair of electron gates 16 are disposed downstream of the first anode 12 for controlling the electron flow 13 so as to vary the amount of electrons impingeing on the phosphor coating 14 by controlling the biasing voltage applied to them and hence the brightness of the light emitted from that particular phosphor strip. The electron gates are conveniently in the form of slots formed e.g. etched, in a high dielectric insulating material such as sio2 (Silicon Dioxide).A thin aluminium coating 17 is provided at the rear of the phosphor coating 14 in order to maximise forward emission of light by reflecting any emissions caused by the electrons impinging the phospor screen, which emit rearwardly away from the phosphor screen and away from the viewer. The final acceleration to the electron flow 13 is induced by the application of a positive high voltage (e.g. 10 kV)to the outer edges 18 and the aluminium coating 17 on the rear of the phosphor screen 14.

In more detail, as shown in fig. 3, the phosphor coating is made up of strips of suitable phosphor e.g. rare earth phosphor, such as those readily available commercially under the designations: P55, P56, P1.

Alternating strips of phosphor for producing Red, Green, or Blue light, 17R, 17G, 17B, separated from each other by narrow black matrix guard bands 20, are used for a colour display. Other phosphor strip combinations e.g.

for monochrome display only, or for providing either one of monochrome or full-colour display by switching between different combinations of phosphor strips, may also be used though.

The LCD screen module 4 is of,generally conventional construction with suitable LCD material 21 sandwiched between front and rear plates of glass 22, 23, except that the LCD material 21, is in the form of an array of narrow LCD strip elements 24 each extending the full width of the display 25 (only a small part shown in Fig.

3 on a greatly enlarged scale).

The LCD screen module 4 is shielded from the vacuum module 2 by an electrostatic screen 26 e.g. of Indium Tin Oxide (ITO), provided on the front face 19 of the front glass panel 9a of the vacuum module 4 in order to prevent unwanted electrical interference which could result in unintentional switching of the LCD elements 24. The LCD screen module is secured to the vacuum module 4 by means of a substantially translucent adhesive bond layer 27 e.g. of refractive index matched epoxy resin.

In use of the display device 1 the LCD elements are normally in a substantially opaque condition thereby substantially obscuring the phosphor screen elements 17. One LCD strip element 24a is switched to a clear condition by the application of a suitable voltage thereto so that those parts 17a of the phosphor screen elements 17 directly behind it can now be seen. Thus although the full length of each phosphor screen strip element 17 is illuminated by the respective flood of electrons directed thereonto by its associated electrode means 10, only the revealed part 17a thereof, can be seen, each of these revealed parts 17a thereby constituting one "pixel" of the full screen image display 25.

After a very short interval e.g. 64 usecs said one LCD strip element 24a is switched back to its opaque condition and the next LCD strip element 24b is switched to its clear condition (if necessary the necessary voltage can be applied to the next element before the previous one has become opaque in order to allow for the finite time delay in changing an LCD element to its clear condition). At the same time the electron flow 13 to each of the phosphor strip elements 17 is modulated as required in order to provide the necessary phosphor illumination required for the pixels 17b defined by those parts of the phosphor strip elements overlaid and exposed by said next LCD strip element 24b. Naturally where interlacing is used then it would be a different one e.g. the next but one LCD strip element 24bI which requires to be cleared next.

A complete screen display can thus be built up by synchronising the necessary changes in the illumination of the phosphor screen strip elements 17 with the "clearing" of successive LCD strip elements 24. It will be appreciated that by means of simply changing the switching timing and/or the number of LCD elements switched at the same time and/or the order of switching of the LCD element in an array, it is readily possible to use a single display device of the present invention to operate in accordance with two or more different "line standards" for video image display.

It will also be understood that various different electronic control means may be used in the device of the invention depending on the video image source used and/or the type of video image to be displayed. Fig. 4 shows schematically the basic electronic control circuits including, gate electrode drive and decode circuits, row LCD drive control circuits, video decode circuits, video output analog drive amplifier synchronisation circuits and power supply modules.

In general, apart from the actual column (electrode) element and now (LCD) element switching control means, the other control circuitry used is essentially the same as in conventional video image display apparatus.

Thus the input to the display can be either a specific interface such as R.B.G. with composite sync on the green channel (composite sync is horizontal and vertical sync pulses combined) or a typical TV tuner input. This input information is separated into the component parts of R.G.B. and the sync is separated from the green channel signal by either a transistor or integrated circuit which is termed the video or sync stripper. The sync is further separated using a sync separator into horizontal and vertical pulses which control the line time and frame time in the case of interlaced TV pictures where there is an odd and even field in each frame (50 Hz or 60 Hz). There is generally a need to make the pulses as clean as possible and this is usually accomplished by simple transistor or integrated circuit combinations with some buffer or gain amplifiers.

The Drive pulse generator is again used in conventional manner to control the overall timing of the display. It usually involves what is termed a flywheel sync pulse generator which operates so that if certain incoming sync pulses are lost or degraded the flywheel will maintain picture synchronisation until they are restored.

The raw input video picture information is amplified to a working level usually 5 volts (peak to peak), in the video signed processing section which also has user adjustable control means for adjusting gain, black level etc. In this section additional known processing means e.g. for gamma correction may be incorporated, but this is generally only used for high performance screen display applications.

In conventional CRT screen displays the highly focussed electron beam can cause severe burn damage to the phosphor - if not indeed to the glass faceplate itself - if scanning should fail for any reason so that the beam remains focussed on one part of the screen. In order to prevent such damage there are utilized phosphor protection circuits for interrupting the beam. Whilst such problems are much lesser in the case of the present invention, phosphor protection circuits may still be usefully included. These can though be significantly simpler than for conventional CRT displays as only the amount of energy imparted to the screen need be monitored. The circuits in this invention are mean energy level detection circuits and in any event it is unlikely that the phosphor could be significantly burnedin this present invention due to the lower local energy levels.

The designs of the high voltage and low voltage power supplies (HVPS and LVPS) are substantially conventional.

The Video output drive amplifiers are based on conventional high bandwidth video amplifer designs giving something like 150 Mhz bandwidth at 60 volt peak to peak swing, devised to achieve maximum sharpness of the displayed images, especially alphanumeric characters, by switching between the maximum (+60 volts) and minimum (zero volts) voltages very quickly i.e. with a high slew rate.

The video modulator and RGB selector are used in conventional manner to amplify the isolated colour information in the video signal used for driving the individual R, G, and B electrode units, though, of course, in the present invention there is not just one or three electrode elements but a large number thereof.

In order to achieve the required control of the large number of R, G and B electrode units there is used a column electrode control means. This may be realized in various different forms using suitable known logic circuit elements. Thus in one form of the invention there may be used a series of logic elements such as counters and shift registers which act as "steering" elements to switch on the relevant electrode unit via analogue amplifiers/switches. The sync pulses are used to start a counter which maintains the correct relationship between the incoming video picture signal information and the relevant column electrode element in the screen. The drive of the column electrodes is via a simple amplifier configuration applying a DC potential to the electron gates.In one embodiment one amplifier pair will be used to pass the information to the relevant column electrode electron gates steered to by the logic/analogue switches. In another embodiment the logic will address an individual column electrode electron gates amplifier from an array of individual amplifiers each connected to a respective one of the individual column electrode elements.

A generally similar approach is used for controlling the row electrodes in that the required AC potential has to be applied to the relevant LCD row element at a voltage level and frequency depending on the material chosen (eg 250Hz and 30 Volts AC). The relevant LCD row element can again be either steered to by a series of logic elements from a single analogue amplifier/switch drive unit or the corresponding one of a number of amplifier drives in an array thereof is addressed by suitable logic means. Conveniently both column and row electrodes control means utilise individually addressable amplifier devices in an integrated device in order to facilitate manufacture and assembly.

Fig. 5 shows schematically the power interface to the outside world showing low voltage and high voltage power supply circuits.

The display device of this invention can, of course, be used for a wide range of customer applications and Fig. 6 shows schematically a two channel video input user interface with both multiplexing and additive and exclusive mixing circuits.

Claims (9)

1. A flat panel display device comprising a generally planar phosphor screen vacuum tube having one of a column or row array of electrodes for sequentially illuminating column or row segments of the screen by driving electrons thereonto, and an LCD screen extending across the front of the phosphor screen in a substantially parallel plane and having one of an array of row or column LCD elements formed and arranged orthogonally to said phosphor,screen segments, for screening said phosphor screen in a generally opaque condition and sequentially selectively switchable to a generally clear condition for exposing respective parts of said phosphor screen column or row segments, respectively, whereby in use of the device there is displayed an image made up of pixel elements each of which is defined by overlaying electrode and LCD elements.
2. 2. A flat panel display device as claimed in claim 1 which device has a columnar array of electrodes and a row array of LCD elements.
3. 3. A display device as claimed in claim 1 or claim 2 wherein said screen segments are in the form of strips.
4. 4. A display device as claimed in any one of claims 1 to 3 wherein said screen segments have a black matrix guard band structure therebetween.
5. 5. A display device as claimed in any one of claims 1 to 4 wherein said electrodes comprise a cathode for emitting electrons and at least one anode for accelerating said electrons towards said phosphor screen.
6. 6. A display device as claimed in any one of claims 1 to 5 wherein said LCD elements have a generally clear condition with a light transmission in the region of at least 30% and an opaque condition with a light transmission in the region of less than 1%.
7. 7. A display device as claimed in any one of claims 1 to 6 which includes connector means for connecting said individual electrodes to control and power supply means formed and arranged for sequentially driving the electrodes and for sequentially switching the LCD elements between their opaque and clear conditions.
8. 8. A display device as claimed in any one of claims 1 to 7 which has a substantially rectangular screen without curved corners.
9. 9. A display device substantially as described hereinbefore with particular reference to Figs. 1 to 6 of the accompanying drawings.