FLAT LIGHT SOURCE

Abstract: A flat light source comprising two sheets of material at least one of which is transparent, the sheets being overlaid one upon the other and sealed together to form a gas-tight enclosure. One or more gas discharges can be initiated within the enclosure between a pair of electrodes along a predetermined path, the gas discharge path or paths comprising a plurality of adjacent elongate portions extending between and generally parallel to the sheets. The electrodes may be exposed within the enclosure and connected to a DC or AC electrical power source. Alternatively however the electrodes may be electrically insulated from the enclosure but connected to a high frequency AC electrical power source. The enclosure may be filled with low pressure gas in the manner of conventional discharge tubes.

Description: FLAT LIGHT SOURCE The present invention relates to a flat light source. Plat light sources are known for illuminating for example display panels, the light sources comprising a series of discrete discharge tubes arranged adjacent to one another and energised by an external circuitry incorporating ballast or resistive loads. Light sources of this type are inevitably bulky and require a space behind the display screen which is of considerable depth. In addition, conventional discharge tubes require a high over- voltage to initiate discharge and the time taken to fully initiate a discharge (the strike time) is considerable. The strike time can be reduced by- increasing the over-voltage or using internal or external conductive strips extending along the length of each discharge tube. The known light sources are however difficult to operate over a wide temperature range and if the light output of the light source is to be controllable complex and bulky external dimming circuitry is required. The known light sources are not generally two terminal devices. Because of the abovementioned disadvantages it has proved difficult to provide a flat light source of acceptable size for use in for example back-lighting liquid crystal displays. It is an object of the present invention to provide a flat light source which obviates or mitigates the abovementioned problems. According to the present invention there is provided a flat light source comprising two sheets of material at least one of which is transparent, the sheets being overlaid one upon the other and sealed - 2 - together to form a gas tight enclosure within which at least one gas discharge can be initiated between a pair of electrodes along a predetermined path, the gas discharge path or paths comprising a plurality of adjacent elongate portions extending between and generally parallel to the sheets, and the enclosure being filled with low pressure gas. Preferably the electrodes are exposed within the enclosure and connected to a DC or AC electrical power source. Alternatively however the electrodes may be electrically insulated from the enclosure but connected to a high frequency AC electrical power source. The enclosure may be divided into a plurality of parallel channels by spacers sandwiched- between the sheets. Each channel—-ι ~ =- : ' be " associated with a respective pair of electrodes," o alternatively the channels may be interconnected end to end to define a single passageway between a single pair of electrodes. An additional resistive, capacitive or inductive load may be placed in series with one or both of the electrodes in each pair. Electrical impedances of a. capacitive, resistive or inductive form may be positioned so as to define further electrodes spaced apart along the discharge path or paths. The impedances reduce the magnitude of the applied voltage required to initiate a discharge and can be arranged so as to avoid any need for variable loads to assist discharge initiation or dimming of the light output. The impedances also have the important practical functions of stabilising the gas discharges and allowing spacers to be used which do not touch the top and bottom sheets at all points. In order to improve the luminous efficiency of the light source, one or all of the surfaces exposed to the gas discharge may be coated with phosphor or fabricated from a phosphor containing material. Surfaces other than that of the transparent sheet may be coated with a reflective material. Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a plan view of a first embodiment of the present invention; Fig. 2 is a sectional view taken on line 2-2 of Fig. 1; Fig. 3 illustrates features of a second embodiment of the present invention incorporating an alternative electrode structure- to that of the embodiment of Figs. 1 and 2; Fig. 4 is a plan view of a third embodiment of the present invention; Fig. 5 is a sectional view on the line 5-5 of Fig. 4; Figs. 6 and 7 illustrate alternative spacer structures for use for example in the embodiment of Fig. 4; Fig. 8 illustrates a diffuser for use for example in embodiments of the invention; Fig. 9 illustrates the provision of a resistor in series with an electrode of an embodiment of the present invention; Figs. 10 and 11 show alternative structures incorporating electrodes within the enclosure of embodiments of the present invention; Fig. 12 illustrates a structure in which an electrode is defined on a side surface of an enclosure defined by an embodiment of the present invention; Figs. 13 and 14 are circuit diagrams illustrating the connection of an embodiment of the invention to AC and DC power sources respectively; and Figs. 15 and 16 illustrate alternative embodiments of the invention connected to AC and DC power sources respectively. Referring to Figs. 1 and 2, the illustrated embodiment of the invention comprises a lower sheet of material 1 and an upper transparent sheet of material 2. The sheets 1 and 2 are spaced apart in parallel and sealed around their peripheries by a sealing material 3 so as to define a gas-tight enclosure, the enclosure being divided into four parallel channels 4 by spacers 5. Electrodes 6 are exposed within the channels 4 so that an electrical discharge may be initiated between the pair of electrodes positioned at opposite ends of the respective channel 4. The channels are filled with a low pressure gas which may be of the type used in for example conventional discharge tubes and the surfaces of the channels may be coated with phosphor so as to increase the luminous efficiency of the device. The surfaces of the channels 4 other than the underside surface of the upper sheet 2 may be coated with a reflective material so as to improve the projection of light through the transparent upper sheet 2. When a discharge is initiated in each of the channels 4 simultaneously a large area flat light source is defined. Such a light source may be used for example to back-light a liquid crystal display. The number and length of channels may be selected as necessary to provide the desired area and outline shape for that area. In the embodiment of Figs. 1 and 2 the electrodes 6 extend into electrical contact with the gas within the channels 4. The electrodes 6 may however be electrically insulated from the channels 4 as illustrated in Fig. 3 providing a high frequency signal is applied between the terminals connected to electrodes 6. Referring now to Figs. 4 and 5, in the illustrated embodiment a lower sheet 7 is separated from an upper sheet 8 by a series of spacers 9, the upper and lower sheets being sealed together around their peripheries by a seal 10. A single pair of electrodes 11 is provided, one electrode being positioned at each end of a circuitous single discharge path defined by the nine channels 12 which are connected end to end by virtue of the spacers 9, one end of each spacer stopping short of the adjacent portion of the seal 10. As in the embodiment of Figs. 1 and 2 the channels 12 are filled with a low pressure gas. When a discharge is initiated between the two electrodes 11 light is emitted from each of the channels 12 and thus a large area flat light source is defined. Fig. 5 illustrates the connection of the electrode 11 to an external terminal 13. The electrode 11 is deposited on the upper surface of the lower sheet 7 and connected to the terminal 13 which is also deposited on the lower sheet 7. An insulating layer 14 is then laid over the junction between the electrode 11 and the terminal 13 and the seal 10 is then positioned so that the seal 10 and " insulating layer 14 prevent direct electrical contact between the terminal 13 and the gas within the channel 12. I the embodiment of Figs. 4 and 5 the spacers 9 may be simple strips of rectangular cross section as shown in Fig. 6. With such an arrangement however the top edges of the spacers 9 are visible when the light source is viewed through the upper sheet 8. To reduce the visible effects of the positioning of the spacers 9 various techniques may be used. For example the spacers 9 may be made of a transparent material such as glass and may be made as narrow as possible adjacent the upper sheet 8. For example 0 spacers having a triangular cross section rather than the rectangular cross section as shown in Fig. 6 could be used. As an alternative when the spacers 9 are fabricated from a transparent material the spacers 9 may be made relatively wide close to the £ upper sheet 8 so that light transmitted, into., the spacer 9 is directed towards the upper sheet S—,. In such an arrangement the - spacer 9 could have- a triangular cross section with one face of the triangle flat against the underside of the upper 0i sheet 8. Other techniques for adjusting the..visual impact of the spacers 9 may be used, for example by surface treatment, adjustment to the detailed geometrical shape, construction of the spacers- from semi-transparent or diffusing material etc. A 5 further method is to curve the surfaces of spacers 9, adjacent to the upper and lower blocks, and to coat the surface adjacent to the lower block with a reflective material. Fig. 7 shows an alternative spacer structure 0- which substantially eliminates the visual impact of the spacers when the light source is viewed through the upper transparent sheet 8. In the arrangement of Fig. 7 each spacer comprises a portion 15 of triangular cross section which is spaced from the 5 upper sheet 8. Thus the spacers are not in contact with the upper sheet 8. Small "pips" 16 of a dielectric material are screen printed on the underside of the upper sheet 8 and contact the upper edges of the spacer portions 15. The functional effect of the pips 16 may be achieved in other ways, for example by using discrete thin support elements such as glass fibres extending across the top of the spacer portions 15. As a further alternative, the pips 16 may be simply omitted providing the upper sheet 8 is sufficiently rigid to prevent it being sucked downwards in an irregular manner by the differential pressure across its thickness. Fig. 8 illustrates a further structure which produces a more uniform light source. In the arrangement of Fig. 8 the upper sheet 8 -.of—the ■ light- source (the lower sheet 7 not being shown) supports'^ diffusing panel 17 on edge spacers 18. This produces a very uniform light source but at the expense of increasing the overall thickness of the device. Alternatively the diffuser may be in contact with the upper block or incorporated as part of the upper bloc . It will be noted from the above that it is not necessary for the spacers 9 to be sealed at their upper and lower edges to the upper and lower sheets 7 and 8. Depending on the length, width and overall dimensions of the light source the barriers represented by the spacers between adjacent discharge channels may vary considerably. It is simply necessary to provide sufficiently extensive barriers to ensure that a discharge in one channel does not prevent the initiation of a discharge in an adjacent channel. in the arrangement of Figs. 4 and 5 the electrodes 11 are directly connected to the terminals 13. Accordingly if any ballast resistors are required these must be provided external to the enclosure defined by the light source. As an 5 alternative however and as illustrated in Fig. 9 a resistor 19 may be deposited on the lower sheet 7 so as to be connected in series between the electrode 11 and the terminal 13. The resistor 19 is electrically insulated from the discharge channel 12 by the layer 10 14 of insulating material. It will of course be appreciated that the resistor 19 could be placed beneath the seal 10 or alternatively on the surface of the lower sheet 7 external to the seal 10. The voltage required to initiate a discharge in -^ one of the channels *__- ~ can be reduced and the overall stability ^of_the-..gas-discharge or discharges improved by arranging a series—of" electrodes along the length of the channels. Such an arrangement is illustrated in Fig. 10 in which the top surface of the lower 2-0 sheet 7 is shown as supporting between the electrodes 11 a series of secondary electrodes 20 each individually connected by a series of electrical conductors 21 and 22 to an external resistive, inductive or capacitive ballast (not shown) . It 5 shall be appreciated that the electrodes 20 and 11 are exposed to the gas discharge through respective holes in the adjacent dielectric. The electrodes are not located in open channels in the dielectric. The electrical conductors 21 and 22 are electrically 0 insulated from the gas in the channel with which they are associated by a layer of insulating material 23. The electrical conductors 21 and 22 are linked electrically together, after each series ballast has been inserted, by a further set of conductors 5 external to the discharge. It is possible to place a plurality of electrodes 20 across the width of each channel. Detailed circuits for use with a structure such as that illustrated in Fig. 10 are described below. Referring now to Fig. 11, this illustrates a structure which enables a resistive element 24 to be placed on the lower sheet 7 in series with each electrode 20 and each electrical conductor 21 or 22. The electrical conductors 21 or 22 connect all the resistive elements 24 in rows extending perpendicular to the channels. The resistors 24 are electrically insulated from discharges by the layer of insulating material 23. The electrical supply to the device will be appreciated from the detailed circuits described below. ~ - ~ - The formation^- ^ of " the electrodes, resistors, conductors and insulation layers on the lower sheet 7 may be achieved in any convenient manner. For example components may be deposited by printing, vapour deposition or any other conventional techniques including physical insertion of discrete components where appropriate. After coating the lower block with phosphor it may be necessary to clean the electrode surfaces. The electrodes may be cleaned more easily by increasing the thickness of the electrodes to make them project above the level of the dielectric material. Alternatively the cleaning process may be eliminated by placing the electrodes beneath the spacer elements, such that the electrodes remain in electrical contact with the gas, with the spacers having a maximum width in the vicinity of the electrodes. The phosphor coating may then be sprayed in such a fashion so as to shield the electrodes from any phosphor coating. In the embodiments described ' above all the various components have been supported on the lower sheet 7. This means that the area used by these components is relatively large when the device is viewed through the upper transparent sheet 8. This area may be reduced however by using surfaces perpendicular to the two sheets to support the various electrodes and other electrical components. Such an arrangement is shown in Fig. 12. Electrodes : 25 are for example silk screen printed on the upper sheet 8 and lower sheet 7 and on to the surface- 26 perpendicular thereto. An internal conductive connection 27 is made between the electrode 25 on the lower sheet and the perpendicular surface 26. The _ electrodes on the upper and lower sheets and resistors 28 are connected together externally by linking conductors 29. A single load resistor could be used common to both the upper and lower block electrodes. Figs. 13 and 14 illustrate the circuit of an embodiment constructed to comprise the components described with reference to Figs. 9 and 10. Ballast resistors 30 are provided as described with reference to but not illustrated in Fig. 10. The perimeter of ' the gas filled enclosure is indicated by dotted line 31 and the position of spacers dividing the enclosure into three independent discharge channels are indicated by dotted lines 32. Fig. 13 illustrates the circuit for AC energisation and Fig. 14 the circuit for DC energisation. Figs. 15 and 16 illustrate the circuits of embodiments similar to those illustrated in Figs. 13 and 14 but incorporating the resistors 24 as illustrated in Fig. 11. Current limiting resistors 33 are also provided. The perimeter of the enclosure is indicated by dotted lines 34 and spacers separating the enclosure into three independent discharge channels are indicated by dotted lines 3 * 5. The current limiting resistors 33 could, of course, be incorporated within the perimeter of the enclosure as could the external conductors shown in Figs. 15 and 16 thereby producing a two terminal device.

Claims: CLAIMS : 1. A flat light source comprising two sheets of material at least one of which is transparent, the - sheets being overlaid one upon the other and sealed together to form a gas tight enclosure within which at least one gas discharge can be initiated between a pair of electrodes along a predetermined path, the gas discharge path or paths comprising a plurality of - adjacent elongate portions extending between and generally parallel to the sheets, and the enclosure being filled with low pressure gas. 2. A flat light source according to claim 1, wherein the electrodes are exposed within the enclosure and arranged for connection to_.a->DG-or AC electrical power source. 3. A flat light source accordinq._.to claim 1, wherein the electrodes are electrically insulated from the enclosure and arranged for connection to a high frequency AC electrical power source. 4. A flat light source according to any preceding claim, wherein the enclosure is divided into a plurality of parallel channels by spacers sandwiched between the sheets. 5. A flat light source according to claim 4, wherein each channel is associated with a respective pair of electrodes. 6. A flat light source according to claim 4, wherein the channels are interconnected end to end to define a single passageway between a single pair of electrodes. 7. A flat light source according to any preceding claim, wherein an additional resistive, capacitive or inductive load is connected in series with one or both of the electrodes in each pair. 8. A flat light source according to any preceding claim, wherein electrical impedances of a capacitive, resistive or inductive form are positioned so as to define further electrodes spaced apart along the discharge path or paths. 9. A flat light source according to any preceding claim, wherein one or all of the surfaces exposed to the gas discharge are coated with phosphor or fabricated from a phosphor containing material. 10. A flat light source according to any one of claims 1 to 8, wherein surfaces other than that of the transparent sheet are coated with a reflective material. 11. A flat light source substantially as hereinbefore described with reference ^ to the accompanying drawings.

Inventors: IPSON STANLEY STUART, IPSON, STANLEY, STUART
Applicants: SIDEFACT LTD, SIDEFACT LIMITED
Application year: 1987
Main classification: H01J61
IPC codes: H01J61, H01J65