GB2233139A - Electroluminescent device - Google Patents

Abstract

The device comprises a laminated structure provided on a support 1, said laminated structure comprising a first conductive layer 3, a dielectric layer 5, and an electroluminescent layer 7; and a second conductive layer provided on and covering a minor part of said electroluminescent layer. The second conductive layer may be in part a conductive ink so that the device provides a Scratch pad upon which a user may write or draw with the conductive ink. On application of an electric field the device will then glow in the areas near the user's marks. <IMAGE>

Description

ILLUMINATED SCRATCH PAD This invention relates to electroluminescent lamps, particularly those in which a voltage is applied across a phosphor layer thereby causing it to glow.

Electroluminescent lamps are well known and consist essentially of a thin layer of Phosphor in a dielectric matrix sandwiched between two conductive layers of which at least one is transparent or translucent. An electric field, usually alternating, is applied between the two conductive layers which causes the Phosphor to glow.

Normally a layer of material with a high dielectric constant is placed between the Phosphor layer and one of the conductive layers in order to enhance the electric field across the Phosphor layer without increasing the potential difference between the two conductive layers.

The Phosphor layer typically consists of a dispersion of zinc sulphide in a binder having the required dielectric properties, and the dielectric layer consists of a dispersion of a high dielectric material such as barium titanate in a suitable binder, such as cyanoethyl cellulose which may be plasticized for example with cyanoethyl phthalate. With this arrangement the Phosphor layer is sensitive to moisture and the electroluminescent lamp is therefore enclosed in an encapsulating envelope of a thermoplastic material which is highly impervious to moisture.

An alternative method of construction of such a device involves using a solvent free UV-curable binder for the phosphor and dielectric layers. This has the advantage that the UV-curable binder is highly impervious to moisture itself and obviates the need for an encapsulating layer of plastic. Alternatively a layer of UV-curable binder may be coated over the top of the phosphor and dielectric layers to protect them from moisture.

Such devices, especially when made by the latter method, are often used for point of sale displays and similar illuminated notices. However such a device presents a large evenly illuminated area and, in order to display information and the like, opaque or coloured material must be placed on top of it so as to mark out selected areas of the illuminated device. If it is desired that a logo or product name be bright on dark background then the majority of the device must be masked out resulting in a relatively dim display. Otherwise the devices are limited to use in situations where the logo or product name is dark on a light background and thus stands out less.

According to a first aspect of the present invention there is provided an electroluminescent device comprising a laminated structure provided on a support means, said laminated structure comprising a first conductive layer, a dielectric and light generating.

layer; and a second conductive layer provided on and covering a minor part of said light generator layer, whereby in use, an electric field is provided between said first and second conductive layers and light is generated in said light generating layer proximate said second conductive layer.

According to a preferred feature of the present invention the second conductive layer comprises a conductive border around said light generating layer.

A second aspect of the present invention provides the combination of an electroluminescent device according to the first aspect, and means for applying a further electroconductive layer to make electrical contact with, or to partially overlap, said second conductive layer. The applying means may be used to form areas of conductive ink contiguous with said conductive border, said areas of conductive ink being optionally applied by the user of said device. A further optional feature is a grid of conductive filaments over the light generating layer and electrically connected to the border.

The present invention will be further described hereinafter with reference to the following detailed description of preferred exemplary embodiments and the accompanying drawings in which: Figure 1 is a separate cross-section of a device according to the present invention; Figures 2 and 3 are plan views of devices according to the present invention.

Figure 1 shows a support 1, which may be plastic, card or part of a box, onto which is deposited a conductive layer 3, a dielectric layer 5 and a phosphor layer 7. A conductive border 9 is provided around the edge of the phosphor layer 7 and a voltage is applied between the conductive layer 3 and the conductive border 9 from source 11 via wires 13.

The dielectric layer 5 forms an insulating barrier between the phosphor layer 7 and the conductive layer 3. It is preferably coloured white to reflect the light emitted from the phosphor layer and thereby increase the brightness of the device.

The conductive layer 3 may be a thin metallic foil, more particularly aluminium, or might be a printed layer of a silver-containing, or other, conductive ink. The dielectric and phosphor layers are similar to those described in the prior art. The conductive border 9 may be opaque and similar to the conductive layer 3 or might alternatively be transparent, for example made from an ink containing indium/tin oxide or carbon. A grid of conductive filaments 19, shown in Figure 3, may optionally be provided across the surface of the phosphor layer. The voltage source 11 is normally an AC source providing 125 volts rms at approximately 400 Hz. The voltage level may be varied depending on the thickness of the layers, and voltages as low as 40 V rms or as high as 400 V rms are possible.For the device to operate the alternating electric field in the phosphor layer must be large enough to raise phosphor atoms to a higher energy level which then decays emitting a photon.

In use the user draws on the surface of the phosphor layer using a conductive ink and those areas that appear to intersect the conductive border, or the conductive grid, will glow.

Alternatively shapes, such as numbers or letters, could be cut from conductive plastic and stuck on the phosphor layer. Again those areas that appear to intersect the border or the grid, ie. that are in electrical contact with the voltage source, will glow. As can be seen from Figure 2 the letters H and i which connect to the border 9 are illuminated while the dot 17 of the i which does not intersect, or otherwise connect to, the border 9 is not illuminated. In Figure 3 the letters H and i and the dot are all in contact with the conductive grid and therefore all illuminated.

The device according to the present invention has a wide variety of applications. It may be used as an illuminated sign at the point of sale in a retail outlet, and in this use has the advantage over the prior art that the information that is written on it is brighter than the surrounding and therefore more eye-catching and easier to read. The device is also suitable for use as an illuminated game for children in which they can draw shapes etc. or play games such as noughts and crosses which will then be illuminated and therefore pleasing and eye-catching. The device is particularly suitable for displaying messages at night time, the messages, being illuminated, are particularly clear and visible.

Constructions where the phosphor layer is contained in, or covered by, a layer of UV-curable lacquer are particularly suited to the present invention since the UV-curable lacquer is hard and impervious and may therefore be written on and cleaned repeatedly.

The conductive ink may be made of non-toxic substances such as water/glycerine/metal salts and may contain a food dye if coloured inks are required. For industrial uses the inks may be made from indium/tin oxide or carbon and preferably made such that they may be removed by wiping with a rag or tissue. While the voltage between the two conductive layers is quite high, approximately 125 volts rms, the currents involved are small and the device may therefore be powered by a battery and an inverter and is safe for use by children.

Claims (12)

1. An electroluminescent device comprising a laminated structure provided on a support means, said laminated structure comprising a first conductive layer, a dielectric and light generating layer; and a second conductive layer provided on and covering a minor part 9f said light generator layer; whereby in use, an electric field may be provided between said first and second conductive layers and light may be generated in said light generating layer proximate said second conductive layer.

2. A device according to claim 1, wherein said dielectric and light generating layer comprises a light generating layer and a separate dielectric stratum.

3. A device according to claim 1 or 2, wherein said second conductive layer includes a conductive border around said light generating layer.

4. A device according to claim 3 wherein, said second conductive layer includes a grid of conductive filaments over said light generating layer.

5. A device according to any one of the preceding claims wherein said support is provided adjacent said first conductive layer and said second conductive layer is provided on the reverse side of said laminated structure.

6. A device according to any of claims 1 to 5, wherein said light generating layer comprises a thin layer of phosphor.

7. A device according to claim 6, wherein said phosphor is dispersed in a layer of UV curable lacquer.

8. A device according to claim 5, wherein said light generating layer is covered by a layer of W curable laquer.

9. A device according to claim 7 or 8, wherein said layer of W curable laquer is substantially 50 jm thick.

10. The combination of an electroluminescent device according to any one of claims 1 to 9, with means for applying conductive ink to form electrically conductive areas contiguous with said second conductive layer.

11. A device according to claim 10, wherein said conductive ink may be readily removed from the surface of said conductive layer.

12. An electroluminescent device constructed and arranged to operate substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.