DISPLAY DEVICE

Abstract: An electrophoretic display device utilizes transparent spheres (16) whose diameter is similar to that of visible light in place of the conventional pigment particles whereby to enhance the retro-reflective effect of the device. The spheres (16) may be glass or plastics or a combination of both with a specific gravity similar to that of the suspension medium (15) in which they are contained.

Description: DISPLAY DEVICE The present invention relates to display devices and more particularly to electrophoretic or die-lectricphoretic display device-s. Electrophoretic display devices are known and a feature of these devices is that they are passive, i.e. .they do not emit light rather they reflect or transmit incident light . An object of the present invention is to . provide an electrophorectic or dielectricphoretic display device with enhanced reflectance in the direction of illumination-. In order that the present invention be more readily understood, an embodiment thereof will now be described by -way of example with reference to the accompanying drawing which shows a cross-section through an electrophoretic display device-. ■ - An electrophoretic display device 10 comprises a non-conductive substrate 11 to which is applied an electrode 12 and an electrode 13 spaced from the electrode 12. The space between the electrode 12 and the electrode 13 is filled by a liquid " material 15 containing small particles 16. When an electric field is applied across the space by a voltage applied to the electrode 12 and electrode 13, the particles migrate to either the electrode 12 or the αlectrodo 13. Either or both of the electrodes 12, 13 can be an array so as to produce any desired pattern depending on the disposition and shape of the or each array. In this embodiment, the device is designed for viewing in the direction of the arrow A in which case the electrode 13 will be formed of a transparent material and provided with a transparent protective cover 17. The particles iβ are specifically- selected for their reflective properties and it has been found that they should be optically transparent * in at least part of the visible spectrum. Further, they should have a diameter similar to or larger than the wavelength of visible light, e.g. from 0.5 to 20 microns.. It is advantageous if they have a specific gravity similar to that of the liquid material so that th ' ey exhibit neutral buoyancy in the liquid material and can move relatively easily under the action of ' n electric field. These t-wo desiderata point to glass' or plastics particles being used. A combination of glas--- and plastics is also possible such- as glass coated with plastics. The pre erred plastics are polyamide, poly- i ide, polyester, polypropylene or polycarbonate. Preferably the particles are spherical but may be either solid or hollow spheres. The refractive index of the material of the spheres should preferably be higher than that of the liquid material. Such particles are known to exhibit good reflectance in the direction of illumination. The electrophoretic activity can be enhanced by adding a surfactant to the liquid material and/or by forming electrets within the particles. The above construction may be used as " an addressable sign such as a road sign, a warning display or an information panel and has the additional advantages that dye absorption on reflective glass particles would be lower than absorption on conventional organic pigments. This provides increased perceived contrast. Also, chemical and light-induced degradation is lower for glass particles than for organic pigments. Thus, the life of the device would be increased. With glass particles, it may be necessary to process them so that they exhibit an electrophoretic effect. A number of processes are available such as exposing molten glass to an electrical discharge and cooling the glass to trap charged particles in the ' glass matrix. Alternatively, glass at .room temperature could be exposed to ionising radiation such as cathode rays or X-rays to form charged 1 particles in the glass.- Both these processes form electrets but it is also possible to activate the surface of the glass particles chemically and then coat. the particles with long chain molecules to cause a charge to be present. - UR£ ^

Claims: - - - CLAIMS : 1. A display device comprising spaced electrodes and electrophoretically active particles in a liquid suspension medium disposed between said electrodes, the particles having a refractive index greater than that of the suspension medium characterised in that the particles are transparent to light in at least part of the visible spectru and have an external diameter similar to or larger than ' the wavelength of visible light. 2. A display device according to claim 1, characterised in that the particles are spheres and are of glass or plastics materials or a combination thereof. 3. A display device according to claim 1 or 2 , -characterised in that the particles are hollow. 4« A display device according to claim 1,~2 or 3 - characterised in that the particles have a specific gravity similar to that of the suspension medium. 5« A display device according to any one of the preceding claims characterised in that the particles include electrets to enhance the electrophoretic activity. 6. A display device according to any one of the preceding claims, characterised in that the liquid suspension medium includes a surfactant to enhance the electrophoretic activity. O

Inventors: BASSETT PETER JOHN, PEETERS JOSEPHUS BONIFACIUS, VERHEIJEN AUGUSTINE WILHELMUS
Applicants: BASSETT PETER JOHN, PATSCENTRE BENELUX NV SA, PEETERS JOSEPHUS BONIFACIUS, VERHEIJEN AUGUSTINE WILHELMUS, PATSCENTRE BENELUX NV
Application year: 1982
Main classification: G02F1
IPC codes: G02F1, G03G17