LIGHT MODULATORS

• US 20050213191A1
• Filed: 03/22/2005
• Published: 09/29/2005
• Est. Priority Date: 03/23/2004
• Status: Active Grant

• Current Assignee: E Ink Holdings Incorporated
• Original Assignee: E Ink Holdings Incorporated
• Inventors: Paolini, Richard J. Jr., Whitesides, Thomas H., McCreary, Michael M.
• Granted Patent: US 7,327,511 B2
• Time in Patent Office: Days
• US Class Current: 359/296
• CPC Class Codes: E06B 2009/2464 featuring transparency control by applying voltage, e.g. LCD, electrochromic panels
• CPC Class Codes: G02F 1/167 by electrophoresis
• CPC Class Codes: G02F 1/16757 Microcapsules
• CPC Class Codes: G02F 1/1677 Structural association of cells with optical devices, e.g. reflectors or illuminating devices
• CPC Class Codes: G02F 1/1681 having two or more microcells partitioned by walls, e.g. of microcup type
• CPC Class Codes: G02F 2001/1678 characterised by the composition or particle type
• CPC Class Codes: G02F 2201/12 electrode

Abstract

Various improvements in electrophoretic media and displays intended for use in light modulators are described. These improvements include index matching of the suspending fluid to a continuous phase surrounding the fluid, index matching of a capsule wall to a binder, planarization of a layer containing electrophoretic capsules before application of adhesive thereto, methods for concentrating electrophoretic particles into limited areas of sidewalls of electrophoretic capsules or microcells in the light-transmissive state of the display, and, in the case of light modulators comprising an electrophoretic layer sandwiched between two transparent plates, forming at least one of the plates so as to absorb electromagnetic radiation which adversely affects the electrophoretic layer.

Claims

1. An electrophoretic medium comprising a suspending fluid and a plurality of electrically charged particles disposed in the suspending fluid and capable of moving therethrough on application of an electric field to the suspending fluid, the suspending fluid and electrically charged particles being present as a plurality of discrete droplets, the electrophoretic medium further comprising a continuous phase surrounding the droplets, wherein the difference between the refractive index of the suspending fluid and the continuous phase is not greater than about 0.07.

2. An electrophoretic medium according to claim 1 wherein the difference between the refractive index of the suspending fluid and the continuous phase is not greater than about 0.05.

3. An electrophoretic medium according to claim 1 wherein the continuous phase comprises a plurality of capsule walls surrounding the droplets.

4. An electrophoretic medium according to claim 1 wherein the continuous phase comprises a polymeric binder in direct contact with the suspending fluid.

5. An electrophoretic medium according to claim 1 wherein the continuous phase comprises a carrier medium having a plurality of closed cavities formed therein, the droplets being confined within the cavities.

6. An electrophoretic medium according to claim 1 wherein the suspending fluid comprises a mixture of a hydrocarbon and chloronaphthalene.

7. An electrophoretic display comprising a layer of an electrophoretic medium according to claim 1 and electrode means arranged to apply an electric field to the electrophoretic medium, the electrode means being arranged to drive the electrophoretic medium to a non-light-transmissive state, in which the particles occupy a major proportion of the area of the layer, thereby rendering the layer substantially non-light-transmissive, and a transmissive state, in which the particles occupy only a minor proportion of the area of the layer, thereby rendering the layer substantially light-transmissive.

8. An electrophoretic medium comprising a suspending fluid and a plurality of electrically charged particles disposed in the suspending fluid and capable of moving therethrough on application of an electric field to the suspending fluid, the suspending fluid and electrically charged particles being present as a plurality of discrete droplets, each droplet being surrounded by a capsule wall, the electrophoretic medium further comprising a polymeric binder phase surrounding the capsule walls, wherein the difference between the refractive index of the capsule walls and the polymeric binder is not greater than about 0.07.

9. An electrophoretic display comprising a layer of an electrophoretic medium according to claim 8 and electrode means arranged to apply an electric field to the electrophoretic medium, the electrode means being arranged to drive the electrophoretic medium to a non-light-transmissive state, in which the particles occupy a major proportion of the area of the layer, thereby rendering the layer substantially non-light-transmissive, and a transmissive state, in which the particles occupy only a minor proportion of the area of the layer, thereby rendering the layer substantially light-transmissive.

10. In a process for forming an electrophoretic medium, which process comprises:

(a) providing a substrate;

(b) applying to the substrate a coating material comprising a plurality of capsules in a liquid phase, each of the capsules comprising a suspending fluid and a plurality of electrically charged particles disposed in the suspending fluid and capable of moving therethrough on application of an electric field to the capsule, and subjecting the coating material to conditions effective to cause formation of a coherent solid layer containing the capsules on the substrate; and

(c) applying an adhesive layer over the capsule-containing layer on the substrate,

the improvement which comprises planarizing the capsule-containing layer on the substrate after step (b) but before step (c).

11. A process according to claim 10 wherein the planarization is effected by coating the capsule-containing layer with a material having a refractive index which does not differ from that of the capsule layer by more than about 0.1.

12. A process according to claim 10 wherein the planarization is effected by applying mechanical pressure on the capsule-containing layer.

13. An electrophoretic display comprising a layer of an electrophoretic medium comprising a suspending fluid and a plurality of electrically charged particles disposed in the suspending fluid and capable of moving therethrough on application of an electric field to the suspending fluid, and electrode means arranged to apply an electric field to the electrophoretic medium, the electrode means being arranged to drive the electrophoretic medium to a non-light-transmissive state, in which the particles occupy a major proportion of the area of the layer, thereby rendering the layer substantially non-light-transmissive, and a transmissive state, in which the particles occupy only a minor proportion of the area of the layer, thereby rendering the layer substantially light-transmissive, wherein the suspending fluid and electrically charged particles being present as a plurality of discrete droplets, the electrophoretic medium further comprising a continuous phase within which the droplets are confined, the droplets having an aspect ratio not greater than about 0.5.

14. An electrophoretic display according to claim 13 wherein the continuous phase comprises a plurality of capsule walls surrounding the droplets.

15. An electrophoretic display according to claim 13 wherein the continuous phase comprises a polymeric binder in direct contact with the suspending fluid.

16. An electrophoretic display according to claim 13 wherein the continuous phase comprises a carrier medium having a plurality of closed cavities formed therein, the droplets being confined within the cavities.

17. A method of driving an electrophoretic display, the display comprising a layer of an electrophoretic medium having first and second surfaces on opposed sides thereof, and electrode means arranged to apply an electric field to the electrophoretic medium, the electrode means being arranged to drive the electrophoretic medium to a non-light-transmissive state, in which the particles occupy a major proportion of the area of the layer, thereby rendering the layer substantially non-light-transmissive, and a transmissive state, in which the particles occupy only a minor proportion of the area of the layer, thereby rendering the layer substantially light-transmissive, the electrophoretic medium comprising a suspending fluid and a plurality of electrically charged particles disposed in the suspending fluid and capable of moving therethrough on application of an electric field to the suspending fluid, the suspending fluid and electrically charged particles being present as a plurality of discrete droplets, the electrophoretic medium further comprising a continuous phase within which the droplets are confined,

the method comprising:

while the display is in its non-light-transmissive state, applying via the electrode means a direct current electric field, thereby moving the particles adjacent one of the first and second surfaces of the electrophoretic medium; and

thereafter applying via the electrode means an alternating current field, thereby moving the particles laterally within the electrophoretic medium and causing the display to assume its light-transmissive state.

18. An electrophoretic display comprising a laminar carrier medium having end walls and side walls defining a plurality of closed cavities formed therein, the sidewalls extending substantially normal to the thickness of the laminar carrier medium, each of the cavities having confined therein a suspending fluid and a plurality of electrically charged particles disposed in the suspending fluid and capable of moving therethrough on application of an electric field to the suspending fluid, wherein at least a portion of each side wall is electrically conductive.

19. An electrophoretic display according to claim 18 further comprising a light-transmissive front electrode provided at or adjacent one major surface of the laminar medium and at least one rear electrode at or adjacent the other major surface of the laminar medium, and therein the conductive side walls are electrically connected to at least one of the rear electrodes but are insulated from the front electrode.

20. A light modulator comprising two light-transmissive panels spaced from one another and an electrophoretic medium disposed between the two panels, the electrophoretic medium having a light-transmissive state and a dark state in which its light transmission is lower than in its light-transmissive date, wherein the electrophoretic medium is sensitive to at least one wavelength of electromagnetic radiation, and at least one of the light-transmissive panels comprises an absorber for electromagnetic radiation of this wavelength.

21. An electrophoretic display comprising a layer of an electrophoretic medium comprising a suspending fluid and a plurality of first particles, which are electrically charged and colored, and disposed in the suspending fluid and capable of moving therethrough on application of an electric field to the suspending fluid, and electrode means arranged to apply an electric field to the electrophoretic medium, the electrode means being arranged to drive the electrophoretic medium to a non-light-transmissive state, in which the fist particles occupy a major proportion of the area of the layer, thereby rendering the layer substantially non-light-transmissive, and a transmissive state, in which the first particles occupy only a minor proportion of the area of the layer, thereby rendering the layer substantially light-transmissive, wherein the electrophoretic medium also comprises a plurality of second particles, which are electrically charged and substantially transparent, disposed in the suspending fluid and capable of moving therethrough on application of an electric field to the suspending fluid.