Electrophoretic device having a transparent light state

1. A light attenuator comprising:

a cell comprising a first substrate and a second substrate spaced apart from said first substrate, and

a layer between said substrates containing an electrophoretic ink, a surface of said layer adjacent said second substrate comprising a monolayer of closely packed transparent protrusions projecting into said layer, said protrusions having surfaces defining a plurality of depressions in the volumes there between, said ink including charged particles of at least one type, the particles being responsive to an electric field applied to said cell to move between:

a first extreme light state, in which particles are maximally spread within the cell to lie in the path of light through the cell and to strongly attenuate light transmitted through the cell;

over the surface of said protrusions, which deflect said particles from their path from the first substrate to the second substrate;

to a second extreme light state in which said particles are maximally concentrated within each said depression to remove them from the path of light through the cell and to substantially transmit light through the cell.

2. A light attenuator as claimed in claim 1 , wherein each of said substrates comprises a respective electrode defining an electrophoretic cell.

3. A light attenuator as claimed in claim 1 , wherein said electrophoretic cell is metastable having one or more light states intermediate said first and second light states.

4. A light attenuator as claimed in claim 1 , wherein said protrusions are either: monodisperse or have a distribution of sizes and/or shapes.

5. A light attenuator as claimed in claim 1 , wherein said protrusions comprise surfaces which are at least partly defined by non-planar structures.

6. A light attenuator as claimed in claim 5 wherein the protrusions non-planar structures are in discrete form.

7. A light attenuator as claimed in claim 1 wherein in said second light state, each protrusion provides a discrete aperture area that is substantially free of said charged particles and transparent to specular light, said charged particles in said second light state surrounding said aperture areas defining a contiguous strongly light attenuating area.

8. A light attenuator as claimed in claim 1 wherein the particles are responsive to an electric field applied to said cell to move between two unidirectional sets of intermediate light states, a first set of intermediate light states transitioning from said first light state and providing a range of increasing light transmittance levels as charged particles vacate an increasing aperture area on each protrusion's surface as a second light state is approached, and a second set of intermediate light states transitioning from said second light state and providing a range of decreasing light transmittance levels as concentrated charged particles spread adjacent said first electrode, decreasing aperture areas as said first light state is approached.

9. A light attenuator as claimed in claim 1 wherein said electrophoretic cell includes one or more of the following polymer elements: protrusions, space-filling polymer, and cavity walls; said elements being optically isotropic and refractive index matched to a suspending fluid of said electrophoretic ink, preferably within 0.01 of each other, more preferably within 0.005, and most preferably within 0.002.

10. A light attenuator as claimed in claim 9 wherein said polymer elements comprise either: a non-polar, amorphous, polymer; or a polar, amorphous polymer.

11. A light attenuator as claimed in claim 1 wherein less than about 10% of said protrusions provide cell-gap spacers, determining the cell gap of the electrophoretic cell.

12. A light attenuator as claimed in claim 2 wherein said electrophoretic cell is divided into a monolayer of discrete or semi-discrete cavities, said cavities conforming to the curvature of said protrusions on one side and the planar surface of said second electrode on another side.

13. A light attenuator as claimed in claim 1 wherein said charged particles include one of: a dye colorant, the charged particles being refractive index matched to the electrophoretic ink suspending fluid; a pigment colorant; or comprise one of positively or negatively charged black particles.

14. The light attenuator as claimed in claim 1 wherein the light attenuator is incorporated as a layer in one of a mirror, a window, a light shutter, a light modulator, a variable light transmittance sheet, a variable light absorptance sheet, a variable light reflectance sheet, a monochrome display, a color display, an electrophoretic skin, an electrophoretic paint layer, an electrophoretic sun visor for a vehicle and a see-through display.

15. A device according to claim 14 including a reflective surface wherein said electrophoretic ink layer is disposed adjacent said reflective surface, and in a first light state said charged particles directly mask the reflective surface.

16. A light attenuator as claimed in claim 1 wherein the depressions are in contiguous form.

17. A light attenuator as claimed in claim 5 , wherein a maximum separation of adjacent non-planar structures at a plane incorporating said depressions, does not exceed 250 microns and more preferably does not exceed 150 microns to avoid a perceivable optical defect.