Electronic skin having uniform gray scale reflectivity

• US 20090284691A1
• Filed: 05/16/2008
• Published: 11/19/2009
• Est. Priority Date: 05/16/2008
• Status: Active Grant

• Current Assignee: Manning Ventures, Inc.
• Original Assignee: Manning Ventures, Inc.
• Inventors: Marhefka, Duane W., Barua, Sankar, Venkataraman, Nithya
• Granted Patent: US 8,310,630 B2
• Time in Patent Office: Days
• US Class Current: 349/86
• CPC Class Codes: G02F 1/1347 Arrangement of liquid crystal layers or cells in which the final condition of one light beam is achieved by the addition of the effects of two or more layers or cells
• CPC Class Codes: G02F 1/13718 based on a change of the texture state of a cholesteric liquid crystal
• CPC Class Codes: G02F 1/1391 Bistable or multi-stable liquid crystal cells G02F1/141 takes precedence

Abstract

The invention features electronic skin including an active layer formed of bistable cholesteric liquid crystal material and articles comprising the skin, the electronic skin having uniform gray scale reflectivity. Also featured is method for producing suitable reduction pulses that will provide a display (e.g., the electronic skin) with uniform gray scale reflectivity. Reduction pulses of narrow width are used to create uniform levels of gray in the electronic skin to overcome display imperfections that cause discontinuity in the gray scale reflectivity.

Claims

1. Electronic skin comprising:

electrically conductive layers;

at least one active layer of bistable cholesteric liquid crystal material, each said active layer being disposed between adjacent said conductive layers, wherein said cholesteric liquid crystal material of each said active layer has a pitch length effective to reflect light of a predetermined color and each said active layer has an area that is susceptible to discontinuities in gray scale reflectivity; and

an outer layer of transparent material adjacent one of said conductive layers;

wherein each said active layer reflects light at a gray scale having uniform reflectivity in said area.

2. The electronic skin of claim 1 comprising a light absorbing layer that is adapted to absorb light passing through said active layer.

3. The electronic skin of claim 1 comprising a second outer layer disposed adjacent another one of said conductive layers, said outer layer and said second outer layer being composed of flexible glass or polymer enabling said electronic skin to conform to a fixed three dimensional shape.

4. The electronic skin of claim 1 wherein said cholesteric liquid crystal material is dispersed in a polymer matrix.

5. The electronic skin of claim 1 comprising two stacked said active layers, wherein said predetermined colors of said active layers are different than each other.

6. The electronic skin of claim 1 comprising three stacked said active layers, wherein said predetermined colors of said active layers are different than each other.

7. The electronic skin of claim 1 comprising a single substrate or substrate component disposed between adjacent said active layers, wherein said single substrate includes said conductive layers on both sides thereof and said substrate component includes two substrates having said conductive layers on outside surfaces thereof and index of refraction matching material disposed between said substrates, wherein said single substrate and said substrates of said component are composed of flexible and transparent glass or plastic.

8. The electronic skin of claim 1 wherein said electrically conductive layers are patterned or unpatterned.

9. The electronic skin of claim 2 wherein all of said conductive layers are transparent, except for one of said conductive layers closest to said light absorbing layer which is transparent or opaque.

10. An article having electronically switchable colors, said article comprising:

electronic skin including electrically conductive layers, at least two stacked active layers of bistable cholesteric liquid crystal material, each said active layer being disposed between adjacent said conductive layers, said cholesteric liquid crystal material of said active layers having pitch lengths effective to reflect light of different predetermined colors than each other, each of said active layers having a display area that is susceptible to discontinuities in gray scale reflectivity, an outer layer of transparent material adjacent one of said conductive layers,

a layer of light absorbing material adapted to absorb light passing through said active layers, said light absorbing layer being black or a back color; and

means for applying erasing and reduction voltage pulses to said conductive layers for each of said active layers,

wherein said erasing pulses are effective to place said active layers in a focal conic or planar texture, and

wherein said reduction pulses are effective to enable said active layers to reflect light at a series of gray scale levels having uniform reflectivity in said display areas, enabling said electronic skin to display gray scale colors.

11. The article of claim 10 comprising a support surface on which said electronic skin is attached.

12. The article of claim 11 wherein said support surface has a contour and said electronic skin is flexible and can conform to said contour of said support surface.

13. The article of claim 10 comprising three of said active layers, wherein said predetermined colors of said active layers are a different one of red, green and blue in any order.

14. The article of claim 10 wherein said cholesteric liquid crystal of each of said active layers is dispersed in a polymeric matrix.

15. The article of claim 10 wherein said electrically conductive layers are patterned or unpatterned.

16. The article of claim 10 comprising a single substrate or substrate component disposed between adjacent said active layers, wherein said single substrate includes said conductive layers on both sides thereof and said substrate component includes two of said substrates having said conductive layers on outside surfaces thereof and index of refraction matching material disposed between said substrates, wherein said single substrate and said substrates of said component are composed of flexible and transparent glass or plastic.

17. The electronic skin of claim 10 wherein all of said conductive layers are transparent, except for one of said conductive layers closest to said light absorbing layer which is transparent or opaque.

18. A method for providing an active layer of bistable cholesteric liquid crystal material with uniform gray scale reflectivity, comprising:

plotting electrooptic response curves for said active layer showing response of reflectance to reduction pulse voltage, each said curve corresponding to a different reduction pulse width;

identifying uniformity regions on said curves at which said active layer has a uniform gray scale reflectivity;

selecting a voltage that intersects said identified uniformity regions on said curves;

choosing a series of gray scale levels for said active layer that can achieve a substantially linear decrease in reflectance in response to increasing reduction pulse number; and

selecting gray scale reduction pulse widths at said selected voltage which, based on said pulse widths of said curves that intersect said selected voltage, will produce gray scale reduction pulses that achieve each said gray scale level in the series at uniform reflectivity.

19. The method of claim 18 comprising providing a display including said active layer disposed between adjacent electrically conductive layers and a light absorbing layer adapted to absorb light passing through said active layer, said light absorbing layer being black or a back color,

wherein said gray scale levels of the series are in a sequential decreasing order, comprising driving said display to a selected one of said gray scale levels by applying to said conductive layer all of said reduction pulses in order from a first reduction pulse, which reduces reflectivity from a preceding maximum reflectivity level, to said gray scale reduction pulse for said selected gray scale level.

20. The method of claim 19 wherein said gray scale levels are spaced such that a decrease in reflectance of each said gray scale level from an immediately preceding said gray scale level is similar for all said gray scale levels of the series.

21. The method of claim 19 wherein said reduction pulses are unipolar or bipolar.

22. The method of claim 19 wherein said reduction pulses are an AC square waveform.

23. The method of claim 19 comprising changing said gray scale level of said display to a gray scale level of higher reflectivity by:

applying erasing voltage pulses effective to place said liquid crystal material in said planar texture at said maximum reflectivity; and

conducting said step of driving said display to said selected gray scale level, where said selected gray scale level is said gray scale level of higher reflectivity.

24. The method of claim 19 wherein said display includes at least two stacked said active layers of bistable cholesteric liquid crystal material, each said active layer being disposed between adjacent said conductive layers, said cholesteric liquid crystal material of said active layers having pitch lengths effective to reflect light of different predetermined colors than each other, said layer of light absorbing material adapted to absorb light passing through said active layers, said light absorbing layer being black or a back color.

25. The method of claim 24 comprising two of said active layers, comprising providing said display with gray scale colors at said uniform gray scale reflectivity by applying said gray scale reduction pulses to said electrically conductive layers for one or both of said active layers.

26. The method of claim 24 comprising three of said active layers, comprising providing said display with gray scale colors at said uniform gray scale reflectivity by applying said gray scale reduction pulses to said electrically conductive layers for one, two or all three of said active layers.

27. A method for providing an active layer of bistable cholesteric liquid crystal material with uniform gray scale reflectivity, comprising:

plotting electrooptic response curves for said active layer showing response of reflectance to reduction pulse voltage, each said curve corresponding to a different reduction pulse width;

identifying a uniformity region on each of said curves at which said active layer has a uniform gray scale reflectivity;

choosing a series of gray scale levels for said active layer; and

selecting voltages and pulse widths which, based on said identified uniformity regions of said curves, will produce gray scale reduction pulses that produce said gray scale levels in the series.

28. The method of claim 27 wherein said gray scale levels are chosen to achieve a substantially linear decrease in reflectance in response to increasing reduction pulse number.

29. The method of claim 27 comprising identifying a uniformity line that intersects all of said curves at said uniformity region of each said curve, identifying intersection points where each of said gray scale levels intersects said uniformity line, wherein said gray scale reduction pulses comprise pulse widths and voltages on or near each of said intersection points.

30. The method of claim 29 comprising providing a display including said active layer disposed between adjacent electrically conductive layers and a light absorbing layer adapted to absorb light passing through said active layer, said light absorbing layer being black or a back color,

wherein said gray scale levels of the series are in a sequential decreasing order, comprising driving said display with uniform gray scale reflectivity to a selected one of said gray scale levels by applying to said conductive layers all of said reduction pulses in order from a first said reduction pulse, which reduces reflectivity from a preceding maximum reflectivity level, to said gray scale reduction pulse for said selected gray scale level.

31. The method of claim 30 wherein each said gray scale level is produced by applying one or more of said reduction pulses.

32. The method of claim 30 wherein said display includes at least two stacked said active layers of bistable cholesteric liquid crystal material, each said active layer being disposed between adjacent said conductive layers, said cholesteric liquid crystal material of said active layers having pitch lengths effective to reflect light of different predetermined colors than each other, said layer of light absorbing material adapted to absorb light passing through said active layers.

33. The method of claim 32 comprising two of said active layers, comprising providing said display with gray scale colors at said uniform gray scale reflectivity by applying said gray scale reduction pulses to said electrically conductive layers for one or both of said active layers.

34. The method of claim 32 comprising three of said active layers, comprising providing said display with gray scale colors at said uniform gray scale reflectivity by applying said gray scale reduction pulses to said electrically conductive layers for one, two or all three of said active layers.