Cholesteric polarizer for liquid crystal display and overhead projector

• US 5,325,218 A
• Filed: 12/31/1992
• Issued: 06/28/1994
• Est. Priority Date: 12/31/1992
• Status: Expired due to Fees

• Current Assignee: 3M Company
• Original Assignee: 3M Company
• Inventors: Tritle, Gary L., Willett, Stephen J.
• Primary Examiner(s): Gross, Anita P.
• Application Number:
  US07/999,230
• Time in Patent Office: 544 Days
• Field of Search: 359/40, 359/53, 359/65, 359/64, 359/73, 353/122, 353/DIG. 3, 353/DIG. 5, 353/119
• US Class Current: 349/74
• CPC Class Codes: G02F 1/133514 Colour filters
• CPC Class Codes: G02F 1/133533 Colour selective polarisers G02F1/1347 takes precedence
• CPC Class Codes: G02F 1/133536 Reflective polarizers G02F1/13362 takes precedence
• CPC Class Codes: G02F 1/133543 Cholesteric polarisers
• CPC Class Codes: G02F 1/133621 providing coloured light G02F1/133617, G02F1/133533 take precedence
• CPC Class Codes: G02F 1/13473 for wavelength filtering or for colour display without the use of colour mosaic filters
• CPC Class Codes: G02F 2201/343 cholesteric liquid crystal reflector
• CPC Class Codes: Y10S 353/03 Transparent
• CPC Class Codes: Y10S 353/05 Transparency holder accessories

Abstract

A liquid crystal display (LCD) panel employs a cholesteric polarizer which passes light of a first circular polarization, but reflects light of a second circular polarization. The LCD panel may be monochrome or color; in the color embodiments, several different color filters are used, each having one or more cholesteric polarizers tuned to a specific bandwidth in the visible spectrum. These polarizers replace dyed (neutral or dichroic) polarizers of prior art LCD panels, and provide improved color purity. The invention may be enhanced by providing a light recycling scheme whereby the handedness of the circularly polarized light reflected by the polarizers is changed and redirected back to the polarizers. This recycling scheme may also be adapted to provide an overhead projector having a high output polarized light source, particularly useful in conjunction with an LCD panel.

Claims

1. A display device adapted to display color images, comprising:

first, second and third circular polarizers which pass circularly polarized light in a bandwidth within the visible spectrum, each of said polarizers being generally planar;

first, second, third, fourth and fifth quarter-wave plates for converting circularly polarized light into linearly polarized light, each of said quarter-wave plates being generally planar;

first, second and third addressable, liquid crystal cells for selectively modulating the phase of linearly polarized light;

lens means for collecting and collimating light;

said first circular polarizer being interposed between said lens means and said first liquid crystal cell, and tuned to circularly polarize light of a first primary color;

said first quarter-wave plate being interposed between said first circular polarizer and said first liquid crystal cell;

said first liquid crystal cell being tuned to selectively rotate linearly polarized light of said first primary color;

said second circular polarizer being interposed between said first and second liquid crystal cells, and tuned to circularly polarize light of all three primary colors;

said second quarter-wave plate being interposed between said second circular polarizer and said first liquid crystal cell;

said third quarter-wave plate being interposed between said second circular polarizer and said second liquid crystal cell;

said second liquid crystal cell being tuned to selectively rotate linearly polarized light of a second primary color;

said third circular polarizer being interposed between said second and third liquid crystal cells, and tuned to circularly polarize light of all three primary colors;

said fourth quarter-wave plate being interposed between said third circular polarizer and said second liquid crystal cell;

said fifth quarter-wave plate being interposed between said third circular polarizer and said third liquid crystal cell;

said third liquid crystal cell being tuned to selectively rotate linearly polarized light of a third primary color; and

analyzer means for rendering visible modulated light exiting said third liquid crystal cell.

2. The display device of claim 1 further comprising second lens means, adjacent said analyzer means, opposite from said fifth quarter-wave plate, for condensing light which passes through said analyzer means.

3. The display device of claim 1 wherein at least one of said polarizers:

passes light in said bandwidth which is circularly polarized with a first right/left handedness; and

reflects light in said bandwidth which is circularly polarized with a second right/left handedness.

4. The display device of claim 1 wherein at least one of said polarizers comprises a cholesteric polarizer tuned to said bandwidth.

5. A display device adapted to display color images, comprising:

first, second and third circular polarizers which pass circularly polarized light in a bandwidth within the visible spectrum, each of said polarizers being generally planar;

first, second and third addressable, liquid crystal cells for selectively modulating the phase of linearly polarized light;

first, second, third, fourth and fifth quarter-wave plates for converting circularly polarized light into linearly polarized light, each of said quarter-wave plates being generally planar;

lens means for collecting and collimating light;

said first circular polarizer being interposed between said lens means and said first liquid crystal cell, and tuned to circularly polarize light of a first primary color;

said first quarter-wave plate being interposed between said first circular polarizer and said first liquid crystal cell;

said first liquid crystal cell being tuned to selectively rotate linearly polarized light of said first primary color;

said second circular polarizer being interposed between said first and second liquid crystal cells, and tuned to circularly polarize light of said first primary color and of a second primary color;

said second quarter-wave plate being interposed between said second circular polarizer and said first liquid crystal cell;

said third quarter-wave plate being interposed between said second circular polarizer and said second liquid crystal cell;

said second liquid crystal cell being tuned to selectively rotate linearly polarized light of said second primary color;

said third circular polarizer being interposed between said second and third liquid crystal cells, and tuned to circularly polarize light of said second primary color and of a third primary color;

said fourth quarter-wave plate being interposed between said third circular polarizer and said second liquid crystal cell;

said fifth quarter-wave plate being interposed between said third circular polarizer and said third liquid crystal cell;

said third liquid crystal cell being tuned to selectively rotate linearly polarized light of said third primary color; and

analyzer means for rendering visible modulated light exiting said third liquid crystal cell.

6. The display device of claim 5 further comprising second lens means, adjacent said analyzer means, opposite from said fifth quarter-wave plate, for condensing light which passes through said analyzer means.

7. The display device of claim 5 wherein at least one of said polarizers:

passes light in said bandwidth which is circularly polarized with a first right/left handedness; and

reflects light in said bandwidth which is circularly polarized with a second right/left handedness.

8. The display device of claim 7 further comprising:

a fourth circular polarizer adjacent to said first circular polarizer, tuned to circularly polarize light of said second and third primary colors;

a light source at a known effective distance from the display device; and

a back reflector for said light source;

wherein said lens means comprises a convergent Fresnel lens having a focal length which is approximately equal to said effective distance.

9. The display device of claim 5 wherein at least one of said polarizers comprises a cholesteric polarizer tuned to said bandwidth.

10. A display device adapted to display color images, comprising:

first, second and third circular polarizers for passing circularly polarized light in a bandwidth within the visible spectrum, each of said polarizers being generally planar;

first, second and third addressable, liquid crystal cells for selectively modulating the phase of linearly polarized light;

first, second, third, fourth and fifth quarter-wave plates for converting circularly polarized light into linearly polarized light, each of said quarter-wave plates being generally planar;

lens means for collecting and collimating light;

said first circular polarizer being interposed between said lens means and said first liquid crystal cell, and tuned to circularly polarize light of a first primary color and a second primary color;

said first quarter-wave plate being interposed between said first circular polarizer and said first liquid crystal cell;

said first liquid crystal cell being tuned to selectively rotate linearly polarized light of said first primary color;

said second circular polarizer being interposed between said first and second liquid crystal cells, and tuned to circularly polarize light of said first primary color and of a third primary color;

said second quarter-wave plate being interposed between said second circular polarizer and said first liquid crystal cell;

said third quarter-wave plate being interposed between said second circular polarizer and said second liquid crystal cell;

said second liquid crystal cell being tuned to selectively rotate linearly polarized light of said second primary color;

said third circular polarizer being interposed between said second and third liquid crystal cells, and tuned to circularly polarize light of said second and third primary colors;

said fourth quarter-wave plate being interposed between said third circular polarizer and said second liquid crystal cell;

said fifth quarter-wave plate being interposed between said third circular polarizer and said third liquid crystal cell;

said third liquid crystal cell being tuned to selectively rotate linearly polarized light of said third primary color; and

analyzer means for rendering visible modulated light exiting said third liquid crystal cell.

11. The display device of claim 10 further comprising second lens means, adjacent said analyzer means, opposite from said fifth quarter-wave plate, for condensing light which passes through said analyzer means.

12. The display device of claim 10 wherein at least one of said polarizers:

passes light in said bandwidth which is circularly polarized with a first right/left handedness; and

reflects light in said bandwidth which is circularly polarized with a second right/left handedness.

13. The display device of claim 10 wherein at least one of said polarizers comprises a cholesteric polarizer tuned to said bandwidth.

14. An overhead projector having high output polarized light, comprising:

a base having a stage area;

a projector head mounted to said base, proximate said stage area;

a light source in said base;

means for directing light from said light source to said stage area, defining an optical path;

means for reflecting light, said light reflecting means located adjacent said light source and opposite said stage area with respect to said optical path;

first lens means for collecting and collimating light from said light source toward said stage area;

second lens means, interposed between said first lens means and said stage area, for condensing light from said first lens means and directing it through said stage area toward said projector head;

polarizer means, located between said first and second lens means, for passing light of a first polarization, and for reflecting light of a second polarization; and

means for removing said polarizer means from said base.

15. The overhead projector of claim 14 wherein said polarizer means comprises at least one circular polarizer which passes light that is circularly polarized with a first right/left handedness, and reflects light that is circularly polarized with a second right/left handedness.

16. The overhead projector of claim 14 further comprising a quarter-wave plate interposed between said polarizer means and said second lens means.

17. The overhead projector of claim 14 wherein said removing means comprises a drawer which is slidably attached to said base, said polarizer means being located in said drawer.