Cholesteric polarizer and the manufacture thereof

• US 5,506,704 A
• Filed: 01/10/1994
• Issued: 04/09/1996
• Est. Priority Date: 01/11/1993
• Status: Expired due to Term

• Current Assignee: US Philips Corporation (Koninklijke Philips N.V.)
• Original Assignee: US Philips Corporation (Koninklijke Philips N.V.)
• Inventors: Lub, Johan, Broer, Dirk J.
• Primary Examiner(s): Lerner, Martin
• Application Number:
  US08/179,420
• Time in Patent Office: 820 Days
• Field of Search: 359/63, 359/64, 359/65, 359/103, 359/105, 359/490, 359/492, 359/500, 252/585, 252/299.7
• US Class Current: 349/98
• CPC Class Codes: G02B 5/3016 involving passive liquid crystal elements optical properties of liquid crystals G02F1/0063; polarising elements associated with active liquid crystal devices G02F1/133528
• CPC Class Codes: G02F 1/133536 Reflective polarizers G02F1/13362 takes precedence
• CPC Class Codes: G02F 1/133543 Cholesteric polarisers
• CPC Class Codes: G02F 1/13362 providing polarized light, e.g. by converting a polarisation component into another one

Abstract

A broadband cholesteric polarizer is described, as well as a method of manufacturing such a polarizer. Said polarizer comprises an optically active layer of a polymer material having a cholesteric order, said material being oriented so that the axis of the molecular helix extends transversely to the layer. In accordance with the invention, the polarizer is characterized in that the pitch of the molecular helix in the layer is varied in such a manner that the difference between the maximum pitch and the minimum pitch is at least 100 nm. The optically active layers are preferably provided on substrates which bring about a conversion of circularly polarized light into linearly polarized light. Three different methods of manufacturing such broadband polarizers are described.

Claims

1. A cholesteric polarizer comprising an optically active layer of a polymer material, the polymer material having a cholesteric order and a molecular helix, the molecular helix having an axis extending transversely to the layer, wherein a pitch of the molecular helix in the layer varies with a difference between a maximum pitch and a minimum pitch of at least 100 nm, further characterized in that the optically active layer is present on a substrate of a stretched synthetic resin film, the degree of stretching and the thickness of the film being selected in such a manner that the optical retardation of the film is approximately 0.25 times the wavelength of the band reflected during operation of the polarizer.

2. A cholesteric polarizer comprising an optically active layer of a polymer material, the polymer material having a cholesteric order and a molecular helix, the molecular helix having an axis extending transversely to the layer, wherein a pitch of the molecular helix in the layer varies with a difference between a maximum pitch and a minimum pitch of at least 100 nm, further characterized in that the optically active layer is present on a substrate of two stretched synthetic resin films of different composition, the directions of stretching of both films extending substantially transversely to each other and the degree of stretching of both films being selected in such a manner that, due to a difference in dispersion between the films, the net retardation of the substrate is substantially equal to 0.25 times the wavelength over a substantial portion of the reflected band of the polarizer.

3. A method of manufacturing a cholesteric polarizer characterized in that a mixture of chiral and nematogenic monomers, each having a different reactivity, is provided in the form of a layer between two parallel substrates, after which actinic radiation is applied in accordance with a radiation profile whose intensity varies over the layer, so that the mixture is polymerized to an optically active layer of polymer material having a cholesteric order.

4. A method as claimed in claim 3, characterized in that the number of reactive groups of the chiral monomer differs from that of the nematogenic monomer.

5. A method as claimed in claim 4, characterized in that the mixture also comprises a dye having an absorption maximum whose wavelength corresponds substantially to the wavelength of the actinic radiation used.

6. A method as claimed in claim 3, further comprising removing the substrates from the optically active layer.

7. A method as claimed in claim 3, characterized in that the mixture also comprises a dye having an absorption maximum whose wavelength corresponds substantially to the wavelength of the actinic radiation used.

8. A method of manufacturing a cholesteric polarizer characterized in that a mixture of chiral and nematogenic monomers, each having a different reactivity, is provided on a substrate in the form of a layer, after which actinic radiation is applied in accordance with a radiation profile whose intensity varies over the layer, so that the mixture is polymerised to an optically active layer of polymer material having a cholesteric order.

9. A method as claimed in claim 8, characterized in that the mixture also comprises a dye having an absorption maximum whose wavelength corresponds substantially to the wavelength of the actinic radiation used.

10. A method as claimed in claim 8, characterized in that the number of reactive groups of the chiral monomer differs from that of the nematogenic monomer.

11. A method as claimed in claim 8, further comprising removing the substrate from the optically active layer.

12. A method of manufacturing a cholesteric polarizer comprising the steps of:

providing an optically active layer of polymerised liquid crystalline material having a cholesteric order on a substrate, providing on a surface of the optically active layer of polymerised liquid crystalline material a film of reactive monomers which diffuse in the layer and cause a concentration gradient in the layer as a result of diffusion, and polymerising the monomers.

13. A method as claimed in claim 12, characterized in that the film comprises a mixture of monomers having different diffusion rates in the layer.

14. A method as claimed in claim 13, characterized in that at least a part of the monomers comprises two or more polymerisable groups.

15. A method as claimed in claim 12, characterized in that at least a part of the monomers comprises two or more polymerisable groups.

16. A lighting device comprising a socket for an electric light source, a reflector, and a cholesteric polarizer which is provided with a quarter-wave plate, characterized in that said cholesteric polarizer comprises an optically active layer of a polymer material, the polymer material having a cholesteric order and a molecular helix, the molecular helix having an axis extending transversely to the layer, wherein a pitch of the molecular helix in the layer varies with a difference between a maximum pitch and a minimum pitch of at least 100 nm, further characterized in that the optically active layer is present on a substrate of a stretched synthetic resin film, the degree of stretching and the thickness of the film being selected in such a manner that the optical retardation of the film is approximately 0.25 times the wavelength of the band reflected during operation of the polarizer.

17. A lighting device comprising a socket for an electric light source, a reflector, and a cholesteric polarizer which is provided with a quarter-wave plate, characterized in that said cholesteric polarizer comprises an optically active layer of a polymer material, the polymer material having a cholesteric order and a molecular helix, the molecular helix having an axis extending transversely to the layer, wherein a pitch of the molecular helix in the layer varies with a difference between a maximum pitch and a minimum pitch of at least 100 nm, further characterized in that the optically active layer is present on a substrate of two stretched synthetic resin films of different composition, the directions of stretching of both films extending substantially transversely to each other and the degree of stretching of both films being selected in such a manner that, due to a difference in dispersion between the films, the net retardation of the substrate is substantially equal to 0.25 times the wavelength over a substantial portion of the reflected band of the polarizer.