Diffuse reflector and method

• US 20040136038A1
• Filed: 12/01/2003
• Published: 07/15/2004
• Est. Priority Date: 12/15/2000
• Status: Active Grant

• Current Assignee: De La Rue International Limited (De La Rue PLC)
• Original Assignee: De La Rue International Limited (De La Rue PLC)
• Inventors: Holmes, Brian William, Drinkwater, Kenneth John
• Granted Patent: US 7,251,077 B2
• Time in Patent Office: Days
• US Class Current: 359/15
• CPC Class Codes: G02B 5/0221 the surface having an irregular structure G02B5/0226 takes precedence
• CPC Class Codes: G02B 5/0252 using holographic or diffractive means
• CPC Class Codes: G02B 5/0268 characterized by the fabrication or manufacturing method
• CPC Class Codes: G02B 5/0284 used in reflection
• CPC Class Codes: G02B 5/32 Holograms used as optical elements
• CPC Class Codes: G02F 1/133374 for displaying permanent signs or marks
• CPC Class Codes: G02F 1/133553 Reflecting elements associated to illuminating devices G02F1/133605
• CPC Class Codes: Y10S 359/90 Methods

Abstract

A diffuse reflector comprises a substrate (20) having an asymmetric, reflective, periodic surface relief microstructure formed such that, under illumination by a multiplicity of wavelengths in a plane of incidence or illumination lying along the axis of asymmetry, substantially all the said wavelengths are preferentially scattered or guided into a single, common viewing zone offset from the direction of specular reflection hereinebefore defined.

Claims

1. A diffuse reflector comprising a substrate having an asymmetric, reflective, periodic surface relief microstructure formed such that, under illumination by a multiplicity of wavelengths in a plane of incidence or illumination lying along the axis of asymmetry, substantially all the said wavelengths are preferentially scattered or guided into a single, common viewing zone having an axis offset from the direction of specular reflection as hereinbefore defined.

2. A reflector according to claim 1, wherein the common viewing zone is centred on a direction closer to a normal to the substrate than to the plane of the substrate.

3. A reflector according to claim 2, wherein the single, common viewing zone is centred substantially on the normal to the substrate.

4. A reflector according to any of claims 1 to 3, wherein the said wavelengths are diffracted such that the single, common viewing zone is defined by one of the two first order diffracted components.

5. A reflector according to any of the preceding claims, wherein the surface relief microstructure comprises a relatively coarse asymmetric surface relief on which is provided a finer surface relief diffuse scattering structure.

6. A reflector according to any of the preceding claims, wherein the surface relief microstructure is embossed into the substrate.

7. A reflector according to any of the preceding claims, wherein the substrate is provided with a reflection enhancing layer.

8. A reflector according to claim 7, wherein the substrate is metallised.

9. A reflector according to claim 7 or claim 8, wherein the reflection enhancing layer is provided with micro holes.

23. A display device comprising a transparent display generator;

and a diffuse reflector according to any of claims 1 to 9 or fabricated according to any of claims 10 to 22 located behind the display generator.

24. A display device according to claim 23, when dependent on claim 7, wherein the reflection enhancing layer is partially transparent, the device further comprising a back light source for generating back light which can be transmitted through the reflection enhancing layer.

25. A display device according to claim 23 or claim 24, wherein the display generator comprises a spatial light modulator such as a liquid crystal display.

26. A display device according to claim 23 or claim 24, wherein the display device constitutes safety wear, clothing, a cats eye, flat-panel display, e.g. lap-top computers, cellular telephone displays, wrist watches, calculators, personal digital assistants, etc. or is a substrate for documents such as, labels, tickets, banknotes, tags, transaction cards, cheques, brand enhancement/packaging features, documents of value, notices, road signs and the like.

27. A diffuse reflector according to any of claims 1 to 9, which is either adhered to a polymeric carrier film or is transferred from a carrier onto a final substrate using an adhesive system.

10. A method of fabricating a diffuse reflector, the method comprising projecting a holographic image of a diffuser on to a planar recording member oriented in the plane of the projected image while illuminating the recording member with a coherent reference beam, wherein the image and reference beam are projected on to the recording member on the same side of a normal to the plane of the recording member, to create an interference pattern in the recording member;

and forming a surface relief microstructure defining an asymmetric, diffuse reflector using the interference pattern.

11. A method according to claim 10, wherein the holographic image is derived from a holographic recording of a transmissive diffuser using a reference beam which is the conjugate of the reference beam used to project the holographic image.

12. A method according to claim 11, wherein the step of holographically recording a transmissive diffuser further comprises integrating one or more images into the diffuser image.

13. A method according to claim 12, wherein the or each image is defined by a mask.

14. A method according to claim 12 or claim 13, wherein the image is one of a logo, indicia or graphic design.

22. A method according to any of claims 10 to 21, further comprising enhancing the asymmetric property by angled reactive ion-beam milling or etching the recording medium.

15. A method of fabricating a diffuse reflector, the method comprising

i) fabricating a relatively coarse, asymmetric, periodic structure in a recording member;

and

ii) forming a diffusely scattering, finer microstructure on the periodic structure, the resultant surface relief of the recording member forming an asymmetric, diffuse reflector.

16. A method according to claim 15, wherein, between steps (i) and (ii), the coarse structure is coated with a photo responsive material, step (ii) comprising exposing the photoresponsive material to a diffuse light field.

17. A method according to claim 16, wherein the diffuse light field is generated directly from a diffuser or indirectly using a holographic image of a diffuser.

18. A method of fabricating a diffuse reflector, the method comprising focussing two beams of coherent light on to a recording member through respective first and second focussing elements such that the two focussed beams overlap on the recording member, the focussing elements being designed such that the overlapping beams generate an interference pattern pixel in the recording member;

repeating this process for a plurality of pixels; and

forming a surface relief microstructure defining an asymmetric, diffuse reflector using the interference pattern.

19. A method according to claim 18, wherein the pixels are arranged in a regular array.

20. A method according to claim 18 or claim 19, wherein one of the focussing elements is a holographic optical element.

21. A method according to any of claims 18 to 20, wherein both beams impinge on the recording member from the same side of a normal to the recording member.