Diffuse reflector and method
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 hereinbefore defined.
1. A method of fabricating a diffuse reflector, the method comprising using an off-axis hologram to project 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 wherein the planes of interference are substantially inclined towards the plane of the recording medium thereby forming a surface relief microstructure defining an asymmetric, diffuse reflector using the interference pattern.
2. A method according to claim 1 , 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.
3. A method according to claim 2 , wherein the step of holographically recording a transmissive diffuser further comprises integrating one or more images into the diffuser image.
4. A method according to claim 3 , wherein the or each image is defined by a mask.
5. A method according to claim 3 , wherein the image is one of a logo, indicia or graphic design.
6. A method according to claim 1 , further comprising enhancing the asymmetric property by angled reactive ion-beam milling or etching the recording medium.
7. A method according to claim 1 , wherein the diffuse image beam projected from the off-axis transmission hologram has an angular distribution required to form, when overlapped with the reference beam at the plane of the recording member, grating periodicities ranging from 500 lines per mm to 1500 lines per mm.
8. A method of fabricating a diffuse reflector, the method comprising focussing two beams of coherent light on to a planar recording member through respective first and second focussing elements such that the two focussed beams overlap on the same side of a normal to the plane facing the reference and object beams, to form an interference pattern pixel whose planes of interference are inclined to the plane facing the reference and object beams; repeating this process for a plurality of pixels; and forming a surface relief microstructure defining an asymmetric, diffuse reflector using the interference pattern.
9. A method according to claim 8 , wherein the pixels are arranged in a regular array.
10. A method according to claim 8 , wherein one of the focussing elements is a holographic optical element.
11. A method according to claim 8 , wherein the object beam has a diffusion angle greater than 20 degrees, within the plane of dispersion such that the reflector has a substantially achromatic response.