Optical: systems and elements – Diffraction – From grating
Patent
1998-06-09
2000-03-28
Spyrou, Cassandra
Optical: systems and elements
Diffraction
From grating
359569, 359900, 264 131, 264 25, G02B 518, B29D 1100
Patent
active
06043936&
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a diffractive structure.
In many applications, it is desirable to have a diffractive structure having reflective properties which do not depend upon specific limited illumination and viewing angles to produce colour.
For example, in security films, security holograms conventionally consist of a thermoformed plastic layer having a reflective aluminium film deposited on the thermoformed layer. The hologram is formed by surface relief. The absence of any absorption within the structure, and the angular sensitivity of a holographic image, means that under diffuse illumination (such as dull daylight or rooms lit by many lamps), the hologram cannot be seen or can only be seen from within a very narrow range of viewing angles.
As a further example, diffractive pigments, such as those disclosed in JP-A-63/172779, would benefit from the use of a diffractive structure which does not appear "washed out" and lacking in colour under normal outdoor viewing conditions. JP-A-63/172779 discloses a pigment which consists of a multiplicity of particles each of which carries on its surface grooves that form a diffraction grating. Since the gratings on the particles suffer strong angular dependence and have no intrinsic absorption, the diffractive colour effect will only be visible under strong, highly directional, illumination such as direct sunshine or spotlight illumination. Under diffuse illumination (for example on an overcast day), the pigment as disclosed in JP-A-63/172779 would appear grey.
In EP-A-0303355, there is disclosed a hologram/diffractive medium having a plurality of periodically-spaced stepped structures each of which is distributed depthwise in the medium.
An object of the present invention is to provide a diffractive structure that will produce colour over a wide range of viewing and illumination angles.
According to a first aspect of the present invention, there is provided a diffractive structure, the structure comprising: a substantially planar substrate; and, a set of facets formed in or on said substrate, the plane or planes in which the facets lie being arranged at a non-zero angle to the plane of the substrate; the facets having a diffraction grating formed thereon having a periodicity of 500 nm or less.
According to a second aspect of the present invention, there is provided a method of manufacturing a diffractive structure as described above, the method comprising the steps of: (A) producing a mould by machining a substrate with repeated passes of a cutting tool, the tool cutting the substrate deeper on each pass of the tool thereby to produce a cut face having machining lines; (B) repeating step (A) to cut a further similar lined face opposite said first face thereby to produce a groove having machining lines on each opposed face; (C) producing a master from said mould; and, (D) producing the diffractive structure from said master, each facet having formed thereon a diffraction grating corresponding to the machining lines of the grooves in the mould.
According to a third aspect of the present invention, there is provided a method of manufacturing a diffractive structure as described above, the method comprising the steps of: (A) producing a mould by anisotropic etching in a silicon substrate to produce a plurality of facets on the mould; (B) coating the mould with a resist layer; (C) writing the fine structure of the diffraction grating directly into the resist with an electron beam or an ion beam; (D) producing a master from said mould; and, (E) producing the diffractive structure from said master.
Preferred features of the present invention are set out in the claims below.
Thus, in an example of the invention, a prismatic surface structure consisting of an array of substantially planar facets is formed in a polymer layer. These facets are typically in the region of 1 micron to 100 microns across and are disposed at a predetermined angle to the plane of the polymer layer. A grooved surface, a ruled array of tetrahedra, square pyramids or a corner cube structure (
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De La Rue International Limited
Jr. John Juba
Spyrou Cassandra
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