Transparent and semitransparent diffractive elements,...

Details Transparent and semitransparent diffractive elements,... Transparent and semitransparent diffractive elements,...

2000-11-27

2002-09-17

Spyrou, Cassandra (Department: 2872)

Optical: systems and elements

Holographic system or element

Having particular recording medium

C359S569000, C359S576000

Reexamination Certificate

active

06452698

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to a improvement and semitransparent diffractive elements and more particularly to a transparent and semitransparent type holograms and their making process. These diffractive elements are themselves transparent or semitransparent in visible (VIS) and/or near infrared (NIR) spectral region and yet are also endowed with the characteristics of a reflection type elements being observed under suitable angle. It means that reproduction in the transparent or semitransparent element of the present invention is effected only within specific reproduction angle range, while no hologram is recognised at other ordinary angles. This leads to the advantage that there is no visual obstruction of the article on which the diffractive element is laminated.
FIG. 1
shows the basic constitution of the transparent or semitransparent diffractive element according to the present invention.
STATE OF THE ART
Demand for holograms has grown not only as the way of the record of sound or information but as the elements used in such activities of human beings as advertisement, security sector, safety technique, protection of product originality, money counterfeit protection etc. It is well known that one of the following replication technologies is usually used for mass production of any diffractive elements in suitable polymer materials—hot embossing, injection moulding and casting.
Relief microstructure (master copy) is produced by one of the many high resolution fabrication technologies, the most commonly used being holographic exposure of suitable photosensitive material, including chalcogenides (U.S. Pat. No. 3,825,317), direct writing with focused laser and e-beam, optical photolithography with subsequent wet or dry etching.
In most cases, a nickel shim or stamper is electroformed or replica is produced through casting into epoxy resin. These replicas are used for own mass production of copies into polymers using injection moulding (CD fabrication), casting (production of gratings for spectrophotometers) or hot embossing, for example into transparent foil (M. T. Gale: J. of Imaging Science and Technology 41 (3) (1997) 211).
Transparent polymeric materials such as polyethylene with index of refraction n=1.5-1.54, polypropylene n=1.49, polystyrene, 1,6, polyvinyl chloride 1.52-1.55, polyester resin 1.52-1.57 etc. (for more examples see U.S. Pat. No. 4,856,857) or copolymers (for correction of index of refraction) can be used for transparent or semitransparent holograms and other diffractive elements production. Low refraction index value of these polymers or copolymers prepared from them determines their own reflectance (R about 4%), hence the holographic effect of diffractive structure developed in layers of these polymers is insufficient (U.S. Pat. No. 4,856,857). Under the term “holographic effect” used in the following text we will understand the phenomenon, that the hologram is very intensive in reflected light at suitable angle of observation. Low reflected intensity and thus the drawback of poor brightness of diffractive element recorded in the polymer layer is usually passed by forming a thin metallic film (generally Al) on the relief forming face of transparent polymeric layer (M. Miller: Holography—theoretical and experimental fundamentals and their application, SNTL, Prague 1974 (in Czech); M. T. Gale: J. of Imaging Science and Technology 41 (3) (1997) 211).
Strong improvement of brightness achieved at the cost of loss of the transparency is the main drawback of such technique. Transparency or at least semitransparency of diffractive element is required or desired in many applications (for example protective diffractive elements on banknotes, identity cards with photo etc.). Some technical applications of diffractive elements are directly conditioned by transparency or semitransparency of created element (for example microlense array for CCD cameras, polarising filters etc.).
It is further known that to preserve (or to decrease only partly) the transparency of diffractive element and at the same time to improve holographic effect of the hologram recorded in the polymeric layer (further called layer
1
), it is necessary to cover layer
1
by other transparent layer (further called layer
2
) of different material (further called holographic effect enhancing material) which has in general different index of refraction n (i.e. higher or lower) than material of the transparent layer
1
(U.S. Pat. No. 4,856,857, U.S. Pat. No. 5,700,550, U.S. Pat. No. 5,300,764). The higher difference in index of refraction of polymeric bearing layer
1
and holographic effect-enhancing layer
2
, the higher holographic effect can be achieved (U.S. Pat. No. 4,856,857).
It is as well known that very thin layer (with thickness to the limit 20 nm) of suitable metal (e.g. Cr, Te, Ge) can be used as such layer
2
deposited on the transparent
1
in which a hologram has been hot-formed. Such very thin metallic layer being used, relatively high transparency is preserved. Relatively strong enhancing of holographic effect can be achieved when the index of refraction of deposited metallic layer is either significantly lower (e.g. Ag n=0.8; Cu=0.7) or significantly higher (e.g. Cr n=3.3, Mn n=2.5, Te n=4.9) than index of refraction of transparent layer
1
(n about 1.5), (U.S. Pat. No. 4,856,857). Such thin metallic layers are deposited at transparent, diffractive element bearing layer
1
by vacuum deposition technique. The drawback of the application of thin metallic layer as holographic effect enhancing material is relatively high melting point of these materials and therefore difficult evaporating of many of these metals. An additional drawback is high absorption coefficient of metals. Already slight deviations in the thickness of evaporated metal layer implicate significant deviations in the transmissivity of the system (layer
1
—bearing diffractive element+layer
2
—metal) and moreover upper limit of the permissible thickness is very low (it depends on the metal, but in general it must not exceed 20 nm (U.S. Pat. No. 4,856,857)). According to our measurements evaporation of either 10 nm thick Cr layer or 4 nm thick Ge layer on the polymeric layer decreases its transmissivity down to about 30% (see FIG.
2
).
In the present art, oxides of metals (e.g. ZnO, PbO, Fe
2
O
3
, La
2
O
3
, MgO etc.) halogenide materials (e.g. TlCl, CuBr, ClF
3
, ThF
4
etc.) eventually more complex dielectric materials (e.g. KTa
0.65
NB
0.35
O
3
, Bi
4
(GeO
4
)
3
, RbH
2
AsO
4
etc.) are used single or possibly in several layers deposited criss-cross as holographic effect enhancing layers (U.S. Pat. No. 4,856,857). The drawback of the application of these materials is the fact that their index of refraction values are very close to the index of refraction of transparent polymeric layer
1
(e.g. index of refraction values are 1.5 for ThF
4
, 1.5 for SiO
2
, 1.6 for Al
2
O
3
, 1.6 for RhH
2
AsO
4
etc.) (U.S. Pat. No. 486,857). Accordingly, an amplification of holographic effect is relatively low. Many of these materials require again relatively high temperature for their evaporation and not least some of them are quite expensive or hardly preparable, what obstructs their mass application.
Further it is known, that binary chalcogenides of zinc and calcium as well as compounds Sb
2
S
3
and PbTe (U.S. Pat. No. 4,856,857), eventually multilayer systems of these chalcogenides with oxides or halides (U.S. Pat. No. 5,700,550) or multilayer system ZnS and Na
3
AlF
6
(U.S. Pat. No. 5,300,784) can be used as holographic effect enhancing. These materials are endowed with satisfactory index of refraction values (e.g. 3.0 for Sb
2
S
3
, 2.6 for ZnSe, 2.1 for ZnS). But short wavelength absorption edge of many of these materials (e.g. Sb
2
S
3
, CdSe, CdTe, ZnTe) lies within near IR region only and these materials are characterised by high values of absorption coefficient in VIS. Similarly with metal layer used as layer
2
, only very thin layers

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