Optical: systems and elements – Single channel simultaneously to or from plural channels – By surface composed of lenticular elements
Patent
1995-12-12
1998-01-27
Nelms, David C.
Optical: systems and elements
Single channel simultaneously to or from plural channels
By surface composed of lenticular elements
359622, 359623, G02B 2710
Patent
active
057127310
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a security device and methods of making such security devices.
BACKGROUND OF THE INVENTION
Many different proposals have been made in the past for designing security devices for affixing to security documents such as banknotes, credit cards and the like to assist in authenticating such security documents during their use. Typical examples, include holograms and other optical variable devices.
One particular type of security device which has been used for many years is the security thread. Recently, in U.S. Pat. No. 4,892,336, a development of the security thread device was described. In this case, the security thread was provided as a transparent substrate, one side of which carried a set of lenses and the other side of which carried microprinting which could be viewed through the lenses. Typically, the microprinting comprises strips of different colours such that when the thread is viewed at different angles, different colours will be perceived. One of the problems with this approach is the need for a very precise register between the microlenses and printing. In fact, in U.S. Pat. No. 4,892,336 this need for precise register is put forward as one of the advantages of that invention in that it makes it very much more difficult to counterfeit such security devices. On the other hand, for a security device to be useful commercially, genuine devices must be relatively easy to manufacture since otherwise production costs will be prohibitive.
U.S. Pat. No. 4,765,656 also describes a security device made using a lenticular screen and in this case the microimages are formed by direct laser writing through the microlenses which are already in situ in the device. Again, this approach is not suited to mass production techniques although it does achieve exact register between the lenses and images.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, a security device comprises a regular two dimensional array of substantially identical printed microimages, each having a dimension up to about 250 .mu.m and which, when viewed through a two dimensional array of substantially spherical microlenses having substantially the same pitch as the microimage array, each microlens having a diameter in the range substantially 50-250 .mu.m, generate one or a number of magnified versions of the microimage depending on the extent to which the array of microimages and the array of microlenses are in register.
We have realised that a new security device can be constructed which is suited to mass production techniques by utilising the effects of a two dimensional array of spherical microlenses.
Generally, each microlens will have identical optical properties. The viewing condition also assumes that the microimage objects are situated near the focal plane of the microlenses. In a first "ideal" example of this system, the pitches of the images and lenses are identical and the two are accurately aligned and the microimages are near the focal planes of the microlenses. When this system is viewed from a given direction each lens images the corresponding point on the image underneath it to form a very highly magnified image of the underlying microimage element. If there is a difference in pitch between the two arrays a set of moire fringes will appear where the repeat element is the object in the array when across the moire fringe successive elements image successive portions of the object to build up an image which will repeat every time the mismatch equals an integer number of lenses. A pitch mismatch between a microlens array and microimage array can also conveniently be generated by angularly misaligning the arrays which will also generate a moire repeating image of the object. It should be appreciated that most of the practical embodiments of this invention will use the misaligned condition of the arrays to form images as this arrangement has particularly advantageous properties.
The term spherical lenses means that the surface form of each microlens approximates to a portion of
REFERENCES:
patent: 3860955 (1975-01-01), Shinozaki
patent: 4509824 (1985-04-01), Yamasaki et al.
patent: 4765656 (1988-08-01), Becker
patent: 4892336 (1990-01-01), Kaule
patent: 5363240 (1994-11-01), Miyashita
Drinkwater Kenneth J.
Hudson Philip M. G.
Mack Ricky
Nelms David C.
Thomas De La Rue Limited
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