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Holographic images have been in use to protect security documents since the beginning of the 80’s. They were first used to protect credit cards. Since that holographic and optical image technologies have improved dramatically. Now the term “hologram” is used to describe any optical security image, classified as DOVID (Diffractive Optical Variable Image Device). All DOVIDs present an image which changes colors when tilted in various angles. In all cases, the DOVID is a collection of microscopic peaks and valleys (interference patterns) on the plate surface, which are reproduced on the surface of micro embossing shims using electro-forming processes. This is used to emboss directly onto polymer films or onto specialized coatings applied to polymer or paper webs.

Several types of diffractive images have been developed and are available on the market. There are two major categories of DOVID techniques: – classical holograms – grating images

1. Classical holograms

2D/3D a multi-plane hologram contains two or more two-dimensional image planes placed one behind another in 3D-arrangement giving parallax effect. The foreground plan is brighten and consist the main information. The background plane usually consists patterns as text or logos. The depth between foreground and background is a few millimeters.

3D a classic hologram using a 3 dimensional model. Same size sculpted stable products or models coloured in grey scale.

Stereogram a hologram produced by recording many individual frames of a sequence of images. The original sequence may be photographic film, movie footage, and video or computer graphics. Holographic stereograms are usually 3D and can show a short movement sequence.

2. Grating holograpgic images There are several types of grating images available on the market. They are known by the trade names: DotMatrix, Lidogram, Kinegram, Alphagram, Movigram, Gyrogram, Exelgram, etc.

They differ in the engraving method, the resulting image resolution, brightness, and animation capabilities. The gratings are engraved on the plate either by a laser photo-optical method, or by direct grating engraving. When light touches the grating, it is diffracted at a specific angle and the light is separated into all the rainbow components.

Dot-Oriented Devices (Dot-matrix) a computer generated holographic images in which the image is optically written dot by dot. Every “dot” is an elementary grating with different frequency and orientation. The gratings are engraved on the photo-resist plate by two laser beams. They interfere to create an interference pattern in each dot. By varying the angle between laser beams as well as the orientation of the beams, interference lines in a dot are controlled.

Typical resolution varies between 300 to 3000 dots per inch.

Pixel-Oriented Devices a computer generated hologram in which the image is written using an electron beam lithographic technology. Each diffraction groove is written separately by a focused E-beam, scanning across electron-sensitive resist layer. The pixelgrams consist of a regular matrix of microscopic rectangular diffractive elements (pixels). Each pixel is composed of numerous diffraction grooves that continuously vary in period and azimuth. Thanks to the high resolution of E-beam technology, the resolution of the pixelgram is extremely high.

Track-Oriented Devices a very sophisticated computer generated hologram in which the image is written using an electron beam lithographic technology. The exelgrams is composed of line-shaped diffraction elements (diffractive tracks). In these tracks the diffraction patterns are frequency and angle modulated. The exelgram overcomes some of the limitation of the pixelgram technology, like spurious diffraction effects from the edge of the pixels and other.

Vector-Oriented Devices another type of computer generated holographic images in which the image is ‘written’ by lines instead of dots. This image elements are composed of lines with different period and orientation. Every element can be seen only in one direction of observation.

Combined holographic images The most complex and secure form of DOVIDs is combining two or more separate techniques together.

The counterfeiters usually use the following methods to fake the holograms: Imitation use other printing techniques to make the optical device like hologram.

Using of hot-foil stamping: -combination of pigment or holographic pattern foils -silver or holographic pattern foil with over-printing with inks -silver or holographic pattern foil with embossing

Using of different holograms. In the 1990’s several hologram companies became selling stock “security holograms”. Stock image holograms can be found easily in the Internet and purchased by anyone for any purpose. They consist of elements like security, void, genuine, original and are often used to counterfeit documents.

The imitations are low-level fakes and can be easily detected by the general public.

Copying mechanical or optical replication. If the counterfeiter is crackjack, the result can be very satisfactory. To prevent this possibility the producer must protect holographic relief and deny access to contact copying. These precautionary measures must be made during the process of mass production of holograms.

Remaking (reorigination) making of new hologram origination. For this reason there are many additional security effects, which can be added to the image. Most of them can help the usual user or the expert to differ original image from the faked.

There are three levels of authentication: 

General public level: The image consists of different optical effects for immediate recognition. Special effects – cinematic effects, high resolution line patterns, 3D effects, switch effects, expand effects, true-color images, grayscale images, animation effects.

For example Holographic Pastel Colors:

Inspector level (policemen, conductors, bankers, customs officials, etc.): The image consists of effects for close inspection by naked eye or by simple tools (magnifying glass, low magnification microscope, special detectors, etc.).

For example Polarization Flip Effect:

Special effects – Micro-text, symbols. Special optical techniques have been developed to incorporate hidden information into the ‘hologram’. This information can only be ‘read’ using proprietary readers.

Expert level (Central Bank inspectors, detectives, manufacturers, etc.)

Special effects – Nano-text, secret hidden features, coded images, machine-readable images.

Special tools (special readers, special microscopes, etc.) or special machine readability.

Holographic Images

All types of holograms (DOVIDs) are a collection of microscopic peaks and valleys (interference patterns) on the plate surface. The holographic interference pattern can be recorded in a photoresist that after development contains the pattern as a microscopic surface relief. From this relief pattern a nickel replica is derived by electroplating. Thin nickel replica is used to press this surface pattern into a plastic foil at an elevated temperature. This process is called hologram embossing. The embossed pattern in the foil is provided with a very thin reflective layer of aluminum or another metal, like gold or chromium, which transforms the transmission hologram into a reflection hologram. In spite of the thinness of the metal layer, such embossed reflection holograms are completely opaque. Semitransparent alternatives can be achieved by the application of high-refractive-index layer (ZnSe, TiO2) as well as partially metallized layers, which render so-called semitransparent overlays or see-through holograms.

Holograms can be applied to a product as an adhesive label, hot-stamped onto an item, used as a thread or tape, or used as an over-laminate of a product.

The production of holograms gets through the following steps:

Origination

The first step is making the original hologram image called Master hologram. This process differs holography production from standard printing processes. The process needs special optical equipment, which records original image onto photo-resist. After developing glass plate with photo-resist, the result is Master hologram.

Data Preparation

E-Beam-Litography

Quality Control

Vacuum

Recombining

Electro-forming and recombining

Electro-forming – the process converts image from photo-resist or plastic to metal (nickel). Recombining – the process of multiplying a single image from nickel original to plastic base. Step by step:

Converting image from photo-resist to nickel; Multiplying a single image from a nickel original to a plastic base;

Converting multiple images from plastic to nickel. This product is called a Master shim;

After that the Master shim is used as an original for the reproduction of Work shims.

Work shims are used as instruments for embossing holograms.

Electro-forming and recombining is a complete pre-press process for mass production of holograms.

Embossing and finishing operations

Embossing is a kind of printing process where by means of high pressure and temperature micro-relief from Work nickel shim transforms to base materials (foils). In security holography base materials are special multilayer foils. After embossing, depending on the end product, embossed foil gets through some additional operations: laminating, die cutting, coating, slitting, and rewinding.

End products can be:

Self-adhesive labels;

Hot stamping foil;

Laminate.

Special operations

For high security applications are known different additional steps like numbering and selective demetallization.

 Modern holography dates from 1947, when Dennis Gabor, a scientist researching the ways to improve the resolution of the electron microscope, developed what he called holography. In fact, in 1948 he coined the term hologram from the Greek words “holos”, meaning “whole,” and “gramma”, meaning “message”. Gabor did not have at his disposal coherent light source needed for continuing his work. Thirteen years later, in 1960, the LASER was invented and the serious work in holography began. In 1971 Dr. Dennis Gabor was awarded the Nobel Prize in Physics for his work in holography.

Two researchers, Emmett Leith and Juris Upatnieks, from the University of Michigan, in 1962, theorized that holography could be used as a three dimensional visual presentation. These individuals decided to apply Gabor’s theory with the newly invented laser light sources. The result was the first laser transmission hologram of 3D objects. These transmission holograms produced images with clarity and realistic depth. Unfortunately, these transmission holograms required laser light to view the holographic image. The work of Leith and Upatnieks led to standardization of the equipment used to make holograms. Today, thousands of laboratories and studios possess the necessary equipment: a continuous wave laser, optical devices (lens, mirrors and beam splitters) for directing laser light, a film holder and an isolation table on which exposures are made. Stability is absolutely essential because movement as small as a quarter wave-length of light during exposures of a few minutes or even seconds can completely spoil a hologram. The basic off-axis technique that Leith and Upatnieks developed is still the staple of holographic methodology.

In 1960 the pulsed-ruby laser was developed by Dr. T.H. Maimam of the Hughes Aircraft Corporation. This laser system (unlike the continuous wave laser normally used in holography) emits a very powerful burst of light that lasts only a few nanoseconds (a billionth of a second). It effectively freezes movement and makes it possible to produce holograms of high-speed events, such as a bullet in flight, and of living subjects. The first hologram of a person was made in 1967, paving the way for a specialized application of holography: pulsed holographic portraiture.

Like so many scientific advancements, holography was simultaneously being developed by other scientists. It was a Russian, Uri Denisyuk, who, in 1962, brought the work of Gabriel Lippmann (1908 Nobel Laureate) to holography and produced the first white-light reflection holograms that could be viewed in ordinary incandescent light bulb.

The 1967 World Book Encyclopedia Science Yearbook contained what is arguably the first mass-distributed hologram, a 4″x3″ transmission view of chess pieces on a board. An article describing the production of the hologram and basic information about the history of holography accompanied it. A .05 watt He-Ne laser was used on a nine-ton granite table in a 30-second exposure to make the original from which all the copies were produced.

The next important steps in the development of the holography were made by Stephen Benton, who, in 1968, invented white – light transmission holography. White-light holography allows the image to be viewed in ordinary white light. Undoubtedly, it was the brilliant rainbow colors that attracted artists to this new holographic technology. This type of hologram can be viewed in ordinary white light creating a “rainbow” image from the seven colors which make up the white light. This type of hologram was called “rainbow hologram”.

The most important result of Stephen Benton’s work was that it led to the ability to mass-produce holographic images using optical embossing techniques. Embossing allowed the images to be reproduced by a press that stamped the image onto plastic surfaces.

With this technique, developed by Michael Foster in 1974 and brought to commercial viability by Steve McGreww in 1979, holographic information is transferred from light sensitive glass plates to nickel embossing shims. The holographic images are “printed” by stamping the interference pattern onto plastic. The resulting hologram can be duplicated millions of times for a few cents apiece. Consequently, embossed holograms are now being used by the publishing, advertising, packaging, banking and security industries.