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.