Visual features of classical and high-tech security holograms are described and their advantages and shortbacks discussed. New holographic features, improving both security hologram visual appeal and its anticounterfeiting value are described. 2D/3D backgrounds can be highly improved using multicolor textures, smooth changing color pattern and variable background depth. Complex grating based images with a lot of colors and smooth transitions between them highly improves visual and security value of grating based optically variable devices. Modulation of grating parameters may create distinguishing effect of glossy 3D surface. Combining various holographic features increases security holograms value very significantly.
The term 'computer generated hologram' (CGH) describes a diffractive structure strictly calculated and recorded to diffract light in a desired way. The CGH surface profile is a result of the wavefront calculation rather than of interference. CGHs are able to form 2D and 3D images. Optically, variable devices (OVDs) composed of diffractive gratings are often used in security applications. There are various types of optically and digitally recorded gratings in security applications. Grating based OVDs are used to record bright 2D images with limited range of cinematic effects. These effects result form various orientations or densities of recorded gratings. It is difficult to record high quality OVDs of 3D objects using gratings. Stereo grams and analogue rainbow holograms offer 3D imaging, but they are darker and have lower resolution than grating OVDs. CGH based OVDs contains unlimited range of cinematic effects and high quality 3D images. Images recorded using CGHs are usually more noisy than grating based OVDs, because of numerical inaccuracies in CGH calculation and mastering. CGH based OVDs enable smooth integration of hidden and machine- readable features within an OVD design.
In diffractive optically variable image devices (DOVIDs) used as anticounterfeiting elements new features appear, which are directly adapted from traditional security paper techniques such as microprints or guilloche. Usually, they are used in combination with standard optically variable diffractive effects. Modern versatile DOVID printers and microlithographic UV and e-beam techniques allow to obtain a wide range of optically variable effects and hidden features. In this paper, we present various possibilities of employing guilloche in security DOVIDs. The first is an animated guilloche widely used in DOVIDs composed of diffraction gratings such as KinegramsTM and KineformsTM as well as in high resolution dot-matrix elements. The second possibility is the multicolor guilloche, used by means of 2D rainbow holograms. Such an optically variable guilloche is visible in wide range of observation angles, however, it is darker than grating structures. Increased visibility may be achieved using computer generated holograms (CGH) structures what results in widely visible and bright DOVIDs. With CGH structures it is also possible to combine kinematic guilloche with hidden features visible using a special reader and to introduce completely new kinds of effects resulting in a multicolor guilloche with colors changing in cyclic way and others special effects guilloches showing the kinematic effects and/or various color changing effects.
A new optically variable device characterized by both rainbow holographic effects and two kinds of hidden elements is presented. Both components are recorded as computer generated Fourier holograms. The first hidden element is a slit or small image formed by a line of text or other image contained in a slit-shaped frame. Under normal illumination conditions the first element shows rainbow holographic and kinematic effects. The second element is a darker image visible at an angular range different than that of the first element. This second image is less visible under usual white light illumination conditions, but appears sharply in laser light. The advantage of the optically variable device proposed is that it gives both rainbow holographic effect and hidden Fourier-plane images at the same time from the whole hologram area. In this way, the need to use special encoded subfields which attract attention is eliminated.
We consider several aspects of security properties of holographic optically variable devices that are used for document protection. Features of optically recorded rainbow holograms, stereograms, holograms with pixel structure, and volume holograms are discussed from the point of view of optics. Similar approach is used with respect to optical properties of computer generated stereograms and digital holograms with pixel structure. Optical properties which are considered include diffraction efficiency, uniformness of the recording, angular range of horizontal parallax, multiplicity of recording, spatial separation of recording planes, color visual perception, spectral range, as well as transmission and reflection properties. The second group of security properties is connected with employed technology. Technological aspects of a hot stamping foil structure, self-destructive labels, and other are considered. Security properties of holographic optically variable marks are also analyzed in terms of novelty of employed techniques, the level of scientific and technological complexity, as well as resistance to imitating.
Optically recorded Fourier holograms readable by special laser detectors are frequently used as 'hidden' elements on optically variable devices (OVDs) used for security purpose. However, they are not difficult to identify and reproduce in holographic laboratory. In this paper we propose the use of the phase only computer generated Fourier hologram for security purposes. Its main feature is a possibility of recording a referenceless Fourier hologram which may form a nonsymmetric image. Such an element may be viewed even in the sunlight and it is impossible to manufacture it using standard analog holographic techniques. To obtain the best quality the hologram should be calculated using iterative Fourier-transform algorithm (IFTA) and manufactured using multiple-phase-step or continuous-phase technologies.