Dot-matrix Hologram Research Articles

Dot-Matrix Hologram Rendering Algorithm and its Validation through Direct Laser Interference Patterning

The fight against forgery of valuable items demands efficient and reasonably priced solutions. A security tag featuring holographic elements for anti-counterfeiting is one of them. However, the content and colours of a diffraction image that would be seen by an observer are often counterintuitive in the design stage. Here, we propose an original algorithm based on the conical diffraction formalism, which can be used to describe the variations of a diffraction image with respect to all aspects of observation. We validate the output of the algorithm by comparing it to test holograms, which we have produced by employing direct laser interference patterning (DLIP) in electrochemically grown nickel foil. We have employed a motorized femtosecond laser system to micro-machine arrays of 65 µm × 65 µm sized diffraction gratings with a defined orientation and pitch on the order of 1 µm. Based on completed diffraction efficiency measurements, we determined optimal ablation parameters, i.e. 57.4 mJ/cm2 fluence per pulse and 1100 pulses/pixel. Furthermore, we show how accurate the proposed algorithm is through measured diffraction spectra as well as captured diffraction images of test holograms produced using the obtained parameters. Finally, we showcase anti-counterfeiting tag prototypes with complex holographic effects, i.e. colour reconstruction, animation effects, and image multiplexing. The proposed algorithm can severely shorten the time between design and production of a holographic tag, especially when realizing it via a competitive origination technology—DLIP.

Anticounterfeiting method for a dot-matrix hologram composed of grating dots with different fringe orientations

A method for identifying a dot-matrix hologram composed of grating dots with different fringe orientations is proposed. Two neighboring grating dots, Dot 1 and Dot 2, are selected. The intersection angle between the two sets of fringes of the two grating dots must be smaller than a threshold angle, e.g., 50 deg. The overlapping region of Dot 1 and Dot 2 can show a moire pattern composed of many stripes. Some of the stripes contain the same fringe orientation as that of Dot 1, some of the stripes contain the same fringe orientation as that of Dot 2, and some of the stripes contain special fringe orientations different from those of Dot 1 and Dot 2. The stripes which contain special fringe orientations are called transition stripes here, and the number of transition stripes (transition stripe number) is used to identify Dot 1 and Dot 2. Because the transition stripe number for a dot pair is sensitively affected by the distance between the dot pair and the grating brightness distributions of the two dots, it is difficult to correctly reproduce the transition stripe numbers for the grating dot pairs on a dot-matrix hologram, i.e., counterfeiting the hologram is not easy.

Implementation of Augmented Reality by Using the Image-switching Effect on a Dot Matrix Hologram

Implementation of Augmented Reality by Using the Image-switching Effect on a Dot Matrix Hologram

Identifying a dot-matrix hologram with the fringe-overlapping phenomena of grating dots

Dot-matrix holograms created by two-beam writers contain many grating dots. Because the phase difference between two laser beams for interference cannot be controlled accurately, the fringe positions of grating dots are randomly determined. Therefore, fringe positions are a good kind of tool to identify dot-matrix holograms. In this paper, a number difference between two special fringes of a target grating dot is used to identify a dot-matrix hologram. The two special fringes are determined by three grating dots with parallel fringes. The first special fringe is corresponding to a fringe pair with the best matching for the fringes of the target grating dot and the fringes of the second grating dot. The second special fringe is corresponding to a fringe pair with the best matching for the fringes of the target grating dot and the fringes of the third grating dot. An experiment has proved the proposed method practical and feasible. Because reproducing a grating dot with a specified fringe number difference is difficult, the proposed method is excellent for anti-counterfeiting.

Identifying a dot-matrix hologram by checking the intersecting angles of its gratings

Three features about the intersecting state of two gratings are used to identify dot-matrix holograms created by two-beam writers in this paper. The first feature is the intersection angle of the fringes of two gratings. The second feature is the ratio of the pitches of two lowest-order moiré-patterns formed by overlapping two gratings. The third feature is the length ratio of the two diagonal lines of a parallelogram constructed by two neighboring fringes of a first grating and two neighboring fringes of a second grating. All the three features are equivalent, i.e. they are all relative to the intersection angle of two gratings. Because the grating orientations of grating dots cannot be accurately reproduced for the orientation uncertainties of the components used by a two-beam writer, the intersection angles of the gratings of grating dots cannot be easily counterfeited. Therefore, the proposed methods are practical and feasible for the anti-counterfeiting applications of dot-matrix holograms.

Principles of Creation and Reconstruction of Dot-Matrix Holograms

Principles of Creation and Reconstruction of Dot-Matrix HologramsA method for calculation of dot-matrix holograms is presented, whose distinction from other methods is simplicity of the formulas for the parameters of each pixel element in a hologram. At the Innovation Spectroscopy Center of DU a device for recording dot-matrix holograms has been designed, with a program using the pro-posed method for their calculation. Holograms 50x50 mm in size well corresponding to the initial graphic design were obtained.

Identifying a dot-matrix hologram by the position-error curves of its grating dots

Dot-matrix holograms created by two-beam writers usually contain grating-dot position errors. The position errors may result from lens aberrations, rotation mechanisms, or beam direction inaccuracy, so they are not controllable. When the components of a two-beam writer are not reinstalled, the position errors of the grating dots created by the writer can be reproduced. Hence, position-error curves derived from the regression of the data points on position-error diagrams have some high-end applications. Two applications of position-error curves of dot-matrix holograms are proposed. The first application is to determine whether a dot-matrix hologram was created by a specified two-beam writer or not. The second application is to determine if two dot-matrix holograms were created by the same two-beam writer with the same component installation.

Dot-matrix hologram with an encrypted figure

A dot-matrix hologram contains many two-dimensional grating dots, which may have different features, e.g., two-dimensional positions, grating pitches, grating orientations, and dot sizes. In this paper, such features are used to encode binary data to hide a figure almost without changing the hologram appearance. Because many encoding methods are available, decoding the hidden figure without authorization can be very difficult. The encrypting scheme presented here possesses both feasible and confidential features.

Identifying a dot-matrix hologram by the deviations of the fringe positions of its grating dots

Dot-matrix holograms created by two-beam writers usually contain two-dimensional grating dots. Although the dot positions may be accurate, the fringe positions of the dots are randomly determined. The random fringe positions are good tools to identify a hologram. A grating position-deviation coefficient derived from the fringe position difference of two grating dots with the same pitch and the same orientation is defined and it is used as data to identify a dot-matrix hologram. Experiments have proved the proposed method practical and feasible. Because the fringe position difference between the two grating dots with the same pitch and the same orientation is difficult to reproduce, it is excellent for anticounterfeiting.

Using random features of dot-matrix holograms for anticounterfeiting

The images on a dot-matrix hologram contain many two-dimensional (2D) dots with different grating orientations and different grating pitches. Because the zeroth-order light nondiffracted by different grating structures has the same progress direction, the nondiffracted light can be diffracted to a 2D spot spectrum by the 2D dot structure. The 2D spot spectrum depends on grating depths and dot sizes. Although ordinary noises are troublesome for 2D spots, noises caused by special dot arrangement defects or special grating moiré fringes are useful in checking holograms. Since the features of grating depths, dot sizes, dot arrangement defects, and grating moiré fringes can be randomly changed on a case by case basis, 2D spot spectra in different cases are different. The aforementioned random features are used to identify dot-matrix holograms.

Printing via hot embossing of optically variable images in thermoplastic acrylic lacquer

A novel printing process via hot embossing of either grating or micro-mirror microstructures has been demonstrated in thermoplastic acrylic lacquer. Embossing experiments were performed in the temperature range 100–150 °C and at 80 kN force. The range of microstructures has included a dot-matrix hologram, grating-based optically variable devices (OVDs) and a micro-mirror based OVD. High quality replicas of each type of device have been fabricated using this process. Embossed replicas of grating-based OVDs have shown optical effects including image switching and color movement. For devices based on micro-mirror arrays, the embossed replicas have shown an optically variable switch between a portrait and a non-portrait image. Printing via an embossing process offers the possibility of incorporating optically variable devices into documents without the use of hot stamping foil. This is particularly relevant for documents based on polymeric substrates such as credit cards and polymer banknotes.

Hiding techniques to enhance anticounterfeiting capacity of dot-matrix holograms

The images on a dot-matrix hologram consist of many 2-D grating dots with different pitches and orientations. Only some specified pitches and 256 specified orientations are usually employed. The difference between any two pitches to show different colors is more than 0.1 µm; therefore, it is possible to hide several extra pitches in a specified pitch. If all the extra pitches are almost equal to the specified pitch, grating dots with extra pitches will be of nearly the same color as the specified pitch. When a laser beam is incident on the dot-matrix hologram with hidden pitches, data for the hidden pitches will be decoded by the diffraction pattern of the grating dots of the hologram. A specified grating orientation can be used to hide extra grating orientations similarly. What's better, mixing the above two hiding techniques is possible.

Compact, reliable asymmetric optical configuration for cost-effective fabrication of multiplex dot matrix hologram in anti-counterfeiting applications

Summary This study presents a compact optical configuration that can generate a multiplex dot-matrix hologram with complex interlaced images for anti-counterfeiting applications on valuable paper. Varying the orientation of the interference plane can enable the multiplex hologram to be recorded without changing the interference angle of two laser beams. With its simple asymmetric optical setup, a multiplex hologram with many interlaced images can be efficiently fabricated, increasing the cost of imitation. Experimental details of the asymmetric optical setup are also described.

Dot-matrix hologram with hidden image

A dot matrix hologram with a hidden image is disclosed, wherein the hidden image technology is applied to a dot matrix hologram. Although the hidden image technology has been used in a conventional hologram, it has not been used in a dot matrix hologram. Although the above two kinds of hologram both can show hidden images with illuminating laser light, the disclosed dot matrix hologram with a hidden image and the conventional hologram with a hidden image use different working principles and have different detailed characteristics to create hidden image regions.