We present diffractive second-line security features based on the moiré phenomenon, that are designed for use in Optically Variable Devices (OVDs). After a short introduction of our 2D and 1D moire methods, we first present the integration of line-based, 1D animated moire patterns into OVDs. These covert features are verified using a printed, high-resolution screen, which causes the covert information to become visible. When the screen is moved back and forth, the covert information, for example a text, appears to move dynamically in a well defined way. We then present diffractive OVDs where specially designed 2D moire features have been integrated into graytone images. Such an integration has the advantage that the area in which the second-line security feature appears can be used simultaneously for a visually attractive first-line effect rather than just having a homogeneous background. The integrated diffractive
moire features are verified with a 2D microlens array through which the OVD is viewed; as the verifier or the OVD together with the verifier is moved, one observes dynamic visual effects. A special form of integrating a diffractive moire-feature into an OVD is shown in the last part of the paper, where the 2D microlens verifier is used in a fixed combination together with the information layer that consists of diffractive microstructures. Such a diffractive moire magnifier feature is characterized by the unique visual impression that it creates where projected images appear to move as the sample is tilted.
Smartcard technologies, combined with biometric-enabled access control systems, are required for many high-security government ID card programs. However, recent field trials with some of the most secure biometric systems have indicated that smartcards are still vulnerable to well equipped and highly motivated counterfeiters. In this paper, we present the Kinegram Secure Memory Technology which not only provides a first-level visual verification procedure, but also reinforces the existing chip-based security measures. This security concept involves the use of securely-coded data (stored in an optically variable device) which communicates with the encoded hashed information stored in the chip memory via a smartcard reader device.
Optical effects in Optically Variable Devices (OVDs) are generally grouped in three different categories depending on how the effects can be verified: First-line features, second-line features, and third-line features. In this paper, we present novel examples for different classes of second-line security features designed for use in OVDs. The first class consists of features based on polarization effects. The second class consists of features which are based on special forms of very small text or images, that are invisible to the naked eye. The third class encompasses moire features that are based on engineered diffractive microstructures. These second-line security features can be checked by using simple verification devices like a polarizer, a magnifier glass, or a binary amplitude screen.
The simultaneous engineering of diffractive structures and amterial systems enables innovative appraoches for realizing iridescent color shifts for Optically Variable Devices (OVDs) for document secrity. In a first exmaple, we dmonstrate the interplay between a color-shifting coating, whichfunctions inreflection,a nd embossed gratings,w ith effects inthe first and hgher diffraction orders. In a second exmpale, we demosntrate novel surface-releif, zero-order gratingw hich show a color shift, e.g. green to red, uponrotation by 90 degrees. In a third example, we explore the use of combination gratings to realize color-shifting and color-contrast features. These color-shifting features are integrated within OVD designs in which an observer can easily check the behavior of the feature and verify the authenticity of the document.
We present the results of the application of zero-order diffraction gratings for optical variable devices (OVD's) for document security. Zero-order gratings have periods which are smaller than the wavelength of light; to describe accurately the optical properties of the zero-order gratings, we have applied rigorous electromagnetic theory, which we have compared to experimental measurements. We studied the diffractive behavior of zero-order grating varies locally in a predetermined manner. In the latter case, the resulting surface profile can exhibit variations in the diffraction properties, for example, a moire pattern. Furthermore, we have developed diffractive surface-reliefs which are a combination of a high-frequency, zero-order grating with large-period gratings, the addition of the zero-order grating to a large-period grating results in a surface relief with novel diffractive properties.
We have studied the use of achromatic features in Optically Variable Devices (OVD) for document security applications. We present various forms of matt structures as we have implemented them in OVD designs. By tailoring the scattering characteristics of the surface relief, we have created OVDs which appear in various intensities of white or gray, and whose brightness can vary as the viewing conditions are changed. Furthermore, we have realized surface reliefs which appear bright and colorless when viewed within a predetermined solid angle and appears dark in all other viewing directions. The gratings appear bright and colorless when viewed from one side of the grating normal; however, when these gratings are rotated by 180 degrees in their plane, the gratings appear dark. We will show gratings of this type, where the surface reliefs have been engineered so that the bright and colorless appearance covers an enlarged solid angle.
We designed, fabricated and characterized a micro-optical beamshaping device, intended to optimize the coupling of an incoherent, linearly extended high-power diode-laser into a multimode fiber. The device uses two aligned micro-optical elements (DOEs) in combination with conventional optics. With a first prototype we achieved an overall efficiency of 28 %. Straightforward improvements, like antireflective coatings and the use of graytone elements, should lead to an efficiency of about 50 %. The device is compact and the fabrication is suited for mass production at low cost. We applied three different technologies for the fabrication of the micro-optical elements and compared the performance. The technologies were: direct laser writing, multiple projection photolithography in combination with reactive ion etching (RIE) in fused silica, and high-energy-beam-sensitive (HEBS) glass graytone lithography in photoresist. We found that the refractive type elements (graytone) yield better efficiency for large deflection angles, while diffractive elements give intrinsically accurate deflection angles.
We present a comparison of three different technologies for the fabrication of micro-optical elements with arbitrary surfaces. We used direct laser writing in photoresist, binary mask lithography in combination with reactive ion etching in fused silica, and High-Energy- Beam-Sensitive (HEBS) glass graytone lithography in photoresist. We analyzed the efficiencies and the deflection angles of different elements in order to quantify the performance of the different technologies. We found that higher effencies can be achieved with refractive type elements, while precise deflection angles can be obtained more easily with diffractive elements.
Diffractive optical elements (DOEs) are characterized by a large negative dispersion. This property can be exploited to correct chromatic aberration in hybrid optical systems. The diffraction efficiency of phase DOEs decreases when the wavelength deviates from the design wavelength. Consequently, DOEs are usually applied only in optical systems where the spectral bandwidth is limited. Thus the dilemma that the chromatic correction capability of the DOE can not be fully exploited in optical systems with large spectral bandwidths where the correction could be most useful. It has been shown that by careful selection of the dispersion properties of two different optical materials it is possible to achromatize the diffraction efficiency of the DOE in a sandwich configuration, Fig. la.1 The dispersion properties of the two materials must be matched such that refractive index change at the interface, An(2), remains proportional to the wavelength, A, i.e.AIzn() = constant. The introduction of a second surface profile removes the above constraint on the dispersion properties of the materials. Such an approach has been described where an air gap has been inserted between the two materials and the depth of the surface profile for each material are optimized to minimize the variation of the phase modulation with wavelength, Fig. lb.2 Better performance can be achieved by placing the second surface relief on the exterior of one of the sandwiched materials, Fig. ic. The advantages are reduced shadowing effect, less Fresnel reflection losses, and much better fabrication tolerances. Furthermore, the second surface profile has removed the materials limitation. Materials that are much better suited to fabrication can be used, thus allowing for mass fabrication at reduced costs.
The continuing trend towards miniaturization of integrated circuits requires increasing efforts and new concepts to clean wafer surfaces from dust particles. We report here about our studies of the `steam laser cleaning' process first described by Tam and coworkers. In order to remove submicron particles from a surface, first a thin liquid layer is condensed onto the substrate from the gas phase, and is subsequently evaporated momentarily by irradiating the surface with a short laser pulse. We have investigated the nucleation and growth of gas bubbles in the liquid, by which the whole process is started, with optical techniques like light scattering and surface plasmon resonance spectroscopy. The experiments indicate that the temperature where nucleation sets in is surprisingly low, which facilitates the application of this phenomenon for cleaning purposes. On the basis of these results and in order to study the cleaning effect for the particularly interesting surface of silicon in a quantitative way, we have deposited well-characterized spherical polymer and silica particles of different diameters from several ten to hundred nanometers on commercial Si wafers and have studied systematically the cleaning efficiency of the explosive evaporation process. The results show that steam laser cleaning is a promising and suitable method for removing sub-micron particles from semiconductor surfaces.
Fan—out phase gratings are micro—optical elements vhicli split an incoming beani into au arra of light beanis with equal power. whereas the diffracted beams are focused by a Fourie-transforin lens. These elements are used. for example. iii optical iiitercoiuiects. in multidetector systems. and in parallel optical processing [ I I. We present different concepts for the fabrication of lwbrid elements which combine the faii-out and focusing function. The combination of the refractive and diffractive function results in a monolithic element with miniaturized dimensions and has therefore a high potential for applications in optical microsystems. To achieve this functionality. we used two different designs and different fabrication technologies. The hybrid elements were fabricated by injection moulding. double-sided photolithographv. and direct laser writing. Single elements as well as large arrays of elements have been fabricated.
Christoph-Thomas Germer, Christoph Isbert, Dirk Albrecht, Joerg-Peter Ritz, Andreas Schilling, Andre Roggan, Karl Wolf, Gerhard Mueller, Heinz-Johannes Buhr
Purpose: To evaluate gadolinium-DTPA-enhanced MRI for follow-up monitoring of laser-induced thermotherapy (LITT) and to determine of a useful examination schedule. Materials and Methods: LITT Of the liver was performed in 55 rabbits using a Nd:YAG laser (4 w power output, 840 s exposure time). Gd-DTPA MRI and histological examinations were performed at different times (0 - 168 days). Results: Laser- induced lesions underwent regeneration and volume size reduction (69% after 168 days). The correlation coefficient (MR vs macroscopic analysis) for the mean lesion diameter was r equals 0.96. Histology of lesions comprised the four zones that correlated best with MRI findings. Coagulation necroses immediately after LITT were seen as an area of no enhancement on Gd-DTPA MRI. Circular enhancement was first seen 72 - 96 hours after LITT, which was due to early mesenchymal proliferation. Conclusions: Gd-DTPA MRI is a good monitoring procedure for LITT. MRI should be performed 24 and 96 hours after LITT.
Christoph Isbert, Christoph-Thomas Germer, Dirk Albrecht, Andreas Schilling, Antje Heiniche, Andre Roggan, Karl Wolf, Gerhard Mueller, Heinz-Johannes Buhr
The purpose of the study was to evaluate Gd-DTPA-enhanced MRI for the follow-up of laser-induced thermotherapy (LITT) in the treatment of liver metastases. Interstitial laser-induced thermotherapy (LITT) was performed in 55 chinchilla bastard rabbits with a Nd:YAG laser and a specially developed diffuser tip applicator. The animals were examined at different times using MRI and histological methods. T1-weighted spin-echo sequences were made with gadolinium-DTPA and submitted to correlation analysis. The analysis yielded a good correlation (r equals 0.96) between the largest lesion diameters. Lesions showed discontinuous regeneration kinetics with a 41% volume reduction 1 month after LITT. MRI of the lesion directly after LITT showed a hyperintense transition zone. Following the application of intravenous contrast medium, transition zone enhancement was first seen 72 - 96 hours after LITT. Even after 6 months, the laser-induced lesion is visualized as a non-enhanced area. Lesion regeneration followed the principles of wound healing. It is concluded that gadolinium-DTPA- enhanced MRI yielded definitive criteria for the follow-up assessment of LITT.
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