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Positive photoresists are in wide use for rainbow hologram origination. The media have high contrast and very narrow linear part of an exposure curve. The Gaussian intensity profile of a scanning spot gives over-exposure in the center of rainbow line and under-exposure next to its periphery. A rainbow lines lose brightness and edge sharpness. The device developed was equipped with 6_6_VIS pi-shaper for 405 nm. The device optical scheme and a number of practical aspects are considered. A circular spot even with flattop gives non-uniform exposure across a rainbow line. Two solutions were studied: M-shape spot and a flattop square spot. Both can be done with Pi-shapers. M-spot gives better results for smooth lines, while square aperture is better for sharp tracks. Pi-shaper required special attention for the alignment of the system components especially to tilts and shifts. The system developed allows high-speed hologram origination, adequate security features for expert authentication and prevents the possibility of being counterfeited by most recording systems. |
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