From Event: SPIE Optical Engineering + Applications, 2018
Most automated holographic origination systems work in “stop-and-repeat” mode thus making framed fringe structure. It means that the final holographic image consists of “bricks”. So, origination systems giving non-fragmented, continuous fringe structure is useful for expert verification. “Light pen” origination systems (LP) make continuous rainbow lines scanning a photoresist plate with a focused laser spot. Intensity distribution across the spot effects greatly on diffraction efficiency and image quality. Beam shaping, not exactly gauss-to-flat, has a critical value for such systems.
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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A. F. Smyk and A. V. Shurygin, "Beam shaping in "light pen" holographic originating system," Proc. SPIE 10744, Laser Beam Shaping XVIII, 107440D (Presented at SPIE Optical Engineering + Applications: August 20, 2018; Published: 14 September 2018); https://doi.org/10.1117/12.2319019.