Rainbow holograms like those used to protect credit cards and currency can be created with an ordinary inkjet printer, reducing production time and cost. The technique was developed at the St. Petersburg National Research University of Information Technologies, Mechanics and Optics (ITMO University). It involves colorless ink made of nanocrystalline titania, which can be loaded into an inkjet printer and deposited onto microembossed paper, resulting in unique patterned images. The ink makes it possible to print custom holographic images on transparent film in a matter of minutes, instead of days as with the use of conventional methods. An inkjet-printed rainbow hologram. Courtesy of ITMO University. "The conventional way of preparing a hologram is incredibly time-consuming and consists of several stages," said ITMO researcher Aleksandr Yakovlev. "First, one needs to create a master hologram, which is usually laser recorded on a thin layer of photosensitive polymer. The polymer is then dried and [washed] out to get rid of unexposed parts. The resulting stencil is then transferred to a metallic matrix, which eventually serves to emboss holographic microrelief on the surface of a transparent polymer film." Obtaining a holographic image can take up to several days, Yakovlev said. And to prepare a master hologram, rigid requirements must be met, including temperature control and vibration isolation. "The peculiarity of our ink lies in its high refractive index in [the visible spectrum]," said senior research associate Alexander Vinogradov. "The use of nanocrystalline ink forms a layer with high refractive index that helps preserve the rainbow holographic effect after the varnish or a polymer layer is applied on top." The research was published in Advanced Functional Materials (doi: 10.1002/adfm.201503483).