New process allows inkjet printers to produce rainbow holograms
Credit card and banknote-style security holograms are an effective form of anti-counterfeiting technology, as they're very difficult to replicate. Every time a new batch is made, however, a "master hologram" has to be created first, to act as a template. These masters can take days to produce, using complex, expensive equipment. That could be about to change, however, as scientists at Russia's ITMO University have developed a quick-and-easy hologram production method that utilizes a regular inkjet printer.
Ordinarily, to create a master hologram, the desired image is first shone onto a thin photosensitive polymer film using a laser. Once the unexposed polymer is dried out and removed, the remainder is used to create a stencil of the image. That stencil is reproduced within a metallic matrix, which is subsequently used to emboss holographic microreliefs on the surface of a transparent polymer film – that's where all the copies based on the master come from.
By contrast, the ITMO technique simply requires an inkjet printer, a colorless nanocrystalline ink, and a pre-microembossed material such as a sheet of paper or polymer film. In its unaltered form, the refractive index of that embossed surface causes it to reflect light in a holographic rainbow-like pattern.
The ink has a high refractive index, protecting the rainbow sheen in the areas where it's applied by the printer. Once the whole surface is subsequently covered with a clear varnish that has a low refractive index, however, the rainbow effect disappears everywhere else. In other words, the only places in which the rainbow remains visible is in the lines that form the desired image.
According to the university, the process takes just a matter of minutes, can be used to produce holograms of any size, and should considerably reduce printing costs. One thing that remains to be seen, however, is whether it might also make life easier for counterfeiters.
The research was led by Aleksandr Yakovlev, and was recently described in a paper published in the journal Advanced Functional Materials.
Source: ITMO University