Encoding Hidden Information onto Surfaces Using Polymerized Cholesteric Spherical Reflectors

Abstract

The omnidirectional Bragg reflection of cholesteric liquid crystals molded into spheres turns them into narrow-band retroreflectors with distinct circular polarization. It is shown that these cholesteric spherical reflectors (CSRs) can encode information onto surfaces for far-field optical read-out without false positives, as the selective retroreflectivity allows the background to be easily subtracted. In order to hide the encoding from detection by the human eye, the retroreflection band is tuned to the near-UV or IR spectra, allowing ubiquitous deployment of CSRs in human-populated environments. This opens diverse application opportunities, for instance, in supporting safe robotic navigation and in augmented reality. A key breakthrough is our ability to permanently embed CSRs in a binder such that undesired scattering and reflections are minimized. This is achieved by realizing CSRs as shells that are polymerized from the liquid crystalline state. The resulting shrinkage around an incompressible fluid deforms the thinnest region of each shell such that it ruptures at a well-defined point. This leaves a single small hole in every CSR that gives access to the interior, allowing complete embedding in the binder with optimized refractive index, minimizing visibility.

Linking physical objects to their digital twins via fiducial markers designed for invisibility to humans

“…Any CSR fiducial is therefore physically unique, and so is its near-field optical response. A less beneficial aspect of the blue-shifted reflections at θ > 0 is that they also set design limitations for CSR fiducials that should operate in the IR spectrum and need to be unobtrusive to humans [18,19]. If λ 0 is too close to red, we see colors for reflections at θ > 0 (figures 5(h)-(i)), yet if it is too far into the IR, illumination and imaging requires more complex and more expensive equipment.…”

“…This would compromise the requirement of unobtrusiveness of CSR fiducials. Fortunately, the refraction at the air-binder interface limits the maximum angle θ to values around 65 • , allowing the design of coatings for IR operation that do not give rise to visible reflections, but the parameter window needs to be carefully evaluated [19]. With p tuned for near-UV (blacklight) retroreflection, this issue is absent since θ > 0 reflections are always blue-shifted, thus they are outside the visible spectrum [19].…”

“…Shell CSRs are particularly useful in this respect, as the binder can fill also their interior and thus minimize visibility. Geng et al recently developed a scalable method to puncture shells en masse [19], allowing exchange of the original core fluid with the index matching binder, as well as removal of stabilizers from the production process. Near perfect index matching was achieved already with the common UV-curing glue norland optical adhesive (NOA) as binder [19].…”

“…We recently presented a new material component that we call cholesteric spherical reflector (CSR) [18,19] that would allow the realization of fiducial markers that robots can easily detect and analyze, with no impact on the environment as we see it. CSRs exhibit selective omnidirectional retroreflectivity, with distinct right-or left-handed circular polarization, in a narrow wavelength band that can be tuned to the invisible ultraviolet (UV) or infrared (IR) spectra to ensure that they are undetectable by the human eye.…”

“…The circular polarization of the reflections allows exceptionally simple image segmentation, i.e. removal of the background, thereby removing the risk of false positives [19].…”