Retroreflective Material Helps Robots to Analyze Surroundings

JAKE SALTZMAN, NEWS EDITOR
jake.saltzman@photonics.com

A material developed and introduced by researchers at the University of Luxembourg will help robots analyze and understand their surroundings. The material relies on retroreflective spheres made from cholesteric liquid crystals, which turn into a solid state by the process of polymerization.

According to members of a team led by Jan Lagerwall, principal investigator of the study and a professor in the Department of Physics and Materials Science, the material can be used to introduce tailor-made graphical information in the environment, which is invisible to humans but easily readable by robots.

As automatic and autonomous machines such as self-driving cars and drones become more prevalent, the likelihood that these machines will interact with humans also increases. These interactions occur both in and outside industrial workplaces.

“As beneficial as this transition to ubiquitous automation could be, it also comes with significant challenges of many types,” Lagerwall said. Despite the state-of-the-art sensors and computing technologies that many automated machines incorporate, the machines must make sense of environments created by humans.

“It is simply not easy to make sense of the busy, complex, and messy world that we humans create and live in, full of signals, some important, some only distracting, and others yet being pure noise,” Lagerwall said.

The new materials-based approach is an alternative to a focus on sensory inputs and computational power that are often combined to allow robots access to human-populated environments. In the work, the cholesteric spherical reflectors (CSRs) function in a way that is like the retroreflectors found in certain cars, road signs, and clothing. The CSRs, like retroreflectors, send light back to the source regardless of the direction along which they are illuminated.

Retroreflective spheres made from cholesteric liquid crystals that turn into a solid state by the process of polymerization are poised to help robots understand their surroundings. Courtesy of the University of Luxembourg.

There are two differences between the CSRs used in the work and these retroreflectors, however. First, the reflection is limited to a narrow wavelength range — this explains why the human eye cannot see them. Second, the reflection is circularly polarized, in the same way as each of the two movies shown simultaneously in a 3D movie theater are circularly polarized, in opposite ways.

"If you ever took off your goggles while at a 3D cinema you will have noticed that the human eye cannot distinguish different polarizations, as both our eyes then see both movies, and we simply experience a strange shadow effect. The goggles contain circular polarizers, one right-handed and the other left-handed, ensuring that our right eye sees only the movie for the right eye, the left only the movie for the left eye,” the scientists explained.

In applications that use the new material, a robot designed to read out CSR-encoded information would have two cameras, both of which would operate in the ultraviolet and/or infrared regions in which the CSRs reflect. Each camera would have a circular polarizer that is a different type than that of its counterpart, just like 3D cinema glasses. The robot could then subtract one image from the other, so that all visual information that is not circularly polarized (which is all content except the CSRs) would cancel out because this information appears identical to the two cameras.

“But the CSRs remain, as they are visible only to one camera but not to the other,” the scientists said. “This allows the robot to identify the CSR-encoded information extremely rapidly, with minimum computing power, and without risk of false positives.”

Researchers in Lagerwall’s group collaborated with Matthew Schwartz, a professor at New Jersey Institute of Technology (NJIT), on the work; in 2018, that collaboration reported on how liquid crystal shells enable a new type of sensor device. Last year, the Experimental Soft Matter Physics Group led by Lagerwall demonstrated the capability of liquid marbles to act as platforms to control the self-assembly of a bio-derived polymer into a cholesteric liquid crystalline phase.

The new work was published in Advanced Functional Materials (www.doi.org/10.1002/adfm.202100399).