Recently, researchers from the USA and China have explained the mechanism behind the structural color that appears when light is reflected off a thin film of microscale concave interfaces. The team also proposed a range of possible applications of the technology, including smart road signs that enhance the pattern recognition abilities of autonomous vehicles.
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Pigments, bioluminescence, and complex micro-and nanostructures are the three main sources of color in nature. Pigmentary colors are produced by color molecules (such as dyes) that absorb light within a certain range of wavelengths and reflect (or refract) the non-absorbed light to display color. The application of pigments and dyes has become an indispensable part of our daily lives.
Periodic Structures Create Colors
In contrast, structural colors result from the reflection of white light from complex, periodic micro-and nanostructures invisible to the human eye. Plants, insects, and animals exhibit an incredible variety of structural colors that serve a wide variety of purposes, such as camouflage, improving reproduction, and communication. Compared to the pigmentary colors, structural colors do not photobleach and can be created by using common, non-colored materials such as bioplastics or glass. They can be eco-friendly and fade-resistant, which makes them promising candidates for a wide range of applications, including sensors, displays, optical or optoelectronic devices, coatings, and security labels.
Recently, Dr. Lauren Zarzar, an assistant professor of chemistry, materials science, and engineering at Pennsylvania State University in the USA, together with a team of collaborators discovered a new structural coloration mechanism that combines total internal reflection and interference at concave optical interfaces to generate brilliant rainbow colors.
Total Internal Reflection and Interference at Microscale Concave Interfaces
Total internal reflection (TIR) is a phenomenon occurring when an incident light beam hits an interface between a high refractive index medium (such as water) and a lower refractive index medium (such as air) at a particular angle in such a manner that all the incident light is reflected from the interface back into the high refractive index medium. This is the effect that allows optical fibers to carry optical signals over large distances with low loss.
What Dr. Zarzar and her colleagues have discovered is that an array of hemispherical, colorless oil droplets deposited on a flat transparent surface can produce brilliant colors under white light illumination. In this case, the droplet array acts as multiple microscale concave interfaces (MCIs) where the hemispherical surface geometry allows the incident light to be reflected within the droplet two, three, or more times through TIR, before exiting the droplet.
The interference between the incident and exiting light beams determines whether a droplet will exhibit color or not. The color that droplets produce also depends on the size and curvature of the droplets, and the refractive index of the material used to fabricate the MCIs.
Dr. Zarzar's team developed a model that can be used to predict the resulting color depending on the structural and optical parameters of the MCIs. The researchers envisage that these design principles can be applied to fundamental research in optics and as well as the development of colloidal paints, novel display technology, and optical sensing.
How to Make Autonomous Vehicles Safer
Now, building on the research at Penn State, an international team of scientists led by Qiaoqiang Gan, professor of electrical engineering at the University of Buffalo, refined the MCI-based structural color technology and explored its possible applications in retroreflective films and tunable reflective displays that could be visualized by infrared and visible light cameras or detected by laser detection and ranging (LIDAR) sensors.
Currently, LIDAR sensors and imaging cameras are used in autonomous vehicles to scan the surrounding environment and identify other vehicles, pedestrians, and traffic signs. However, despite the use of advanced pattern recognition algorithms, these imaging systems can misidentify insufficiently illuminated or physically altered (for example, by placing stickers) traffic signs.
As Prof. Gan explains, the challenge for autonomous vehicles lies in the fact that the light reflected from conventional traffic signs contains too little information for reliable pattern recognition.
Passive Tunable MCI Display as a Traffic Sign
As a proof-of-concept, Prof. Gan's team created a passive tunable retroreflective display that uses MCI-based structural color technology. Each pixel of the display had an MCI structure controlled by a dielectric elastomer actuator. By changing the shape of the individual actuators, the researchers were able to control the illumination angle and the reflectivity of each pixel.
In trials, the tunable retroreflective device was used to display a conventional stop sign pattern. The resulting highly reflective pattern on the display was visualized by a visible light camera and an infrared LIDAR sensor. The experimental data showed that the MCIs can generate strong reflected signals detectable at a wide range of angles by both the camera and the LIDAR sensor. This combined strategy allowed enhanced and unambiguous pattern recognition.
The retroreflective MCI technology has been patented in the USA and China, with the University of Buffalo and Fudan University as the patent-holders. Currently, the researchers are testing different materials for the MCIs that would enhance the performance of the device and enable designing smart traffic signs, for safer and more reliable autonomous transportation.
References and Further Reading
M. Allen (2021) Could microscale concave interfaces help self-driving cars read road signs? [Online] www.physicsworld.com Available at: https://physicsworld.com/a/could-microscale-concave-interfaces-help-self-driving-cars-read-road-signs (Accessed on 20 October 2021).
C. Hsu (2021) This rainbow-making tech could help autonomous vehicles read signs [Online] www.buffalo.edu Available at: http://www.buffalo.edu/news/releases/2021/08/021.html (Accessed on 20 October 2021).
Rada, J., et al. (2021) Multiple concentric rainbows induced by microscale concave interfaces for reflective displays. Applied Materials Today, 24, 101146. Available at: https://doi.org/10.1016/j.apmt.2021.101146
Li, K., et al. (2021) Designable structural coloration by colloidal particle assembly: from nature to artificial manufacturing. iScience, 30, 24, 102121. Available at: https://doi.irg/10.1016/j.isci.2021.102121
Goodling, A. E., et al. (2019) Colouration by total internal reflection and interference at microscale concave interfaces. Nature, 566, 523–527. Available at: https://doi.org/10.1038/s41586-019-0946-4