At their root, reflections happen whenever light abruptly leaves one transparent medium and enters another with a different refractive index. The strongest effects are seen at interfaces between air and materials like glass and optical polymers. When light from a headlamp reflects off the polycarbonate lens of its housing, the reflected light is lost and decreases the headlamp’s brightness and efficiency. When light reflects from the inside of the windshield into the driver’s eyes at night it causes glare and distraction. And when light reflects off of an instrument cluster or indicator it reduces the contrast and readability. The range of effects caused by unwanted reflections range from annoyances to serious dangers.
The first approach to reducing unwanted reflections is to carefully position the transparent surfaces to geometrically reduce the likelihood, intensity, and consequences of reflections. Antireflective coatings (ARCs) are specialty films that provide another means of glare reduction. The most advanced conventional ARCs involve dozens of layers of different materials, each one tens of nanometers thick. Aside from being difficult to fabricate at large sizes, they suffer from technical challenges associated with strain mismatch between the substrate and coating materials and a strong angle dependence of the reflection reduction.
As a solution to unwanted reflections, META offers a nanopatterned surface technology inspired by nature. Moths are most active in low light conditions where every photon counts. Light that is reflected from the surface of a moth’s eye poses two problems; it reduces the transmitted light available for the animal to see by, and the reflection can alert and attract predators. To combat these twin threats, moths’ eyes have evolved a unique approach to minimizing light reflection. The surface of the eye’s lens is covered with a densely packed array of nanoscale cones that taper upwards to points. Most transparent surfaces suffer reflections because of the abrupt change in refractive index between air and the lens material. The taper of the cones in the moth’s eyes means that the effective refractive index changes gradually from that of air at the tips of the cones, to that of the lens at the base. This gradual change in the refractive index leads to a strong decrease in the reflected light and increase in the transmitted light.
META uses Rolling Mask Lithography (RML) to produce anti-reflective moth eye surfaces that reduce reflections from glass or polymer substrates. RML allows the cone angle and spacing to be reproducibly controlled and optimized to match the intended application. RML moth eye structures not only offer improved anti-reflection characteristics, they do so with far less angle dependence than traditional ARCs, so surfaces are non-reflective no matter the viewing angle.