The eyes of a moth reflect almost none of the light that strikes their surface. This feature evolved to aid the insect’s survival, making the moth eye a nearly perfect anti-reflective surface in the natural world. This unique biological structure has inspired engineers and material scientists to replicate its design at the nanoscale to solve reflection and glare problems in human technology. The concept of biomimicry, adapting nature’s designs for technological application, finds an example in the development of low-reflection coatings based on the moth eye.
Anatomy of the Compound Eye
The moth’s visual apparatus is a compound eye, a structure fundamentally different from the single-lens system found in the human eye. The compound eye is composed of thousands of repeating units known as ommatidia, which are individual light-collecting columns. In some moth species, the number of ommatidia can reach up to 30,000, each topped with its own transparent corneal lens.
Each ommatidium functions as an independent optical unit, contributing to the overall mosaic image perceived by the moth. The outer surface of this corneal lens is the interface between the air and the eye’s internal structures, where light reflection would ordinarily occur. This surface is where the moth’s anti-reflective properties originate.
The Anti-Reflective Nanostructure
The ability of the moth eye to suppress reflection is due to a precise physical structure on the cornea’s surface, not a chemical coating. This structure is a periodic array of minute, conical protrusions, often referred to as “corneal nipples” or the “moth-eye array.” These pillars are sub-wavelength structures, meaning their height and spacing are smaller than the wavelength of visible light, typically measuring between 100 to 300 nanometers.
The geometric arrangement of these conical shapes eliminates reflection by creating a smooth, gradual transition in the refractive index between the air and the denser material of the cornea. When light encounters a smooth surface, the abrupt change in refractive index causes a portion of the light wave to be immediately reflected, known as Fresnel reflection. The moth-eye nanostructure prevents this by acting like a blended material where the refractive index slowly increases from that of the air at the cone tips to that of the cornea at their base.
This continuous change in the effective refractive index prevents the sudden optical impedance mismatch that causes reflection. Instead, the light wave is gently guided into the eye, maximizing the light transmitted through the surface. This mechanism provides broad-spectrum anti-reflection, meaning it works effectively across a wide range of light wavelengths and incident angles, unlike traditional multi-layer interference coatings.
Biological Advantage of Low Reflection
The evolution of the non-reflective eye provides the nocturnal moth with two survival advantages. The first benefit is related to vision, as the low-reflection surface maximizes the amount of available light that enters the eye. Since moths are active in low-light environments, absorbing nearly 100% of the incident light is necessary for visual acuity and navigation.
The second advantage is stealth, as the absence of a reflective surface helps the moth avoid detection by predators. A shiny eye would reflect ambient light, making the insect an easy target for nocturnal hunters like bats or spiders. The matte, non-glinting surface allows the moth to remain visually inconspicuous, merging with the darkness.
Biomimicry in Technology
The principles of the moth-eye nanostructure have been adopted in biomimicry to engineer anti-reflective surfaces for numerous human applications. By replicating the sub-wavelength array of conical pillars onto synthetic materials, engineers can create films that drastically reduce surface reflection. For example, anti-glare coatings are manufactured for electronic display screens, such as those on smartphones, computer monitors, and televisions. These coatings increase the screen’s readability in bright conditions by reducing the reflection of surrounding light.
The technology has also been applied to maximize energy capture in photovoltaic devices like solar panels. Applying a moth-eye-like texture directly to the surface of a solar cell ensures that light transitions efficiently into the semiconductor material instead of reflecting away, thereby increasing the overall light transmission and power generation efficiency. Engineered films mimicking the moth eye have demonstrated a visible light reflectance of only 0.4%, allowing for superior optical performance over a wide range of angles and wavelengths compared to traditional anti-reflection methods.