Optical performances of antireflective moth-eye structures. Comparison with standard vacuum antireflection coatings for application to outdoor lighting LEDs

Protection windows or lenses for outdoor white LED lighting generate high optical losses. Moth-eye structures elaborated by plasma etching offers a simple and low cost way for conferring broadband antireflective properties to protection optics. In this study, we optimized this technology and compared it to "standard" antireflective coatings elaborated by electron-beam evaporation: MgF2 single layer and broadband SiO2/ZrO2 multilayers. First glass substrates have been treated in order to compare nominal antireflective performances of these three vacuum technologies. Reactive ion etching process was then optimized for maximising transmission in normal and angular incidence in the wavelength range of white LEDs (400-750 nm). Best results with moth-eye structure were achieved on glass substrate etched on both sides: we measured 97,5% of white LED light transmission comparatively to 92% with untreated substrate. In a second time, the influence on antireflective properties of different ageing conditions generated during outdoor normative tests have been studied. Spectrophotometry and surface energy measurement highlighted the degradation of MgF2 coating under high humidity ageing conditions: we noticed a decrease of 1% of antireflective performance. Only moth-eye structures gave high environmental stability as they kept their white LED light transmission after ageing tests. In a third time, we introduced a novel comparison approach for such "technological study": These three antireflective vacuum solutions were confronted from an economical and environmental impact point of view. Moth-eye structures elaborated by plasma etching offer a simple and low cost technology. Nevertheless, the use of greenhouse gas in this reactive ion etching process could be problematic for a large-scale industrialization.