Optical Properties of CaF2 Anti-Reflection Coating On ZnS for 8~12 µm Infrared Region

Abstract

Technologies for pedestrian safety are increasingly emphasized by Automakers in advance of autonomous driving vehicles. A Night Vision System attached behind the front grille can reduce fatal accidents, especially during the nighttime, however, consumers may hesitate to adopt such systems on account of their high price. High-cost Germanium is used in commercial Night Vision System windows, and therefore replacing it with a cheaper infrared window material can lead to a more affordable system. To achieve this, Zinc Sulfide (ZnS), which has about 70% transmittance in the Long-Wavelength Infrared region of 8~12 μm, was selected for the window substrate material. In this study, we designed, fabricated and characterized a single layer cost-effective anti-reflection coating on a ZnS window substrate using Calcium Fluoride (CaF2). The CaF2 coating was fabricated by E-beam evaporation technique, with Quarter wavelength anti-reflection thickness (QAR). It was characterized by FT-IR, SEM and a thermal camera test module. We found that CaF2 both side coated the ZnS window and exhibited about 10~15% higher transmittance than the ZnS window substrate. In addition the CaF2 coating stably bonded to the ZnS substrate without any internal defects. A thermal camera based window test also showed better detection performance with the CaF2 Coating than a bare ZnS substrate window, which was calculated using the output voltage of the microbolometer thermal sensor.