IVESTIGATION OF BRITTLENESS OF IG6 LENSES USED IN INFRA-RED OPTICAL SYSTEMS

In recent years the use of chalcogenide glasses expanded rapidly, especially IG6 (by Vitron) as a substrate for lenses in night vision systems. The advantage of using IG6 lenses is in their having good transmission in the IR range, low dependency of refractive index on temperature, and a thermal expansion coefficient close to that of aluminum. Moreover, compared to conventional crystalline materials that are used for lenses in IR systems nowadays, such as germanium and silicon, IG6 can be casted, a manufacturing process that will make it cheaper to produce these lenses in the future. IG6 transmits more than 65% in the IR range without coating, and an Anti-Reflective Coating (ARC) on both sides can improve this transmission to more than 99%. The coating is accomplished using PVD process methods, under heating in vacuum furnace. However, some lenses break as a result of the coating process.

In this study we try to understand the root cause of lens breakage, by performing various tests to characterize IG6 lenses before and after the coating process. Mechanical, optical, and XRD tests failed to indicate significant changes in structure and properties due to the coating process, and pointed to defects in the original lenses as the main cause of lens breakage. IR pictures of lenses before any treatment showed varying crack densities and sizes, which in some cases seemed to be critical, and which may have led to subsequent fracture of the lenses. We also observed that IG6 is more sensitive to thermal shock, than conventional materials due to low mechanical properties, which demands lower temperature gradients in the coating process.