This disclosure provides systems, methods and apparatus for providing illumination by using a light guide to distribute light. In one aspect, a passivation layer is attached to the light guide of an illumination device. The passivation layer may be an optically transparent moisture barrier and may have a thickness and refractive index which allows it to function as an anti-reflective coating. The passivation layer may protect moisture-sensitive underlying features, such as metallized light turning features that may be present in the light guide. The light turning features may be configured to redirect light out of the light guide. In some implementations, the redirected light may be applied to illuminate a display.