Coatings on automotive bulbs are becoming increasingly more important. Environmentally friendly amber and red filter layers can be used on bulbs for signaling functions. Infrared reflective layers enhance the efficiency of halogen bulbs. Styling needs can be addressed with bluish bulbs matching the color impression of Xenon headlamps or silver / neutral colored amber filters for turn signal applications. Aside from aesthetic reasons, there is evidence that the modified spectral output of coated bulbs may provide visual benefits over standard tungsten halogen lamps for example during nighttime driving.
Experience has shown that poorly engineered color-coated bulbs may fade in color or cause glare for other road participants by light scattering and color separation. Additionally insufficient road illumination for the driver himself is another negative side effect that can be observed with poorly engineered coatings.
In this contribution, a survey of various coating types, the underlying physical principles responsible for their effect and the technology used to realize such coatings are presented. Specific requirements in the automotive environment will be addressed. Several examples of thin film coatings on the bulb surface applied by different coating technologies and the characterization thereof are given. These include coatings designed to increase visibility while driving at night or under specific weather conditions or coatings that give bulbs a silver appearance when unlit while ensuring that intensity and color of the emitted light meets the legal requirements when the lamp is switched on.
One of the future trends currently under discussion is active infrared night vision which requires a light module blocking the visible light and transmitting the near-infrared radiation. An infrared filter coating on the bulb surface which eliminates the need of an additional volume filter element will be discussed.