This report investigates the use of single and multi-layer coatings on replaceable headlamp bulbs and how such coatings can affect the performance of bulbs in terms of light scattering, which can contribute to glare, and spectral separation in headlamps. Tests were developed to investigate the effects of absorptive and interference (multi-layer) coatings on bulbs, and on bulbs in headlamp systems. These tests provide validation for a proposed bulb color separation test, which establishes limits for spectral separation within the boundaries of SAE J578 white color requirements. The bulb color separation test provides a definitive selection criterion to identify bulbs that cause excessive light scatter (glare) and/or spectral separation in an optical system.
The introduction and sale of tinted replaceable headlamp bulbs first appreared in the late 1990s. The lamps initially sold used standard replaceable capsules, which were coated with interference (multi-layer) coatings to produce a "HID color." A survey of bulbs that were being sold in the aftermarket showed that some of the bulbs did not meet performance specifications for light output or wattage. In the SAE Lighting Committee a Coated Bulb Task Force was formed. The Task Force was charged to investigate the effects of coatings applied on replaceable headlamp bulbs. The findings of the Task Force are presented in this information report. The findings have also been presented at the SAE 2004 World Congress and have been published (1).