Coated with a special heat-resistant yellow-ceramic filter, this is the end view of a 400 watt mercury vapor street light lamp which was introduced in the later 1950s. These lamps remained in production (at least by GE) until the early 1970s and were cataloged in the same (BT-37 bulb) style. These lamps operated on standard 400 watt mercury ballasts and were intended for installations in street lights in hazardous areas (railroad crossings, freeway merges, airports, etc). For more information, please contact: http://www.groups.yahoo.com/group/vintage_streetlights