Here is a photo of a rare mercury vapor lamp made from the late 1950's through the early 1970's that was used in standard street light fixtures to denote "caution" and hazardous areas. Some of these lamps had an external ceramic coating (GE); or an actual internal yellow phosphor (Sylvania) and Westinghouse with a translucent yellow-orange coating on the outside of the bulb which provided saturated yellow light just like the aforementioned others. These light bulbs were electrically interchangeable with standard 400W mercury lamps. They were eventually phased out by the early 1970s on account of the more energy-efficient high pressure sodium lamps (yellow-light-emitting) that are practically used everywhere for street lighting purposes now. For more information about nostalgic street lighting please refer to my website: http://www.vintagestreetlights.com and.. http:/groups.yahoo.com/group/vintage_streetlights.