These were used in streetlights where color rendition was of most significance. The best known installation of these purple coated mercury lamps was in 1961 by the Los Angeles Bureau of Street Lighting inside the famous "three-star" rectangular fixtures that still adorn Hollywood Blvd. The original lamps were 700W mercury (purple coated). The lamp shown is 175 watts (BT-28 bulb) of the same vintage however the (Sylvania) lamps originally employed were of a much larger (BT-46 bulb). The reason for such transparent lamp (purple) coatings was to provide ultimate color rendition on the surface below. That provided well within the lamp industry until deluxe white (DX) phosphored lamps were introduced in 1964 with the same results and with no coating! The lamp shown is of a lower wattage, made in 1962. For much more information about vintage streetlights, mercury lamps and their accessories, please refer to: http://www.groups.yahoo.com/group/vintage_streetlights