These light bulbs were produced between 1958-late 1960s for bright yellow illumination within 400 watt standard mercury vapor street light luminaries to indicate "caution" or "danger". These lamps are now hard to find and typically were used for railroad crossings, highway interchange on-ramps/exits and other vehicular dangerous vicinities. These light bulbs inserted inside standard mercury vapor streetlights with the same electrically-compatible ballast. These lamps are very rare! Most were coated with a yellow/orange transparent dye however the Sylvania versions were a pure yellow phosphor. See Joe's street lighting group/website at: http://www.groups.yahoo.com/group/vintage_streetlights for more information. Thank you.