A newly designed high pressure sodium lamp

In experiments on high pressure sodium lamps, we found that lamp efficacy is increased about 10% and that lamp extinction characteristics improve remarkably when the filling pressure of inert gas xenon is increased from the conventional 20 torr up to 350 torr. The improved efficacy is thought to be due to the reduction in thermal conduction loss caused by the lowered thermal conductivity of the discharge gas. The improved extinction characteristics are caused by the considerable lowering of lamp reignition peak voltage due to the same reason. Utilizing these effects, we designed a new type of high pressure sodium lamp to increase lamp efficacy while still using ordinary high pressure mercury lamp ballast. The new lamp uses the ballast of 400 W mercury lamps, requires 360 W of input, and has 138 lm/W of lamp efficacy. For the starting device, we designed an improved bimetal switch that is built in the lamp bulb.