Low Pressure Sodium
The Low Pressure Sodium lamp (LPS) was developed in Europe (1930s). The LPS lamp is not a true HID lamp. The LPS operates like a low-pressure mercury/argon tube (Fluorescent). In Europe these lamps became real popular. LPS never really became popular in North America.
The LPS lamp has an inner bulb and an outer bulb. LPS lamps use neon/argon gas to start up and contains sodium to produce yellow light. The bulb needs a special inner bulb because standard glass / quartz glass can't withstand metallic sodium. The lamp may be started by an ignitor or a ballast with high enough peak / OCV voltage.
Light output is monochromatic 589 nm, with a color temperature 1800K. This is one reason LPS never really became popular in North America. Everything under this light source appears yellow/gray. Some cities/states are starting to use LPS lamps to decrease light pollution.
The LPS bulb was improved by putting a reflective coating on the outer bulb to reflect heat back at itself. LPS is the most efficient HID lamp (100-183 LPW). LPS bulbs are commonly available in 18 - 180 watts and only available in tubular shapes.
Low pressure sodium usage:
Non-color critical lighting
Outdoor lighting
Security lighting
Road lighting
Outer bulb: Glass bulb usually with a reflective coating to increase efficiency.
Arc tube: Where light is generated. Contains Sodium and argon/neon gas.
Glass dimples / nipples: Collects metallic sodium while cooling. Keeps sodium from migrating to the ends of the arc tube.
Nipple / Tip: Where chemicals / gas are added in the manufacturing process.
Pinch seal: The arc tube seal with two electrode leads.
Electrodes: Special coated filament electrodes.
Drawing and picture by: Paul Erik Hirvonen
