Sodium vapor lamp
|
|
A sodium vapor lamp is a gas discharge lamp which uses sodium in an excited state to produce light. There are two varieties of such lamps: low pressure and high pressure.
Low pressure / LPS / SOX
Na-lamp-2.jpg
A LPS at full power
LPS Lamps (Low Pressure Sodium), also known as SOX Lamps (Sodium OXide), consist of an outer vacuum envelope of glass coated with an infrared reflecting layer of indium-tin oxide, a semi-conductor material that allows the visible light wavelengths out and keeps the infrared (heat) back. It has an inner borosilicate 2 ply glass U shaped tube containing sodium metal and a small amount of neon and argon gas to start the gas discharge, so when the lamp is turned on it emits a dim red/pink light to warm the sodium metal and within a few minutes it turns into the common bright orange/yellow color as the sodium metal vaporizes. These lamps produce a virtually monochromatic light in the 589 nm wavelength. As a result, objects have no color rendition under a LPS light, only the reflection of the 589 nm light.
LPS lamps are the most efficient electrically-powered light source — up to 200 lm/W. As a result they are widely used for outdoor lighting such as streetlights and security lights where color rendition is less important. LPS lamps are available with power ratings from 18 watts up to 180 watts.
High pressure / HPS
Na-HPS-lamp.jpg
An HPS at full power
High pressure Sodium (HPS) lamps are smaller and contain some other elements (for example, mercury), produce a dark pink glow when first struck, and produce a pinkish orange light when warmed up. These lamps produce continuous spectrum light (not monochromatic), hence colors of objects under them can be distinguished. This leads them to be used in areas where good color rendering are important (for example, to identify the color of a fleeing suspect's car).
High pressure Sodium lamps are quite efficient — about 100 lm/W, up to 150 lm/W. Therefore they are widely used for outdoor lighting such as streetlights and security lighting where color rendition is more important.
Because of the extremely high chemical activity of the high pressure sodium arc, the arc tube is typically made of translucent aluminum oxide (alumina). This construction led General Electric to use the tradename "Lucalox" for their line of high-pressure sodium lamps.
Light pollution considerations
For placements where light pollution is of prime importance (for example an observatory parking lot), low pressure sodium is preferred. As it emits light on only one wavelength, it is the easiest to filter out.