What are sodium lights useful for?
Sodium vapor lamps are not new. They have been used for a number of years in places where good light was needed along with low cost of operation. Thus starting from street lighting, to floodlighting and interior lighting in commercial establishments, sodium vapor lamps have been used for decades.
There are two basic types of sodium lamps, the high pressure sodium lamps and low pressure sodium lights. The difference between the hps lamp and the lps lamps is in their output and the color rendering.
The way a sodium vapor lamp works is very simple. The lamp as such contains a small amount of some other vapor like argon, xenon, neon or even mercury which will start discharging light immediately when current is passed into the lamp. There is also sodium metal inside the light that gets warmed up when the other vapor inside the lamp starts discharging light. Once the sodium reaches optimum temperature, it takes over. That is why when a sodium vapor lamp is switched on, initially it glows reddish or pink first before switching to the typical yellow. The pink light is from neon or argon, while if the light is white or blue, it means that the lamp uses mercury to warm up the sodium.
A low pressure sodium light only emits light in a very small bandwidth. This means that the color of objects is never true under a low pressure sodium light. Many reception areas are lit by low pressure sodium lights because it gives good light, but the narrow spectrum of the light ensures that dust and dirt does not show up too well.
A high pressure sodium light emits light in a broader bandwidth. This is because of two reasons. When sodium vapor is pressurized it starts to increase the bandwidth of the light emitted. Secondly the starter gas used is mercury which continues to produce light. Mercury produces whitish light which when combined with the yellow of sodium gives a nearly sunlight-like spectrum of light.
Commercial establishments that require very good lighting generally go for these high pressure sodium light because they last an average of 20,000 hrs. Mercury vapor lamps do give out better light and last almost as long, but many cities prefer the yellow of the sodium vapor lamps because it causes lesser light pollution than the white light of a mercury vapor lamp.