Sodium Vapour Lamps
Low and High Pressure Lamps
One of the most remarkable discharge lamps is the low pressure sodium is best known for its yellow/orange monochromatic light. It has also been the most efficient light source available for more than 90 years, and only now being matched with LED technology. Now no longer manufactured these lamps still demonstrate the art of engineering and priciples in physics.
The pictures on the left shows an early SO/H 60W with vacuum Dewar and a 26W SOX the last type manufactured. These two lamps are to some extent interchangeable even though separated by over 60 years.
This is a 90W SOX running on an auto-leak transformer. This type of transformer produces the high starting voltage required to strike the lamp then limits the current during operation. These were later replaced by choke / ignitor combinations reducing the weight and cost. Finally electronic inverter ballasts were available offering further efficiency improvements.
The image here is of the same lamp turned off but illuminated with an external sodium light source. If the sodium is still in vapour state the discharge tube looks cloudy. This is a demonstration of sodium absorption. Sodium vapour will produce its characteristic double yellow line spectrum when excited electrically in the discharge tube, but will also readily absorb its own emission lines when in a stable vapour state. This has implications on the lamp design and resultant efficiency. One final point, the absorption is resonant and the appearance here is from re-emitted light from the sodium vapour but at reduced output from the incident light.
Another type of sodium lamp available until the late 1980 was the linear sodium lamps. These were slightly more compact and the same shape a a fluorescent lamp. They were slightly less efficient than the 'U'-shaped discharge lamps and more expensive to manufacture. This is the smallest lamp rated at 60W but there were at least to other popular SLI/H ratings of 140W and 200W.
High Pressure Sodium Lamps
The, so called, high-pressure sodium lamp also contained mercury, but the sodium dominates in the discharge spectrum. Increasing the lamp vapour pressure causes spectral line broadening This improves the continuation of the spectrum reducing the monochromatic nature of the low-pressure sodium lamp. The issue was the arc could not be contained in a glass or quartz tube due to the aggressive chemical nature of hot sodium. The arc tube is made with sintered aluminium oxide baked into a sapphir crystal.
The spectral images below are for low-pressure and high-pressure sodium lamps. Note the dark line in the low-pressure sodium double is hard to resolve but this also broadens with pressure and can become much wider than the low pressure lines. In some high-pressure sodium lamps light absorption become visible in white light when the lamp extinguish.