Low Pressure Sodium Lamps
Low pressure sodium vapour lamps (LPS, SOX) have been around since the 1930s and fall into the class of high intensity discharge lamps (HID).
There are two varieties of sodium lamps, high pressure (HPS) and low pressure (LPS);
High pressure sodium lamps (which are most common in today's street lighting) emit a broader spectrum of light than that of the low pressure sodium lamp which is the focus of this article.
Low pressure sodium lamps are highly efficient sources of light capable of efficiencies upto 180 lumens per Watt.
The 35W BY22d produces approximately 4700 lumens (luminous flux) with an luminous efficacy of 129lm/W.
By comparison, a 40W incandescent globe produces around 11lm/W.
The downside to sodium light, is its very poor colour rendering (an index of 0). Colours are indistinguishable under sodium light.
With that said, Sodium lamps are still in use in carparks and street lighting, they do offer excellent contrast at night together with their high efficiency.
An interesting aspect of the Low Pressure Sodium lamp, it's light output is monochromatic (one colour) at 589nm (technically two distinct peaks very close together at 589nm and 589.6nm), a characteristic yellow light.
The monochromatic light from an LPS can be used to demonstrate some interesting scientific experiments with atomic absorption and emission. One such experiment is observing a burning flame appearing dark when viewed under sodium light.
Another Sodium lamp that I have in the collection, is the Russian DNaS-18 spectral lamp, which is used in scientific equipment.
- Written by Flavio Spedalieri -
