High Pressure Sodium Vapor (HPS) lights, similar to LPS lights, are a specific type of gas-discharge light (also known as a High Intensity Discharge, HID or arc light). The principal difference between low and high pressure sodium lights is the operating pressure inside the lamp. As indicated by the name, “high” pressure sodium vapor lights operate at a higher internal pressure. The arc tube is made of aluminum oxide and the sodium metal is combined with several other elements like Mercury which counter-balances the yellow glow with some white to light blue emissions.
Low Pressure Sodium Vapor (LPS) lights are a specific type of gas-discharge light (also known as a High Intensity Discharge, HID or arc light). The bulb principally contains solid sodium metal inside a borosilicate glass tube that vaporizes once the lamp is turned on. During start (while the sodium is still in solid form) the lamp emits a dim reddish/pink glow. Once the metal is vaporized the emissions become the characteristic bright yellow associated with sodium vapor lamps. The spectrum of visible emissions from an LPS light is actually very close together (589 and 589.6 nm, virtually monochromatic) resulting in the colors of illuminated objects being nearly indistinguishable.
Both low and high pressure sodium lights require ignition which is typically provided by a voltage pulse or a third electrode (an additional metal part) internal to the bulb. Starting is relatively simple with small tubes but can require significant voltage with larger lights. Sodium vapor lighting typically requires a “warm-up” period in order to evaporate the internal gas into plasma. Additionally, as the light heats up it requires additional voltage to operate which is balanced by a ballast (a magnetic or electric device designed to provide the light constant current). As sodium vapor lights age, more and more voltage is required to produce the same amount of light until eventually the voltage exceeds the fixed resistance provided by the ballast and the light goes out (fails). The lights become less efficient over time because they must use more and more voltage to produce the same lumen output as the light degrades. That said, HPS lights in particular maintain fairly good light output (roughly 80%) at their typical end-of-life (24,000 operating hours).
Sodium vapor lighting has been around since the middle of the 20th century (in commercial production since the 1930s) and it generally represents a high efficiency way to provide lighting over a vast area. Sodium lights operate in a range where the human eye is very sensitive and so there is less power required to achieve the same lighting effect. For this reason they are very efficient. Additionally, despite their long warm-up period (5-10 minutes), low pressure sodium lamps will re-ignite immediately in the event of a power interruption. It is particularly useful for outdoor lighting where energy efficiency is at a premium (such as with municipalities lighting the streets or other common areas like parking lots. LPS and HPS lights are much more efficient as well as longer lasting than incandescent bulbs, many fluorescent bulbs, and most high intensity discharge lamps in general. It is only recently with the advent of affordable and prevalent LED lighting that they are being consistently surpassed in terms of energy efficiency and lifespan.