What is a High Pressure Sodium Light?

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.

What is Low pressure sodium Light?

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.

Commonalities Between LPS & HPS Lights:

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).

What is LED Light?

LED stands for light emitting diode. A diode is an electrical device or component with two electrodes (an anode and a cathode) through which electricity flows - characteristically in only one direction (in through the anode and out through the cathode). Diodes are generally made from semi-conductive materials such as silicon or selenium - solid state substances that conduct electricity in some circumstances and not in others (e.g. at certain voltages, current levels, or light intensities). When current passes through the semiconductor material the device emits visible light. It is very much the opposite of a photovoltaic cell (a device that converts visible light into electrical current).