Lighting Comparison: LED vs High Pressure Sodium/Low Pressure Sodium
Ever wonder what’s better: high pressure sodium lights (and the related low pressure sodium lights) or light emitting diodes (LEDs)? Well here’s a head-to-head comparison of the two followed by an in-depth discussion of each technology in turn.
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 a 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).
LED Lighting:
What is a Light Emitting Diode (LED)
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).
Qualitative Comparison Between LED & LPS/HPS
What is the Difference Between Sodium Vapor and LED Lights?
The two different technologies are entirely different methods of producing light. Sodium vapor bulbs contain metals that are evaporated into inert gas within the glass casing while LEDs are a solid state technology. Both technologies are very efficient. The difference is that sodium vapor lights were the most efficient technology of the 1970s while LEDs are the modern day equivalent. Although sodium vapor lighting beats virtually every other technology in terms of energy efficiency (which is why it was chosen to illuminate the streets of so many cities), it loses out to LEDs. Both LEDs and sodium vapor lights emit electromagnetic radiation across a small portion of the visible light spectrum, however, LEDs waste much less energy producing waste heat and they also provide an incredibly better variety of high color rendering index options to the user (thus eliminating the monochromatic black appearance of objects illuminated by LPS and HPS bulbs).
Lighting Comparison: LED vs High Pressure Sodium/Low Pressure Sodium 2
Lighting Comparison: LED vs High Pressure Sodium/Low Pressure Sodium 1
