History of Street Lighting in The United States - Means of Electric Light Generation - High Pressure Sodium

High Pressure Sodium

Around 1970, a new streetlight was put into service: The high pressure sodium (HPS) light. It was initially disliked by most residents because of its orange glow, but the sodium vapor streetlight has since become the dominant type on American roadways and most people have become accustomed to the orange/yellow glow. It is by far the most efficient light source available. Color-corrected sodium vapor lamps exist but are expensive. These "color corrected" HPS lamps have lower life and are less efficient.

There are two types of sodium vapor streetlights: high-pressure (HPS) and low-pressure (LPS). Of the two, HPS is the more-commonly used type, and it is found in many new streetlight fixtures. Sometimes, older (pre-1970) fixtures may be retrofitted to use HPS lights as well. Virtually all fixtures that are converted to HPS have previously been lit with mercury vapor. Examples of retrofitted fixtures for HPS use include the GE Form 400, first generation GE M400 and M250 and the second-generation Westinghouse OV-15 and OV-25 Silverliner (although later versions of this model were available from the factory as HPS units) as well as the Line Materials Unistyle 400 and 175/250.

LPS lights are even more efficient than HPS, but produce only a single wavelength of yellow light, resulting in a CRI of zero, meaning colors cannot be differentiated. LPS lamp tubes are also significantly longer with a less intense light output than HPS tubes, so they are suited for low mounting height applications, such as under bridge decks and inside tunnels, where the limited light control is less of a liability and the glare of an intense HPS lamp could be objectionable.

HPS lamps have slightly different electrical requirements than do the older MV lamps. Both HPS and MV lamps require a transformer or ballast to change the voltage and regulate the current, however, HPS lamps also require an electrical "starter" circuit—much like older fluorescent lamps in residential use. MV lamps do not require a separate "starter" circuit because they have a special starter element within the bulb used for striking the arc. MV lamps slowly dim over time, and a twenty-year-old lamp may emit a very pleasing, but useless, soft green glow, rather than the powerful blue-white light of a new MV lamp. The yellow-spectrum HPS lamps also slowly dim over time but are known for "cycling," where the lamp cycles on and off when it has reached the end of its life cycle. When cycling, the arc within the lamp extinguishes and the lamp must cool down before the starter circuit initializes a new arc. This has been the most recognized downfall of HPS. Some HPS lamps start to burn a pinkish/reddish color at the end of their life (usually when already cycling), or also burn out at the end of their life cycle whether they cycle or not. HPS fixtures can contain a special photocell or ballast that can sense a cycling lamp and shut off the fixture to prevent damage to the ignitor and ballast.

HPS lamps by and large have the same rated lifespan as do MV lamps, and they do give increased light and efficiency at lower wattages. Usually, when an MV light is replaced, it is usually replaced with a HPS light of a lower wattage, for example, a 175 watt MV fixture will get replaced with a 100 or 150 watt HPS fixture as that will meet or exceed the lumen output of the 175 watt MV fixture. At end of life MV lamps just become dimmer and sometimes color shifting towards the green end of the spectrum and continue to consume the same amount of electricity. HPS lamps begin to suffer end-of-life cycling before the amount of useful light becomes visibly diminished, or even just burn out. HPS lights come in wattages of 35, 50, 70, 100, 150, 200, 250, 310, 400, 600, 750, and 1,000 watt sizes, while LPS lights come in wattages of 18, 35, 55, 90, 135, and 180 watt sizes