High-pressure sodium lamps (sometimes called HPS lights) are commonly used as plant grow lights. They have also been widely used for outdoor area lighting such as streetlights and security. Understanding the change in human color vision sensitivity from photopic to mesopic and scotopic is essential for proper planning when designing lighting for roads.



High-pressure sodium lamps are quite efficient—about **0 lm/W—when measured for photopic lighting conditions. The higher power lamps (**0 W) have an efficiency of **0 lm/W.



Because the high-pressure sodium arc is extremely chemically reactive, the arc tube is typically made of translucent aluminum oxide. This construction led General Electric to use the trade name "Lucalox" for their line of high-pressure sodium lamps.



Xenon at a low pressure is used as a "starter gas" in the HPS lamp. It has the lowest thermal conductivity and lowest ionization potential of all the non-radioactive noble gases. As a noble gas, it does not interfere with the chemical reactions occurring in the operating lamp. The low thermal conductivity minimizes thermal losses in the lamp while in the operating state, and the low ionization potential causes the breakdown voltage of the gas to be relatively low in the cold state, which allows the lamp to be easily started.



High-pressure sodium lamps have a broader spectrum of light than the low-pressure lamps, but still poorer color rendering than other types of lamps. Low-pressure sodium lamps only give monochromatic yellow light and so inhibit color vision at night.