High density discharge lamps are those that have compact arc tubes that contain gases and metal salts. They operate at high pressures and temperatures. High pressure sodium lamps fall into this category. Their arc tubes contain sodium, and the metal salt included is typically mercury. The gas that is present is usually xenon, which is used for starting the lamp.
High pressure sodium lamps, invented in 1931, use sodium in an excited state to produce light. They operate by utilizing the gas and sodium mixture as a conductor. Electric current then vaporizes the conductor and causes light.
One challenge in the production of these lamps is to find a material that sodium does not corrode. Aluminum oxide is the modern solution to this problem. This material, which is used for the arc tube, often appears to be milky white but is actually about 95% transparent.
High pressure sodium lamps are generally considered long-life, low maintenance, and relatively inexpensive lighting options. They generally turn about 50% of their consumed electricity into visible light. They are not as efficient as low pressure sodium lamps, which are often regarded as one of the most efficient lamps available.
Low pressure sodium lamps have two disadvantages, however. First, they require significantly larger bulbs. Second, they are almost completely incapable of rendering color.
To the human eye, the light produced by high pressure sodium lamps tends to appear yellowish-white. On a light spectrum, the light is actually orange to red. In comparison with some other options, high pressure sodium lamps also have poor color rendition, limiting the ability to show true color to the human eye.
That does not prevent these lamps, which can last for tens of thousands of hours, from being popular. They are used throughout the world for various purposes. In horticulture, high pressure sodium lamps are generally used in the later stages of plants’ lives. They can also be found in industrial settings, such as warehouses and factories, and are commonly used for outdoor lighting.
In Australia, these tend to be the most commonly used lamps for major road lighting. It has been observed that the yellow light they produce is absorbed by foliage instead of reflected, which results in low brightness of surrounding areas. Greater lighting effects have been achieved by shortening poles and placing the lamps closer together.
High pressure sodium lamps are not designed for immediate on/off action. If they are turned off or a power outage occurs, they typically require a cooling period of 3 to 15 minutes before they can produce light again. This makes them less popular for indoor residential use.