High Intensity Discharge (HID) light is emitted from a plasma discharge rather than a filament. Types of HID lamps include mercury vapor, metal halide, high-pressure sodium and low-pressure sodium lighting. Like fluorescent light, the HID system uses a ballast to start the light and specific gases to produce the light. Metal electrodes conduct electrical energy to the light-emitting gas. As the lamp heats, the metal and glass expand at differing rates.

Its use of dissimilar materials and vapor make HID lighting vulnerable to failure. The stress of prolonged cycles causes separation of the components. Failure can also result from escaping gases, ambient temperature, vibration & filament deterioration. HID lighting efficiency varies depending on the gases used. HID lamps’ color rendering also varies widely - from the bluish-white cast of mercury vapor lamps to the yellow light of low-pressure sodium.

Light Quality and Color Perception

Lighting quality is central to creating a more inviting, effective ambience. A good example of how light quality affects our perception is the High Pressure Sodium (HPS) lamp. Among the HID lighting types, HPS lamps have often been the first choice in outdoor commercial lighting, because they have a relatively long, reliable lifespan. However, the distinctive, unnatural yellow light of HPS lamps has a poor Color Rendering Index (CRI) of approximately 22, making it difficult to correctly distinguish color.

HID Advantages and Disadvantages

For years HID lighting was the dominant choice for many commercial uses, as it offered reasonably long life and high efficiency. But HID lighting has been overtaken by more energy and cost-conscious lighting choices like induction and LED lighting for an ever-expanding range of commercial applications. HID lighting remains more complicated to manufacture, consumes a lot of energy, and has a high rate of depreciation and color shift. The construction and limited lifespan of HID result in on-going costs due to lamp and ballast purchases, and man-hour allocations for replacement and maintenance.