The Low-Down on High Intensity Discharge (HID) Lamps

HID lamps offer bright, efficient alternatives to filament lamps like incandescents and halogens. They’re found illuminating parking lots, streets, and indoor arenas, among other commercial, industrial, and outdoor locations. What is an HID lamp, you ask? Read on to find out…

All HID (High Intensity Discharge) lamps produce light by passing electricity through a gas (called an arc discharge) between two electrodes at either end of the lamp’s arc tube. There are four different kinds of HID lamps, each with its own unique properties:

1. Mercury Vapor is the oldest of the HID lamps, and is on the way out. New federal laws have begun prohibiting the manufacturing and importation of this product. A mercury vapor lamp’s rated life ranges anywhere from 16,000 to 24,000 hours, but with an output of only 25-60 lumens per watt and a poor CRI of 50, there are better choices out there.
2. Low Pressure Sodium lamps are classified as HID, but lack a compact, high intensity arc. Instead, the long stretched-out arc has more in common with a fluorescent light. Low pressure sodium lamps are the most efficient of HID lamps, producing the most lumens per watt (up to 150 l/w), but their stark yellow color produces extremely poor color rendition (a CRI of 0),  limiting their use to lighting streets, tunnels, and parking lots. They last anywhere from 14,000 to 18,000 hours.
3. High Pressure Sodium light sources have become increasingly more popular over the years, and with their efficacy of 50-140 lumens per watt, we can see why. Compared to low pressure sodium lamps, these lights produce a slightly less severe yellowish white light, making them a more versatile light source. Their lifetime ranges from 16,000 to 24,000 hours, which makes them a smart choice for most outdoor lighting applications.
4. Metal Halide lamps have a CRI that ranges from fair to very good (between 65 and 90) – the best color rendering index of the bunch. This high CRI means that metal halide lamps thrive in many applications that require white light with good color rendering – they’re replacing high pressure sodium lamps in some applications and certain MH lamps are even used in retail displays!  The lamp itself is very similar to a mercury vapor lamp, but the added metal halide gas provides a higher light output, more lumens per watt (65-115 l/w), and a better color rendering. The biggest drawback to a metal halide lamp is its rated-life, which is capped at about 20,000 hours.

Properties of All HID Lamps

Ballasts: Ballasts, used to regulate the voltage and amperage supplied to an HID lamp, are required for the lamp to function properly. Each lamp requires different starting and operating conditions, depending on the type of lamp and its wattage. The Department of Energy (DOE) has recently issued new regulations banning mercury vapor ballasts (obviously), and requiring pulse-start ballasts for metal halides of 150-500W. The goal here is to increase efficiency as much as possible – one day we may see standards so high that only electronic ballasts will be able to comply.

Warm-Up Time: So, we know every HID lamp needs a little help on the startup with a ballast, but they also require a little grace to work up to their full lumen output. Therefore, these lights are not suited for things like motion sensors. They’re most suitable for staying on hours at a time. The various warm-up times for HID lamps are as follows:

• Mercury Vapor: 5 to 7 minutes
• Low Pressure Sodium: 7 to 15 minutes
• High Pressure Sodium: 2 to 4 minutes
• Metal Halide: 2 to 5 minutes (With improved pulse-start technology, can be as low as 2 minutes)

Re-Strike Time: Just like warm-up time, HID lamps need a bit of time to re-strike after a power interruption. The time it takes to re-strike varies by light source:

• Mercury Vapor: 10 minutes
• Low Pressure Sodium: 3 to 12 seconds
• High Pressure Sodium: 1 minute
• Metal Halide: 10 to 20 minutes (With improved pulse-start technology can be as low as 4 to 5 minutes)