High Intensity Discharge Lamp (HID)

Primelite continues our look at the different types of light bulbs. From Incandescent to LED, lighting has changed immensely over the past 200+ years. In the past we’ve taken a look at Incandescent Bulbs, Reflector Lamps (Incandescent), Tungsten-Halogen Lamps, Fluorescent Lamps and Compact Fluorescent Lamps (CFL). This week we’ll explore High Intensity Discharge Lamp (HID).

What is a High Intensity Discharge Lamp.

High-intensity discharge lamps (HID lamps) are a type of electrical gas-discharge lamp which produces light by means of an electric arc between tungsten electrodes housed inside a translucent or transparent fused quartz or fused alumina arc tube. This tube is filled with both gas and metal salts. The gas facilitates the arc’s initial strike.

Once the arc is started, it heats and evaporates the metal salts forming plasma, which greatly increases the intensity of light produced by the arc and reduces its power consumption. HID lamps use an electric arc to produce intense light. Like fluorescent lamps, they require ballasts. They also take up to 10 minutes to produce light when first turned on because the ballast needs time to establish the electric arc. Because HID lamps take awhile to establish, they are most suitable for applications in which they stay on for hours at a time. Due to the intense light they produce at a high efficacy, HID lamps are commonly used for outdoor lighting and in large indoor arenas.

HID lamps provide the highest efficacy and longest service life of any lighting type. It can save 75%-90% of lighting energy when it replaces incandescent lighting.

The three most common types of high-intensity discharge lamps are:

Mercury Vapor Lamps
Metal Halide Lamps
High-Pressure Sodium Lamps

Mercury Vapor Lamps
Mercury vapor lamps, the oldest types of high-intensity discharge lighting, are used primarily for street lighting.Mercury vapor lamps provide about 50 lumens per watt. They cast a very cool blue/green white light. Most indoor mercury vapor lamps in arenas and gymnasiums have been replaced by metal halide lamps. Metal halide lamps have better color rendering and a higher efficacy. However, like high-pressure sodium lamps, mercury vapor lamps have longer lifetimes (16,000-24,000 hours) than metal halide lamps.

Metal Halide Lamps
Metal halide lamps produce a bright, white light with the best color rendition among high-intensity lighting types. They are used to light large indoor areas, such as gymnasiums and sports arenas, and outdoor areas, such as car lots.

Metal halide lamps are similar in construction and appearance to mercury vapor lamps. The addition of metal halide gases to mercury gas within the lamp results in higher
light output, more lumens per watt, and better color rendition than from mercury gas alone. Metal halide lamps have shorter lifetimes (5,000-20,000 hours) than mercury vapor and high-pressure sodium lamps.

High-Pressure Sodium Lamps
High-pressure sodium lighting is becoming the most common type of outdoor lighting. High-pressure sodium lamps have more lumens per watt—an efficiency exceeded only by low-pressure sodium lamps. They produce a warm white light. Like mercury vapor lamps, high-pressure sodium lamps have poorer color rendition than metal halide lamps but longer lifetimes (16,000-24,000 hours).

At the end of life, many types of high-intensity discharge lamps exhibit a phenomenon known as cycling. These lamps can be started at a relatively low voltage. As they heat up during operation, however, the internal gas pressure within the arc tube rises and a higher voltage is required to maintain the arc discharge. As a lamp gets older, the voltage necessary to maintain the arc eventually rises to exceed the voltage provided by the electrical ballast. As the lamp heats to this point, the arc fails and the lamp goes out. Eventually, with the arc extinguished, the lamp cools down again, the gas pressure in the arc tube is reduced, and the ballast can once again cause the arc to strike. The effect of this is that the lamp glows for a while and then goes out, repeatedly.

Sometimes the quartz tube containing mercury can explode releasing up to 30 mg of mercury vapor into the atmosphere. This quantity of mercury is potentially toxic, but the main hazard from broken lamps is glass cuts. The manufacturer Philips recommends the use of a mercury vacuum cleaner, ventilation or respiratory protection, eye protection, and protective clothing when dealing with broken lamps. Mercury lamps also require special waste disposal, depending on location

Series: Primelite looks at the light bulb:

• Incandescent Bulbs
• Reflectorized
• Tungsten-Halogen
• Fluorescent Bulbs
• Compact Fluorescent Lamps (CFLs)
• High-Intensity Discharge (HID)
• Light Emitting Diodes (LEDs)

Sources:
Wikipedia – High-Intensity Discharge Lamps
US Dept. of Energy / Energy.Gov – High-Intensity Discharge Lighting Basics

Photos:
Mercury Vapor Lamp – Wikipedia
Metal Halide Lamp – Wikipedia