Gas Discharge Lamps
Gas-discharge lamps are a family of artificial light sources that generate light by sending an electrical discharge through an ionized gas. Typically, such lamps use a noble gas (argon, neon, krypton and xenon) or a mixture of these gases. Most lamps are filled with additional materials, like mercury, sodium, and/or metal halides. In operation the gas is ionized, and free electrons, accelerated by the electrical field in the tube, collide with gas and metal atoms. Some electrons in the atomic orbitals of these atoms are excited by these collisions to a higher energy state. When the excited atom falls back to a lower energy state, it emits a photon of a characteristic energy, resulting in infrared, visible light, or ultraviolet radiation.
Gas-discharge lamps offer long life and high efficiency, but are more complicated to manufacture, and they require electronics to provide the correct current flow through the gas.
There are three groups of gas discharge lamp, namely:
- Low pressure discharge
- High pressure discharge
- High intensity discharge
Low Pressure Discharge
Low-pressure lamps have working pressure much less than atmospheric pressure. They include:
- Fluorescent lamps, we have discussed in detail in the previous section.
- Low pressure sodium lamps, the most efficient gas-discharge lamp type, producing up to 200 lumens per watt, but at the expense of very poor color rendering. The almost monochromatic yellow light is only acceptable for street lighting and similar applications. Low pressure sodium lamps also have long life span.
High Pressure Discharge
High-pressure lamps operate under slightly less to greater than atmospheric pressure. They include:
- Metal halide lamps.
- High pressure sodium lamps
- High pressure mercury-vapor lamps.
The above three lamps are also well known as High Intensity Discharge (HID) lamps. The definition of high intensity discharge is related to the special type of electrode used in the lamps. Compared to other lamp types, relatively high arc power exists for the arc length.
High Intensity Discharge
High-intensity discharge (HID) lamps in general are very energy efficient and tend to have the longest service life of any lighting type. They can save 75%–90% of lighting energy when they replace incandescent lamps.
HID lamps use an electric arc (relatively high arc power for the arc length as compared to other lamp types) to produce intense light. Like fluorescent lamps, they require ballasts. They also take up to ten minutes to produce light when first turned on, because the ballast needs time to establish the electric arc. Because of the intense light they produce at a high efficacy, HID lamps are commonly used for outdoor lighting and in large indoor arenas. Since the lamps take awhile to establish, they are most suitable for applications where they stay on for hours at a time. They are not suitable for use with motion detectors. These are the three most common types of HID lamps:
- High pressure mercury vapor lamps
- Metal halide lamps
- High-pressure sodium lamps
We will explain each one in detail in the next section.
High Pressure Mercury Vapor Lamps
High pressure mercury vapor lamps (often called as 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.
Significant energy savings are also possible by replacing old mercury vapor lamps with newer high-pressure sodium 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.
Some Metal Halide Applications
Metal halide lamps have shorter lifetimes (5,000–20,000 hours) compared to both mercury vapor and high-pressure sodium lamps.
High-Pressure Sodium Lamps
High-pressure sodium lighting—a type of high-intensity discharge lighting—is becoming the most common type of outdoor lighting. High-pressure sodium lamps have an efficacy of 50–140 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).
Disposal of HID lamps
It is also important to note that all high intensity discharge (HID) lamps, which include high pressure mercury vapor bulbs, metal halide and high-pressure sodium bulbs, are mercury-containing lightbulbs and should be recycled.
The chart below compares various types of high-intensity discharge lamps.
|High-Intensity Discharge Lighting Type||Efficacy
|Color Rendition Index
|Mercury vapor||25–60||16,000–24,000||50 (poor to fair)||3200–7000 (warm to cold)||Outdoors|
|Metal halide||70–115||5000–20,000||70 (fair)||3700 (cold)||Indoors/outdoors|
|High-pressure sodium||50–140||16,000–24,000||25 (poor)||2100 (warm)||Outdoors|
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