The Various Forms of High Intensity Discharge Lighting|
Article- November 2012 By Larson Electronics.com
Larson Electronics 1000 Watt Metal Halide Flood Light
HID lamps are one of the most powerful types of luminary available. Their high intensity makes them a favorite choice for applications where a large amount of area must be illuminated, but not all HID lighting systems are them same, which can cause some problems unless fully understood.
HID lamps produce light by using a high voltage electrical arc to ignite gases and metallic salts within a glass bulb. Once this ignition takes place a prodigious amount of light is produced in a relatively small area. The initial ignition voltage required to produce ignition is quite high, however, once ignition takes place it is only necessary to maintain a much lower voltage to sustain it. The types of gases and salts used in the lamp have a direct bearing on the color and quality of the light produced, which has resulted in a few main types of HID lighting which have achieved prominence over the years.
The wide variances in voltages required to operate an HID lamp necessitates the use of a ballast to manage these voltages. These ballasts provide the high initial voltage spike necessary to cause ignition, then provide the much lower current needed for constant operation. Older style magnetic ballasts were effective, but reduced performance due to their slow warmup times and tendency to wear out fairly quickly. Newer electronic ballasts have greatly reduced the amount of time it takes an HID unit to reach full power and provide a much longer operational life.
Here we will go over these various types and touch on their different advantages and drawbacks.
The oldest and once most common form of HID lighting is the mercury vapor lamp. As their name implies, these lamps rely on mercury vapor to produce their light. The mercury in these lamps is a liquid at normal temperatures, and becomes vaporized at higher temperatures. These lamps also contain a small amount of argon gas which helps to facilitate the ignition process. When current is applied to the mercury vapor lamp, the argon gas is ionized, creating an electrical arc between electrodes contained within the lamp. This arc heats the mercury, which in turn vaporizes and causes another arc between a second primary of electrodes, then creating the high intensity output associated with these lamps. This process takes several minutes, which means these lamps are slow starting and cannot be quickly turned off and on; they require a cool down period between on and off cycles. The light produced by these lamps is bluish in color and renders colors poorly, making these lamps more effective in applications such as street or parking lot lighting. Mercury vapor lamps are fairly efficient, producing about 60 lumens per watt, but also contain toxic mercury making them more difficult to dispose of. As of 2008 the United States government has banned the production of mercury vapor lamps, as a result, expect to see these lamps become obsolete in the very near future.
High pressure sodium lamps produce light that orange/yellow in color. Similar in operation to other HID lamps, HPS lamps contain xenon gas along with a mixture of mercury and sodium which is ignited to produce light. HPS lamps are extremely efficient, producing around 150 lumens per watt, however, their color quality is so poor that their use is limited to applications such as street lighting and exterior building and parking lot illumination. Areas illuminated by HPS take on a reddish orange tinted appearance, and contrasting and object details are poorly rendered as well. HPS lamps tend to lose a lot of efficiency as they age, and problems with flickering and erratic on/off operation occur as well. As with mercury vapor lamps, HPS lamps contain toxic mercury, increasing the hazards of disposal.
Low pressure sodium lamps are the most efficient form of lighting available producing approximately 200 lumens per watt. LPS lamps produce light over a very short part of the light spectrum in the reddish orange range. They have the worst color rendering and contrasting properties, and distinguishing colors such as red, orange yellow and white under the illumination from an LPS lamp can oftentimes be nearly impossible. These lamps contain sodium, neon and argon gases and ignition and operations the same as with other HID lamps. Just as with MV and HPS lamps, LPS lamps are slow to warm up and cannot be quickly shit off and restarted. Their extremely poor color rendering drastically limits their practical use, and most applications see them utilized as street or parking lot illumination.
Metal halide lamps fall into a practical middle ground among HID lamps. They provide very good efficiency, around 115 lumens per watt, produce high intensity light output, and the quality of the light produced is much better than other forms of HID lamps. Metal halide lamps contain mercury vapor, and “metal halides”, more commonly known as salts, such as sodium-iodide. They operate similarly to other HID lamps, but have a shorter warmup period, and in newer and more advanced MH systems a fast restrike is possible, allowing the lamps to recover quickly from shut down without the long cooling off and warm up period common to other HID lamps. The light produced by MH lamps is much whiter, bordering on bluish, and is much more effective in correctly reproducing colors and detail contrasts. Metal halide is by far the most practical form of HID lighting for general illumination and can be commonly seen illuminating stadiums, warehouses, factories and other locations where large areas must be illuminated and good color quality required.
Of all the drawbacks to HID lighting, the mercury they contain is probably the most significant. HID lamps require proper disposal procedures when they reach the end of their useful life, and larger fixtures can pose a contamination problem in more confined areas should the lamp shatter or break. Despite this and the various shortcomings of HID lighting, it remains one of the overall most effective forms of lighting available.