There have been many types of lamps invented over the years, we will start with the earliest and work progressively to the present day technologies.
Incandescent Light Bulb
The incandescent light bulb, incandescent lamp or incandescent light globe produces light by heating a filament wire to a high temperature until it glows. The hot filament is protected from oxidation in the air with a glass enclosure that is filled with inert gas or evacuated. In a halogen lamp, filament evaporation is prevented by a chemical process that redeposits metal vapor onto the filament, extending its life. The light bulb is supplied with electrical current by feed-through terminals or wires embedded in the glass. Most bulbs are used in a socket which provides mechanical support and electrical connections.
Incandescent bulbs are manufactured in a wide range of sizes, light output, and voltage ratings, from 1.5 volts to about 300 volts. They require no external regulating equipment, have low manufacturing costs, and work equally well on either alternating current or direct current. As a result, the incandescent lamp is widely used in household and commercial lighting, for portable lighting such as table lamps, car headlamps, and flashlights, and for decorative and advertising lighting.
Incandescent bulbs are less efficient than several other modern types of light bulbs, with most varieties converting less than 10% of the energy they use into visible light (with the remaining energy being converted into heat). Some applications of the incandescent bulb deliberately use the heat generated by the filament. Such applications include incubators, brooding boxes for poultry, heat lights for reptile tanks, infrared heating for industrial heating and drying processes, and the Easy-Bake Oven toy. But waste heat can also significantly increase the energy required by a building's air conditioning system.
Incandescent light bulbs are gradually being replaced in many applications by other types of electric lights, such as fluorescent lamps, compact fluorescent lamps (CFL), cold cathode fluorescent lamps (CCFL), high-intensity discharge lamps, and light-emitting diodes (LEDs). These newer technologies improve the ratio of visible light to heat generation. Some jurisdictions, such as the European Union, are in the process of phasing out the use of incandescent light bulbs in favor of more energy-efficient lighting.
Metal Halide Lamps
A metal-halide lamp is an electric light that produces light by an electric arc through a gaseous mixture of vaporized mercury and metal halides (compounds of metals with bromine or iodine). They are members of the high-intensity discharge (HID) family of gas discharge lamps. Developed in the 1960s, they are similar to mercury vapor lamps, but contain additional metal compounds in the arc tube, which improve the efficiency and color rendition (whiteness) of the light.
Metal-halide lamps have high luminous efficacy of around 60 - 75 lumens per watt, about twice the efficiency of mercury vapor lights and 3 to 5 times that of incandescent lights, moderate bulb life (6,000 to 15,000 hours, a little shorter than mercury lamps) and produce an intense white light. As one of the most efficient sources of high CRI white light, metal halides are the fastest growing segment of the lighting industry. They are used for wide area overhead lighting of commercial, industrial, and public spaces, such as parking lots, sports arenas, factories, and retail stores, as well as residential security lighting and automotive lighting. However, they are nothing like as efficient as the latest LED technology.
The lamps consist of a small fused quartz or ceramic arc tube which contains the gases and the arc, enclosed inside a larger glass bulb which has a coating to filter out the ultraviolet light produced. Like other HID lamps, they operate under high pressure (4 to 20 atmospheres) and require special fixtures to operate safely, as well as an electrical ballast. They also require a warm-up period of several minutes to reach full light output, so they are not typically used for residential room lighting, which is turned off and on frequently.
Dangers Using Metal Halide Lamps
At the end of life, metal-halide lamps exhibit a phenomenon known as cycling. These lamps can be started at a relatively low voltage but as they heat up during operation, the internal gas pressure within the arc tube rises and more and more voltage is required to maintain the arc discharge. As a lamp gets older, the maintaining voltage for 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. This causes the lamp to glow for a while and then goes out, repeatedly. In rare occurrences the lamp explodes at the end of its useful life.
Modern electronic ballast designs detect cycling and give up attempting to start the lamp after a few cycles. If power is removed and reapplied, the ballast will make a new series of startup attempts.
Risk of lamp explosion
All HID arc tubes deteriorate in strength over their lifetime because of various factors, such as chemical attack, thermal stress and mechanical vibration. As the lamp ages the arc tube becomes discoloured, absorbing light and getting hotter. The tube will continue to become weaker until it eventually fails, causing the breakup of the tube.
Although such failure is associated with end of life, an arc tube can fail at any time even when new, because of unseen manufacturing faults such as microscopic cracks. However, this is quite rare. Manufacturers typically "season" new lamps to check for manufacturing defects before the lamps leave the manufacturer's premises.
Since a metal-halide lamp contains gases at a significant high pressure, failure of the arc tube is inevitably a violent event. Fragments of arc tube are launched, at high velocity, in all directions, striking the outer bulb of the lamp with enough force to cause it to break. If the fixture has no secondary containment (e.g. a lens, bowl or shield) then the extremely hot pieces of debris will fall down onto people and property below the light, likely resulting in serious injury, damage, and possibly causing a major building fire if flammable material is present.
The risk of a "nonpassive failure" of an arc tube is very small. According to information gathered by the National Electrical Manufacturers Association (www.nema.org), there are approximately 40 million metal-halide systems in North America alone, and only a very few instances of nonpassive failures have occurred. Although it is not possible to predict, or eliminate the risk, of a metal-halide lamp exploding, there are several precautions that can be taken to reduce the risk:
Using only well designed lamps from reputable manufacturers and avoiding lamps of unknown origin. Inspecting lamps before installing to check for any faults such as cracks in the tube or outer bulb. Replacing lamps before they reach their end of life (i.e. when they have been burning for the number of hours that the manufacturer has stated as the lamp's rated life). For continuously operating lamps, allowing a 15 minute shutdown for every 7 days of continuous operation. Relamp fixtures as a group. Spot relamping is not recommended. Also, there are measures that can be taken to reduce the damage caused should a lamp fail violently:
Ensuring that the fixture includes a piece of strengthened glass or polymeric materials between the lamp and the area it is illuminating. This could be incorporated into the bowl or lens assembly of the fixture. Using lamps that have a reinforced glass shield around the arc tube to absorb the impact of flying arc tube debris, preventing it from shattering the outer bulb. Such lamps are safe to use in 'open' fixtures. These lamps carry an "O" designation on the packaging reflective of American National Standards Institute (ANSI) standards.
Mercury Vapor Lamp
A mercury-vapor lamp is a gas discharge lamp that uses an electric arc through vaporized mercury to produce light. The arc discharge is generally confined to a small fused quartz arc tube mounted within a larger borosilicate glass bulb. The outer bulb may be clear or coated with a phosphor; in either case, the outer bulb provides thermal insulation, protection from the ultraviolet radiation the light produces, and a convenient mounting for the fused quartz arc tube.
Mercury vapor lamps (and their relatives) are more energy efficient than incandescent and most fluorescent lights, with luminous efficacies of 35 to 65 lumens/watt. Their other advantages are a long bulb lifetime in the range of 24,000 hours and a high intensity, clear white light output. For these reasons, they are used for large area overhead lighting, such as in factories, warehouses, and sports arenas as well as for streetlights. Clear mercury lamps produce white light with a bluish-green tint due to mercury's combination of spectral lines. This is not flattering to human skin color, so such lamps are typically not used in retail stores. "Color corrected" mercury bulbs overcome this problem with a phosphor on the inside of the outer bulb that emits white light. They offer better color rendition than the more efficient high or low-pressure sodium vapor lamps.
Sodium Vapour Lamps
A sodium vapor lamp is a gas discharge lamp that uses sodium in an excited state to produce light. There are two varieties of such lamps: low pressure and high pressure. Low-pressure sodium lamps are the most efficient electrical light sources, but their yellow light restricts applications to outdoor lighting such as street lamps. High-pressure sodium lamps have a broader spectrum of light but poorer color rendering than other types.
Because sodium vapor lamps cause less light pollution than mercury-vapor lamps, many cities that have large astronomical observatories employ them.
Low Pressure Sodium Lamps
Low-pressure sodium (LPS) lamps have a borosilicate glass gas discharge tube (arc tube) containing solid sodium, a small amount of neon, and argon gas in a Penning mixture to start the gas discharge. The discharge tube may be linear (SLI lamp) or U-shaped. When the lamp is turned on it emits a dim red/pink light to warm the sodium metal and within a few minutes it turns into the common bright yellow as the sodium metal vaporizes. These lamps produce a virtually monochromatic light averaging a 589.3 nm wavelength (actually two dominant spectral lines very close together at 589.0 and 589.6 nm). As a result, the colors of illuminated objects are not easily distinguished because they are seen almost entirely by their reflection of this narrow bandwidth yellow light.
LPS lamps have an outer glass vacuum envelope around the inner discharge tube for thermal insulation, which improves their efficiency. Earlier types of LPS lamps had a detachable dewar jacket (SO lamps). Lamps with a permanent vacuum envelope (SOI lamps) were developed to improve thermal insulation. Further improvement was attained by coating the glass envelope with an infrared reflecting layer of indium tin oxide, resulting in SOX lamps.
LPS lamps are the most efficient electrically powered light source when measured for photopic lighting conditions�up to 200 lm/W, primarily because the output is light at a wavelength near the peak sensitivity of the human eye. As a result they are widely used for outdoor lighting such as street lights and security lighting where faithful color rendition was once considered unimportant. Recently, however, it has been found that under mesopic conditions typical of nighttime driving, whiter light can provide better results at lower power.
LPS lamps are more closely related to fluorescent than high intensity discharge lamps because they have a low�pressure, low�intensity discharge source and a linear lamp shape. Also like fluorescents they do not exhibit a bright arc as do other HID lamps; rather they emit a softer luminous glow, resulting in less glare. Unlike HID lamps, which can go out during a voltage dip, low pressure sodium lamps restrike to full brightness rapidly. LPS lamps are available with power ratings from 10 W up to 180 W; however, longer bulb lengths create design and engineering problems.
Another unique property of LPS lamps is that, unlike other lamp types, they do not decline in lumen output with age. As an example, mercury vapor HID lamps become very dull towards the end of their lives, to the point of being ineffective, while continuing to consume full rated electrical use. LPS lamps, however, do increase energy usage slightly (about 10%) towards their end of life, which is generally around 18,000 hours for modern lamps.
Light Pollution Considerations
For placements where light pollution is of prime importance, such as an astronomical observatory parking lot, or a large city nearby an astronomical observatory, low pressure sodium is preferred. Such lamps emit light on just one dominant spectral line (with other far weaker lines), and therefore is the easiest to filter out. One consequence of widespread public lighting is that on cloudy nights, cities with enough lighting are illuminated by light reflected off the clouds. As sodium vapor lights are often the source of urban illumination, this turns the sky a tinge of orange. If the sky is clear or hazy, the light will radiate over large distances, causing large enough cities to be recognizable by an orange glow when viewed from outside the city.
High Pressure Sodium
High-pressure sodium (HPS) lamps are smaller and contain additional elements such as mercury, and produce a dark pink glow when first struck, and an intense pinkish orange light when warmed. Some bulbs also briefly produce a pure to bluish white light in between if the mercury achieves its high pressure arc discharge characteristic before the sodium is completely warmed. The sodium D-line is the main source of light from the HPS lamp, and it is extremely pressure broadened by the high sodium pressures in the lamp. On account of this broadening and the emissions from mercury, more colors can be distinguished compared to a low-pressure sodium lamp. This leads them to be used in areas where improved color rendering is important, or desired. Thus, its new model name SON is the variant for "sun" (a name used primarily in Europe and the UK). HPS Lamps are favored by indoor gardeners for general growing because of the wide color-temperature spectrum produced and the relatively efficient cost of running the lights.
High pressure sodium lamps are quite efficient�about 100 lm/W�when measured for photopic lighting conditions. The higher powered versions of 600 W have an efficacy of even 150lm/W. They have been widely used for outdoor area lighting such as streetlights and security. Understanding the change in human color vision sensitivity from photopic to mesopic and scotopic is essential for proper planning when designing lighting for roads.
Because of the extremely high chemical activity of the high pressure sodium arc, the arc tube is typically made of translucent aluminum oxide. This construction led General Electric to use the tradename "Lucalox" for their line of high-pressure sodium lamps.
Xenon at a low pressure is used as a "starter gas" in the HPS lamp. It has the lowest thermal conductivity and lowest ionization potential of all the non-radioactive noble gases. As a noble gas, it does not interfere with the chemical reactions occurring in the operating lamp. The low thermal conductivity minimizes thermal losses in the lamp while in the operating state, and the low ionization potential causes the breakdown voltage of the gas to be relatively low in the cold state, which allows the lamp to be easily started.
End of life
At the end of life, high-pressure sodium lamps exhibit a phenomenon known as cycling, which is caused by a loss of sodium in the arc. Sodium is a highly reactive element and is easily lost by reacting with the arc tube, made of aluminum oxide. The products are sodium oxide and aluminum:
6 Na + Al2O3 ? 3 Na2O + 2 Al
As a result these lamps can be started at a relatively low voltage, but as they heat up during operation, the internal gas pressure within the arc tube rises, and more and more voltage is required to maintain the arc discharge. As a lamp gets older, the maintaining voltage for the arc eventually rises to exceed the maximum voltage output 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, typically starting at a pure or bluish white then moving to a red-orange before going out.
More sophisticated ballast designs detect cycling and give up attempting to start the lamp after a few cycles, as the repeated high-voltage ignitions needed to restart the arc reduce the lifetime of the ballast. If power is removed and reapplied, the ballast will make a new series of startup attempts.
LPS lamp failure does not result in cycling; rather, the lamp will simply not strike or will maintain its dull red glow exhibited during the start-up phase.