Today's Specials

Reading Lamps
Many high intensity reading lamps use halogen bulbs. The light can be pointed directly at the reading material without disturbing others.
Decorator Lighting
Some chandeliers use halogen bulbs for a clean and crystal-like appearance. Halogen bulbs also come in colors for special decorator lighting effects.
Spotlights
Halogen bulbs are used in spotlights for stage performances including plays and concerts. The intense light can easily be directed.Security Lighting
Halogen bulbs produce bright lights to illuminate large areas for security purposes as bright light can discourage thieves and burglars.
Photography
Halogen Bulbs are used for photography and video production. The intense light helps reproduce colors more effectively.

Halogen bulbs are incandescent lamps with construction and operation roughly similar to the classical light bulbs. They are generally much smaller than their 'cousins', have increased lifetimes and are able to produce high light outputs with elevated color temperatures, as opposed to all other types of incandescent lamps. Halogen bulbs are basically made of a tungsten filament encased inside a transparent envelope. The preferred material for the envelope is generally quartz, since glass is unable to sustain the temperature at which the filament operates (up to 2,500 degrees Celsius), while the inert atmosphere is made of a mix of argon and nitrogen, and a small amount of a halogen element, such as iodine or bromine.

Typical incandescent light bulbs have relatively short lifetimes, lasting for only 750 to 1,000 hours of normal use. This is mostly because the high temperature of the filament allows tungsten vapor to escape in the inert atmosphere of the device. Over time, this process will eventually lead to a thinning of the filament, until it will no longer be able to support the electric current being passed through it and it will stop working.

Additionally, incandescent light bulbs are extremely inefficient, converting less than 2.5 percent of the electric energy into light, while the rest is lost through heat. Another common problem with classical light bulbs is that they have low color temperatures. 

Halogen bulbs, through their special build, are able to bring several improvements in these three issues. Halogen bukbs are known to combine with tungsten if the temperature of the medium is sufficiently high. Through this property the generated tungsten vapor is recycled back to the filament rather than simply being deposited on the inner walls of the envelope.

Secondly, by running the filament at a much higher temperature, halogen bulbs are able to release more light, reaching efficiency rates as high as 3.5 percent. At the same time the color temperature of the emitted light is increased, getting closer to white, as opposed to the yellowish-white light generated by classical incandescent light bulbs.

MR16 halogen bulbs will be familiar to some readers as a reflector lamp originally used in slide projectors. Its small size, durable construction, and wide range of intensities and beam spreads have since made MR16 halogen bulbs a useful tool in lighting design as well. MR16 halogen bulbs now are commonly used for highlighting display objects (e.g., in museums, galleries, and retail spaces) and for accenting architectural and landscape features.
The Great Internet Light Bulb Book, Part IIncandescent including halogen light bulbs Copyright (C) 1996, 2000, 2005, 2006 Donald L. Klipstein (Jr) (don@donklipstein.com) Freely distributing copies of this entire document or un-HTML-ized text versions thereof is permitted and encouraged. History Basic principles Luminous efficiency Vacuum vs. gas-filled bulbs How bulbs burn out Why bulbs often burn out when you turn them on Why burnout is sometimes so spectacular How bad a current surge bulbs draw when turned on Making bulbs last longer Long-life bulbs Reduced power Soft start devices DC vs. AC operation Why making bulbs last longer often does not pay How to minimize lighting costs Halogen bulbs The halogen cycle Lifetime and efficiency of halogen bulbs Halogen bulb failure modes Use of halogen bulbs with dimmers Ultraviolet from halogen bulbs History of Incandescent Bulbs It is widely regarded that Thomas Alva Edison invented the first reasonably practical incandescent lamp, using a carbon filament in a bulb containing a vacuum. Edison's first successful test occurred in 1879. There were earlier incandescent lamps, such as one by Heinrich Goebel made with a carbon filament in 1854. This incandescent lamp had a carbonized bamboo filament and was mentioned as lasting up to 400 hours. At least some sources regard Goebel as the inventor of the incandescent lamp. Joseph Wilson Swan began trying to make carbon-based incandescent lamps in 1850 and made one in 1860 that was workable except for excessively short life due to poor vacuum. He made more successful inccandescent lamps after better vacuum pumps became available in the mid 1870's. Since that time, the incandescent lamp has been improved by using tantalum and later tungsten filaments, which evaporate more slowly than carbon. Nowadays, incandescent lamps are still made with tungsten filaments. Basic Principles The filament of an incandescent lamp is simply a resistor. If electrical power is applied, it is converted to heat in the filament. The filament's temperature rises until it gets rid of heat at the same rate that heat is being generated in the filament. Ideally, the filament gets rid of heat only by radiating it away, although a small amount of heat energy is also removed from the filament by thermal conduction. The filament's temperature is very high, generally over 2000 degrees Celsius, or generally over 3600 degrees Fahrenheit. In a "standard" 75 or 100 watt 120 volt bulb, the filament temperature is roughly 2550 degrees Celsius, or roughly 4600 degrees Fahrenheit. At high temperatures like this, the thermal radiation from the filament includes a significant amount of visible light. For more info on incandescent lamps, try these pages at Bulbs.com: Incandescent Lamp Workings The incandescent lamp top page at "Lightbulb University" at Bulbs.com. Luminous Efficiency In a 120 volt, 100 watt "standard" bulb with a rated light output of 1750 lumens, the efficiency is 17.5 lumens per watt. This compares poorly to an "ideal" of 242.5 lumens per watt for one idealized type of white light, or 683 lumens per watt ideally for the yellowish-green wavelength of light that the human eye is most sensitive to. Other types of incandescent light bulbs have different efficiencies, but all generally have efficiencies near or below 35 lumens per watt. Most household incandescent bulbs have efficiencies from 8 to 21 lumens per watt. Higher efficiencies near 35 lumens per watt are only achieved with photographic and projection lamps with very high filament temperatures and short lifetimes of a few hours to around 40 hours. The reason for this poor efficiency is the fact that tungsten filaments radiate mostly infrared radiation at any temperature that they can withstand. An ideal thermal radiator produces visible light most efficiently at temperatures around 6300 Celsius (6600 Kelvin or 11,500 degrees Fahrenheit). Even at this high temperature, a lot of the radiation is either infrared or ultraviolet, and the theoretical luminous efficiency is 95 lumens per watt. Of course, nothing known to any humans is solid and usable as a light bulb filament at temperatures anywhere close to this. The surface of the sun is not quite that hot. There are other ways to efficiently radiate thermal radiation using higher temperatures and/or substances that radiate better at visible wavelengths than invisible ones. This is covered by Part II of the Great Internet Light Bulb Book, Discharge Lamps. The efficiency of an incandescent bulb can be increased by increasing the filament temperature, which makes it burn out more quickly. Vacuum vs. gas-filled bulbs At first, incandescent bulbs were made with a vacuum inside them. Air oxidizes the filament at high temperatures. Later, it was discovered that filling the bulb with an inert gas such as argon or an argon-nitrogen mixture slows down evaporation of the filament. Tungsten atoms evaporating from the filament can be bounced back to the filament by gas atoms. The filament can be operated at a higher temperature with a fill gas than with a vacuum. This results in more efficient radiation of visible light. So why are some bulbs still made with a vacuum? The reason is that a fill gas conducts heat away from the filament. This conducted heat is energy that cannot be radiated by the filament and is lost, or wasted. This mechanism reduces the bulb's efficiency of producing radiation. If this is not offset by the advantage of operating the filament at a higher temperature, then the bulb is more efficient with a vacuum. One property of thermal conduction from the filament to the gas is the strange fact that the amount of heat conducted is roughly proportional to the filament's length, but does not vary much with the filament's diameter. The reason this occurs is beyond the scope of this document. However, this means that bulbs with thin filaments and lower currents are more efficient with a vacuum, and higher current bulbs with thicker filaments are more efficient with a fill gas. The break-even point seems to be very roughly around 6-10 watts per centimeter of filament. (This can vary with filament temperature and other factors. The break-even point may be higher in larger bulbs where convection may increase heat removal from the filament by the gas.) Sometimes, premium fill gases such as krypton or xenon are used. These gases have larger atoms that are better at bouncing evaporated tungsten atoms back to the filament. These gases also conduct heat less than argon. Of these two gases, xenon is better, but more expensive. Either of these gases will significantly improve the life of the bulb, or result in some improvement in efficiency, or both. Often, the cost of these gases makes it uneconomical to use them. How light bulbs burn out Due to the high temperature that a tungsten filament is operated at, some of the tungsten evaporates during use. Furthermore, since no light bulb is perfect, the filament does not evaporate evenly. Some spots will suffer greater evaporation and become thinner than the rest of the filament. These thin spots cause problems. Their electrical resistance is greater than that of average parts of the filament. Since the current is equal in all parts of the filament, more heat is generated where the filament is thinner. The thin parts also have less surface area to radiate heat away with. This "double whammy" causes the thin spots to have a higher temperature. Now that the thin spots are hotter, they evaporate more quickly. It becomes apparent that as soon as a part of the filament becomes significantly thinner than the rest of it, this situation compounds itself at increasing speed until a thin part of the filament either melts or becomes weak and breaks. Why bulbs often burn out when you turn them on Many people wonder what goes on when you turn on a light. It is often annoying that a weak, aging light bulb will not burn out until the next time you turn it on. The answer here is with those thin spots in the filament. Since they have less mass than the less-evaporated parts of the filament, they heat up more quickly. Part of the problem is the fact that tungsten, like most metals, has less resistance when it is cool and more resistance when it is hot. This explains the current surge that light bulbs draw when they are first turned on. When the thin spots have reached the temperature that they would be running at, the thicker, heavier parts of the filament have not yet reached their final temperature. This means that the filament's resistance is still a bit low and excessive current is still flowing. This causes the thinner parts of the filament to get even hotter while the rest of the filament is still warming up. This means that the thin spots, which run too hot anyway, get even hotter when the thicker parts of the filament have not yet fully warmed up. This is why weak, aging bulbs can't survive being turned on. Why burnout is sometimes so spectacular When the filament breaks, an arc sometimes forms. Since the current flowing through the arc is also flowing through the filament at this time, there is a voltage gradient across the two pieces of the filament. This voltage gradient often causes this arc to expand until it is across the entire filament. Now, consider a slightly nasty characteristic of most electric arcs. If you increase the current going through an arc, it gets hotter, which makes it more conductive. Obviously, this could make things a bit unstable, since the more conductive arc would draw even more current. The arc easily becomes conductive enough that it draws a few hundred amps of current. At this point, the arc often melts the parts of the filament that the ends of the arc are on, and the arc glows with a very bright light blue flash. Most household light bulbs have a built-in fuse, consisting of a thin region in one of the internal wires. The extreme current drawn by a burnout arc often blows this built-in fuse. If not for this fuse, people would frequently suffer blown fuses or tripped circuit breakers from light bulbs burning out. Although the light bulb's internal fuse will generally protect household fuses and circuit breakers, it may fail to protect the more delicate electronics often found in light dimmers and electronic switching devices from the current surges drawn by "burnout arcs". How bad a current surge bulbs draw when turned on It is fairly well known that a cold light bulb filament has less resistance than a hot one. Therefore, a light bulb draws excessive current until the filament warms up. Since the filament can draw more than ten times as much current as usual when it is cold, some people are concerned about excessive energy consumption from turning on light bulbs. The degree of this phenomenon has become a matter of urban folklore. However, the filament warms up very rapidly. The amount of energy consumed to warm up a cold filament is less than it would consume in one second of normal operation. Making bulbs last longer Long-life bulbs Many light bulbs are made to operate with a slightly lower filament temperature than usual. This makes the bulbs last much longer with a slight reduction of efficiency. Reduced Power Reducing the voltage applied to a light bulb will reduce the filament temperature, resulting in a dramatic increase in life expectancy. One device sold to do this is an ordinary silicon diode built into a cap that is made to stick to the base of a light bulb. A diode lets current through in only one direction, causing the bulb to get power only 50 percent of the time if it is operated on AC. This effectively reduces the applied voltage by about 30 percent. (Reducing the voltage to its original value times the square root of .5 results in the same power consumption as applying full voltage half the time.) The life expectancy is increased very dramatically. However, the power consumption is reduced by about 40 percent (not 50 since the cooler filament has less resistance) and light output is reduced by reduced by about 70 percent (cooler filaments are less efficient at radiating visible light). Soft-start devices Since bulbs usually burn out during the current surge that occurs when they are turned on, one would expect that eliminating the surge would save light bulbs. In fact, such devices are available. Like the diode-based ones, they are available in a form that is built into caps that one could stick onto the tip of the base of a light bulb. These devices are "negative temperature coefficient thermistors", which are resistors having a resistance that decrease when they heat up. When the bulb is first started, the thermistor is cool and has a moderately high resistance that limits current flowing through the bulb. The current flowing through the thermistor's resistance generates heat, and the thermistor's resistance decreases. This allows the current to increase in a fairly gradual manner, and the filament warms up in a uniform manner. However, this extends the life of the bulbs less than one might think. If the filament has thin spots that cannot survive the current surge that occurs when the bulb is turned on, then the filament is already in very bad shape. At this time, the thin spots are significantly hotter than the thicker parts of the filament and are evaporating rather rapidly. As described earlier, this process is accelerating. If the thin spots are protected from surges, the life of the bulb would be extended by only a few percent. Additional life extension occurs only because the thermistor keeps enough resistance to result in enough heat to keep it fairly conductive. This resistance slightly reduces power to the bulb, extending its life somewhat and making it slightly dimmer. DC vs. AC operation As tungsten atoms evaporate from the filament, a very small percentage of them are ionized by the small amounts of short-wave ultraviolet light being radiated by the filament, the electric field around the filament, or by free electrons that escape from the filament by thermionic emission. These tungsten ions are positively charged, and tend to leave the positive end of the filament and are attracted to the negative end of the filament. The result is that light bulbs operated on DC have this specific mechanism that would cause uneven filament evaporation. This mechanism is generally not significant, although it has been reported that light bulbs sometimes have a slight, measurable decrease in lifetime from DC operation as opposed to AC operation. In a few cases, AC operation may shorten the life of the bulb, but this is rare. In rare cases, AC may cause the filament to vibrate enough to significantly shorten its life. In a few other rare cases involving very thin filaments, the filament temperature varies significantly throughout each AC cycle, and the peak filament temperature is significantly higher than the average filament temperature. Ordinarily, one should expect a light bulb's life expectancy to be roughly equal for DC and AC. Why making bulbs last longer often does not pay You may have heard that the life expectancy of a light bulb is roughly inversely proportional to the 12th or 13th power of the applied voltage. And that power consumption is roughly proportional to voltage to the 1.4 to 1.55 power, and that light output is roughly proportional to the 3.1 to 3.4 power of applied voltage. This would make the luminous efficiency roughly proportional to applied voltage to the 1.55 to 2nd power of applied voltage. Now, if a slight reduction in applied voltage results in a slight to moderate loss of efficiency and a major increase in lifetime, how could this cost you more? The answer is in the fact that the electricity consumed by a typical household bulb during its life usually costs many times more than the bulb does. Bulbs are so cheap compared to the electricity consumed by them during their lifetime that it pays to make them more efficient by having the filaments run hot enough to burn out after only several hundred to about a thousand hours or so. Here is an example with actual numbers (using U.S. dollars, in 1996): Suppose you have 10 "standard" 100 watt 120 volt bulbs with a rated lifetime of 750 hours. Such bulbs typically cost around 75 cents in the U.S. The electricity used by all ten of these bulbs is 1 kilowatt, which would typically cost about 9 cents per hour (approximate U.S. average). Over 750 hours, this would cost (on an average) $67.50 for the electricity plus $7.50 for 10 bulbs, or $75. Now, suppose you use these bulbs with 110 volts instead of 120. These bulbs would consume about 87.8 watts instead of 100. However, they would only produce 76 percent of their normal light output (and this is a slightly optimistic figure). To restore the original light output, you need 13 of these bulbs. (And this will fall very slightly short.) Using 13 bulbs that consume 87.8 watts apiece results in a power consumption of 1141 watts. Over 750 hours at 9 cents per KWH, this would cost $77. This is more than the $75 cost of running 10 bulbs at full voltage even if the bulbs never burn out at 110 volts. At 110 volts instead of 120, the life expectancy of the bulbs may be tripled. One third of 13 times 75 cents is about $3.25, which adds to the $77 cost of electricity to result in an average total cost of $80.25 for 750 hours. This example should explain why you often get the most light for the least money using standard bulbs rather than longer-lasting ones. How to minimize lighting costs Higher wattage bulbs tend to be more efficient than lower wattage ones. One reason for this is the fact that thicker filaments can be operated at a higher temperature, which is better for radiating visible light. Another reason is that since higher wattage bulbs would lead you to use fewer bulbs, you buy fewer bulbs and the cost of bulbs becomes less important. To optimize cost effectiveness in this case of higher wattage light bulbs, the filaments are designed to run even hotter to improve energy efficiency to reduce your electricity costs. Smaller bulbs use less electricity apiece, making the cost of the bulb more important. This is why lower wattage bulbs are often designed to last 1500 to a few thousand hours instead of 750 to 1000 hours. Designing the bulbs to last longer reduces their light output and energy efficiency. To minimize your cost of both electricity and bulbs, you should use as few bulbs as possible, using higher wattage bulbs. To get the same amount of light with lower wattage bulbs, you need both more electricity and more bulbs. An even better way to reduce your lighting costs is to use fluorescent, compact fluorescent, or HID (mercury, metal halide, or sodium) lamps since these are 3 to 5 times as efficient as incandescent lamps. Halogen Bulbs The halogen cycle, What are halogen bulbs? A halogen bulb is an ordinary incandescent bulb, with a few modifications. The fill gas includes traces of a halogen, often but not necessarily iodine. The purpose of this halogen is to return evaporated tungsten to the filament. As tungsten evaporates from the filament, it usually condenses on the inner surface of the bulb. The halogen is chemically reactive, and combines with this tungsten deposit on the glass to produce tungsten halides, which evaporate fairly easily. When the tungsten halide reaches the filament, the intense heat of the filament causes the halide to break down, releasing tungsten back to the filament. This process, known as the halogen cycle, extends the life of the filament somewhat. Problems with uneven filament evaporation and uneven deposition of tungsten onto the filament by the halogen cycle do occur, which limits the ability of the halogen cycle to prolong the life of the bulb. However, the halogen cycle keeps the inner surface of the bulb clean. This lets halogen bulbs stay close to full brightness as they age. In order for the halogen cycle to work, the bulb surface must be very hot, generally over 250 degrees Celsius (482 degrees Fahrenheit). The halogen may not adequately vaporize or fail to adequately react with condensed tungsten if the bulb is too cool. This means that the bulb must be small and made of either quartz or a high-strength, heat-resistant grade of glass known as "hard glass". Since the bulb is small and usually fairly strong, the bulb can be filled with gas to a higher pressure than usual. This slows down the evaporation of the filament. In addition, the small size of the bulb sometimes makes it economical to use premium fill gases such as krypton or xenon instead of the cheaper argon. The higher pressure and better fill gases can extend the life of the bulb and/or permit a higher filament temperature that results in higher efficiency. Any use of premium fill gases also results in less heat being conducted from the filament by the fill gas, meaning more energy leaves the filament by radiation, meaning a slight improvement in efficiency. Lifetime and efficiency of halogen bulbs A halogen bulb is often 10 to 20 percent more efficient than an ordinary incandescent bulb of similar voltage, wattage, and life expectancy. Halogen bulbs may also have two to three times as long a lifetime as ordinary bulbs, sometimes also with an improvement in efficiency of up to 10 percent. How much the lifetime and efficiency are improved depends largely on whether a premium fill gas (usually krypton, sometimes xenon) or argon is used. Halogen Bulb Failure Modes Halogen bulbs usually fail the same way that ordinary incandescent bulbs do, usually from melting or breakage of a thin spot in an aging filament. Thin spots can develop in the filaments of halogen bulbs, since the filaments can evaporate unevenly and the halogen cycle does not redeposit evaporated tungsten in a perfect, even manner nor always in the parts of the filament that have evaporated the most. However, there are additional failure modes. One failure mode is filament notching or necking. Since the ends of the filament are somewhat cool where the filament is attached to the lead wires, the halogen attacks the filament at these points. The thin spots get hotter, which stops the erosion at these points. However, parts of the filament even closer to the endpoints remain cool and suffer continued erosion. This is not so bad during continuous operation, since the thin spots do not overheat. If this process continues long enough, the thin spots can become weak enough to break from the weight of the filament. One major problem with the "necked" ends of the filament is the fact that they heat up more rapidly than the rest of the filament when the bulb is turned on. The "necks" can overheat and melt or break during the current surge that occurs when the bulb is turned on. Using a "soft-start" device prevents overheating of the "necks", improving the bulb's ability to survive "necking". Soft-start devices will not greatly extend the life of any halogen bulbs that fail due to more normal filament "thin spots" that run excessively hot. Some halogen bulbs may usually burn out due to filament end necking, and some others may usually burn out from thin, hot spots forming in the filament due to uneven filament evaporation/recovery. Therefore, some models may have a significantly extended life from "soft-starting" and some other models may not. It is generally not a good idea to touch halogen bulbs, especially the more compact, hotter-running quartz ones. Organic matter and salts are not good for hot quartz. Organic matter such as grease can carbonize, leaving a dark spot that absorbs radiation from the filament and becomes excessively hot. Salts and alkaline materials (such as ash) can sometimes "leach" into hot quartz, which typically weakens the quartz, since alkali and alkaline earth metal ions are slightly mobile in hot glasses and hot quartz. Contaminants may also cause hot quartz to crystalize, weakening it. Any of these mechanisms can cause the bulb to crack or even violently shatter. If a quartz halogen bulb is touched, it should be cleaned with alcohol to remove any traces of grease. Traces of salt will also be removed if the alcohol has some water in it. Since the hotter-running quartz halogen bulbs could possibly violently shatter, they should only be operated in suitable fully enclosed fixtures. Use of Halogen Bulbs with Dimmers Dimming a halogen bulb, like dimming any other incandescent lamp, greatly slows down the formation of thin spots in the filament due to uneven filament evaporation. However, "necking" or "notching" of the ends of the filament remains a problem. If you dim halogen lamps, you may need "soft-start" devices in order to achieve a major increase in bulb life. Another problem with dimming of halogen lamps is the fact that the halogen cycle works best with the bulb and filament at or near specific optimum temperatures. If the bulb is dimmed, the halogen may fail to "clean" the inner surface of the bulb. Or, tungsten halide that results may fail to return tungsten to the filament. Halogen bulbs have sometimes been known to do strange and scary things when greatly dimmed. Halogen bulbs should work normally at voltages as low as 90 percent of what they were designed for. If the bulb is in an enclosure that conserves heat and a "soft-start" device is used, it will probably work well at even lower voltages, such as 80 percent or possibly 70 percent of its rated voltage. However, do not expect a major life extension unless soft-starting is used. Even with soft-starting, do not expect to more than double or possibly triple the life of any halogen bulb already rated to last 2,000 hours or more. Even with soft starting, the life of these bulbs will probably not continue to improve much as voltage is reduced to less than about 90 percent of the bulb's voltage rating. Dimmers can be used as soft-start devices to extend the life of any particular halogen bulbs that usually fail from "necking" of the ends of the filament. The bulb can be warmed up over a period of a couple of seconds to avoid overheating of the "necked" parts of the filament due to the current surge that occurs if full voltage is applied to a cold filament. Once the bulb survives starting, it is operated at full power or whatever power level optimizes the halogen cycle (usually near full power) The dimmer may be both "soft-starting" the bulb and operating it at slightly reduced power, a combination that often improves the life of halogen bulbs. Many dimmers cause some reduction in power to the bulb even when they are set to maximum. (A suggestion from someone who starts expensive medical lamps by turning up a dimmer and reports major success in extending the life of expensive special bulbs from doing this.) Ultraviolet from Halogen Bulbs There is some common concern about the ultraviolet output of halogen bulbs, since they operate at high filament temperatures and the bulbs are made of quartz instead of glass. However, the filament temperature of halogen bulbs rated to last 2,000 hours or more is only slightly greater than that of standard incandescent lamps, and the UV output is only slightly higher. Halogen fixtures typically have a glass or plastic shield to confine any possible bulb explosions, and these shields absorb the small traces of shortwave and mediumwave UV that gets through the quartz bulb. Higher temperature photographic and projection bulbs are different. The much higher filament temperature of shorter life bulbs results in possibly significant hazardous UV. For maximum safety, use these bulbs in fixtures or equipment designed to take these bulbs, and in a manner consistent with the fixture or equipment instructions. For those who want to take special precautions against UV, a UV blocking clear filter gel such as the GAM no. 1510 or Rosco "UV Filter" (03114) may be a practical solution. This filter gel withstands use moderately close to halogen lamps and withstands heat to maybe 100 to 150 Celsius or so. This filter gel can be placed immediately outside the glass shield of most fixtures, although the tubular shield in many popular 300 watt torchiere lamps gets too hot for the filter gel. The GAM 1510 and Rosco "UV Filter" is available at some theatrical supply shops. Written by Don Klipstein (Jr). Back to Don's lighting index page. Back to Don's home page Sam's and Don's D-Lamp FAQ Gas Discharge Lamps, Ballasts, and Fixtures Principles of Operation, Circuits, Troubleshooting, Repair Version 1.35 Copyright (C) 1996,1997,1998,1999 Samuel M. Goldwasser Donald L. Klipstein --- All Rights Reserved --- Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied: This notice is included in its entirety at the beginning. There is no charge except to cover the costs of copying. Introduction Gas discharge lamp basics The use of electrically excited gas discharges significantly predates the invention of the incandescent lamp. Physics labs of yesteryear as well as today have use of a variety of gas filled tubes used for numerous purposes involving light generation including spectroscopy, materials analysis, studies of gas dynamics, and laser pumping. Look through any scientific supply catalog and you will see many different types of gas filled tubes in all shapes and sizes. Gas discharge lamps are used in virtually all areas of modern lighting technology including common fluorescent lighting for home and office - and LCD backlights for laptop computers, high intensity discharge lamps for very efficient area lighting, neon and other miniature indicator lamps, germicidal and tanning lamps, neon signs, photographic electronic flashes and strobes, arc lamps for industry and A/V projectors, and many more. Gas discharge automotive headlights are on the way - see the section: "HID automotive headlights". Because of the unusual appearance of the light from gas discharge tubes, quacks and con artists also have used and are using this technology as part of expensive useless devices for everything from curing cancer to contacting the dead. Unlike incandescent lamps, gas discharge lamps have no filament and do not produce light as a result of something solid getting hot (though heat may be a byproduct). Rather, the atoms or molecules of the gas inside a glass, quartz, or translucent ceramic tube, are ionized by an electric current through the gas or a radio frequency or microwave field in proximity to the tube. This results in the generation of light - usually either visible or ultraviolet (UV). The color depends on both the mixture of gasses or other materials inside the tube as well as the pressure and type and amount of the electric current or RF power. (At the present time, this document only deals with directly excited gas discharge lamps where an AC or DC electric current flows through the gas.) Fluorescent lamps are a special class of gas discharge lamps where the electric current produces mostly invisible UV light which is turned into visible light by a special phosphor coating on the interior of the tube. See: Fluorescent Lamps, Ballasts, and Fixtures for more info. The remainder of this document discusses two classes of gas discharge lamps: low pressure 'neon' tubes used in signs and displays and high intensity discharge lamps used for very efficient area and directional lighting. Safely Working with Gas Discharge Lamps and Fixtures Fixtures for gas discharge lamps may use up to 30,000 V while starting depending on technology. And, they are often not isolated from the power line. Neon signs are powered by transformers or electronic ballasts producing up to 15,000 V or more. Thus, the only safe way to work with these is to assume that they are potentially lethal and treat them with respect. Hazards include: Electric shock. There is usually little need to probe a live fixture. Most problems can be identified by inspection or with an ohmmeter or continuity tester when unplugged. Discharge lamps and fixtures using iron ballasts are basically pretty inert when unplugged. Even if there are small capacitors inside the ballast(s) or for RFI prevention, these are not likely to bite. However, you do have to remember to unplug them before touching anything! Neon signs using iron transformers are also inert when unpowered - just make sure they are off and unplugged before touching anything! However, those using electronic ballasts can have some nasty charged capacitors so avoid going inside the ballast module and it won't hurt to check between its outputs with a voltmeter before touching anything. Troubleshooting the electronic ballast module is similar to that of a switchmode power supply. See the document: Notes on the Troubleshooting and Repair of Small Switchmode Power Supplies The pulse starters of some high intensity discharge lamps may produce up to 30 kV during the starting process. Obviously, contact with this voltage should be avoided keeping in mind that 30 kV can jump over an inch to anyplace it wants! Nasty chemicals: Various toxic substances may be present inside high pressure discharge lamps (sodium and mercury) and neon signs (some phosphors). Contact with these substances should be avoided. If a lamp breaks, clean up the mess and dispose of it properly and promptly. Of course, don't go out of your way to get cut on the broken glass! WARNING: Metallic sodium reacts with water to produce hydrogen gas, an explosive. However, it is unlikely that the inner tube of a sodium vapor lamp would break by accident. Ultra-Violet (UV) light: High intensity discharge lamps generate substantial UV internally, often the particularly nasty UV-B variety. Unless designed to generate UV (for medicinal purposes, photoengraving, or whatever), the short wave radiation will be blocked by the outer glass envelope and/or phosphor coating. However, should the outer envelope break or be removed, the lamp will still operate (at least for a while - some have a means of disabling themselves after a few hours or less of exposure to air). DO NOT operate such a lamp preferably at all but if you do, at least take appropriate precautions to avoid any exposure to the UV radiation. And take care around sharp sheet metal! Neon Technology Neon Lights and Signs Neon technology has been around for many years providing the distinctive bright glowing signs of commerce of all kinds before the use of colored plastics became commonplace. Neon tubes have electrodes sealed in at each end. For use in signs, they are formed using the glass blower's skill in the shape of letters, words, or graphics. Black paint is used to block off areas to be dark. They are evacuated, backfilled, heated (bombarded - usually by a discharge through the tube at a very high current) to drive off any impurities, evacuated and then backfilled with a variety of low pressure gasses. Neon is the most widely known with its characteristic red-orange glow. Neon may be combined with an internal phosphor coating (like a fluorescent tube) to utilize neon's weak short-wave UV emissions. A green-emitting phosphor combines with neon's red-orange glow to make a less-red shade of orange. A blue-emitting phosphor may be used to result in a hot-pink color. Neon may be used in tubing made of red glass to produce a deep red color. Other colors are usually produced by tubing containing argon and mercury vapor. The mercury is the active ingredient, the argon produces negligible radiation of any kind but is important for the "neon" tubing to work. Clear tubing with mercury/argon glows a characteristic light blue color. Such tubing is often phosphor-coated on the inside, to utilize the major short-wave UV emission of low-pressure mercury. In this way, much of the "neon" tubes in use are a kind of fluorescent lamp. Phosphor-coated tubing with mercury can glow blue, blue-green, slightly white-ish green, light yellow, bright pink, light purple, or white. Use of mercury vapor with colored tubing (with or without phosphors) can provide a lime-green or deep blue or deep violet-blue. Nowadays, nearly all "neon" tubing contains neon or mercury vapor (with argon), whether with or without phosphors and/or colored glass. Well in the past, various colors were obtained (generally at reduced efficiency) by using different gases. For example, helium can produce a white-ish orange light in shorter length, smaller diameter tubing. Hydrogen in this case makes a lavender-hot-pink color. These gases glow more dimly with duller color shades in larger tubing. Krypton makes a dull greenish color. Argon makes a dimmish purple color. Nitrogen (generally in shorter length tubing) makes a grayish purple-pink color. Xenon, which is expensive, generally glows with a dim bluish gray color, along with the glass tubing giving a slight dim blue fluorescence from very short wave UV from the xenon discharge. Krypton also often causes a dim blue glass fluorescence. For general information on neon signs and technology including a neon FAQ, see: The Internet's Neon Shop Power Supplies for Neon Extremely high voltage power supplies are used to power neon signs. In the past, this was most often provided by a special current limited HV line transformer called a neon sign or luminous tube transformer. The output is typically 6,000 to 15,000 VAC at 15 to 60 mA. One such unit can power 10s of feet of tubing. This transformer acts as its own ballast providing the high voltage needed for starting and limiting the running current as well. Warning: the output of these transformers can be lethal since even the limited current availability is relatively high. As with everything else, the newest neon sign power supplies use an electronic AC-AC inverter greatly reducing the size and weight (and presumably cost as well) of these power supplies by eliminating the large heavy iron transformer. Small neon lamps inside high-tech phones and such also use solid state inverters to provide the more modest voltage required for these devices. Neon Sign Installation (From: Clive Mitchell (clive@emanator.co.uk)). The voltage required to light a run of neon tube is variable according to diameter, gas type, pressure and number of tubes in circuit. For a 15 kV transformer and neon gas you could run: 33 feet of 10 mm tube, 45 feet of 12 mm tube, 60 feet of 15 mm tube, 78 feet of 20 mm tube, 102 feet of 25 mm tube. Deduct one foot of tube for every pair of electrodes (tube section). These figures are based on a chart in "Neon Techniques And Handling" which is the traditional neon reference. The larger the diameter of the tube, the lower the voltage required, and the dimmer it will be. Transformers come with different current ratings. For larger diameter tubes, you can increase brightness by using a higher current. Don't attempt to run too much tube on a transformer, since it can cause breakdown of the insulation and destroy the transformer. Don't attempt to run too little tube on a transformer, since it can cause overheating and burn-out. It is absolutely imperative that proper neon sign cabling and insulators are used, and that all local regulations are strictly followed. If you are intending to work with neon tubing, you should learn as much as possible first, since neon poses both a shock and serious fire risk if installed incorrectly. The lengths quoted above may vary according to the transformer you use. The transformer manufacturers usually provide their own loading charts on request. Anyone using this information does so at their own risk, and I cannot be held responsible for any horrible smouldering deaths experienced by incompetent dabblers, etc. (From: Kenny Greenberg (kenny@neonshop.com)). The neon circuit is not so simple. In a standard AC circuit neon acts like a diac - high breakover voltage followed by fast drop in resistance. Neon sign transformers are designed to 'leak' and thus self-regulate. You have a combined resistive and reactive circuit. But take heart, it's all been figured out. :-) There are a few variables: A 'purely' neon filled tube (generally in the red range) has a higher voltage requirement than an argon-mercury tube (whose discharge is usually providing UV for phosphor with a wide range of colors. The voltage requirement varies inversely with the tubing diameter. That is large diameters of a lower voltage requirement than small diameters. The voltage requirement varies directly with tubing length. The number of units (or pairs of electrodes) increases the voltage requirement because the electrodes have a voltage drop. Wiring methods and length will also contribute to the formula but that's a whole 'nuther discussion. You can download a free Neon Voltage Calculator for Windows. An old tech method for determining the voltage requirement is to use a Variac on a large neon transformer. Bring the voltage down to where the neon just flickers. This should be at a point approximately 78% of the required voltage. A better way involves using a milliameter to measure open circuit and closed circuit current and an rms voltmeter to measure actual operating voltage. Problems With Neon These fall into two categories: Power supply - like fluorescent ballasts, the high voltage transformers can fail resulting in reduced (and inadequate) voltage or no power at all. Since they are already current limited, overheating may not result and any fuse or circuit breaker may be unaffected. The use of a proper (for safety if nothing else) high voltage meter can easily identify a bad transformer. If a high voltage probe is not available, position (with power off!) the ends of well insulated wires connected to the outputs of the transformer a fraction of an inch apart (about 1/32" per 1,000 V of transformer rating) and apply the power from a safe distance. If a hot arc results, the transformer is likely good (at least when cold). Neon tubes - these may lose their ability to sustain a stable discharge over time as a result of contamination, gas leakage, or electrode damage (either from normal wear or due to excessive current). Check for obvious damage such as a cracked tube or cracked seals around the electrodes or badly deteriorated electrodes. A previously working tube that now will not strike or maintain a stable discharge on a known good transformer will need to be replaced or rebuilt. Comments on Little Neon Bulbs and Tubes The comments below relate to the little neon bulbs used as indicators, for voltage regulation or limiting, and other applications in all sorts of electronic equipment. (From: Mark Kinsler (kinsler@frognet.net).) Neon lamps can be used for voltage limiters and oscillator elements and just about anywhere else that a non-linear element is needed. The tremolo circuit in the classic Fender guitar amplifier uses a neon lamp relaxation oscillator. The neon lamp is heat-shrinked to a CdS photocell in the volume control circuit. Less well-known is the fact that you can make a pretty reasonable computer logic element out of them: I believe that this was tried sometime in the 1940's. Another cool use is as a radiation sensor: you bias the lamp so that it almost turns on, after which any incident radiation: radio waves (as in police radar), light, or gamma radiation will kick the lamp on. There were various circuits in the 1950's that used neon lamps to detect uranium, fight nuclear destruction, or escape the newly-developed police radar guns. And finally, there's the mystery elevator button. Again, you bias the lamp so that it almost, but not quite, turns on. If you enclose the lamp properly, it'll stay off until you touch it. The electric field variation from your touch will turn the thing on, and it'll stay on. Such lamps are used in some self-service elevators: once the lamp is fired, the low voltage across it is sensed by the ancient logic circuits of the elevator controller and it'll send the elevator to the appropriate floor. These were a lot of fun in the 1960's. I think the controllers used vacuum tubes. The problem with neon lamps is that they're not so reliable. Their turn-on voltage isn't particularly stable. This means that oscillators have a tendency to drift as the lamps age or when ambient radiation changes. I suspect that the computers are slow and cranky, and the radiation detector isn't anything you'd wish to stake your life or drivers' license on. Still, they're great fun, and I have a fine time with them. One other use: hang a neon lamp across a telephone line to detect the ring signal. Place it in series with a piezo beeper, and you've got a reliable telephone ringer. High Intensity discharge Lamps High Intensity Discharge (HID) Lamp Technology These have been used for a long time in street, stadium, and factory lighting. More recently, smaller sizes have become available for home yard and crime prevention applications. Like other gas discharge lamps, these types require a special fixture and ballast for each type and wattage. Unlike fluorescents, however, they also require a warmup period. There are three popular types: High pressure mercury vapor lamps contain an internal arc tube made of quartz enclosed in an outer glass envelope. A small amount of metallic (liquid) mercury is sealed in an argon gas fill inside the quartz tube. After the warmup period, the arc emits both visible and invisible (UV) light. High pressure mercury vapor lamps (without color correction) produce a blue-white light directly from their discharge arc. Phosphors similar to those used for fluorescent lamps can be used to give these a color closer to natural light. (Without this color correction, people tend to look like cadavers). Mercury vapor lamps have the longest life of this class of bulbs - 10,000 to 24,000 hours. The technology was first introduced in 1934 and was the first of the commercially viable HID lamps. Metal halide lamps are constructed along similar lines to mercury vapor lamps. However, in addition to the mercury and argon, various metal halides are included in the gas fill. The most popular combination is sodium iodide and scandium iodide. A few versions of this lamp have lithium iodide as well. A much less common version has sodium iodide, thallium iodide, and indium iodide. The use of these compounds increases the luminous efficiency and results in a more pleasing color balance than the raw arc of the mercury vapor lamp. Thus, no phosphor is needed to produce a color approaching that of a cool white fluorescent lamp with more green and yellow than a mercury vapor lamp (without correction). Some metal halide lamps have a phosphor that adds some orange-ish red light, but not much, since the metal halide arc does not emit much UV. High pressure sodium vapor lamps contain an internal arc tube made of a translucent ceramic material (a form of aluminum oxide known as "polycrystalline alumina"). Glass and quartz cannot be used since they cannot maintain structural strength at the high temperatures (up to 1300 degrees C) encountered here, and hot sodium chemically attacks quartz and glass. Like other HID lamps, the arc tube is enclosed in an outer glass envelope. A small amount of metallic (solid) sodium in addition to mercury is sealed in a xenon gas fill inside the ceramic arc tube. Some versions of this lamp use a neon-argon mixture instead of xenon. Basic operation is otherwise similar to mercury or metal halide lamps. High pressure sodium vapor lamps produce an orange-white light and have a luminous efficiency much higher than mercury or metal halide lamps. Since hot liquid sodium often eventually leaches through things and can get lost this way, sodium lamps have a surplus of sodium in them. Proper lamp operation depends on the sodium reservoir being within a proper temperature range. Mercury vapor lamps are roughly as efficient as fluorescent lamps. Metal halide lamps are much more efficient, generally around 50 to 75 percent more efficient than fluorescent lamps. High pressure sodium lamps are roughly twice as efficient as fluorescent lamps. Unlike fluorescent lamps, HID lamps will give full light output over a wide range of temperatures. This often makes HID lamps more suitable than fluorescent lamps for outdoor use. When cold, the metallic mercury or sodium in the arc tube is in its normal state (liquid or solid) at room temperature. During the starting process, a low pressure discharge is established in the gases. This produces very little light but heats the metal contained inside the arc tube and gradually vaporizes it. As this happens, the pressure increases and light starts being produced by the discharge through the high pressure metal vapor. A quite noticeable transition period occurs when the light output increases dramatically over a period of a minute or more. The entire warmup process may require up to 10 minutes, but typically takes 3 to 5 minutes. A hot lamp cannot be restarted until it has cooled since the voltage needed to restrike the arc is too high for the normal AC line/ballast combination to provide. Problems With High Intensity Discharge Lamps While HID lamps have a very long life compared to incandescents (up to 24,000 hours), they do fail. The ballasts can also go bad. In addition, their light output falls off gradually as they age. For some types, light output may drop to half its original value towards the end of their life. A lamp which is cycling - starting, warming up, then turning itself off - is probably overheating due to a bad bulb or ballast. A thermal protector is probably shutting down the fixture to protect it or the arc is being extinguished on its own. However, make sure that it is not something trivial like a photoelectric switch that is seeing the light from the lamp reflected from a white wall or fence and turning the fixture off once the (reflected) light intensity becomes great enough! Sodium lamps sometimes "cycle" when they have aged greatly. The arc tube's discolorations absorb light from the arc, causing the arc tube to overheat, the sodium vapor pressure becomes excessive, and the arc cannot be maintained. If a sodium lamp "cycles", the first suspect is an aging bulb which should be replaced. Sodium lamp "cycling" used to be very common, but in recent years the lamp manufacturers have been making sodium lamps that are less prone to cycling. If you have more than one fixture which uses **identical** bulbs, swapping the bulbs should be the first test. If the problem remains with the fixture, then its ballast or other circuitry is probably bad. Don't be tempted to swap bulbs between non-identical fixtures even if they fit unless the bulb types are the same. Warning: do not operate an HID lamp if the outer glass envelope is cracked or broken. First, this is dangerous because the extremely hot arc tube can quite literally explode with unfortunate consequences. In addition, the mercury arc produces substantial amounts of short wave UV which is extremely hazardous to anything living. The outer glass normally blocks most of this from escaping. Some lamps are actually designed with fusable links that will open after some specified number of hours should air enter the outer envelope. Thus, an undetected breakage will result in the lamp dying on its own relatively quickly. Troubleshooting a Discharge Lamp Fixture (From: Greg Anderson (a3a30878@bc.sympatico.ca).) The following applies directly to high pressure sodium lamps. It may also also be used for metal halide and mercury vapour lamp problems as long as references to the starter are ignored. (Metal halide and mercury vapour lamps do not have starters, except for "instant re-light" metal hhalide lamps.) The starter produces about 2 to 5 kV spikes to ionize the gas in the lamp. The starter normally has a triac across the ballast and a diac trigger cct. When open cct voltage is across the lamp, the diac fires the triac to short the ballast, the triac then opens. This "kick" produces the voltage spike. Once the gas ionizes, the lamp impedance drops then gradually increases as the lamp warms up. The lamp running voltage is about 1/2 of the open cct voltage With the lamp removed and power on, you can normally hear a good starter "ticking". The open cct voltage is stamped on the ballast and is between about 150 and 350 Vac, depending on lamp wattage and ballast. Also, a capacitor is often connected in series with lamp to improve peaking and ballast action. Steps to follow: Bypass the photo cell - It may be bad Check connections - water, salt, and bird poop are not good for wiring Check the capacitor, if installed - normally they blow-up when bad Check for open/shorted ballast. Power up and check for starter "ticking" REMOVE starter from cct and measure open cct volts Check/Replace lamp Check/replace lamp socket Replace starter Replace complete fixture. Replace electrician. :) Repairing a starter is not economically viable and often proves that electronic devices contain smoke and sometimes fire. Ballasts and Bulbs Should be Matched! HID bulbs generally need specific ballasts, and any given ballast can usually safely and effectively operate only one type or a few types of HID bulbs. The bulb wattage must be matched to the ballast. A smaller bulb will usually be fed a wattage close to what the proper bulb takes, and will generally overheat and may catastrophically fail. Any catastrophic failures may not necessarily happen quickly. A larger bulb will be underpowered, and will operate at reduced efficiency and may have a shortened lifetime. The ballast may also overheat from prolonged operation with an oversized bulb that fails to warm up. See The Discharge Lamp Mechanics Document (rather technical) for why it can be bad to underpower an arc discharge lamp. Even if the ballast and bulb wattages match, substitutions can be limited by various factors including but not limited to different operating voltages for different bulbs. Examples are: Pulse-start sodium lamps often have a slightly lower operating voltage than metal halide and mercury lamps of the same wattage, and ballasts for these sodium bulbs provide slightly more current than mercury and metal halide ballasts for the same wattage would. The higher current provided by the pulse-start sodium ballast can overheat mercury and metal halide lamps. Mercury and metal halide lamps may also "cycle" on and off in lower voltage sodium ballasts, such as many 50 to 100 watt ones. Metal halide lamps have an operating voltage close to that of mercury lamps in many wattages, but have stricter tolerances for wattage and current waveform. Metal halides also usually need a higher starting voltage. Most metal halide lamps 100 watts or smaller require a high voltage starting pulse around or even over 1,000 volts. 175 to 400 watt metal halide lamp ballasts can power mercury lamps of the same wattage, but the reverse is not recommended. Mercury lamps 50 to 100 watts will work on metal halide ballasts, but hot restriking of mercury lamps 100 watts or smaller on metal halide lamps may be hard on the mercury lamp since the starting pulse can force current through cold electrodes and the starting resistor inside the mercury lamp. 1,000 watt mercury lamps come in two operating voltages, one of which is OK for 1,000 watt metal halide ballasts. A few wattages of pulse-start sodium (150 watts?) come in two voltages. A low voltage lamp in a high voltage ballast will be underpowered, resulting in reduced efficiency, possible reduced lamp life, and possible ballast overheating. A high voltage lamp in a low voltage ballast will usually cycle on and off, operate erratically, or possibly overheat. This will usually result in greatly reduced lamp life in any case. One class of sodium lamps is made to work in mercury fixtures, but these only work properly with some mercury ballasts, namely: 'Reactor' (plain inductor) ballasts on 230 to 277 volt lines. 'High leakage reactance autotransformer' ballasts, preferably with an open circuit voltage around 230 to 277 volts. NOT 'lead', 'lead-peak' nor any metal halide ballast! These sodium lamps may suffer poor power regulation and accelerated aging in the wrong mercury ballasts, especially after some normal aging changes their electrical characteristics. Also, these lamps may overheat and will probably have shortened life with pulse-start sodium ballasts. Many sodium lamps require a high voltage starting pulse provided only by ballasts made to power such lamps. Operation of Discharge Lamps on DC Sometimes, one may want to run a discharge lamp on DC. There are two possible reasons: Only DC power is available. To reduce flicker. Sometimes, the lamp performs differently for electricity flowing in one direction than the other. In addition, the positive and negative ends of the arc can make different amounts of light, resulting in a flicker rate equal to the AC frequency rather than twice the AC frequency. However, end flicker is usually not significant. In HID lamps, the total arc size is generally small. Only if the fixture has a reflector that causes some areas to receive light from only one end of the arc should end flicker be significant. In most multi-tube fluorescent fixtures, the tubes