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.
And take care around sharp sheet metal!
- 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.
- 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.
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
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
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:
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
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.
(From: Clive Mitchell (firstname.lastname@example.org)).
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:
These figures are based on a chart in "Neon Techniques And Handling" which is
the traditional neon reference.
- 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).
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.
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.
- 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.
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
(From: Kenny Greenberg (email@example.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:
You can download a free
Neon Voltage Calculator for Windows.
- 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.
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
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
(From: Mark Kinsler (firstname.lastname@example.org).)
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 (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 warm-up period.
There are three popular types:
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.
- 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 warm-up
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
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 warm-up 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
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 (email@example.com).)
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
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.
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
- 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 are usually in
series pairs with the two tubes in any pair oriented in opposite directions.
This generally reduces end flicker effects, especially in fixtures with
Bulbs should perform close enough to identically in both directions, unless
the bulb is near the end of its life. In such a case, one electrode deteriorates
enough to affect performance before the other does. However, this generally
indicates a need to replace the bulb rather than to attempt to make it flicker
If you want to rectify the AC to provide the bulb with DC, use a bridge
rectifier after the ballast. Most ballasts, including all "iron" types, require
AC of the proper voltage and frequency to work. Do this only if only two wires
feed the bulb. Otherwise, diodes in the bridge rectifier may short parts of the
ballast to each other, at least for half the AC cycle. Problems can also occur
with fluorescent ballasts with filament windings. Only fully isolated filament
windings or separate filament transformers should be used if you rectify the
output of a ballast with filament windings. Also, the bridge rectifier must
withstand the peak voltage provided by the ballast.
If the power supply is DC of adequate voltage, you need a resistor ballast or an
electronic ballast specifically designed to run your lamp from the available DC
voltage. "Iron" ballasts only limit current when used with AC. Preheat
fluorescent lamps operated from DC supplies and without special ballasts need
both the usual "iron" ballast to provide the starting "kick" and a resistor to
In addition, most discharge lamps are only partially compatible with DC, and
some are not compatible at all.
Mercury vapor and fluorescent lamps generally work on DC. However, the life may
be shortened somewhat by uneven electrode wear.
Fluorescent lamps may get dim at one end with DC. Since the mercury vapor
ionizes more easily than the argon, some of it exists as positive ions. This can
cause the mercury to be pulled to the negative end of the tube, resulting in a
mercury shortage at the positive end. This is more of a problem with longer
length and smaller diameter tubes.
Some fluorescent fixtures made for use where the power available is DC have
special switches to reverse polarity every time the fixture is started. This
balances electrode wear and reduces mercury distribution problems.
Mercury vapor lamps generally work OK with DC, but some may only reliably work
properly if the tip of the base is negative and the shell of the base is
positive. This is because the starting electrode does its job best when it is
In addition, if the nearby main electrode is positive, it may cause a thin film
of metal condensation that shorts the starting electrode to the nearby main
electrode. This may make some brands, models, and sizes of mercury lamps unable
to start after some use. The negative main electrode will not release as much
vaporized electrode material, since the electrode material easily forms positive
ions making the electrode material vapor tend to condense on the electrode
rather than condense on nearby parts of the arc tube.
Metal halide and sodium lamps should not get DC. Use these only with ballasts
that give the bulb AC. In metal halide lamps, ions from the molten halide salts
can leach into hot quartz in the presence of a DC electric field. This can cause
strains in the quartz arc tube. At the ends of the arc tube, electrolysis may
occur, releasing chemically reactive halide salt components that can damage the
arc tube or the electrodes. The arc tube may crack as a result.
There are a few specialized metal halide lamps that are made to work on DC.
These often have asymmetrical electrodes and/or short arc lengths. These lamps
often also must be operated only in specific positions, and only with the type
of current they were designed for in order to achieve the proper distribution of
active ingredients within the arc tube and to achieve proper electrode usage.
For example, some of these lamps may go wrong in some way or another with AC.
In high pressure sodium lamps, which contain both sodium and mercury, the sodium
forms positive ions more easily than the mercury does and drifts towards the
negative electrode. The positive end can go dim from a lack of sodium. In
addition, if any part of the arc tube is filled with a mixture containing
excessive sodium and a lack of mercury, heat conduction from that part of the
arc to the arc tube will increase. Furthermore, the hot arc tube may suffer
electrolysis problems over time in the presence of sodium ions and a DC electric
Low pressure sodium lamps should not get DC for the same reasons. The sodium is
likely to drift to the negative end of the arc tube, and hot glass will almost
certainly experience destructive electrolysis problems if exposed to hot sodium
or sodium ions and a DC electric field.
Special purpose HID lamps such as xenon and HMI
The usual general purpose HID lamps are mercury vapor, metal halide, and high
pressure sodium. You can get these at home centers, although usually only in
wattages up to 400 watts. These versions of HID lamps are optimized for high
efficiency, long life, and minimized manufacturing cost.
However, the arc surface brightness of these lamps is roughly equal to the
surface brightness of incandescent lamp filaments and general purpose halogen
lamp filaments. For some applications such as endoscopy and movie projection, it
is necessary to have a much more concentrated light source. This is where
specialized HID lamps such as short arc lamps and HMI lamps come in.
Short arc lamps consist of a roughly spherical quartz bulb with two heavy duty
electrodes spaced only a few millimeters apart at the tips. The bulb may contain
xenon or mercury or both. Mercury short arc lamps have an argon gas fill for the
arc to start in.
In a short arc lamp, the arc is small and extremely intense. The power input is
at least several hundred and more typically a few thousand watts per centimeter
of arc length. The operating pressure in the bulb is extremely high - sometimes
as low as 20 atmospheres, more typically 50 to over 100 atmospheres. These lamps
are an explosion hazard!
Mercury short arc lamps are used when a compact, intense source of UV is needed
or where one cannot have the high voltage starting pulses needed for xenon short
arc lamps. Mercury short arc lamps are slightly more efficient than xenon ones.
The pressure in a mercury short arc lamp does not need to be as high for good
efficiency as in a xenon one, but is still tremendous.
Xenon short arc lamps are more common than mercury ones, since they do not
require time to warm up the way mercury lamps do and have a daylight-like
spectrum. A disadvantage of xenon is the requirement of a very high voltage
starting pulse - sometimes around 30 kilovolts!
Xenon short arc lamps are used for movie projection and sometimes for
searchlights. Lower wattage ones are used in specialized devices such as
HMI lamps are metal halide lamps with a more compact and more intense arc. The
arc is larger and less intense than that of a short arc lamp. Typical power
input is hundreds of watts per centimeter of arc length, but gets to a few
kilowatts per centimeter in the largest ones.
HMI lamps are used in some spotlights. They are used in some endoscopes and
projection applications where the intensity of the HMI arc is adequate since
they cost less and last longer and are more efficient than true short arc lamps.
There are all sorts of HMI and similar lamps, including HTI lamps and the lamps
used in HID auto headlights.
HID Automotive Headlights
First there were gas lamps, then there were electric bulbs, then sealed beam,
then halogen. Now, get ready for - drum roll please! - high intensity discharge
lamps with sophisticated controllers. High-end automobiles from makers like BMW,
Porsche, Audi, Lexus, and now Lincoln are coming equipped with novel headlight
technology. No doubt, such technology will gradually find its way into
mainstream automobiles - as well as other applications for mortals.
Among the potential advantages of HID headlights are higher intensity, longer
life, superior color, and better directivity:
In order to make this practical - even for a $40,000 Lexus - special DC-DC
converter chips have been designed specifically with automotive applications in
mind. These, along with a handful of other basic electronic components,
implement a complete HID headlight control system.
- Light intensity - HID lamps are about 3 times as efficient as halogen lamps.
Thus, even when the efficiency of the DC-DC converter is taken into
consideration, the lower power input can actually result in much brighter
headlights than are possible with halogen bulbs. This reduced power also leads
to cooler operation and less drain on the battery and alternator.
- Lifespan - an HID lamp can be expected to last 2,700 hours or more and thus
covered under the bumper to bumper warranty for 100,000 miles. As a practical
matter, the HID lamp may outlast the automobile. Since warranty replacement of
headlights turns out to be a significant expense, there is strong incentive to
see this long lived technology take off.
- Spectral output - the light from the HID lamp is richer in blue (and more
like daylight) than halogen bulbs. This turns out to enhance reflectivity of
signs and road markings.
- Beam pattern - the small arc size of the HID lamp permits the optical system
to be optimized to direct light more effectively to where it is needed and
prevent it from spilling over to where it is not wanted.
The HID bulb itself is similar in basic design to traditional HID lamps: Two
electrodes are sealed in a quartz envelope along with a mix of solids, liquids,
and gasses. When cold, these materials are in their native state (at room
temperature) but are mostly gases when the lamp is hot. Starting of these lamps
may require up to 20 KV to strike an arc but only 50 to 150 V to maintain it.
Lamps may be designed to operate on either AC or DC current depending on various
factors including the size and shape of the electrodes. A unique set of ballast
operating parameters must be matched to each model HID bulb.
Of all the problems that had to be addressed for HID headlights to become
practical (aside from the cost), the most significant was the warm-up time. As
noted in the section: "High intensity discharge (HID) lamp technology", common
HID lamps require a warm-up period of a few minutes before substantially full
light output is produced. This is, of course, totally unacceptable for an
automotive headlight both for cold start (imagine: "Honey, I have to go cook the
headlights") as well as when they need to be blinked. The warm-up problem was
solved by programming the controller to deliver constant power to the lamp
rather than the more common nearly constant current that would be provided by a
traditional ballast. With this twist along with a special lamp design, the lamp
comes up to at least 75% of full intensity in under 2 seconds. The controller
also provides 'hot strike' capability for blinking (recall that HID lamps
typically cannot be restarted when hot). Thus, restarting a hot lamp is
While this technology is just beginning to appear, expect inroads (no pun
intended) into household, office, store, factory, and other area and work
lighting. The combination of high efficiency, long life, desirable spectral
characteristics, small size, and solid state reliability should result in many
more applications in the near future. The nearly instant starting capability
addresses one of the major drawbacks of small HID lamps.
If you have some time and money to spare:
(From: Declan Hughes (firstname.lastname@example.org).)
OSRAM Sylvania Products Inc.
They have a "sample" for sale at $250.00 for one lamp including the 12 VDC
electronic ballast. 42 W total power, 35 W light power, 3,200/2,800 lm output
(there are two types, D2S and D2R), 2,000 hours rated lifetime, 91/80 lm/W
luminous efficacy, 4,250/4,150 K color temperature, 6,500 cd/cm^2 average
luminance, 4.2 mm arc length, burning position horizontal +/- 10 deg., luminous
flux after 1 sec. = 25%, max. socket temp. = 180 deg C, any errors are mine.
For more info, look in
Don Klipstein's Automotive HID Lamp File.
Substitution of Metal Halide Lamps?
The following was prompted by a request for info on replacing an (expensive) 250
watt metal halide lamp in a video projector with something else.
I would not substitute this lamp, for many reasons below:
The metal halide lamp requires a ballast. The ballast should only run a 250 watt
metal halide lamp of the same arc voltage. You will have to measure the arc
voltage yourself after the lamp warms up, and do this without exposing yourself
to the nasty UV that some of these things emit but which does not pass through
glass. Arc voltages of many specialized metal halide lamps are not widely
published and may or may not be available from the lamp manufacturer.
WARNING: The strike voltage on these may be several kV which will probably
obliterate your multimeter should the arc drop out and attempt to restart while
you are measuring it! Either the operating or strike voltage may obliterate you
should you come in contact with live terminals! (Special metal halides probably
usually only need a couple to a few kV. Xenon metal halide automotive lamps need
6 to 12 kV to strike and 15 to 20 kV for hot restrike. The worst are short arc
xenon that may use up to 30 kV or more.)
Most metal halide lamps are AC types and some are DC and you can only use AC
lamps on AC output ballasts and DC lamps on DC output ballasts. Different metal
halide lamps may have different requirements for starting voltage also.
If you match arc voltage, AC/DC type, and the ballast will start the lamp, you
might be in business but good chance not. Many projector lamps have specific
cooling requirements and some have specific burning position requirements. Metal
halide lamps may prematurely fail (possibly violently!) if they overheat, in
addition to being off-color. If overcooled, they are more like mercury lamps and
will be off-color and have reduced light output. In addition, some metal halide
lamps have a halogen cycle in them to keep the inner surface of the bulb clean,
and that may not work if the lamp is overcooled and not enough of the chemicals
in the bulb get vaporized. This could also even make the lamp fail.
If you get the alternate lamp to operate satisfactorily, the arc may be in a
different location from that of the original lamp. The arc may be of a different
shape or size than that of the original lamp. This can affect your projection.
Your projection may not get much light or may have illumination of only part of
The arc may have a different color or spectrum, which can affect the color
rendering of what's being projected. Metal halide arcs are often not of uniform
color, and if the alternate lamp has a less color-uniform arc than the original
lamp then your pictures may have strange color tints in them.
As for using a halogen instead of metal halide? You will get less light, as well
as problems from the filament having a different shape or size than the original
metal halide arc does. Most likely, the filament is larger or longer than the
arc and this will reduce the percentage of the light being utilized. Should you
try a halogen lamp hack, you will almost certainly have to bypass the metal
halide ballast. And halogen lamps emit more infrared than metal halide lamps of
the same wattage - you might overheat the source of your image (e.g., LCD panel
I would not recommend substituting a projector lamp for all of these reasons.
This should only be tried at your own risk and only by those that are very
familiar with all of the characteristics of the lamps in question - including
being familiar with burning position requirements, cooling requirements, shape
and size of the light-emitting region, etc.
Projector lamps in general, and especially specialized HID lamps, should be used
only in equipment made specifically to use the particular lamps in question, or
by those who know about these things well enough to make their own ballasts and
know the other messy things about these lamps. And you may not save much by
using a different lamp - specialized metal halide lamps are all expensive.
And for anyone shopping for any sort of projector - look into price,
availability, and life expectancy of lamps!
Low Pressure Sodium
(Portions from: Bruce Potter (email@example.com))
Low pressure sodium lamps are the most efficient visible light sources in common
use. These lamps have luminous efficacies as high as 180 lumens per watt.
A low pressure sodium lamp consists of a tube made of special sodium-resistant
glass containing sodium and a neon-argon gas mixture. Since the tube is rather
large and must reach a temperature around 300 degrees Celsius, the tube is bent
into a tight U-shape and enclosed in an evacuated outer bulb in order to
conserve heat. As an additional heat conservation measure, the inner surface of
the outer bulb is coated with a material that reflects infrared but passes
visible light. This material has traditionally been tin oxide or indium oxide.
The electrodes are coiled tungsten wire coated with thermionically emissive
material, and somewhat resemble the electrodes of fluorescent lamps. Unlike most
fluorescent lamps, low pressure sodium lamps have only one electrical connection
to each electrode and the electrodes cannot be preheated.
The gas mixture is a "Penning" mixture, consisting mainly of neon with a small
amount of argon. Depending on who you listen to, this mixture is .5 to 2 percent
argon, 98 to 99.5 percent neon. More argon-rich mixtures around 98-2 may be
favored today since hot glass has some ability to absorb argon from a low
pressure electric discharge. Ideally the mixture should be only a few tenths of
a percent argon, in order to ionize most easily and do so much more easily than
pure neon or pure argon.
A significant surplus of sodium is contained in the glass arc tube since the
glass may absorb or react with some of the sodium. The sodium vapor pressure is
controlled by the temperature of the coolest parts of the arc tube. When the arc
tube reaches a proper temperature, further heating is reduced by the lamp's
efficiency at producing light instead of heat.
The arc tube has dimples in it, which are normally slightly cooler than the rest
of the arc tube. This causes the sodium metal to collect in the dimples instead
of covering a larger portion of the arc tube and blocking light.
The low pressure sodium lamp usually requires 5 to 10 minutes to warm up.
The light of low pressure sodium consists almost entirely of the orange-yellow
589.0 and 589.6 nM sodium lines. This light is basically monochromatic
orange-yellow. This monochromatic light causes a dramatic lack of color
rendition - everything comes out in an orange-yellow version of black-and-white!
This can cause some confusion in parking lots since cars become more alike in
Some basically red and reddish color fluorescent inks, dyes, and paints can
fluoresce red to red-orange from the yellow sodium light and these will stand
out in sodium light with color differing from that of the sodium light.
Another disadvantage of low pressure sodium light is that many objects will look
darker than they would with an equal amount of other light. Red, green, and blue
objects look dark under low pressure sodium light. Most other sources of light
of sodium-like color such as "bug bulbs" have significant red and green output
and will render red and green objects at least somewhat normally.