Xenon Strobe and Flash Safety Hints

This is not guaranteed to have everything you need to know. Please use caution and common sense. Please read this document in its entirety before building, troubleshooting, or repairing xenon flashes or strobes.

Table of Contents

Electric Shock

The main energy storage capacitor(s) of even a small camera flash is typically charged to 250-330 volts. I have seen the photoflash capacitor in one model of camera flash unit get charged to 370-380 volts when used as directed, even though the photoflash capacitor was only rated for 330 volts. This voltage is usually around 300 volts but may be as high as 600 volts in some smaller party strobes and flashing alarm lights. In many professional photographic flash units, the voltage is even higher - often around 500 volts, in excess of 800 volts in some cases. I have one source that I consider reliable state that the capacitors in his large flash unit charge to 930 volts.
The longer flashlamps used in some lasers and photocopiers require even higher voltages. Some of these use voltages over 2000 volts.

Meanwhile, you might be wondering how much current/charge/energy is safe or unsafe. It is generally considered that the amount of energy stored in a capacitor roughly represents how badly it can shock you. Many sources state that a discharge of 10 joules into the human body can be fatal. Some sources state that a shock as little as 1 joule has some chance of being fatal. If the voltage is high enough to burn through your skin, or your skin is especially conductive due to perspiration, then a higher percentage of the capacitor's energy will be dissipated in your vital organs, and such a shock is more likely to be fatal.

It is recommended to avoid shocks of even 1/4 joule.

The energy storage capacitors aren't the only thing that can kill you. The source of high voltage DC that charges them up can also cause dangerous electric shocks.

Furthermore, consider the hazards of high voltage and line voltage AC. There is a popular misconception that steady DC is worst due to its supposed ability to cause muscles to stay contracted. This is largely untrue, and AC actually sometimes causes this. AC of power line frequencies is also more capable of disturbing your heart rhythms than steady DC or AC of higher frequencies.

Please note that the minimum current considered capable of being possibly fatal could be as low as 5 milliamps according to some sources.

Capacitors can surprise you!

Please also note that they may have some charge after being apparently discharged. The flashtube often stops conducting before the capacitor is completely discharged. In addition, capacitors have a nasty trick of not completely discharging immediately even if shorted. Some energy is "trapped" in the capacitor's dielectric by a phenomenon known as "dielectric absorption" and the voltage across the capacitor can increase for several minutes after the capacitor has been shorted for several seconds. Some particularly bad capacitors have been known to regain 20 percent of their original voltage after being briefly shorted.

Another hazard is capacitors in the voltage multiplier circuits that some strobes have. They may not all immediately discharge if the main storage capacitor is discharged. They may partially recharge the storage capacitor afterwards, or may simply be a source of hazardous voltage. In one voltage multiplier circuit, uneven discharge of these capacitors after unpulgging the unit results in significant voltage across the unit's plug.

Overall, be careful of all capacitors in any strobe and do not assume that they are not charged.

Trigger Circuit Hazards

For one thing, being shocked by the trigger circuit may cause you to involuntarily bump into something else, possibly another circuit capable of a deadly shock.

In addition, the trigger circuit usually is connected to one side of the main energy storage capacitor. Touching the other side and the trigger circuit simultaneously is obviously hazardous.

If for some reason the trigger circuit makes a spark to anything connected to the side of the main storage capacitor other than what the trigger circuit is connected to, there could be trouble. Such sparks are sometimes conductive enough to be maintained by the main voltage, and flash into bright, sometimes loud arcs. Usually, these arcs die out as soon as they greatly discharge the main energy storage capacitor. However, dissipating much of the stored energy at the spark point or in the trigger transformer may be quite damaging.

Please note that the trigger circuit typically produces a few kilovolts, which can spark through a few millimeters of air.

How to minimize shock hazards

2. If possible, avoid working alone. A second person may warn you if you are about to do something dangerous.
In the event of cardiac arrest or fibrillation, you have some chance if CPR is applied or someone calls an ambulance. However, CPR is not always successful, even under the best circumstances.

3. Work with only one hand. This greatly reduces the chance of electric shock affecting your heart.

4. In the event you are working on grounded or line-powered equipment, keep yourself insulated from ground by using non-conducting flooring, shoes, work surfaces, etc. Surfaces that are conductive enough to dissipate static electricity (such as plywood) are generally not conductive enough to be a shock hazard at a few hundred volts. However, concrete sometimes is hazardously conductive, and should be covered with wood or other insulating material. Also, using an isolation transformer reduces risks from touching both something live and ground.

Excessively Bright Light

In most cases, things are safer if you test strobes in a brightly lit work area. Brighter ambient light will make your eyes' pupils constrict, which lets less strobe light into your eyes.

This is usually enough to safely work with most camera flashes, party strobes, and stroboscopes.

In the event you need to look at particularly bright flashtubes or you think you might be looking at one as it goes off, you may want to wear goggles used for acetylene welding. These will block most of the visible light, as well as nearly all infrared and ultraviolet. If you have to make any adjustments or do any kind of work on the strobe's circuitry while wearing welding goggles, you should have enough ambient light to easily see what you are doing.

Amber sunglasses that are rated to block UV are also useful to safely look at a flashing flashtube in borderline cases. They block nearly half the visible spectrum and all ultraviolet. Furthermore, not all visible wavelengths are equally bad, and amber sunglasses block the worse violet and blue wavelengths most capable of causing photochemical damage to your eyes. If ambient lighting is nice and bright, amber sunglasses will usually be enough protection to look directly at most studio flashes. (CAUTION - I disclaim any warranty for your eyes.)

Ultraviolet Radiation

If the flashtube is made of glass, such as cheaper camera flash, stroboscope, and party strobe flashtubes, things aren't too bad. Glass absorbs the shortwave UV and most mediumwave UV. Longwave UV easily gets through glass, however.
Longwave UV and the shorter visible violet wavelengths are not completely safe to the human eye. If the lens of the eye is subjected to ultraviolet exposure comparable to that of bright daylight, it is possible for "nuclear cataracts" to form or get worse. This is a dull brown tint in the lens, and is permanent. Of course, casual exposure to a strobe is not a significant problem. Prolonged, heavy exposure for great peroiods of time could be a problem.

UV can be blocked by acetylene welding goggles or amber or yellow UV-blocking sunglasses. Polycarbonate safety goggles block most longwave UV (and all shortwave and mediumwave UV). In the event you want to reduce the UV output of your strobe, consider using a Gam #1510 UV-shield filter gel or a Rosco UV filter gel (03114).These are available at some theatrical supply stores.

If the flashtube is made of quartz, things get nastier. All longwave and mediumwave UV gets through quartz, as well as the germicidal range of shortwave UV (UV-C), and a bit of "vacuum ultraviolet" (VUV). If oxygen is exposed to VUV, ozone forms. You can even smell ozone at close range if the flashtube emits VUV.

There is such a thing as quartz tubing doped with substances to block some ranges of UV. Some quartz flashtubes may emit some germicidal shortwave UV and plenty of mediumwave UV (UVB) and not cause any detectable ozone.

If you are exposed to injurious amounts of mediumwave or shortwave UV, symptoms usually don't appear immediately. Do not assume that there is not a dangerous level of UV if no symptoms of ill effect of exposure arise quickly.

The conjunctiva of the eye is particularly vulnerable to being burned by shortwave UV. Even here, symptoms usually do not appear immediately.

In the event ultraviolet conjunctivitis develops, go immediately to an emergency room and tell a doctor that the problem is due to shortwave ultraviolet exposure. Prompt, proper treatment may reduce any damage - but is not foolproof, sorry! Symptoms may continue to worsen for hours after exposure.

If you are building your own strobe with a quartz flashtube, you must use glass to block the shortwave and mediumwave UV. Most transparent plastics at least two millimeters thick also work, but may be discolored by heavy UV exposure.

It is recommended to minimize the path that VUV can take before being absorbed by glass. The less air it goes through, the less ozone is formed. Even so, quartz flashtubes ventillated by atmospheric air must only be operated in a well ventillated area or a very large room, since ozone is bad to breathe. If one can smell the ozone, it is hazardous to breathe for even a few hours. Large rooms can dissipate ozone, since it breaks down into ordinary oxygen within minutes after leaving its source. (Ozone lasts longer at lower pressures and lower temperatures in the ozone layer of the atmosphere.)

If you are working on a professionally made xenon flash with a quartz tube, remember that any glass shields and lenses that are part of the flash are probably needed for UV blocking purposes.

If you are building your own strobe light and you want to greatly reduce any possible radiation hazards, use a glass shield followed by a Gam #1510 UV-shield gel.
Blocking visible violet through mid-blue further reduces the risk of eye damage with little loss of brightness. At very high brightness, these wavelengths may be more damaging than longer wavelengths due to nasty photochemical effects. These shorter visible wavelengths can be blocked with a yellow filter gel such as Roscolux #10 or #12, or Gam #480. Of these filters, the Roscolux #10 is the brightest.
If you only want a partial blocking of violet and deep blue, use a Gam #470 or Roscolux #310 filter gel. Of these, the Roscolux #310 seems to be slightly brighter and slightly better at blocking violet. These filters attenuate most violet-blue and mid-blue wavelengths by 70-80 percent, and reduce violet wavelengths by about 60 percent. Lighter yellow filters such as Gam #475 and #510, or Roscolux #06 do not attenuate violet much. The Roscolux #07, however, seems to be a good light yellow filter for mildly reducing violet (by about 50 percent) and deep blue (by about 35 percent).
Please beware that the above yellow and light yellow filter gels are not particularly good at blocking UV. The Gam #1510 UV shield gel is strongly recommended in addition to any yellow filters.

Yellow and amber-orange acrylic sheets such as "plexiglas", on the other hand, do absorb UV well.

Heat and Fire Hazards

As for heat hazards other than failure? It is common for some strobe parts to get really hot. In a repeating strobe, the flashtube is likely to get burning hot. Cheap glass flashtubes can safely operate with part of the tube surface close to the melting point of solder. A few better glass flashtubes safely operate with parts of the glass surface well above the melting point of solder. Many quartz flashtubes safely run with the main part of the flashtube tubing at nearly red-hot temperatures!
There are a few popular circuits with power resistors. There are one or two common circuit types where 220-240 volts AC is rectified and this charges the main capacitor, but charging current is limited by a power resistor to allow the flashtube to de-ionize. Please note that most power resistors get burning hot in normal use. At half rated power, most power resistors get egg-frying hot. At full power, most power resistors get bacon-frying hot and some get hot enough to melt solder. Use caution when troubleshooting, and use caution in any relevant homebrew and kit designs!

Other Hazards

I have known parts to explode. This usually happens only in homebrew designs, improper repairs, improperly built kits, adventurous prototypes, and the like. If you wear any sort of safety goggles, you probably won't get seriously hurt. Electronic products lacking picture tubes or other large vacuum/pressurized devices generally don't explode with enough force to seriously injure body parts other than eyes.

As for anything else to go wrong? No warranty that you don't suffer damage to yourself, others, other living things, or to real or personal property whether in ways not mentioned above or otherwise!

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