Arc Lamp Safety

Presented by Orin LaneyAtwood Research

toIEEE Santa Clara Valley PSES

February 26th, 2006

olaney@juno.com

A Nanohistory

• Sir Humphry Davy constructed the first arc lamp (1807), using a battery of 2,000 cells to create a 100-millimetre (4-inch) arc between two charcoal sticks. When suitable electric generators became available in the late 1870s, the practical use of arc lamps began.

Local HistoryIn 1831, San Jose was the first electrified city west of the Rockies. In 1881, arc lighting came to the city.

• The Light Tower, erected at the intersection of Market and Santa Clara Streets in 1881, was a landmark feature of San Jose for 34 years. It stood 237 feet high, and was designed to light downtown San Jose using six carbon arc lamps at the top of the tower -24,000 candlepower.

San Jose Light Tower

In 1889, a Parisian delegation took copious notes of the tower and skulked back to France to build the Eiffel Tower. San Jose would later sue Paris, unsuccessfully. In 1915, the 15-ton tower was felled by a windstorm. A half-sized replica now stands in San Jose's Kelley Park.

And now for something completely different…

Types of Gas Discharge Lamps• Low pressure glow lamps have modest

intensities and low thermal output, can be AC or DC powered for the same lamp. Line output spectra is determined by choice of gas fill.

• Flash lamps are intended for low duty cycles, have modest thermal emission compared to peak power. Pulsed DC operation is normal. 1 to 3 atmosphere fill, broadened output spectra.

• Arc lamps are rated for continuous operation, use different electrode designs for AC and DC, have high thermal output. Light output part gas emission, part electrode black body radiation.

Discharge Regimes

Two Arc Lamp Families

• Xenon arcs typically used for backlighting, projection, high intensity lamps, and other light requirements for human viewing. Color temperature reasonably close to daylight.

• Mercury arcs create high intensity ultraviolet output, uses include imaging processes (printing plates, PWB resist exposure), photochemistry, sterilization. Some xenon is included to start the arc, lamp is fully on when mercury vaporizes.

Lamp Spectra

Xenon Spectrum (gas alone)

Xenon Short Arc

Mercury – Xenon Short Arc

Long Arc vs. Short Arc

• Short arcs provide a small, intense light source that can be precisely focused in optical systems, such as projectors.

• Long arcs have an elongated envelope, and are typically used with elliptical reflectors to simulate sunlight or other purposes that need a wide output angle.

15 KW Short Arc Lamp

Short Arc Intensity Distribution

Short Arc Lamp Construction• Air cooled lamps range from 10W to

10KW, forced air cooling above 75W. • Liquid cooled lamps 7KW to >35KW• Quartz Glass is used because it has the

right optical properties, tolerates thermal gradients & shock, and doesn’t soften till 1300 degrees C (2370 F)

• Anode is tungsten, cathode is thoriated tungsten, molybdenum used at glass seals

Liquid Cooling Ports on 15 KW Short Arc Lamp

Arc Lamp Applications

• Fiber optic illuminators 20 – 100W• Automobile headlamps ~35W• Video projectors 150 – 400W typical• Movie theater projection ~5 to 15 KW• Searchlights ~10 KW to 50 KW• Industrial applications requiring high

intensity or ultraviolet light• Some medical & scientific instruments

Arc Lamp Hazards

• Luminous intensity• Ultraviolet output• Ozone• High voltage• Thermal• Fire• Explosion

Luminous Intensity

• Intensity (power density) is beyond safe limit of eye. It’s like looking at the sun.

• Must view through a filter (for more reasons than just this).

• When viewing is a routine requirement, a filtered window is often built into the apparatus, lest someone lose the filter and be tempted to view the arc directly.

Ultraviolet Output• UV output is very intense, resulting hazard is

comparable to welding or carbon arcs.• UV can bleach exposed warning labels!• Sunburn and cancer hazard for exposed skin.• Many wavelengths present: UV-A, UV-B, and

some UV-C. Each band is hazardous to a different part of the eye. Even brief viewing can create permanent damage.

• OK to view arc through #12 welding glass or sun-viewing filters. Most other dark transparent materials can transmit too much IR, allowing thermal damage to eye.

Ozone

• Created when oxygen is exposed to wavelengths shorter than 240 nanometers.

• O3 is very reactive; powerful bleach and disinfectant. If you can smell it, lungs will be irritated within hours.

• Hot ozone decays rapidly. If vented outside the building, no hazard by the time it exits duct.

• Ozone formation can be controlled by adding certain oxides to quartz envelope.

Starting & Operating Voltages

• Multi-kilovolt ignition pulses an obvious hazard. 15 – 75 KV @ up to 100s of milliamps. Hard on lamp life.

• Operating voltage 12 to 120V (depends on lamp size and design)

• 50W lamp ~ 20 volts @ 2.5 amps• 250W lamp ~ 50 volts @ 5 amps• 10 KW lamp ~80 volts @ 125 amps

Thermal

• Running lamp temperatures can be 900 to 1050 C (1650 to 1920 F) at the envelope.

• Socket design and connecting wires must be of high temperature construction.

• Circulating air can be hot enough to be hazardous.

• IR output is high, can damage absorbent materials without need for thermal path.

Fire Hazard

• Derivative of thermal hazard.• Filter incoming air, avoid dust deposits.• Hot air vent path should avoid combustibles.• Lamp IR output alone can be dangerous, keep

optical path clean too. Movie theater projector example: running film OK (1/24th second per frame), broken film means crisped frame at stop point. That’s why “safety film” replaced nitrocellulose movie film!

Projection Room Fire Door

Explosion• Cold lamp has internal pressure ~ 5 to 10

atmospheres, hot lamp can be double to triple.• Explosion of a running lamp creates high

velocity shards of red hot glass.• Scratches or nicks too small to see can build up

strain, lead to sudden failure.• Do not touch the lamp with bare skin. Skin oils

on a hot lamp permanently etch the quartz (devitrification), causing local overheating. Strain buildup leads to premature, catastrophic failure. Wiping lamp with alcohol before installation can be a useful precaution.

Designing with Short Arc Lamps

Safety Standards

• CIE S 009/E:2002 Photobiological Safety of Lamps and Lamp Systems

• Safety EN 60598-1:1997 Luminaire Safety Standard, General Requirements EN 60598-2

• Other applicable standards are general in nature, not specific to arc lamps per se.

Implementation Issues -Mechanical

• Proper lamp orientation is critical. Internal convection currents are part of the thermal design. Lamp stability & lifetime affected. Horizontal and vertical styles are available.

• Enclosure mounting is required; no human exposure to direct light, provides explosion containment. Access cover interlock req’d.

• Lamps run red hot, need proper cooling.

Implementation Issues - Electrical

• Ballast design is lamp specific, lamps should not be overdriven (excess temperature, reduced life). (max 110%)

• Or underdriven. The cathode runs cooler, which reduces electron emission, which raises the nearby voltage gradient, which accelerates cathode sputtering, which reduces lamp life. (min 85%)

• DC lamps are polarized.

Starting the Lamp• Cold strike requires 15 to 25 KV pulse for

smaller lamps, up to 75 KV for large ones.• Hot restrike requires up to 2X cold strike voltage.• Ignition at strike not guaranteed, might need

multiple tries. Limit tries to 3 to 5; continued failure implies end of lamp life or other problems.

• Some lamp designs have 3rd starting electrode.• Reduced warm up power is easier on lamp life.• Warm up time runs 2 or 3 minutes for xenon, 10

to 15 minutes for mercury.

Driving the Lamp

• AC drive is usually a square wave to minimize peak to average power ratio. Typical frequency from 200Hz to 50KHz.

• Some DC lamps require well filtered power for full lamp life. Series inductor useful.

• Continuous monitoring of lamp voltage and/or current is good practice, allows detection of lamp aging or sudden failures.

Simmer Circuits• Mostly a flash lamp technique.• Simmer circuit maintains low power glow

discharge, avoiding high voltage restrike (hard on lamp life, immediate ignition not guaranteed). Simmer power too low to impact lamp life.

• Used in specialty applications where light output must vary suddenly with exact timing. Best for short off times of arc lamps (electrodes stay hot).

• Shutter or other means must be used if simmer mode light output intolerable or off time too long.

Lamp Shutdown

• Forced air or liquid cooling should continue to run until a safe temperature is reached.

• This helps prolong the integrity of the quartz envelope.

Drop in Output with Age

End of Life Issues• Life expectancy of 750 to >2000 hours typical,

depending on lamp design and usage.• Running voltage slowly rises with age; current

decrease to compensate is good practice.• As electrodes sputter onto quartz walls, light

output reduces, running temperature increases.• Envelope develops stress points.• End of life = 25% decrease in light output, >10%

arc instability, or cessation of operation under normal conditions.

Lamp Replacement

• Replace at 125% of average life, if current drops below spec sheet min, or if voltage rises above spec sheet max.

• Never replace a lamp while it is hot.• Use safety goggles, soft cotton gloves.• Consumer serviceable equipment should use

lamp assemblies that include an integral shroud. Prevents touching and mechanical stress, contains glass if an explosion happens.

Disposal• Manufacturer advice is to throw lamp in trash. Quartz

glass and xenon are inert, tungsten / molybdenum electrodes are considered benign.

• Lamps cannot be rebuilt, no market for used lamps. Possible scrap value in electrodes.

• Remnant explosion hazard probably not an issue where actual bulb is contained in a larger assembly.

• What to do with bare bulbs? Safely shatter the bulb? (how?) Repackage before discarding? Return to manufacturer? If lamp life ~ equipment life, leave installed and toss whole unit?

• Mercury-containing lamps are considered hazardous material – dispose accordingly.

That safely disposes of our topic for tonight…

Acknowledgements• http://www.ushio.co.jp/products/catalog/imp_e/1035

.html• http://www.ushio.nl/index.php?page=spectrum• http://www.pti-nj.com/obb_lamps.html• http://www.hilltech.com/products/uv_components/S

hort_arc_lamps.html• http://en.wikipedia.org/wiki/Xenon_arc_lamp• http://www.britannica.com/eb/article-9009242• http://www.sanjose.com/underbelly/unbelly/Sanjose

/plaza/plaza7.html