Strobe light

Blue Strobe light

A strobe light or stroboscopic lamp, commonly called a strobe, is a device used to produce regular flashes of light. It is one of a number of devices that can be used as a stroboscope. The word originated from the Greek strobos, meaning "act of whirling."

A typical commercial strobe light has a flash energy in the region of 10 to 150 joules, and discharge times as short as a few milliseconds, often resulting in a flash power of several kilowatts. Larger strobe lights can be used in “continuous” mode, producing extremely intense illumination.

The light source is commonly a xenon flash lamp, or flashtube, which has a complex spectrum and a color temperature of approximately 5,600 kelvins. To obtain colored light, colored gels may be used.

Scientific explanation of flashtubes

Strobe lights usually use flashtubes with energy supplied from a capacitor, an energy storage device much like a battery, but capable of charging and releasing energy much faster. Recently, some strobe lights have been found to use rectified mains electricity and no capacitors at all. In a capacitor-based strobe light, the capacitor is charged up to around 300 V. Once the capacitor has been charged, a small amount of power is diverted into a trigger transformer, a small transformer with a high turns ratio. This generates the weak but high voltage spike required to ionize the xenon gas in a flash tube. An arc is created inside the tube, which acts as a bridge for the much bigger pulse to flow down later. Arcs present almost a direct short circuit, allowing the capacitors to quickly release their energy into the arc. This rapidly heats the xenon gas, creating an extremely bright plasma discharge, which is seen as a flash.

A strobe without a capacitor storage device simply discharges mains voltages across the tube once it's fired. This type of strobe requires no charging time and allows for much quicker flash rates, but drastically reduces the lifetime of the flash tube if powered for significant periods of time. Such strobes require a form of current limiting, because as mentioned above, an arc acts as a kind of short circuit. If this current limiting were to be eliminated, the flash tube would attempt to draw high currents from the electricity source, potentially tripping electrical breakers or causing voltage drops in the power supply line.

Individual strobe flashes typically only last around 200 microseconds, but can be sustained for greater or lesser periods of time depending on the strobe's intended use. Some strobes even offer continuous mode of operation whereby the arc is sustained, providing extremely high intensity light, but usually only for small amounts of time to prevent overheating and eventual breakage of the flash tube.

Applications

Stroboscopic effect

A strobe light flashing at the proper period can appear to freeze or reverse cyclical motion.

Special calibrated strobe lights, capable of flashing up to hundreds of times per second, are used in industry to stop the appearance of motion of rotating and other repetitively operating machinery and to measure, or adjust, the rotation speeds or cycle times. Since this stop is only apparent, a marked point on the rotating body will either appear to move backward or forward, or not move, depending on the frequency of the strobe-flash. If the flash occurs equal to the period of rotation (or evenly multiplied, i.e. 2*π*n/ω, where n is an integer and ω the angular frequency) the marked point will appear to not move. Any non integer flash setting will make the mark appear to move forward or backward, e.g. a slight increase of the flash frequency will make the point appear to move backward.

Strobe lighting has also been used to see the movements of the vocal cords in slow motion during speech, a procedure known as video-stroboscopy. A common use of a strobe flash is to optimize a car engine's efficiency at a certain rotational period by directing the strobe-light towards a mark on the flywheel on the engine's main axle. The strobe-light tool for such ignition timing is called a timing light.

Other

Strobe lights are used in scientific and industrial applications, in clubs where they are used to give an illusion of slow motion, and are often used for aircraft anti-collision lighting both on aircraft themselves and also on tall stationary objects, such as television and radio towers. Other applications are in alarm systems, emergency vehicle lighting, theatrical lighting (most notably to simulate lightning), and as high-visibility running lights. They are still widely used in law enforcement and other emergency vehicles, though they are slowly being replaced by LED technology in this application, as they themselves largely replaced halogen lighting. Strobes are used by scuba divers as an emergency signaling device.^[1]

Strobelights are often used in nightclubs and raves, and are available for home use for special effects or entertainment.

History

The origin of strobe lighting dates to 1931, when Harold Eugene "Doc" Edgerton employed a flashing lamp to make an improved stroboscope for the study of moving objects, eventually resulting in dramatic photographs of objects such as bullets in flight.

EG&G [now a division of URS] was founded by Harold E. Edgerton, Kenneth J. Germeshausen and Herbert E. Grier in 1947 as Edgerton, Germeshausen and Grier, Inc. and today bears their initials. In 1931, Edgerton and Germeshausen had formed a partnership to study high-speed photographic and stroboscopic techniques and their applications. Grier joined them in 1934, and in 1947, EG&G was incorporated. During World War II, the government's Manhattan Project made use of Edgerton's discoveries to photograph atomic explosions; it was a natural evolution that the company would support the Atomic Energy Commission in its weapons research and development after the war. This work for the Commission provided the historic foundation to the Company's present-day technology base.^[2]

Internally triggered Strobotrons (light-output optimized thyratrons) were available^[3] as well as flood beam CRT-type, grid-controlled Vacuum stroboscopic light sources with fast phosphors.^[4]

The strobe light was popularized on the club scene during the 1960s when it was used to reproduce and enhance the effects of LSD trips. Ken Kesey used strobe lighting in coordination with the music of the Grateful Dead during his legendary Acid Tests. In early 1966 Andy Warhol's lights engineer Danny Williams pioneered the use of multiple stroboscopes, slides and film projections simultaneously onstage during the 1966 Exploding Plastic Inevitable shows, and at Bill Graham's request, Williams built an enhanced stroboscopic light show to be used at Fillmore West.

Strobe lights and epilepsy

Sometimes strobe lighting can trigger seizures in photosensitive epilepsy. An infamous event took place in 1997 in Japan when an episode of the Pokémon anime, Dennō Senshi Porygon (commonly translated as Electric Soldier Porygon), featured a scene that depicted a huge explosion using flashing red and blue lights, causing about 685 of the viewing children to be sent to hospitals.^[5] These flashes were extremely bright strobe lights. They involved multiple colors with a strobe effect at about 12 Hz. Although 95% of the 685 just complained of dizziness, some were hospitalized. Organizers later said that they did not know about the threshold of strobing.

Most strobe lights on sale to the public are factory-limited to about 10–12 Hz (10–12 flashes per second) in their internal oscillators, although externally triggered strobe lights will often flash as frequently as possible. Studies have shown that the majority of people that are susceptible to the strobing effects can have symptoms at 15 Hz-70 Hz, albeit rare. Other studies have shown epileptic symptoms at the 15 Hz rate with over 90 seconds of continuous staring at a strobe light. There have been no known seizures at or below the 8 Hz (or 8 flashes per second) level. Many fire alarms in schools, hospitals, stadiums, etc. strobe at a 1 Hz rate.

See also

Look up strobe in Wiktionary, the free dictionary.

• Electrotachyscope
• Tachometer
• Zoetrope
• Jerkiness, discontinuity in motion pictures, also called strobing
• Photographic flash, often referred to as a strobe light
• Wagon-wheel effect
• Air-gap flash

References