Xenon flash lamp

Xenon flashlight device

Flash in action.

Lamp IFK-120 Soviet flash units. An electrically conductive coating is applied to the outer surface of the glass tube (third electrode)

Impulse lamp - an electric discharge lamp designed to generate powerful, incoherent short-term light pulses, the color temperature of which is close to sunlight.

Content

1 device
2 Principle of work
3 emission spectrum
4 Flash intensity and duration
5 Application
6 See also

Device

The flash lamp is a sealed tube made of quartz glass , which can be straight or bent in the form of various figures, including a spiral, in the form of a letter $U$ ${\ displaystyle U}$ , or circles, to be placed around the camera’s lens during shadowless photography. The tube is filled with a mixture of noble gases , mainly xenon . The electrodes are soldered to both ends of the tube and connected to a large- capacity electrolytic capacitor (in some cases, connection via a choke ). The voltage on the capacitor plates is from 180 to 2,000 volts , depending on the length of the tube and the composition of the gas mixture. The third electrode is a metallized path along the outer wall of the tube or a thin wire wound around the lamp tube in a spiral indented from the main electrodes.

Then, a high voltage pulse is applied to the third (ignition) electrode, causing ionization of the gas in the tube, the electrical resistance of the gas in the lamp decreases, and an electric discharge occurs between the electrodes of the lamp.

A flash lamp can have only two electrodes, in which case the ignition electrode is aligned with the cathode .

Principle of Operation

The flash occurs after the ionization of the gas and the passage through it of a powerful pulse of electric current. Ionization is necessary to reduce the electrical resistance of the gas so that hundreds of amperes of current can pass through the gas inside the lamp. Initial ionization can be obtained, for example, with a Tesla transformer . A short-time high-voltage pulse applied to the ignition electrode creates the first ions. A current that begins to flow through the gas excites xenon atoms, causing the electrons to occupy orbits with higher energy levels. Electrons immediately return to their previous orbits, emitting a difference in energy in the form of photons. Depending on the size of the lamp, the xenon pressure in the lamp can be from a few kPa to tens of kPa (or 0.01-0.1 atm . Or 10-100 mmHg ).

In practice, an ignition pulse transformer is used for the initial gas ionization. A short high voltage pulse is applied relative to one of the electrodes (most often the cathode) to the ignition electrode, thereby ionizing the gas contained in the lamp and causing the discharge of capacitors to the lamp. The igniting pulse, on average, exceeds the operating voltage of the lamp by 10 times. To ignite a two-electrode lamp, storage capacitors are charged with a voltage higher than the lamp self - breakdown voltage (this parameter is present for all types of flash lamps), as a result of which ionization and discharge in the gas occur.

To ignite a flash lamp, it is important to know its parameters, such as: operating voltage , flash energy , self-breakdown voltage , interval between flashes and load factor .

The flash energy is calculated by the formula: $W={\frac {C\times U^{2}}{2}}$ ${\ displaystyle W = {\ frac {C \ times U ^ {2}} {2}}}$ where

$W$ ${\ displaystyle W}$ - flash energy, J ;

$C$ ${\ displaystyle C}$ - capacitor capacity , Farad ;

$U$ ${\ displaystyle U}$ - electric voltage on the capacitor, Volt .

The passage of electric current through the ionized gas stops as soon as the voltage on the capacitor plates drops to a certain value, the damping voltage $U_{g}$ ${\ displaystyle U_ {g}}$ usually 50-60 volts .

The flash energy formula will look like this: $W={\frac {C\times (U^{2}-U_{g}^{2})}{2}}$ ${\ displaystyle W = {\ frac {C \ times (U ^ {2} -U_ {g} ^ {2})} {2}}}$

The self-breakdown voltage parameter is used to calculate two-electrode lamps.

Also, special attention must be paid to the load factor (dimension - μF × kW · h ). This parameter is not recommended to be exceeded - this will result in an accelerated lamp failure. That is, to work at a given lamp energy and not exceed the operating voltage.

Also, during flash in the lamp, heat is generated. The interval between flashes must be observed. For ordinary glass, the maximum temperature is 200 ° C, for quartz glass - 600 ° C. For high-power lamps, cooling is used - water, sometimes - organosilicon compounds (the most effective cooling).

Scheme of electronic network flash.

The principle of operation of the flash circuit

A large- capacity storage capacitor C ₁ (typical capacitance values are hundreds of microfarads, operating voltage is 300 ... 400 V, depending on the type of flash lamp), connected in parallel with the electrodes of the EL ₁ xenon lamp, is charged from the AC mains through a rectifier ( diodes VD ₁ and VD ₂ with a current limiting resistor R ₁ ) from either a high voltage battery or a low voltage battery and an inverter . At the same time, through the resistors R ₄ and R ₅ , the capacitor C ₂ is charged. A neon lamp HL ₁ , switched on via a voltage divider ( R ₂ , R ₃ ), by its glow signals the readiness of the flash. When the sync contact of the camera (or test button SA ₁ ) is triggered, the capacitor C ₂ closes on the primary winding of the step-up transformer T ₁ , on the secondary winding of which a high-voltage (tens of thousands of volts) pulse is formed, ionizing the gas in the lamp through its ignition contact. The discharge of the capacitor C ₁ through the lamp is accompanied by a bright flash of light. At the end of the flash, the cycle repeats. The next flash is possible only after the capacitor C _{1 is} fully charged, which is reflected by the lighting of the neon lamp HL ₁ in its circuit. Capacitor recharge time (minimum interval between flashes) is limitedpower of the converter and the maximum current that the batteries can give.

Emission Spectrum

As with all ionized gases, the xenon emission spectrum contains various spectral lines . This is the same mechanism that gives a characteristic glow to neon . But in xenon, the spectral lines are distributed over the entire visible spectrum, so that its radiation appears white to a person.

Flash Intensity and Duration

With a short pulse, the number of electrons emitted by the cathode is limited. With a longer pulse, heat dissipation is also limited. Most flash lamps have a pulse duration from microseconds to several milliseconds, with a repetition rate of up to several hundred hertz.

For flash lamps (with high flash energy and long duration between flashes), the pulse power exceeds hundreds of kW.

The radiation intensity of a xenon flash lamp is so high that it can ignite flammable objects in the immediate vicinity of the lamp.

Application

Lamps by operating modes are divided into lighting (used mainly in flash) and stroboscopic. Stroboscopic lamps have much lower flash energy, but the flash frequency can reach several hundred hertz. At frequencies of about 400 Hz , ignition of an electric arc is possible, which is extremely undesirable.

Since the duration of the flash is well controlled and its intensity is quite high, it is used mainly in flash. Also used in high-speed photography, pioneered by Harold Edgerton in the 1930s.

Lamps with a shorter flash duration are used in strobe lights .

Due to the high radiation intensity in the short-wave part of the spectrum (up to UV) and the short flash duration, these lamps are excellent as a pump lamp in a laser . The selection of the gas composition of the lamp allows maximum radiation in the regions of maximum absorption of the working fluid of the laser.

Flash lamps are also used in cosmetology : they are used for photoepilation and skin rejuvenation together with a filter that cuts off the ultraviolet and blue components.