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Xenon flash tube

Xenon flash tubes are light sources that use electrical energy stored in capacitors to produce very high intensity flashes. Their light output is very similar to natural daylight in the visible range, and they have a large amount of ultraviolet and infrared radiation, making them more versatile.

There are many different requirements for flash tubes. Each flash tube design may be suitable for only one of the following tasks:

(1) Aerial or studio photography and photochemical lamps, which require high power.

(2) Amateur and studio photography, general industrial and scientific applications, navigation and airport signal lights, medical research and mapping, etc., which require medium power and reasonably short flashes.

(3) Certain scientific applications require flashes of only a few microseconds.

(4) Use with lasers, which require short arcs and high loads.

(5) Stroboscopic applications, which require the lamp to operate under low power and high frequency conditions, such as checking moving parts, and use in ignition timers and flywheel balancers.

1. Structure of flash tube

Except for high-load flash tubes that use quartz glass, most flash tubes use borosilicate glass. The electrodes and seals must be designed to withstand peak currents that may be as high as several thousand amperes, and the energy dissipation is limited only by cracks and melting on the surface of the discharge tube. In order to maintain a high current density in the positive column area, the discharge tube must be small to compress the arc. In order to form a more concentrated light source, the discharge tube is wound into a spiral or other shape that does not affect the basic performance. The size of the discharge tube, the filling pressure, the electrode design, and the emission coating all have a decisive influence on the efficiency of the lamp, the flash duration, and the life. The radiation emitted by pure xenon is suitable for most monochrome and natural color film photography, but if you want to shoot high-speed moving targets, you can fill it with a mixture of argon and trace amounts of hydrogen, which can produce flashes lasting only a few microseconds. Lamps with shorter inter-electrode distances are filled with 1 to 2 atmospheres of gas, while straight tube or spiral lamps with longer positive columns need to be filled with 50 torr to 200 torr of gas.

Xenon flash tubes come in a variety of shapes, sizes, powers, and light output characteristics. The power load range is wide, with each flash being at least 10 joules.

2. Single flash operation

The voltage for charging the main capacitor C can be supplied by some convenient DC power supply, but the small capacitor C2 needs to be charged through a potential distributor. When switch s is closed, the energy released by C₂ produces a high voltage pulse, which, when applied to the wire wound around the flash tube or to the third electrode, ionizes the gas and causes the energy stored in C1 to escape in the flash tube.

To ensure reliable use of the lamp, the trigger voltage is required to reach 4 kV to 16 kV, with an energy of about 5 microjoules, and its pulse rise time is equivalent to a natural frequency of about four kilohertz. In complete darkness, flash tubes are difficult to trigger, but arc tubes can also ensure ignition under the irradiation of photons, or by increasing the trigger energy and voltage.

The charging resistor R1 has two important functions: it limits the current from the power supply and allows high power output from a low rated current power supply, but this is meaningless when recharging for a long time. Secondly, it must limit the charging current so that when the capacitor C1 has discharged, the flash tube can be deionized and extinguished without causing a flash. The flash can only be produced when the voltage of C1 has risen and exceeded the critical value before deionization is achieved. This is the most important thing in the design of the strobe circuit. Some circuit designs automatically disconnect the capacitor from the charging circuit temporarily after each flash without adopting the above restrictions. Electrolytic capacitors are often used for low-voltage lamps (not more than 1000 volts) because of their size and weight. Paper capacitors have low internal impedance and short strobe duration, so the light efficiency is higher. DC power can be obtained by simple means such as batteries or AC rectifiers. The pulse coil is hollow, and Tesla-type transformers are sometimes equipped with an insulated primary winding. They all have low primary resistance, which can discharge the trigger capacitor quickly. The coil should be placed near the flash tube to avoid high-frequency losses caused by long wires. For safety reasons, the maximum voltage on the trigger capacitor used for photography should not exceed 150 volts, because the capacitor is connected to the camera; similarly, to prevent damage to the contact points, the continuous current should be strictly limited to less than 500 microamperes.

Like other discharge light sources, flashlights cannot be operated in parallel. Each must have its own discharge capacitor and charging resistor, and generally its own trigger coil to ensure normal operation. If the operating voltage can be appropriately increased, it can be used in series, and the triggering method is to apply the trigger pulse to the two tubes in the general way, or to their common contacts.

Due to the voltage drop of the battery and the differences in equipment design, the operating voltage of the flash tube should be considered to have a variable range. The British Standard (BS) 3205 of 1969 stipulates a tolerance of +10% to -20% of the nominal voltage. The working limit of the lamp itself should be slightly larger than the above tolerance. The highest voltage is the hold-off voltage. If it is higher than this voltage, the stray ionizing radiation will cause the lamp to trigger intermittently. The lowest voltage should be set as the minimum ignition voltage. If it is lower than this voltage, the lamp flashes irregularly or cannot ignite. In fact, the working voltage is not a fixed data, but is determined by the ionization effect generated by the trigger pulse (i.e. the voltage and energy of the pulse).

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