Flash Lamp


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flash lamp

[′flash ‚lamp]
(electronics)
A gaseous-discharge lamp used in a photoflash unit to produce flashes of light of short duration and high intensity for stroboscopic photography. Also known as stroboscopic lamp.

Flash Lamp

 

an intermittent, high-intensity light source, utilizing plasma glow arising, for instance, during a condensed spark discharge in an inert gas or during combustion of a metal foil in oxygen. A flash lamp is distinguished from a continuously burning gas-discharge light source by greater values of current density and by higher plasma temperature, which can reach 30,000°K (the temperature of plasma in carbon arc lamps does not exceed 6,000°K). Industrial production of flash lamps in the USSR and abroad was begun in the late 1940’s.

There are two basic designs: flashtubes, with a spark gap l = 1–200 cm, flash energy W = 1—10s joules (J), flash duration τ = 10-4–10-2 sec, luminous efficiency η = 30–50 lumens • sec/J, peak brightness B up to 1010 nits, and flash frequency / up to 100 Hz; and spherical flash lamps, with l = 0.1–1 cm, W = 0.001–10 J, τ = 107–10-5 sec, η = 5–15 lumens-sec/J, B up to 3 X 1011 nits, and /up to 104 Hz. Flash lamps are used in photography, optical position determination and light signali-zation, automated and telemechanical equipment, photochemistry, and printing and for optical pumping of lasers.


Flash Lamp

 

a portable pulsed light source for short, intense illumination of objects being photographed. The flash lamp is connected to a camera by a synchronizing contact, which automatically switches the flash lamps on at the moment when the shutter is fully open.

Two types of flash lamps are in use: single-use (flashbulbs) and multiple-use (electronic flashes). Electronic flashes are widely used. A complete electronic flash includes a gas-discharge flash lamp with a reflector, a power-supply unit (a dry-cell battery or a rectifier that can be connected to an AC line with a voltage of 127 or 220 V), a mounting bracket, and connecting cords.

An electronic flash of the type shown in Figure 1 operates as follows. When the switch S is in position (1) the power supply E (usually about 300 V) charges capacitors C1 and C2 through resistors R1 and R3. Capacitor C2 has a lower capacitance and thus becomes charged sooner than capacitor C1. After capacitor C1 (with a capacitance of 800 microfarads or more) is charged, signal lamp L1 lights up (it is usually a neon lamp, connected in series with the current-limiting resistor R 2), indicating readiness to operate. At the moment when the shutter is fully open, synchronizing contact SC closes, and capacitor C2 is discharged through the primary winding of transformer T. At the same time a voltage pulse of 10–15 kV is generated in the secondary winding of the transformer, ionizing the gas in flash lamp L2. Because of the ionization, the gap between the contacts of lamp L2 becomes conductive and capacitor C1 is discharged through the gap; the discharge is accompanied by a powerful flash of light. After the discharge of capacitor C1 the gas in the flash lamp again becomes nonconductive. At this point the recharging of capacitor C1 begins, and after 5–10 sec the electronic flash is again ready to operate. The energy of flash lamps is 36–100 joules; the duration of the flash is 1/400 to 1/2000 sec. The service life is up to 10,000 flashes.

Figure 1. Schematic diagram of an electronic flash

Flashbulbs for a single flash are small lamps filled with oxygen and metal foil. If a voltage of 3–4 V is applied to the ignition filament, or if a striker pin strikes a percussion cap, the foil ignites and burns in the oxygen atmosphere, releasing a light pulse of about 1/25 sec duration. After firing, the flashbulb must be replaced.

REFERENCES

Vaneev, V. I., and E. K. Sonin. Elektronnye lampy-vspyshki. Moscow-Leningrad, 1959.
Shashin, Iu. V. Elektronika v fotografii, 2nd ed. Moscow-Leningrad, 1966.

S. V. KULAGIN

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