The electronic flash first appeared in the USA during World War 2. Roughly in the 50s of the twentieth century their production began on the European continent. In a short period of time electronic flashes spread rapidly and are currently used by both professional photographers and amateurs. A characteristic feature is their high durability and reliability.
As the source of light for electronic flashes, a xenon discharge lamp, which has an operational life of 10,000 to 5,000,000 flashes, is used. The intensity of the burst is so large that a very small relative aperture is sufficient to shoot at shutter speeds of 1/10,000 sec. Repeatable light parameters make it easy to operate the aperture, depending on the distance of the light to the subject. This distance (beyond the characteristics of a photographed scene) is the only variable factor in assessing the exposure time for the same sensitivity expressed in ISO.
THE CHARACTERISTICS OF ELECTRONIC FLASHES
FLASH POWER - quantity testifying to the power of the burst, is given in Ws (watt second). The flash energy is calculated as follows:
Q - energy Ws ( Watt second )
C - capacitance of capacitor F ( Farad )
U - voltage V ( Volt )
As can be seen from the above formula, the amount of energy depends on the capacitance and voltage to which the capacitor will charge. Faster changes of the energy are obtained by changing the voltage on the capacitor, as its value varies with the square of the voltage. The energy of a flash unit is given mainly with respect to the studio flashes in which various methods of softening the light beam are applied. Therefore, studio flash units are not rated in guide number, which is closely related to the shape of the reflector and placement of the gas tube in it and a method of softening the light.
FLASH GUIDE NUMBER - it is the product of the distance of the flash unit from the photographic subject and aperture. This number depends on the ISO speed of the film (sensor sensivity) and the energy of the flash. For a given flash and the film speed (sensor sensivity) it is a constant, which makes it easier to determine the intensity of the flash exposure. The intensity may be adjusted by varying the distance between the lamp and the illuminated object or by changing the size of the aperture. The guide number of the flash unit with an adjustable angle of coverage (zoom) is given for several focal points, usually at ISO 100, and is from 10 to 100. It is given only for flash with a fixed reflector system. It is impossible to convert guide numbers to flash power and vice versa.
FLASH DURATION - this is the time taken for the flash capacitor to discharge through a discharge lamp. Flash duration in the non-automatic flash is between 1/1000 to 1/500 of a second. In flashes with automatic energy dosage this time is between 1/50,000 sec to 1/300 sec.
COLOR TEMPERATURE - during the discharging of the capacitor, the current flows through the gas discharge lamp and makes the gas (xenon) contained in it glow. The color temperature is approximately 5500K and can be changed using color-correction filters.
RECYCLING TIME - the time required to restore the energy in the capacitor, dependent on the flash power and the method for charging the capacitor, varies from fractions of a second to several minutes.
FLASH POWER STABILITY - within +/- 2% does not cause any visible changes in the exposure.
THE HISTORY OF THE DEVELOPMENT OF THE ELECTRONIC FLASH
Initially, the electronic flash consisted of a bulky power supply and a large reflector. A set weighed around 5 kilograms. In this flash (Fig.1) the current from the power supply charges the capacitor to the supply voltage. Sometime before the capacitor is fully charged, the (neon) ready light turns on. Thus, the charging current flows on until the charging is turned off manually. In the 1960s the first circuit to automatically turn off the charging operation was introduced. Although the flash power was constant, some models were built with a variable number of capacitors to alter the flash output.
Fig.1 Basic scheme for an electronic flash
In the 1970s an energy-lossy system with an automatic flash power dosage was developed. It regulated the energy flow time from the capacitor to the discharge lamp, usually in the range of 1/ 10,000 sec to 1/300 sec, depending on the amount of light reflected from the subject (Fig.2). The part which measures the amount of reflected light was a photocell mounted on the flash unit.
At a fixed f-number and a given film speed, the system regulated the flash duration by means of an additional discharge lamp with a low resistance or a thyristor, a few years later. These elements caused a short circuit and drained the remaining capacitor energy. Some improvement of this system was later made by the introduction of the electronic circuit, which also took into account more aperture values, although not for the full range but usually for two to three values.
Fig.2 Electronic flash with energy-lossy flash power control
Taking photograps at different subject distances, from fractions of a meter to several meters, requires the change of flash duration dozens of time. These requirements led to the creation of a flash energy-saving control system (Fig.3). Cutting the energy supply without any loss after 1/50,000 sec. became possible by using thyristors or IGBTs in the latest solutions. The consequence of this saving is the ability, in favorable conditions, to take about ten times more pictures from the battery while considerably shortening the duration of each charge. All these features are available on modern electronic flashes with power-saving adjustment utilizing a partial discharge of the capacitor.
Fig.3 Electronic flash with energy-saving flash power control
All of the above comments relate to the frontal lighting, when the intensity of light reflected from the subject and falling on the photocell flash is directly proportional to the reflection coefficient and inversely proportional to the square of the distance from the flash to the illuminated object. In many cases, the flash is connected to the camera by a long sync cord to allow side or rear illumination of the photographic subjects. You can also tilt the reflector upwards to use the indirect light reflected off the ceiling. Then, of course, the camera lens sees something different than the photocell built into the flash. This discrepancy was resolved in two ways: by connecting a second photocell placed just off the camera to the flash, or by moving a flash photocell connected with a long cable to the flash.
The further development of this idea was the use of sensors located in the camera. That's how the flash metering and the control system called TTL was born, which, under various names, is now widely used. The TTL Operation method differs in complicated algorithms for the evaluation of reflected light, depending on the manufacturer of photographic equipment.
The problem of exposure using the flash at focal-plane shutter speeds shorter than the minimum synchronization time stated by the camera manufacturer was also solved. The FP High-Speed Sync mode, involving multiple illumination of the frame when shutter curtains are moving, was designed and implemented. This design takes advantage of the energy-saving system.
Fig.4 The flash with FP High-Speed Sync exposure system
The figures above illustrate the changes which take place in the portable flashes from power-up to the exposure of the first photograph. Studio flashes with output up to 200 Ws are made according to schematic diagrams in figures 1 and 3. Flashes of higher output are made only on the basis of the figure 1.
Great progress was achieved also in flash capacitor charging circuits, which manifests itself through the replacement of the high-voltage batteries and electromagnetic inverters by highly efficient and reliable transistor inverters. Several types of high-performance batteries applicable to flash units have been developed.