To put an end to the wild times of Flash Powder, magnesium ribbons and sheets, one had the ingenious idea to put the magnesium inside a glass bulb and light it up with an electric spark. It avoids the smoke and the handling of raw chemicals. But as the quantity of heat released in the process is quite high and the glass quite often does not resist, there is a transparent plastic layer covering the bulb. It does not crack and in this ways it holds the glass tight. Of course it is one bulb one shot and to substitute a bulb that just fired, it is normally still so hot, that burning your fingertips becomes part of the game.
Still today flashbulbs are in production for professional use but they are quite expensive. The advantage is the amount of light that is outputted. Difficult to match with electronic in some situations where, for instance, there is no eletricity outlets available or it is not possible to carry loads of equipment. Caves and nature photography are good examples. A dozen of big flash bulbs weight less than a normal eletronic flash and can light up a theatre.
When one uses electronic flash there is no need to think if the flash will fire exactly when the shutter is open. That is because most of the cameras produced from the 70’s onwards operate only for that kind of flash. To synchronize the shutter with the light for bulbs we must think of it. Here is a page from the book Photo-Technique by H.J.Walls – Focal Press 1954, that explains it in details.
Below I attached the exposure charts for some bulbs. With a GE Nº5, shutter at 1/25, f4 and film 400ASA one can photograph a subject as far as 50 meters (150 feets).
Find more info and bulbs for sale at: Cress Photo . Also Meggaflash is a source for current production of flash bulbs. Even if you are not to purchase from them, the site in itself is really interesting and shows the kind of application where this old technology has still the competitive edge.
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