Photoflash Units

© Brooke Clarke 2011

Background

One Time Flash Methods

The light sources used for photoflash (Wiki: Flash Photography) work have changed over time.  The early units were chemical flash powders (Wiki).  Next came flash bulbs.  The popular type had a bayonet base and the professional type had a standard Edison screw base (the same as a household lamp).  Next came the Flash Cube (Wiki)  that worked with many high volume consumer cameras.  The big advantage of these was that you got four flashbulbs in one unit and it rotated after each shot allowing for a much shorter time between exposures.

Electronic or Strobe Flash

Unlike the above one time methods where you threw away the flash bulb, strobe flash has a great advantage in that the light comes from a tube that can be fired a large number of times.

First Generation

These used a bank of high voltage batteries and had a very short recycle time.  The batteries charged a capacitor that has the flash tube connected directly across the capacitor.  The typical operating voltage was in the 200 to 600 Volt range which was not enough to trigger the flash tube.  In order to start the ionization of the tube a seperate very high voltage (a few thousand volts)  transformer was used which drove a wire wrapped around the flash tube.  Once started the flash lasted until the capacitor voltage drained to below the voltage where the tube would stop conducting.  That's to say that almost all the energy in the capacitor was drained for every flash.  If the battery voltage went down then the flash power would also go down.

Synchronization

It's important the the light from the flash occurs when the shutter is open otherwise some or all of the light is wasted. 
There are two common types of shutter:

Leaf or Diaphragm shutters (Wiki)

These typically are made as part of the lens assembly.  They are common for view and large format cameras.

Focal Plane shutters (Wiki)

Tese are positioned just in front of the film as part of the camera body, so the lens can be made without a shutter thus at a lower cost.  This is common for 35 mm film cameras.  For the faster shutter speeds one curtain starts moving and a short time later the other curtain starts moving.  This way a narrow gap moves across the film.  If a strobe flash went off during one of the faster shutter speeds only a small part of the film would be exposed, and that same problem exists for conventional flash bulbs.

So to use flash with a focal plane shutter either the shutter speed needs to be slow enough so that the shutter is fully open or a special type of flash bulb is used that has a long peak light output (called FP bulbs).

Flash Powder

The first way to do this was to simply remove the lens cap and ignite the flash powder then return the lens cap.

Flash Bulbs

When flash bulbs came out most cameras had leaf or diaphragm shutters (Wiki) and a provision to use a cable release (Wiki) to trip the shutter. So one of the first camera flash units was the Mendelsohn Speedgun (separate web page) for use with the Graflex (Wiki) Press cameras. There were a number of schemes to get the light synchronized with the shutter opening that all related to the use of the shutter release cable.

The next innovation was to incorporate a electrical PC flash contact (Wiki) into the shutter mechanism.  There may be a flash mode switch associated with the electrical contact.  The flash synchronization modes are:

M

For Medium type flash bulbs, the most common and have peak light about 20 to 25 milliseconds after power is applied.

F

For Fast sync. where the bulb is at peak light output in about 5 milliseconds.

FP (Front Curtain Sync)

For Flat Peak flashbulbs.  The contacts are closed at the start of the exposure, also called front curtain sync.  A problem with this is that if there is streaking (say taking a photo of a car at night where the lights cause a streak) the streaks are in front of the car. 

X

For Electronic Xenon flash where the switch closes at the same time as the shutter is fully opened.  For focal plane shutters the shutter speed needs to be slow enough so that the shutter is fully open, i.e. does not work at the faster shutter speeds.

Rear Curtain Sync

Modern digital cameras have Rear Curtain Sync so that the flash fires at the end of the exposure time thus the streaks are behind the car.

Patents

813148 Magnesium Flash-Lamp, Feb 20, 1906, 431/363
813849 Flash-Light Device, Feb 27, 1906, 431/357
814243 Photographic Flash-Light Apparatus, Mar 6, 1906, 431/357

2204424 Photoflash Synchronizer, Samuel Mendelsohn
RE22338 Photoflash Synchronizer, Samuel Mendelsohn
2332587 Testing Device for Camera Synchronizers, Samuel Mendelsohn, Oct 26, 1943

Related

Links

Russian w/Google Translator: E640 info on IGBTs 

Brooke's PRC68, Products for Sale, Photography
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