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[OM] Fast Bright Flash

Subject: [OM] Fast Bright Flash
From: Hughes <hi100@xxxxxxxxxxxxx>
Date: Sun, 27 Apr 2003 22:50:24 -0700 (PDT)
Hi all high speed Flashers,

There is a guy interested in high speed flash (see huecandela.com) who 
recommends the Vivitar
283/285s as doing a particularly good job of short duration flash when set to 
minimum power. 
He claims they get to about 1/16,000sec, and when feeding back the light to the 
sensor it gets to
close to 1/30,000sec.  He uses multiple units synced together (electronically 
not optically I
think).

Really high speed flashes (and true stroboscopes) use a different kind of flash 
tube which has an
internal trigger contact instead of the external wire contact on consumer 
camera flashes. In
consumer camera flashes I would guess the delay before flash fires depends on 
the trigger
autotransfomer and associated capacitor which causes a delay before the trigger 
voltage reaches
the 5kV or so needed to trigger.  Adding a remote optical trigger adds more 
delay. The old self
powered triggers were very slow as they depended on photovoltaic cells. If you 
search on the web
there is lots of do it yourself literature on remote triggers, this appears to 
be a favourite do
it yourself project!    

Auto flashes of course, extend out the flash duration by adding a series 
inductor in the flash
circuit to get durations out to a few milliseconds. This reduces the current 
for the electronic
switches and more importantly the circuitry does not have to respond in a very 
short time to
"quench" the flash in auto mode. In non-auto flashes the flash duration tends 
to be much shorter
(if comparing full power) as it is limited mainly by capacitor and lead 
inductance and circuit
wiring resistance.  The flashes are often designed so the inductor and flash 
tube form aa
approximately critically damped circuit. (see EG&G/PerkinElmar website for 
details)

The F280 (according to the Rochester institute research paper tests) runs at 
about 20kHz, as far
as I remember, so each mini flash is significantly lesss less than 1/20000 sec. 
 According to
Clint this is the same flash tube as used in the T32.

Although the flash duration is often qouted, it is a bit misleading as the 
light output is not at
all constant and looks more like a distorted bell shaped curve when not 
quenched. So effective
movement stopping duration (at full output) can be shorter than quoted.

The high power Metz CT60-4 has a claimed flash duration of ~5mS with the 
shortest duratioon quoted
as 1/20,000.

Regards,
Tim Hughes 
TimHughes@xxxxxxxx





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