At 2000 June 15 - Thursday 20:09, Tom Trottier <TomATrottier@xxxxxxxx>
spoke about *Re: [OM] Lightining...* saying
> Hi Jan,
>
> At 2000 June 15 - Thursday 15:45, Jan Steinman <olympus@xxxxxxxxxxxxxxx>
> spoke about *Re: [OM] Lightining...* saying
>
> > >There is a company selling a "lightining trigger" for cameras. Best as
> > >I've been able to ascertain, it is a couple flash-slave sensors tied to an
> > >amplification circuit which then completes a circuit closure...
> > >
> > >The lightining trigger is designed to trigger the camera's shutter upon the
> > >flash of lightining. They recommend a shutter speed of around 1/2 second
> > >with appropriate lens opening for the scene.
> >
> > It seems to me that this approach would be much too slow to be of any
> > use, unless there are multiple strikes. I don't have the numbers
> > handy, but I seem to recall lightning is a millisecond phenomena,
> > whereas it will take a horizontal cloth shutter 18 mS just to open.
> > Or do I remember wrong?
>
> But lightning strikes last about 300 ms, by my count, enough to catch
> the last 2/3 or so?
>
> Some flash slaves may be sensitive enough to rate of change.
>
> Tom
>From http://www.nofc.forestry.ca/~kanderso/ltgfaq.html
The stepped leader is a small packet of negative charge that descends
from the cloud to the ground along the path of least resistance. In
its path, the leader leaves a trail of ionized gas. It moves in
steps, each typically tens of metres in length and microseconds in
duration. After a step, the leader pauses for about 50 microseconds,
then takes its next step. The leader charge packet sometimes breaks
up to follow different paths, giving lightning its forked appearance.
...
After the return stroke, the lightning flash may end or, if enough
charge in the cloud is collected, a dart leader may come down from
the cloud following a direct path to the surface. In turn, the dart
leader triggers a second return stroke.
A single lightning flash can be comprised of several return strokes.
The average number of return strokes in a lightning flash is 3 or 4,
each stroke typically separated by 40 to 80 milliseconds.
from http://www.forestry.uga.edu/efr/docs/pfor96-003.html
The Lightning Flash
A lightning flash is made up of a number of individual high voltage
strokes averaging four strokes per flash. Each stroke can last many
milliseconds. The duration of the whole flash of lightning is usually
one-half second. The human eye can just catch the individual strokes
in each flash making the lightning appear to flicker.
Tom
------------
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