Re: IR flash

[Bill Baker <wab@xxxxxxxx>]

>> ive just made myself an IR flash by taping a wratten IR filter to a flash.
>
>I would be VERY careful doing this.  When your flash goes off, the filter
>will have to absorb pretty much most of the energy.  I have read of cases
>where the gel bursts into flame when this is done.

I bet.  At the very least the gel will probably melt on the flash head, or
else severely reduce the lifespan of the head through increased thermal
stress.

I've been thinking of rigging up something similar, but I planned to take a
flash diffuser and mount my filter package over the exit aperture, well
removed from the flash head.  And even then I plan on using a cheap old
flash I've got lying around.

Actually, I was thinking of building a two-layer IR flash filter, with a
deep red *theatrical* gel before the IR filter.  Theatrical gels (sez the
Ol' Light Hanger) are designed to absorb wide spectrum, high intensity
light for hours at a time without melting.  Of course they fade pretty
fast, but they're also cheap and easily replaced.  And now that I think
about it, any gel that can withstand the full output of a 1,000-watt
fresnel for hours of dress rehearsals and performances without fading (I
seem to recall they're good for 20-40 hours before becoming noticeably
faded)...can probably survive thousands of microsecond flash pulses.

Or else I'll get off my butt and order a PC patch cord to use with the ol'
Speed Graphic flash gun and IR flashbulbs.  Anyone played with these?  If
you look at the specs of still-available IR flashbulbs, shooting Konica 750
indoors with a #29 seems feasible.  Maybe even an 89a.  Seems like all you
see are landscapes using K750, but I think it would be mighty interesting
to try portraiture with this emulsion.  All you'd need is tons and tons of
IR...and a lot of patience.

I can see the appeal of trying to rig an IR-filtered electronic flash for
TTL metered shooting.  As to the IR sensitivity of TTL meters, when I asked
the Nikon list folks about the spectrum response of the N70 TTL meter no
one seemed to know what the rolloff profile was of the deep red-IR end.  I
had just done a few rolls of HSI with a #25 over the lens and a #29 between
the frame rails, preparatory to calibrating meter compensation for a
#25/#87c combo, when my camera was stolen....


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