IR-flash (was: darkroom -Reply -Reply

["Willem-Jan Markerink" <w.j.markerink@xxxxx>]

On 22 Oct 96 at 14:33, Scott Donkin wrote:

> >Has anyone tried any HIE indoors with flash?  Results ??
> 
> Flash units pump out a fair bit of IR light, so using your flash with HIE
> will work fine.  As a starting point, try, say, 100 ISO, and go from there.
> 
> As a side interest, assuming you are using a little hot-show flash... buy
> yourself an Wratten 89B gel filter (opaque), and put it over the window of
> the flash unit (you may need to cut it up to cover all the window).  Then
> turn off all the lights and take some pics.  You wont see the flash go off,
> but the film will.  Note that you'll have to operate your flash on 'manual'
> - ie, work out flash to subject distances, because the auto feature of any
> flash unit wont work when used in the way I've described.
> 
> Oh... before you go out and buy an 89B gel filter... try developed but
> unexposed tranny film.  According to a few folk, tranny film in this state
> (ie, black) passes very little visible light, but lots of IR... ideal for
> the "flash in the dark" stuff.
> 
> Anyone tried these makeshift filters??

Please note that #89B is far from opaque; even when mounted on a lens
one can see quite clearly through it. Providing bright sunlight of
course. In case of a flash you definately don't block all visible
light, you don't even achieve that with a (normally) absolutely
visible opaque #87C filter. I recently mounted a flexible kind of
#87C material on my flash, and it clearly shows a deep red flash if
you look into it, although you can't see the light bouncing your
subject. 
PS: wonder what fill-flash results in when combining different 
filters on lens and flash (ie light one on the lens, opaque one on 
flash; and dark one on the lens/light one on flash). Maybe an 
experiment worth for the wedding shooters among us?

Anyway: there seems to be a little confusion/discussing in IR
literature as to the why of this visibility; some say that any filter
shows still a little transmission in the visible spectrum (fractions
of promilles), and that the high flash output (stronger than
daylight) makes it seep through. Others say that the human eye can
see up to 1000nm, providing a very high intensity, about a factor
1000 more than needed for the limit at 780nm (the normally accepted
visible limit, as well as 380nm on the bottom end).

In either case: you might not see the flash bounce, but you clearly
can see if light up if you look directly into it. Even mounted on a
MagLite, a #87C still will let you see the bulb. Either by sheer
intensity and a minor visible leak, or by a faint spectral
sensitivity of the human eye.

PS: the 'twin exposed slide' solution is more transmissive than a 
#87C. 
--
Bye,

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      The desire to understand 
is sometimes far less intelligent than
     the inability to understand


<w.j.markerink@xxxxx>
[note: 'a-one' & 'en-el'!]

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Topic No. 17