Re: Invisible light

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

On  9 Apr 99 at 14:32, John Bright wrote:

> IR people,
> 
> I have a need to use infrared flash. That is, I want my flash to be
> invisible to the naked eye. I have successfully used a #87 for this
> previously.
> 
> But what I want to determine is which filter will transmit the widest range
> of infrared light, but which is still invisible to the naked eye. I am
> guessing that maybe an 89b would work. Or maybe a 70? Does anyone know? It
> is OK if the end of the flash glows red, I just don't want the rest of the
> light to be visible.

Since that also depends on the normal illumination of the room (pitch 
black vs daylight through windows), a good answer is not easy.
#87C and #87 only leave a dull faint reddish glow when looking 
straight into the flash, no illumination of the room whatsoever.
What you also should keep in mind are heat problems....not just the 
direct heat from your flash, but the fact that the entire visible 
output will have to be 'burned' as heat inside the filter....that is 
also the reason why my 0.4mm thick #87C can't be used....it would 
(and will!) fade after a dozen shots....the polyester 0.1mm #87 is 
fine, but no gelatine would stand up to this abuse.
 
> Also, I really would like to try the Ilford film because, unlike most of
> you, I want a film which is sensitive to lower wavelengths of light and
> will not give such an infrared look.

If you have a free choice of filters, then that is something you 
still can control....a #21 orange will give a far less pronounced 
IR-effect than a #87C. 
But if you are restricted in filter (like when needing invisible 
flash), then this control is non-existent of course.
Yet *do* keep in mind that the *efficiency* of such a flash output is 
strongly decreased with Ilford vs Kodak....there is no use firing off 
IR-only if the film can only see a tiny part of that spectrum....and 
exactly *that* is the reason why I think most invisible-light 
flashers use Kodak-only....other films hardly make sense, unless you 
compromise the non-visible aspect....in that case I wouldn't even 
bother with #87 or #87C on your flash, but experiment with #70 and 
#89B....



 

--                 
Bye,

Willem-Jan Markerink


      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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