Testing for tranparence to IR

["David N. Horn" <dh@xxxxxxxxxxxxx>]

" Is it posssibe to use Konica 120 infrared film in the plastic toy
Holga camera?"
"Can I use a changing bag to load IR film?"

 To answer either of those questions, you need to determine the
transparency of the substance (body, fabric) to IR.

There are two simple ways to test the items yourself. Far better than
asking for advice. How can anyone speak for a "typical, off-the-shelf
(read: cheap) changing bag"? 

Method one: Load two pieces of unexposed IR film of your choice into
the bag/camera, one wrapped in foil, the other bare. Do this in total
darkness. Then expose the bag/camera to light - at least as much light
as you would normally want to operate under. Then unload the bag/camera
and process the film, again in total darkness.
If both pieces are clear, the item is safe. If only the wrapped one is
clear, the item has failed the test. If both are exposed, you have
goofed. You can skip the control piece if you like.

Method two: Get an IR sensitive photo diode (Radio shack sells them for
a dollar or three). Put it in series with a resistor, a multimeter on
the milliamp or microamp range, and a battery. You should be able to
set it up facing an incandescent light so you get a good strong
reading when the light is on. Now put the substance under test between
the light and the photodiode. Make sure no light gets round the sides.
If the substance is IR-opaque the current should drop to near zero.
Substitute a known IR-opaque material for a check.
I often use this setup together with an oscilloscope to test the
shutter speed on my cameras. I was suprised to find the shutter on
one turn-of-the-century view camera transparent to IR!

Good luck, and please let us know the results of your test.

Dave Horn

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