Victor Knötzel - http://victorknotzel.com
The issue is IR transparency of the holder and slide. Generally, it is more of a problem with super fast, highly sensitive IR film, of which we have none today for large format. However, leaving a film holder in the bright sun will show fogging if there is any to be found.
Interesting observation. B&W IR film sees some fabrics, in particular nylon, as brilliant white, but most carbon-colored (black) plastics are dark - IR absorbing.How much worse would things be for normal photography if film holders were made of a white material?
Unless the shutter creates a bright red reflection, it's not going to fog IR film any more than it fogs normal film.
Things don't suddenly become transparent in IR photography unless they already have some degree of transparency At the end of the day it's the same question as whether white "dark" slides would fog normal film.
Also the response of most IR film drops off greatly before getting to even 900nm... I can't see how they can be sensitive to long wavelengths ~8000nm that penetrate thin surfaces.
G
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