I'm not sure it has anything to do with the IR sensitively of HIE. I think it is because HIE does not have an anti-halation layer, light that comes in through the felt light trap passes through multiple layers of film on the spool exposing it.
Only 35mm HIE needs to be loaded in darkness. 120 HIE can be loaded in subdued light just like any other 120 film. The paper backing protects the film.
I believe that other 35mm IR sensitive film having an anti-halation layer can be loaded in subdued light like ordinary 35mm film. This statement I have only read and do not have first hand experience with like 120 HIE.
it has to do with the IR going through the felt trap. Paper backing is suffiicient to block IR, but the felt isn't.
Remember that HIE is waaaaaaaay more sensitive to IR than any other film. So you can't really compare it to others and draw conclusions based on how it is loaded.
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