Not transmitting or allowing the passage of actinic rays (ultraviolet or other radiation that causes chemical changes).
From Greek a- ('not') + actinic (from aktis, 'ray') + -ic (adjectival suffix). A technical term in photography and chemistry for materials that block active radiation.
Photography darkrooms use adiactinic red lights because these wavelengths don't affect film—the light is visible to humans but 'silent' to the film's photosensitive chemicals!
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