In a manner relating to or produced by actinic radiation, which is the part of light that causes chemical changes.
From actinic (from Greek aktis meaning 'ray') plus the adverbial suffix -ally, combining the Greek root for radiation with the manner-indicating ending.
Actinic radiation is why UV light can give you a sunburn or cause photosynthesis—it's the chemically active part of the light spectrum, and photographers use 'actinic light' to expose their materials.
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