Actinic light is light which produces an identifiable or measurable change when it interacts with matter.

The term was first commonly used in early photography to distinguish light that would expose a film from light that would not. Non-actinic light could be used in a darkroom without risk of exposing light sensitive films, plates or papers.

Early films, plates and papers were sensitive to the high energy end of the visible spectrum from green to UV. Such light was actinic light. Red light was non-actinic.

In the first half of the 20th century, developments in film technology produced films senstive to red and yellow light and extended that through to near infra-red light. In photography, actinic light must now be referenced to the photographic material in question.

Actininc is also applied to medical conditions triggered off by exposure, or excessive exposure to light, especialy UV light.