Photography Use 
The term was first commonly used in early photography to distinguish light that would expose the monochrome films from light that would not. A non-actinic safe-light (e.g. red or amber) could be used in a darkroom without risk of exposing (fogging) light-sensitive films, plates or papers.
Early 'non colour-sensitive' (NCS) films, plates and papers were only sensitive to the high energy end of the visible spectrum from green to UV. This would render a print of the 'red' areas as a very dark tone. Such light was actinic light. Yellow and red light was non-actinic. Xenon flash lamps are generally highly actinic, as is daylight.
In the first half of the 20th century, developments in film technology produced films (orthochromatic and panchromatic) sensitive to red and yellow light and extended that through to near infra-red light. These gave a truer equivalent rendering of colour (hue) as a light, medium or dark tone, according to how we believe the human eye experiences it.
In photography, therefore, actinic light must now be referenced to the photographic material in question.
Medical Use 
Actinic is also applied to medical conditions triggered off by exposure, or excessive exposure to light, especially UV light.
Aquarium Use 
- Wall, E.J. (1890). Dictionary of Photography. London: Hassel, Watson and Viney Ltd.
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