Actinism

Actinism is the property of solar radiation that leads to the production of photochemical and photobiological effects. It is important in chemical photography and x-ray imaging, and causes sunburn and photodegradation of materials. Actinic chemicals include silver salts used in photography and other light-sensitive chemicals.

The word actinism is derived from the Ancient Greek ἀκτίς, ἀκτῖνος ("ray, beam").

In chemical terms, actinism is the property of radiation that lets it be absorbed by a molecule and cause a photochemical reaction as a result. Albert Einstein was the first to correctly theorize that each photon would be able to cause only one molecular reaction. This distinction separates photochemical reactions from exothermic reduction reactions triggered by radiation.

For general purposes, photochemistry is the commonly used vernacular rather than actinic or actino-chemistry, which are again more commonly seen used for photography or imaging.

In medicine, actinic effects are generally described in terms of the dermis or outer layers of the body, such as eyes (see: Actinic conjunctivitis) and upper tissues that the sun would normally affect, rather than deeper tissues that higher-energy shorter-wavelength radiation such as x-ray and gamma might affect. Actinic is also used to describe medical conditions that are triggered by exposure to light, especially UV light (see actinic keratosis).

The term actinic rays is used to refer to this phenomenon.

In biology, actinic light denotes light from solar or other sources that can cause photochemical reactions such as photosynthesis in a species.

Actinic light 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.