Definitions of irradiation:

noun: (Pavolvian conditioning) the elicitation of a conditioned response by stimulation similar but not identical to the original stimulus
noun: the apparent enlargement of a bright object when viewed against a dark background
noun: (physiology) the spread of sensory neural impulses in the cortex
noun: the condition of being exposed to radiation
noun: (medicine) the treatment of disease (especially cancer) by exposure to radiation from a radioactive substance
noun: a column of light (as from a beacon)