The science or practice of measuring the intensity of radiation, light, or solar energy using special instruments.
From actino- (Greek aktis 'ray') + -metry (Greek -metria 'measurement'). Emerged as a scientific discipline in the 19th century.
Actinometry was literally how humans learned to measure sunlight scientifically—before it, people just said the sun was 'hot' or 'bright,' but actinometry let them assign actual numbers to light energy!
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