An instrument that measures the intensity or strength of radiation, light rays, or solar radiation.
From actino- (Greek aktis 'ray') + -meter (Greek metron 'measure'). Developed in the 19th century for measuring light and heat energy.
Actinometers were crucial for early photographers and scientists—they literally measured how powerful the sun's rays were, which helped people figure out correct exposure times before cameras had light meters!
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