'actinometer' originates from Greek, specifically the word 'aktis,' where 'aktin-' meant 'ray' and 'metron' meant 'measure.'

'aktinometron' transformed into the French word 'actinomètre,' and eventually became the modern English word 'actinometer.'

Initially, it meant 'an instrument for measuring rays,' but over time it evolved into its current meaning of 'an instrument used to measure the intensity of solar radiation.'