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

'aktis' transformed into the French word 'actinomètre,' and eventually became the modern English word 'actinometer' through scientific terminology.

Initially, it meant 'an instrument for measuring rays,' and this meaning has largely remained the same in modern usage.