Relating to or involving the measurement of light or radiation; the same as actinometric but with an older or more formal spelling.
From actinometric + -al (variant adjective suffix), creating a longer, more formal variant common in 19th-century scientific writing.
You'll find 'actinometrical' in old Victorian scientific papers—it's the formal, elaborate version that scientists eventually shortened to just 'actinometric' as scientific writing became more efficient!
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