The word of the day is actinometer:
Etymology:
< actino- comb. form + -meter comb. form2.
1.
Originally: an instrument for measuring the heating power of the sun's
radiation. Later more widely: one for measuring the intensity of solar
radiation (cf. actinograph n. 1).First invented by Sir John Herschel (reported in Edinb. Jrnl. Sci. (1825) 3 107).
2. Photogr.
An apparatus by which light intensity (and hence an appropriate
exposure time) can be estimated from the time taken for a piece of
sensitized paper to darken to a standard shade. Now hist.
3.
Any instrument for measuring the intensity of light in or near the
visible range; (also) a chemical system that measures or counts the
number of photons in a beam of light. (OED)
"The quantum yield for the initial disappearance of diazirine (spectrophotometric determination), based on ferrioxalate actinometry, is 2.0
± 0.5."
- Michael J. Amrich and Jerry A. Bell, "Photoisomerization of Diazirine", Journal of the American Chemical Society 86:292 (January 20, 1964)
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