Units used to describe electromagnetic radiation such as Watt, Joule, Quanta, and Einstein. Sometimes you want to convert from one unit to another — if so, consult this Excel spreadsheet. “For marine atmospheres with Sun altitudes above 22°, the quanta/Watt ratio for the region 400–700nm is 2.77 x 1018 quanta/s/Watt to an accuracy of plus or minus a few percent.” This quote and a further discussion of the relationship of quanta to Watts in the water column can be found in Smith and Morel (1974) Limnol. Oceanogr., 19(4), 591–600.