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An einstein is a unit defined as the energy in one mole (6.022×1023) of photons. Because energy is inversely proportional to wavelength, the unit is frequency dependent. This unit is not part of the International System of Units and is redundant with the joule.
In studies of photosynthesis the einstein is sometimes used with a different definition of one mole of photons. As such, photosynthetically active radiation (PAR) is usually reported in microeinsteins per second per square meter (μE m−2 s−1). This usage is also not part of the International System of Units and when used this way it is redundant with the mole.
Since the unit does not have a standard definition and is not part of the SI system, it is usually better to avoid its use. The same information about photosynthetically active radiation can be conveyed using the SI convention by stating something such as, "The photon flux was 1500 μmol m-2 s-1".
- Albrecht Folsing, Albert Einstein: a biography. pg 299. New York. 1997. Viking
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