Einstein (unit)

The einstein, named after physicist Albert Einstein, is a non-SI unit defined as the energy in one mole of photons (${\displaystyle 6.022\times 10^{23}}$ photons).[1][2] Because energy is inversely proportional to wavelength, the unit is dependent on frequency. It is made redundant by the definition of the mole, because it is nothing but the numerical value of Avogadro's number, and the joule, which is the SI unit of energy.

In studies of photosynthesis the einstein is sometimes used with a different definition of one mole of photons.[3] As such, photosynthetically active radiation (PAR) used to be reported in microeinsteins per second per square meter (μE m⁻² s⁻¹). The same information about photosynthetically active radiation can be conveyed using the SI convention by stating something such as, "The photon flux was 1.5 micromol m⁻² s⁻¹."