The einstein (symbol E) is an obsolete unit with two conflicting definitions.
[1][2] Because energy is inversely proportional to wavelength, the unit is frequency dependent.
[3][4][5][6] As such, photosynthetically active radiation (PAR) was formerly often reported in microeinsteins per second per square meter (μE⋅m−2⋅s−1).
Since the unit does not have a standard definition and is not part of the SI system, it has long been considered obsolete.
[3][6] The same information about photosynthetically active radiation can be conveyed using the SI convention by stating something such as "a photon flux of 1500 μmol⋅m−2⋅s−1".