Sometime later, the unit was used differently in studies of photosynthesis to mean one mole of photons, rather than the energy in one mole of photons. As such, photosynthetically active radiation (PAR) was formerly often reported in microeinsteins per second per square meter (μE⋅m ⋅s). This usage is also not part of the SI and when used this way it is redundant with the mole.