einstein per square meter per second einstein per square metre per second mole per second and square meter mol/sm^2 einstein/sm^[2] An irradiance unit which is equal to one einstein per square meter per second. One einstein is one mole of photons, regardless of their frequency. Therefore, the number of photons in an einstein is Avogadro's number. einstein per square meter per second based unit einstein per square meter per second einstein per square metre per second mole per second and square meter mol/sm^2 einstein/sm^[2] An irradiance unit which is equal to one einstein per square meter per second. One einstein is one mole of photons, regardless of their frequency. Therefore, the number of photons in an einstein is Avogadro's number.