The einstein (symbol E) is an obsolete unit with two conflicting definitions. It was originally defined as the energy in one mole of photons (6.022×1023 photons). Because energy is inversely proportional to wavelength, the unit is frequency dependent. This unit is not part of the International System of Units (SI) and is redundant with the joule. If it were still in use, as of the 2019 revision of the SI, its value would be related to the frequency of the electromagnetic radiation by1 einstein = 1 mol × NA h f = 1 mol × 6.02214076×1023 mol−1 × 6.62607015×10−34 J⋅s × f = 3.9903127128934321×10−10 J⋅s × f,