An einstein is a unit defined as the energy content of one mole (N = 6.022*10^23) of photons.

Light can be described as a wave of particles known as photons; these are units of energy, or light quanta.The energy of light varies inversely with the length of the photon waves, i.e. the shorter the wavelength, the greater the energy content.

The energy (E) of a photon is given by the equation E = h*c/λ, where c is the velocity of light, h is Planck’s constant, and λ is the light wavelength. The energy (E) of an einstein is E = N*h*c/λ = 2.7*10^-3/λ, where E is in joule and λ is in metres.

An einstein of red light with a wavelength of 680 nm has an energy of about 176 kjoule