This equation may also be written as I ( λ λ , T ) = 2 h c 2 λ λ 5 e − − h c λ λ k B T , where I ( λ λ , T ) is the amount of energy per unit surface area per unit time per unit solid angle per unit wavelength emitted at a wavelength λ. Wien acknowledges Friedrich Paschen in his original paper as having supplied him with the same formula based on Paschen's experimental observations.