(color online) (a) Schematic of the excitation, emission, and depletion processes of a fluorophore. (b) Schematic profiles of I_sted and γ for the lateral direction in the focus area of experimental case 2. Here, the wavelengths of the excitation and STED beams are 635 nm and 750 nm, respectively. To obtain these two profiles, both I_exc and I_sted distributions on the focal plane are numerically computed by Debye integral method first, with the experimental conditions of case 2. Then, γ is calculated on the basis of Eq. (7). The profiles have been normalized for comparison.