Before revision: It can be calculated from the beam's vacuum wavelength λ0, the radius of curvature R of the wavefront , the index of refraction n (n=1 for air), and the beam radius w (defined at 1/e2 intensity), according to: .
After revision: It can be calculated from the beam's vacuum wavelength λ0, the radius of curvature R of the phase front , the index of refraction n (n=1 for air), and the beam radius w (defined at 1/e2 intensity), according to: .