Scintillation may be the most noticeable for FSO systems. Light traveling through scintillation will experience fluctuations in intensity, even over relatively short propagation paths. The scintillation index, σi2, describes this intensity fluctuation as the normalized variance of intensity fluctuations given by here, I = |E |2: is the irradiance (or intensity) of the signal. The intensity of the scintillation can be measured in terms of the variance of the beamwidth or irradiance σi given by the following: σi2 = 1.23 Cn2 k 7/6L 11/6 Where Cn2 is the refractive index structure, k = 2π /λ is the wavenumber (one expression suggests that longer wavelengths experience less variance), and l is the link gap (m). Where Eq. 26 is mathematically valid for the weak turbulence condition