Abstract: Based on a modified spectrum of refractive index fluctuation that features inner scale,outer scale and a high wave number bump,and the Gaussian aperture model, analytical expressions and interpolation formulas are obtained for the aperture averaging factor associated with optical scintillation of unbounded plane waves and spherical waves respectively,in weak fluctuation regime.These analyses are inclusion of the finite outer scale,which is necessary for quantitative estimate. The results obtained for the modified model turbulence spectrum significantly differ from those obtained for a Kolmogorov turbulence spectrum.