Synthetic GOES-R ABI imagery has been routinely generated at the Cooperative Institute for Research in the Atmosphere (CIRA). Experience has shown that brightness temperatures are quite sensitive to ice crystal size at the top of a thunderstorm at 3.9 µm. Particle size influences values of the asymmetry parameter which in turn influences values of the Legendre coefficients via the Henyey-Greenstein parameterization. Legendre coefficients are used to build a phase function. This function determines how much energy is scattered and in what direction. As a result, obtaining accurate Legendre coefficients is desirable. Legendre coefficients for seven hydrometeor types are used to produce imagery. Pristine ice has been shown to account for the brightness temperatures on tops of thunderstorms. Thus Legendre coefficients for pristine ice are obtained from light scattering calculations; Legendre coefficients for the other six hydrometeors are based on the Henyey-Greenstein formula. We have developed a method to combine Legendre coefficients from the Henyey-Greenstein parameterization and Legendre coefficients from light scattering calculations. Both values are weighted by scattering coefficients. The resulting Legendre coefficients are then used to build the phase function to produce GOES-R ABI 3.9 µm imagery. Lastly, this method may be used to generate scattering phase functions for the remaining shortwave infrared ABI channels.