This paper describes an improvement in the diffraction approximation used to retrieve the size distribution of atmospheric particles from solar aureole radiance measurements. Normalization using total optical thickness based on measurement of the solar disk radiance is replaced with one based on the aureole profile radiance itself. Retrievals involving model calculations for power-law distributions of water droplets show significant improvement using the new algorithm. Tests involving two empirical particle size distributions, one for cirrus and another for aerosols, also show improvement using the new normalization algorithm. Comparisons of the diffraction approximation algorithms with a numerical inversion algorithm found that the accuracy of the latter was higher for two different bimodal aerosol distributions. The role envisioned for the diffraction approximation is in estimating the size distribution of large particles in clouds and especially cirrus.