A simple method to obtain the fourth-order coefficient of the series expansion for the radial distribution of a lenslike medium is described. The coefficient is obtained by measuring the position of a discontinuity of the far-field intensity pattern. The relation between the coefficient and this position is calculated by using either the ray theory or the wave theory. By using a ray model, the position of the discontinuity and the macroscopic intensity distribution is easily obtained.
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Maeda et al. (1977) studied this question.