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Hi! Argument r corresponds simply to the radial coordinate of the input plane - in this case it's the distance from the center of an image. The theory is comprehensively described by J.W. Goodman in the "Introduction to Fourier Optics" (chapters 3-4). You may also find this article by M. Sypek interesting (https://doi.org/10.1016/0030-4018(95)00027-6).
I'm not familiar with this use of an impulse response function for light propagation. What is r in h(r), exactly ? Any pedagogic link to corresponding theory behind this ?