Sheppard, C.J.R.; Wilson, T.: 'Gaussian-beam theory of lenses with annular aperture', IEE Journal on Microwaves, Optics and Acoustics, 1978, 2, (4), p. 105-112, DOI: 10.1049/ij-moa.1978.0023
IET Digital Library, https://digital-library.theiet.org/;jsessionid=1x6buyjse66sp.x-iet-live-01content/journals/10.1049/ij-moa.1978.0023