References

1)"Zone plates with black focal spots" by Salvador Bara Vinas, Zbigniew Jaroszewicz, Andrzej Kolodziejczyk, and Maciej Sypek.

2)"Zone plates for zero axial irradiance" by J. Ojeda-Castaneda and G. Ramirez.

3)"Gabor zone plate with binary transmittance values" by T. D. Beynon, I. Kirk, and T. R. Mathews.

4)Wikipedia page on zone plates.