1. Lens. https://en.wikipedia.org/wiki/Lens_(optics )
2. Objective lens system of Olympus E-30 DSLR Camera. https://commons.wikimedia.org/wiki/File:E-30-Cutmodel.jpg
3. M. Totzeck, W. Ulrich, A. Göhnermeier, W. Kaiser, Pushing deep ultraviolet lithography to its limits. Nat. Photon. 1, 629 (2007)
4. Fresnel lens, https://en.wikipedia.org/wiki/Fresnel_lens
5. G. Andersen, D. Tullson, Broadband antihole photon sieve telescope. Appl. Opt. 46, 3706–3708 (2007)