Editing Gradient-index optics (section)

== History ==
In 1854, [[James Clerk Maxwell|J C Maxwell]] suggested a lens whose refractive index distribution would allow for every region of space to be sharply imaged. Known as the [[Maxwell fish-eye|''Maxwell fisheye lens'']], it involves a spherical index function and would be expected to be spherical in shape as well.<ref>{{cite journal|last1=Maxwell|first1=James Clerk|date=1854|title=Solutions of problems: (prob.&nbsp;3, vol.&nbsp;VIII. p.&nbsp;188)|url=https://gdz.sub.uni-goettingen.de/id/PPN600493962_0009?tify={%22pages%22:[14],%22view%22:%22toc%22} |journal=The Cambridge and Dublin Mathematical Journal|volume=9|pages=9–11}} (reprinted by: {{cite book  |date=1890|editor1-last=Nivin |editor1-first=William Davidson|title=The scientific papers of James Clerk Maxwell|url=https://archive.org/details/scientificpapers01maxw/page/76/mode/2up?view=theater|location=New York |publisher=Dover Publications |pages=76–79}})</ref> This lens, however, is impractical to make and has little usefulness since only points on the surface and within the lens are sharply imaged and extended objects suffer from extreme aberrations. In 1905, [[Robert W. Wood|R. W. Wood]] used a dipping technique creating a gelatin cylinder with a refractive index gradient that varied symmetrically with the radial distance from the axis. Disk-shaped slices of the cylinder were later shown to have plane faces with radial index distribution. He showed that even though the faces of the lens were flat, they acted like converging and diverging lens depending on whether the index was a decreasing or increasing relative to the radial distance.<ref>{{cite book |last1=[[Robert W. Wood|Wood]] |first1=Robert Williams |date=1905 |title=Physical Optics |url=https://archive.org/details/physicaloptics00wooduoft/page/n95/mode/2up |location=New York; London |publisher=Macmillan |page= |pages=71}}<!-- also https://archive.org/details/bub_gb_Ohp5AAAAIAAJ/page/n87/mode/2up--></ref> In 1964, a posthumous book of [[Rudolf Luneburg|R. K. Luneburg]] was published in which he described a [[Luneburg lens|lens]] that focuses incident parallel rays of light onto a point on the opposite surface of the lens.<ref>{{cite book |last=Luneburg |first=Rudolf Karl |date=1964 |title=Mathematical Theory of Optics |location=Berkeley |publisher=University of California Press |isbn=978-0-5203-2826-6 |oclc=1149437946}}</ref> This also limited the applications of the lens because it was difficult to use it to focus visible light; however, it had some usefulness in [[microwave]] applications. Some years later several new techniques have been developed to fabricate lenses of the Wood type. Since then at least the thinner GRIN lenses can possess surprisingly good imaging properties considering their very simple mechanical construction, while thicker GRIN lenses found application e.g. in [[SELFOC Microlens|Selfoc rods]].<ref>{{cite journal |last1=Marchand |first1=E.W. |date=1976 |title=Third-order aberrations of the photographic Wood |journal=Journal of the Optical Society of America |volume=66 |issue=12 |pages=1326–1330 |doi=10.1364/JOSA.66.001326}}<!--|access-date=2010-09-12--></ref>