Talk:Zone plate

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Not Invented by Fresnel The zone plate seems to have been invented by Lord Rayleigh, as evidenced by his notebook entry on 4/11/1871: "The experiment of blocking out the odd Huygens zones so as to increase the light at centre succeeded very well." —Preceding unsigned comment added by 128.112.50.118 (talk) 20:15, 14 April 2008 (UTC)




What does a spiral diffraction grating form? lysdexia 16:33, 22 Oct 2004 (UTC)

I don't think that a spiral diffraction grating forms any sort of coherent image, because it would pass through light at all radii somewhere, rather than like a fresnel lens, only passing through light at radii that will constructively interfere to produce focused images. Laura Scudder 01:22, 6 Nov 2004 (UTC)

[edit] should move this to "zone plate lens"

Zone plates are useful for more than just lenses. They are good for testing video systems, printing processes, image scaling algorithms, alpha blending, gamma correction errors... AlbertCahalan 01:20, 16 Jun 2005 (UTC)

Then you should add descriptions of these uses here. The general rule of thumb is to write on one topic until the article becomes long enough to spin subtopics out as their own articles rather than vice versa. In other words, this article should strive to collect all uses of zone plates until enough material is here for spin-offs, at which point the appropriate spin-off topics should be apparent by the article's organization. So far I only wrote about uses I know of personally, and no one has added to that section since then. --Laura Scudder | Talk 07:29, 16 Jun 2005 (UTC)

Problems with zone plates are:

  • INSANE chromatic aberration (linear z-dispersion with wavelength)
  • Multiple orders of diffraction (the spot that has the 1st order in focus also shows the higher orders unfocused)
  • VERY low efficiency (about 1/100 of the photons actually get where they are supposed to)

[edit] Given formula for radii only approximate

The Formula given for the radius is only valid as an approximation when the radius of the zone plate is much less than the focal distance, otherwise higher order terms come into play. The exact formula can be found by application of Pythagoras: The distance from a point at radius r on the zone plate to the focus, a perpenidcular distance f from the the zone plate, is \sqrt{r^2 + f^2}. Thus the optical path difference (OPD) as r varies is \sqrt{r^2 + f^2} - f. The radius of the nth half-period zone, rn occurs when the OPD is n half-wavelengths: \sqrt{r_n^2 + f^2} - f = n\lambda/2. Expanding this square root with the binomial expansion and taking only the first two terms yields the approximate formula r_n = \sqrt{n \lambda f}, but rearranging without making any approximations gives the correct formula r_n = \sqrt{n \lambda f + \frac{n^2\lambda^2}{4}}.131.111.213.211 14:38, 16 January 2006 (UTC)

[edit] "Video" section

Regarding the discussion in relation to video resolution: Is this really a zone plate? Isn't it just a high-frequency test sample? That is, doesn't it have nothing to do with diffraction? 155.212.242.34 17:52, 29 October 2007 (UTC)

It's both, of course. Yes, it's a test target and not a piece of optics, but it's still the "zone plate" pattern. See [1] (under THE SINUSOIDAL ZONE PLATE) for a reference (and, given that it links here, probably the inspiration for adding the section in the first place). Andrew Rodland 04:50, 4 December 2007 (UTC)