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This article seems to paint Fresnel lenses as some new kind of optics. They're not. The point of Fresnel lenses is to save weight and thickness. In an ordinary
by dreamcompiler 7y ago
This article seems to paint Fresnel lenses as some new kind of optics. They're not. The point of Fresnel lenses is to save weight and thickness. In an ordinary lens, its thickness is proportional to its diameter. In a Fresnel lens, thickness is (almost) constant, irrespective of diameter. So if you need a lens that's a meter in diameter (e.g. for a lighthouse) the weight is enormous if you use ordinary lens technology. Fresnels are lighter because Mr. Fresnel figured out that all the glass in the middle was unnecessary. The angles at which the rays meet the lens surface are what matter, and in his lens the angles are preserved without all that heavy glass in the middle.
- tomjakubowski 7y agoIt sounds like the Fresnel lens was an engineering breakthrough rather than scientific.
- dreamcompiler 7y agoExactly. You said it more succinctly than I did.
- BurningFrog 7y agoWas it an engineering breakthrough in the sense that people had realized it was a desirable design before, but didn't know how to make it before Fresnel (I assume) figured that out?
- flomo 7y agoGood question. When you tour these old lighthouses, the gigantic glass lenses are certainly impressive in their construction. The technology may be well understood today, but I doubt you could procure such a thing. (Well maybe if you're the NRO.)
- ChuckMcM 7y agoYeah, pretty much. That said the ability to shape glass like that was pretty cool. In theory, you can keep making the rings smaller and smaller (and the lens thinner and lighter) until you hit the wavelength of the light you're trying to focus.
- romwell 7y agoA rough ASCII sketch to illustrate this: Regular lens: _____ / \ / \ Fresnel lens: _____ /|/ \|\
- anc84 7y agoThat does not work at all and just confused me. The angles must change on the interrupted parts to follow the general curve of the lens' surface. https://commons.wikimedia.org/wiki/File:Fresnel_lens.svg https://commons.wikimedia.org/wiki/File:Fresnel_lens.svg is a very good illustration.
- crottypeter 7y agoAs a counterpoint the GPs diagram was useful for me (perhaps because I have a background involving some study of optics). Granted the parents picture is way better than the rough ASCIIart
- vanderZwan 7y agoI would consider the insight required to realize that Fresnel lenses work at all to be very much a new step in optics. Geometrically speaking it is not intuitive at all.
- wnkrshm 7y agoThey have their disadvantages though, since the angles between incident light and surface are only mostly preseved in practice: e.g. scattering from single facet radii (due to limited manufacturing precision), the necessity of draft facets that do not serve any optical function and introduce artifacts through unwanted refraction/reflections (and even lateral light transport due to total internal reflection and refraction towards a neighbouring facet). These effects are what usually makes fresnel lenses with a large amount of cuts appear a little blurry or milky, depending on the manufacturing precision. You can reduce the influence of the draft facets to a degree but only if you have a point-like source (e.g. large lens diameter compared to light source size). Artifacts will be more or less visible for different object points, compared to a smooth lens. It's a tradeoff between form factor, efficiency and quality - you wouldn't use them in high-power laser optics or telescope mirrors.
- codeduck 7y agoThe benefit of a Fresnel lense is that it only needs to roughly collimate the light, not to focus an image, whereas for a telescope mirror or lense you want as much incident light to come to the focal plane at the same focal length.