Fresnel lens project with my dad

  1. Hello,
    So my dad and I took apart our old TV that had a Fresnel lens inside. Unfortunately there is a crack near the edge due to a billiard ball accidentally being dropped on it. I was wondering how much this crack would affect the lens' ability to focus the sunlight into a point. When we tested this, it didn't work as well as it did in the video, and had trouble burning dead leaves but we think that since it is winter here in the Northern Hemisphere that might have something to do with it. Any comments or advice would be very helpful. Thanks!

    PS: we were inspired by this video:
     
    Last edited by a moderator: Sep 25, 2014
  2. jcsd
  3. etudiant

    etudiant 922
    Gold Member

    Just one crack should not have much effect at all on the lens's functionality.
    Your description suggests that this lens was not designed to focus light to a point, so it would not be that good at burning leaves. Still, summer noonday sun should improve the performance somewhat.
     
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  4. Redbelly98

    Redbelly98 12,051
    Staff Emeritus
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    Also, no lens has the ability "to focus the sunlight into a point", mainly because the sun is not a point source to begin with. The best you can do is focus the sun onto a small area, as determined by the focal length of the lens.
     
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  5. Alright thanks you guys! Helped a lot.
     
  6. davenn

    davenn 3,977
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    A fresnel lens from a TV/Monitor is used to diffuse light ( spread it out) NOT concentrate light

    Dave
     
  7. sophiecentaur

    sophiecentaur 14,137
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    Where would you find a fresnel lens in a TV? They are common enough in old fashioned OHPs but, as for a TV - are we talking about the lens they used to put in front of 9" tubes to make them look line 12" tubes? That would be a convex (Magnifying) lens, which should focus the Sun OK. It would probably have a focal length of 0.5m (<=2 dioptres) or so, I'd imagine. It couldn't be much shorter than that or the field of view would be too small for more than one person to watch TV.

    (Apologies for trying to tell my astronomer grandma how to suck optical eggs!!)
     
  8. OmCheeto

    OmCheeto 2,197
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    2014 Award

    Found it!

    I've never seen the insides of one of these:

    https://www.youtube.com/watch?v=348PUYaoGm8

    At the end of the video, you see him also pull out a fairly large mirror.
    I'm guessing that there is some kind of miniature TV inside that the mirror and fresnel lens focus on.

    More here if you are interested: Rear-projection television

    In regards to the OP's video, what he did with that gasoline was incredibly stupid. Do not try anything like that, or you may end up without a recognizable face.
     
  9. davenn

    davenn 3,977
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    There's one inside every TV/ monitor that uses backlighting ... so normally LCD type
    it sits behind the LCD panel and it diffuses the light from the back lights to give an even lighting across the whole screen

    the backlight is usually either fluoro tubes or LEDs

    don't think they are used in Plasma screens .... never pulled one apart to find out

    thanks Om

    yes they were also used in the rear projection TVs as your video showed
    I forgot about those ones and was mainly thinking of the other use


    cheers
    Dave
     
  10. sophiecentaur

    sophiecentaur 14,137
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    Ah yes; my memory deliberately edited out the rear projection generation TVs (they were so ghastly). I'm not sure about the optics but it could be a way to achieve a long focal length and big aperture in a short space (a la telephoto lens).
    In the YouTube movie, the lens seems to be a little under 1 Diopter and the focussing is not too fussy. He is working in full, bright sunlight, of course.

    In the LCD backlight system, I always thought it was just a lenticular screen but I didn't think about it in depth. The backlight was a number of strip lights so any lens would be cylindrical.
     
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