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B Directing sunlight into the optical fiber

  1. Jul 19, 2017 #1
    Is it possible to direct sunlight into the optical fiber by magnifying glass and a prism like the image below?
    if yes , what shape of prism should i use to do this?
    Thank you
  2. jcsd
  3. Jul 19, 2017 #2


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    I think a prism bends light the other way...

  4. Jul 19, 2017 #3


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    Hi and welcome.
    A prism is not what you need, imo. (The optics has to be symmetrical about the axis, and a prism only works in one plane. Your idea would actually require a cone and not a prism and it will not produce a focussed spot) What counts is the 'acceptance angle' of the fibre, which is the range of angles of incident light on the end that will actually propagate down the fibre. Whatever the width of the fibre, you need a lens with a focal length that will provide a cone of light that will enter the fibre. The only light that will get into the fibre will be what is within that acceptance cone. If you have a large lens close up, a lot of the light from the spot you have focussed will not get into the fibre. That means you need a long focal length.
    You can measure the acceptance angle by observing a small light source and move it from side to side until you can't see it through the fibre or by looking at the cone of light that emerges from the fibre (on a paper sheet) when you put a light at the other end. That will tell you how big your lens needs to be. The longer the focal length, the more efficient the illumination of the fibre.
    Note, you can overheat the fibre if the lens area is too great and the power that doesn't get into the fibre will dissipate on the way in. So you should mask off the outer part of the lens to restrict the power input to a safe level.
  5. Jul 19, 2017 #4
    You don't need any prism or cone. The lens is all you need.

    This is a good time to learn about etendue: https://en.wikipedia.org/wiki/Etendue
    Since etendue cannot increase, it doesn't help to add extra prisms.

    The fiber has a limited acceptance angle (normally specified by numerical aperture NA).
    NA = n sin(theta)
    where theta is the acceptance half-angle. You should choose a lens with an f-number = 1/(2NA). This will match the bend angle of the lens to the acceptance angle of the fiber.
  6. Jul 19, 2017 #5
    Are you designing a light tube illumination system?
  7. Jul 19, 2017 #6


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    Of course - f number. Brilliant.
  8. Jul 22, 2017 #7
    My goal is to make a powerful light from sun that burns like laser and all light lines are so close and all of them are in same direction
    This is the last thing that i found :
  9. Jul 22, 2017 #8


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    A parabolic reflector is much cheaper, for a given diameter, if you just want raw power. A s/h reflector from an old telescope could be a very reasonable price and it's the sort of thing that amateur astronomers keep, rather than chucking away with an old newtonian telescope.
    It concerns me that the fibre solution could overheat with a few tens of Watts of Power unless you have a high quality fibre. A fibre doesn't actually collimate the light so the beam will spread out at the other end (same cone as on the way in, without a further lens. If you want to produce your intense spot of sunlight at a distance, you are probably better off with a lens / reflector with a very long focal length.
    What is the actual application you had in mind - or is it just that you would like to do it? Unless you really want a very narrow spot of light, it is often better (and easier) to direct the energy onto a slightly defocussed area. Same Power but less Power per square cm.

    PS Safety is important here. Focussing sunlight on a slam spot can produce an image that can damage your eyes. Even it it doesn't seem painfully bright when looking at it, the 'spots' in your vision may not be completely temporary.
  10. Jul 22, 2017 #9


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    I closed this thread because it develops into very dangerous areas. Any experiments with direct sunlight can seriously damage someone's eyesight, in which way processed ever. The more if it is bundled. I can only appeal to everybody to avoid these kind of experiments: fun for a few minutes and blind for the rest of your life. This isn't a good deal. Moreover it bears the risk of an uncontrolled fire. The many wild fires each year tell us how easy a fire can get started and how difficult if not impossible it is to control or extinct.

    I want to take the opportunity and remind everybody who reads this, to be very, very careful with the observation of the upcoming eclipse. Direct observations should be avoided at any costs! Get some of the cheap protection glasses (or foils) which are usually available in these cases. Do not look into the sun without protection. Even usual sun glasses will not be sufficient! It will burn your retina and this will be a lasting damage: no healing, no surgery.
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