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I am interested in collimating infrared LEDs, but I don't know what type of lens to use (eg, concave or convex). Can someone tell me what type of lens to use? I am trying to collimate a regular IR LED for use in laser tag.

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In interpret "collimating" as "generating a beam of parallel rays". Moreover, I assume that the LED is not a laser diode but a LED (generating incoherent light).

Usually, the LEDs are embedded in a plastic structure, such that the front part ends with a sphere. It already acts as a lens, roughly focusing or collimating the beam. It's difficult to further collimate such a beam. I succesfully got better collimated beams from LEDs by grinding the front part, thus making a flat surface for the output of the light, and putting a lens after it.

Now, assuming that you have the LED emitter with a flat plastic surface (or no surface at all). The emission from the active part of the LED is a divergent beam. You must focus it, in order to get a collimated beam. So you need a convex lens. Since the beam is (usually) strongly divergent, I suggest a lens with wide numerical aperture, i.e. the ratio between the focal lenght and the diameter should be betwen 1 and 3. If you want a small diameter, then use a short focal length, of the order of 1 cm. Typically the beam will have the same diameter of the lens, because of the large divergence of the output, so the diameter of the lens determines the diameter of the collimated beam.

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  • $\begingroup$ Re, "It's difficult to further collimate such a beam." Sounds like you've never tried. You'll need an LED, and an ordinary magnifying glass, a dark room, and a white screen. Have fun! $\endgroup$ – Solomon Slow May 31 at 13:21
  • $\begingroup$ Thank you very much! $\endgroup$ – Highvoltagemath May 31 at 13:48
  • $\begingroup$ I'm happy to having been useful. However, usually, here the comments such as "thanks" are avoided. Personally, I thank the other users by up-voting their answers or their comments (the up and down arrows on the left of the answer). $\endgroup$ – Doriano Brogioli May 31 at 13:52
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    $\begingroup$ @DorianoBrogioli, OP doesn't have enough rep to give upvotes yet. He can accept an answer, but we usually recommend to wait 24 hours before accepting any answer to give people in all time zones a chance to answer. $\endgroup$ – The Photon May 31 at 16:17
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You need a lens with a positive focal length: a plano-convex or double-convex lens, typically.

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