What type of lens is used to collimate light? 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.
 A: You need a lens with a positive focal length: a plano-convex or double-convex lens, typically.
A: 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.
