In my area traffic lights with bulbs are being gradually replaced with LED traffic lights. Sometimes these lights have no diffusion lens and the individual LEDs can be seen. Other times the lights have diffusion lenses which give a very smooth, consistent color.

How do these lenses work?

Traffic light in Brookline, MA, with no diffuser:

enter image description here

  • $\begingroup$ Modern LED traffic lights usually have a small light source at the back of the enclosure, a diffusion lens to give a uniform light intensity over the lighted front area, and a Fresnel lens at the front to focus the beam and aim it in the appropriate direction (i.e. at the correct traffic lane). You may be mistaking "individual LEDs" for the individual facets of the Fresnel lens which can often be seen if you are close to the light and positioned on the axis of the beam. There may also be an optical mask in front of the diffuser, if the light displays an arrow etc instead of a full circle. $\endgroup$ – alephzero Jan 24 '17 at 2:47
  • $\begingroup$ @alephzero They are LED lights with an optional diffusion lens. There is no fresnel lens. I have updated my question with photos. $\endgroup$ – Wallace Park Jan 24 '17 at 3:31
  • $\begingroup$ @alephzero those sound more like old school tungsten traffic lights (once you add a reflector). $\endgroup$ – ratchet freak Jan 24 '17 at 10:20
  • $\begingroup$ You can select diffusion properties such that the individual LEDs' outputs overlap to produce a single apparent extended source, or you can choose to diffuse each separately as in your example. It's just a matter of simple optics. $\endgroup$ – Carl Witthoft Jan 24 '17 at 14:44
  • $\begingroup$ Is your question about how diffusers work (i.e. simple optics) or is your question about why they apparently are only used sometimes in traffic lights? $\endgroup$ – hazzey Jan 24 '17 at 17:12

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