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I built a PCB with LEDs on it. The PCB must be protected from the environment by potting it in silicone. I don't trust that silicone to be clear enough for the LEDs to be visible, so I want to use a light guide. I think this is quite common.

How is the gap between the LED and the plastic light guide handled? Are there specific compatibility considerations to look for to minimize the distance? Perhaps just the mechanical distance from the specifications?

Is it for instance applicable to say: "LED sizes are standardized and light guides too, the gap will be very small and the amount of silicone you get there won't matter much for the transparency of most common electric silicones, even after 10 years in the field"

Cross section

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    \$\begingroup\$ Tip: silicon is the chemical element. Silicone is the man-made synthetic polymer. \$\endgroup\$
    – Transistor
    Commented Jan 23 at 15:40
  • \$\begingroup\$ The German word Silizium is translated to silicon, and Silikon to silicone. \$\endgroup\$
    – Uwe
    Commented Jan 23 at 16:18
  • \$\begingroup\$ Alexander, You know more than we do. The picture just shows an unspecified "light guide" which can be almost anything in reality and a cartoon shape for the enclosure that doesn't lend much confidence to saying anything more than cartoon words. The light guide should have a Lagrange invariant you can calculate. Assuming the LED is a SLED (surface emitter, not edge) you should be able to work out its Lagrange invariant, as well. Assuming no matte/diffuser between them (another discussion) you can figure out how to maximize coupling. But cartoons won't cut it. Too many ways. Too little info. \$\endgroup\$
    – periblepsis
    Commented Jan 23 at 17:05
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    \$\begingroup\$ This gap won't stop anything. It will shine through without problem. Make sure there is no silicone in the gap. \$\endgroup\$
    – Fredled
    Commented Jan 23 at 19:09
  • \$\begingroup\$ @fredled "make sure there is no silicone in the gap" -This is exactly my question. Should I handle it by keeping the silicone out? Then it becomes much harder to design... An alternative that I was hoping someone could perhaps verify from experience is "LED sizes are standardized and light guides too, the gap will be very small and the amount of silicone you get there wont matter much for the transparency of most common electric silicones, even after 10 years in the field" \$\endgroup\$
    – Alexander Ohm
    Commented Jan 24 at 11:37

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