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The light would shine down towards the board instead of up and away from it.

This would be useful where for example the board is being used as some sort of capacitative touch pad with LED indicators. The designer would either drill a hole through the board, or maybe even just have a gap in the solder mask and copper underneath and let the light diffuse through the glass fibers.

I've tried searching and can't seem to find any documentation suggesting this exists; is this because it doesn't or because I'm bad at searching?

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The keyword you are looking for is Reverse Mount LED. This is not an uncommon mounting option for LEDs

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These are common and go by names like bottom entry, reverse package, rear-mounted, or similar depending on manufacturer. No drilling is required, the footprint has a hole in it, and the LED is soldered "upside" down.

enter image description here

Here is an example, when soldered down, the LED is actually shining through the hole in the PCB.

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    \$\begingroup\$ Just to be clear, you do need to drill a hole for the LED in the PCB, but that hole should be included in the LED footprint. \$\endgroup\$ – Peter Bennett Jun 5 '15 at 23:30
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I have done this before. I use it with an IR LED and Phototransistor to send and receive data through the board.enter image description here

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Yes, they exist. They're called reverse mount or bottom entry mounting.

Sometimes they ask you to make square through holes, such as these ones from Kingbright:

enter image description here

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    \$\begingroup\$ Lemme just grab my square drill bit :P \$\endgroup\$ – Navin Aug 16 '18 at 7:31

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