What's inside a red LED, probably the most common red light source today.
Reversed Oly OM 50mmm at F/5.6 on Canon 100 macro, on extension tubes on 1.4X tele extender.
Incandescent lamp illuminated the support structure. PS used to bring up the dark areas and sharpen. The led was high brightness clear one with flat area ground and polished on the enclosure.