I’ve thought of make a ultra low current key light like the Lambda light. I use to know a website that listed LEDs that are good at ultra low currents.
For anyone interested in making their own look at very high-efficiency LEDs, those are the ones that often can still produce decent light at ultra low currents. Or now that Lambda Lights isn’t making them anymore maybe he would share the LED he found. Some christmas led lights use the type of inverted V cone lens, |\| that works well for 360° viewing. Steal a lens off those lights.
I was interested in that too, from memory I think it had a 100ohm resitor in my green one but I remember reading he used different resistors with different colour LEDs as some required more power (also gives them a shorter life) to light up.
You could probably run one off a cr123a for 10 years (or probably not, just a wild guess).
If anyone is chasing the keylight nanos I just put 13 unused ones of mine up for sale here: FS: Lambda keylight nano $10 ea, free shipping (Aus only)