As far as I remember it has not been tried, and some alufoil might actually add a tiny amount of luminance to the center bit by recycled light.
There are some leds however, leds with sharply curved domes like the SSL80 but also the 219A, that when focussed a bit above the die, so the focal plane into the dome, produce a round hotspot without visibly loosing luminance. It is an observation that I did but never did any measurements on it.
Yeah I also thought it might increase luminance a little.
I was mostly thinking about this for flat LEDs like the black flat, CFT90, XPL HI, XHP35 HI, etc.
A sharp round spot would be perfect for zoomies without losing candela
Aluminum foil is the only thing I could think of that is super thin, doesnāt burn, and blocks 100% of light.
Idk if anyone else has other ideas.
How about carefully painting the dome with black permanent marker? And well, now that we're here mirror spray paint would be the best option imho because of light recycling phenomena.
Those are COBs, not single die LEDs.
Round COBs are not new, thereās tons from cree and citizen.
They are for producing a lot of lumens, they suck for flashlights because they require very high voltage and also have a huge die size and low intensity.