As many of you have the Thrunite Ti in possession with .04 lumen rating have already discovered, it's much brighter than many would have thought. Just a couple years ago we had a thread based around the 2 lumen mark and others have followed, but what has not (at least to my knowledge) been verified is the absolute lowest lumen rating that one could possibly see by.
Just to be clear, the topic is not what's practical, as that is a variable, but what the lowest that a dark adapted eye of either most people, or even those with extremely sensitive eyes could see by, and at what distance.
My personal interest in this is to find what actual output level would make super-low output level fans like BeaconOfLight, myself and others finally satisfied. I've heard many times about lights that went so low as to be useless, from the Novatac 120, all of Henry's lights, the Titan and countless others. But every time I got my hands on those same lights I realized they were damn bright in context of a super-low output rating and recently my curiosity was once again piqued by the arrival of my programmable Calipsoii ring for my Surefire A2.
Doing a direct comparison so far with my hCRI Clicky and Ti the Calipsoii ring on one emitter is capable of doing what my awkward visual acuity would tell me is aproximately a quarter the output of the Ti, if not less, while the Clicky is nothing short of blinding at it's rated .07-.08.
One light that I know can easily go well below usable levels is the Spy series, but I still have no idea of what the actual lumen rating would be at the Spy or Calipsoii settings, and that's what I'm hoping to find out here.
My suspicion for some time now has been that at least down to .01 lumen, and quite possibly down to .001 is usable depending on distance, setting, general light sensitivity of user, coupled with comfort level at working in low light. I would naturally assume that night hikers, cavers and such would innately understand this topic, hope that all can benefit from the discussion, and moreover hope for some input from the integrating sphere/lux meter owners out there.
Just to be sure, even if the lumen scale is not the best for scientific purposes, it is what most lights are rated by at present and if in the future a better scale is used for this application, maybe we'll all benefit. For the time being, lumens it is, for the layman.
*As this thread goes along I'm going to try and keep updating the title to show the lowest visible lumen rating available.*