Thanks a lot TEEJ for this useful information!
If I correctly understand, a value of 250.000 cd will be traduced in a 500 m distance (using square root rule - 1 lux, instead the inverse square meter rule - 0.25 ANSI standard). That means i.e. SR95S UT beam (250.000 cd) could reach 500 m as effective range (what our eyes can clearly see) vs ANSI range?
I have also noticed that if you double the value of the square root you get the value based on ANSI standard (500x2 = 1.000 m for SR95S UT).
250,000 cd would yield 1 lux at 500 M and 0.25 lux at 1,000 M, as predicted.
Because its 1/4 as bright (lux) at two times the distance, whatever range its at 1 lux, will be a 1/4 lux at double that distance.
I would caveat the "effective range" and "what our eyes can clearly see" phrases you mentioned though.
Essentially, the lux needed to resolve details INCREASES WITH DISTANCE.
Testing performed to calibrate rifle scopes for night use revealed for example that at 200 meters, 1 lux WOULD be enough to shoot a 3' x 2' white paper target, for some people, even a bit less, but others might need closer to 5 lux, but, for everyone, closer to 15 lux was needed to even FIND a rusty ~ 3' steel target.
The contrast of the target and its background CHANGES the needed lux dramatically.
The farther away the target, the more you depend on your fovea to resolve small details (Farther away, things LOOK smaller, etc...) and your fovea has TERRIBLE night vision, and, requires a LOT of light to work.
So there is no one "Minimum Lux Level" that will always be "enough"....and that one lux figure is simply a common rule of thumb that is handy to work with....not a real hard figure.
:wave:
This is one of the reasons why someone might want a light that has a claimed 1,000 meter range, even though their friends chide that you can't see things 1,000 m away in daylight, so who needs a light with that kind of range, etc.
IE: That SR95 UT with its claimed 1,000 M (ANSI) range is probably NOT going to be USED to see things 1,000 M away....but, it CAN be used to see a low contrast target 100 M away...in practice.