Would the diameter and/or depth of the reflector, relative to the emitter, influence the beam converge/diverge points?
I would think that as the starting Lux for the above rules of thumb use lux, you are using the lux you think you have, to pick a point for the lux measurement? Is that like using a word in its own definition?
Or is it like selecting a range for your DMM, where you assume it might be over 100k lux, but less than a million, and use 10 M, or maybe 10 M and then 20 M if you think its closer to 1/2 Mil, and see what you get, and so forth?
Yes, the led to reflection distance will alter the sweet spot. The rule of thumb is not perfect, but useful.
It is an improvement over putting every light at 5 meters.
Altenatively, one needs to read at two distance & see if the number agrees, if not then extend the distance & try again, that would be too much work.
