My 13 year old daughter has been tasked to ascertain the visible distance of a flashlight of 200 lumens on a clear night. The flashlight is a led lenser model number 8407 that is able to focus a narrow beam. I would be grateful if anyone could point me in the right direction that will help me understand at what distance the flashlight would be visible.
As the question asks at what distance the FLASHLIGHT is visible....not what the flashlight can be used to SEE, etc, the ANSI range of the light is not directly usable.
Obviously, a light with a higher cd rating will be MORE visible...but 200 lumens, in of itself is not that usable. For example, the LED Lenser might have about a 1" diameter lens.
That's ~ 0.78 square inches or about 500 mm2.
That means the thing the viewer is trying to resolve is a target about 500 mm2 is size.
There is a point at which 500 mm2 will be too small to resolve, and that will be a practical limit. A point light source though is more easily seen than an object in broad daylight...much as a mote of dust passing through a window's sunbeam becomes quite visible when the light reflects off of it, etc.
For the nautical lights mentioned, there are also minimum size specifications, and part of their purpose is to ensure that the light is large enough to be seen.
Another factor involves the night adapted vision issue.
When night adapted, you lose the use of the fovea, your sharpest field of vision, as it has very little in the way of rods, being primarily cone-based. That means that you may look right AT a distant light, and not see it...but, if your line of sight was a few degrees to the side of the same light, you might be able to see it peripherally. (Peripheral vision has rods, and, most of your night vision capability)
So, for maximum range, we are now talking about the trade off in resolution of a dim target, vs an increase in night vision with a loss in resolution.
At a certain point, there will be a maximum range for this...a point that is the culmination of these factor's effects.
My GUESS is that as a glowing cigarette can be seen a mile away, we are talking about miles not meters.