What else is needed to figure out the throw of a particular light? Seems that most light makers like to give the lumens figure,but are reluctant to show the lux. Can this throw figure be calculated when just the lumens are given? Thanks,HB
Short answer, no.
You can guess if you know emitter, reflector size and drive current but this is still only rough.
Far too many variables, and with the inaccuracies of lumen specs on flashlights… Pretty much impossible I imagine.
Have a link to the light? Perhaps someone has already done a review/mod on it and you can get a rough idea.
I add that what that gives you is an upper bound or limit to what the throw can be if everything else is optimal. It can always be worse if the design is not optimized for throw or if something doesn’t work well.
Thanks! HB
Actually throw is the easier measurement to take - all you need is a half way decent light meter and point the light at the meter at a known distance, measure the best reading -- that's it. Of course there's more details, and establishing a consistent methodology helps, etc.
There is a no equipment way to compare light sources that I was told about when a child. It works for isotropic sources or for light per solid angle comparisons such as throw.
The instrument consists of a piece of paper with oil smeared in the middle. You set down the sources, pointed at each other, hold the paper perpendicular, move it until the more translucent spot in the center has the same brightness as the more reflective part nearer the edges. The intensities are then inversely proportional to the squares of the distances. That is, measure the distances of the sources to the paper, and the ratio of brightnesses is the square of the ratio of distances, with of course the farther one being brighter. (Clearly, repeating while looking at the other side of the paper and averaging will improve accuracy.) One of the people I learned this from was my grandfather, who probably used it for gas or oil lights.
Assuming a parabolic reflector, you can come up with a reasonable estimate of throw if you know the inner diameter of the reflector opening, its depth, and emitter lumens.