On Mar 28, 12:24*pm, Mike Rocket J Squirrel
wrote:
On 3/28/2009 8:38 AM Frank Krygowski wrote:

Again, the units are confusing.

Perhaps the simplest way to understand what lumens measure is to
examine a light bulb package. *The package of household bulbs I just
grabbed said "Total light output: *1600 lumens." *That explains it
adequately.

Reflectors and lenses concentrate those lumens, affecting the number of
lumens per steradian. *Projecting them onto surfaces affects the number
of lumens per square foot or per square meter. *But the fundamental
amount of light is measured in lumens.

Thanks, Frank, most helpful.

So, what units are the most useful to use to compare apparent brightness
of something like a tail light?

I think candela (or, for more detail, "beam candela" or "end candela")
would probably be the most correct unit.
See http://www.lib.uwaterloo.ca/discipli...easurement.pdf
at page 37. A candela is a lumen per steradian.

And what units, if any, are the most
likely to be used by the manufacturer in their advertising? One hopes
there will be some overlap between the two.

Sadly, I doubt it. I think manufacturers are most likely to use
whatever units they think will best sell their product. "Visible for
over a mile" seems to be a favorite.

- Frank Krygowski