Jeff Liebermann wrote:

OK, so FL 1 is the "ANSI FL1 Standard" ("FL"
is "flashlight", I take it) and cd is
"candela". I suppose one has to read the
article [1] to learn what sense to make of
the 400cd specification.

[1] http://www.led-resource.com/ansi-fl1-standard/

Maybe reading the definitions of candelas,
lux, and lumens might help

I don't think so

https://blog.1000bulbs.com/home/whats-the-difference-between-candela-lux-and-lumens
https://www.knivesandtools.com/en/ct/torches-lux-candela-lumen.htm
There are calculators for converting between
these. Be sure to read about "solid angles".

OK, I read the two web pages! (There is nothing
on solid angles what I could see.)

What *I think* I understood is that

LUX: lux is describing how bright the object
will be, or how big an area (floor or wall) you
can enlight, by pointing the flashlight at it.

LUMEN: lumen is how much light that comes from
the light source, only this doesn't take into
account if and how the light is obstructed or
enforced/directed, e.g. because of the
construction of the lamp, what lens there
is and where, etc.

CANDELA: candela takes into account
obstructions (various blockings of the light)
but also the enforcing of it thru a lens.
This means that candela is a good unit to
determine how far away the light will actually
be visible, for example if you waive it to the
sea from the beach of a deserted island...

For bike applications, it seems for commuting
in a well-lit city with traffic, you'd want
candela, for MTB you'd want lux, and for
touring, you'd want a combination

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