how bright should one foglight be?

[BenediktRochow at oaktech.com]

I'd like to fashion rear foglights for some cars
of mine that have no provision for them at all.
What wattage (or rather, how many cd. of brightness -
I'm considering using superbright LEDs) do
fog lights typically have?
Is there something special about the reflector?

thanks,
-gbr

[Thanks for all the QSW buying hints. I think my current plan
is to wait 1-2 wks and then offer between 300 and 800 $.]