Hello Psychomondo,
The Light Measurement Handbook is a wealth of information on this subject.
http://www.intl-light.com/handbook/registered.html
Let's start with candela's. 10 lux measured at 1 meter = 10 candela (cd).
also 10 foot candles (fc) measured at 1 foot = 10 candela (cd).
and 10 candela = 10000 milicandela (mcd).
These are the most useful measurements that are easy for us to use. All that is required is a light meter and a ruler.
Lumens refers to the total light output from the light source in all directions. It is measured in a $15000 integrating sphere that is a bit out of reach for most of us.
Candlepower and candela should be the same thing, but spotlight manufacturers inflate their numbers by 2-4 times the actual values. I am still not sure how they get away with that, but I believe they are reporting best case theoritical values.
Doug (Quickbeam) has throw and overall values for many lights. Light falls of with the square of distance, so
cd = lux * distance squared in meters, or
cd = fc * distance squared in feet
If a light puts out 100 lux, it will have a throw of 10 meters. This is the distance where the light falls down to 1 lux. Doug indicates this in his throw chart. To find the actual lux values, just square the throw values.
The overall values are a way to give more information about the spill of the beam. They are kind of a "poor man's" lumens.
A laser will have a high throw figure and a low overall value.
A flood light will have close to equal values.
Craig at the LED Museum lists output in mcd which can be converted as indicated above.
So, to answer your question, the best way to convert from lumens to lux would be to search the reviews and see if you can find a measured value of the light in question. Failing that, purchase the light and do your own measurments.
Hope this helps.
Tom
