Hella 500 vs. 500FF lights - what's the difference?

Anybody know the difference between these? I have a pair of 500FF's on my 2000 XJ. I'd like to add a third one, and have one as a spare. I see a good deal on a pair of 500's. I know the cover design is different, but how about size? brightness? I haven't actually seen a pair of the 500's except online, and it's hard to tell. I looked on Hella's website, and the dimensions are the same. Not sure what the difference is.

FF stands for the free formed reflector used to disburse the light. They're made in Taiwan as opposed to Germany for the originals. The FF's have better light output.

Hella went from lens type dispersion(500) to a reflector dispersion(500ff). The 500ff are the new design and are supposed have a better pattern. I have never compared the two back to back but i am sure that the difference is minimal. As for you I would just get the third light that matches your other two.