lumen rating is very dubious at best.

if the bike light uses regular AA or AAA batteries, don't count on it being good at all compared to a flashlight that uses Li-Ion or Ni-MH batteries.

actually, look at what battery type and amount the light uses and compare it to it's run time. That should be a fairly good indication of output power. shorter run time = brighter. Larger mAh number + voltage of battery = more power.

if that's a cateye you're looking at, IMO, it's only good as a sub-light.