Mounting light on handlebar with bag installed On my bike I have installed an handlebar bag (the Ortlieb Ultimate 5).
Since I am also considering installing lights on the handlebar, I am afraid they will be simply shielded by the bag.

The clerk of the bike shop where I asked for information assured me the handlebar bags are designed to ensure handlebar lights can be used, but I am not sure and I don't want to try it by buying an expensive light that I cannot then use.

In the picture you can see the bag holder as installed on the handlebar. The bag will exceed its level of few centimeters.
As Chris H notes, the clerk was wrong. And he suggests a good suggestion to raise the light above the bag. However, you can also mount the light below the bag as well. Operating on the same principle as fog lights on cars, low mounted lights can do a better job of showing some road hazards. They are also much less likely to blind drivers and pedestrians (StVZO-compliant handlebar lights are still sadly rare).

There are lights that will mount on the crown of the front wheel fork (where the bolt for caliper brakes and mudguards attaches), but those might still hit the bottom of the bag -- although the Ortlieb bag shown in Christian Lundvig's photo is quite small and thus compatible with fork crown mount lights. 

Here's a google image search for "bicycle lights fork mount". In addition to the fork crown lights, there are also front fork adapters that let you mount handlebar lights on one of the legs of the front fork. 



Here's a blown up image of the top left two -- a fork-mount adapter and a light mounted on said adapter. Note that this placement is just about where ye' olde friction dynamo lights used to be mounted. 



Another option is of course a helmet light: