We've sold Bern helmets for a while now, so we were pretty excited when they developed this light that can attach to the back of most* Bern helmets. You might wonder why a shop that almost 100% sells bikes that come equipped with dynamo lights would recommend getting another, rechargeable light. The simplest answer is, we like having redundant systems, especially where safety is involved. Not to mention the benefit of having lights in multiple places, especially as high up (into the eyeline of motorists) as possible.
The light comes in two pieces, with a mounting bracket that attaches permanently to the helmet with a screw. The light itself then clips onto the bracket so you can remove it fairly easily to recharge it using a mini-USB port (a small cord is provided).
The LED light is red (for rear) and has 3 "on" modes: flash, pulse, and steady. It's turned on by double-tapping the front, then single-tapping to change the mode, then double-tapping to turn it off. We recommend the flashing mode since it preserves the battery longer.
*This may depend on the model and year of your Bern (if you own one already and want to check, turn your helmet to the back and check for the "goggle clip mount," a set of two holes drilled into the helmet's shell. See last photo here for an example).