Like motor vehicle lights, a properly designed cycle light has a trapezoidal beam. This directs most of the light in an even manner on to the road or path ahead of you. Beyond this the light cuts off rapidly, so it doesn't blind oncoming vehicle operators. A nice touch is that to get the beam right, you just point the light straight ahead (as best you can when fitting it), and then, once on the road at night, nudge it up or down a few degrees to fine tune it. The German StVZO specification calls for at least 10 lux of light (10 lumens/m²) in the area up to ten metres ahead of the bike (the yellow section in the drawing). The cut-off is defined as being no more than 2 lux 3.4 degrees above the main beam (the orange section in the drawing). There is no equivalent British specification. Within the European Union, the more tightly-defined German law trumps the British law. Even outside the European Union, the British law is so weakly defined that any StVZO compliant light will meet the British requirement. Since for many decades the British government has shown little interest in cycling as transport, that is unlikely to change. Alas that also means the problem of bad cycle lighting wont go away.
(https://web.archive.org/web/20171115000527/https://blog.wheelies.co.uk/stvzo-approved-bike-lights/) (http://www.light-test.info/en/faq-en/169-stvzo-bike-lamps-regulations)
