beam for off-road riding, or a high- and a low-power beam for road riding. HID lights High-intensity discharge (HID) lights were a brighter/more efficient alternative to filament bulbs prior to the widespread adoption of LEDs. Like halogen systems, they were designed primarily for off-road use, having rotationally symmetrical beams which cast as much light up as down. HID lamps were susceptible to damage from repeated strikes, and in many cases did not relight immediately after shutting down. Likewise, should the battery level fall too low, the lamp would shut down rather than dimming. But the longer battery life than halogens tends to negate these problems, as many riders would simply switch the light on and leave it running throughout the ride. Xenon strobes Xenon strobes were an innovation in bicycle lighting; previously common in industrial applications. They were brighter than LEDs and sometimes used as rear lights. Lighting system Electrical system Batteries, either rechargeable or disposable, are often used to power electric bicycle lights. Where batteries are unwanted a magneto is used. In cycling circles the device is called a dynamo even though the device does not have a commutator. See section Dynamo systems for details. Voltages 3 volt: Often supplied by 2 AA or C/D cell batteries. 6 volt: Sometimes 4 AA batteries or lead-acid batteries are used; also supplied from a bottle- or hub dynamo. 9 volt: Sometimes supplied by a PP3 battery 12 volt: Mostly supplied by sealed lead-acid batteries. The main advantage with this voltage is versatility of the electric system, such as the ability to charge a cell phone battery, though 5 volt USB is also used for that purpose too. It also allows electric air compressors to inflate tires. Some 12-volt dynamos are available too, though their watt rating is underpowered for medium to high watt applications. Light sources Virtually all bicycle lights on the market today, from the cheapest to the most expensive, are based on LEDs. Legal requirements Under the International Vienna Convention on Road Traffic (1968) of the United Nations, a bicycle is a vehicle. Article 44 of the Convention stipulates that: "Cycles without an engine in international traffic shall: (c) Be equipped with a red reflecting device at the rear and with devices such that the cycle can show a white or selective-yellow light to the front and a red light to the rear." In some countries, for example France,