FIAT LUX : Let there be (bike) light

      With the growing traffic in the 20th century,  countries around the world started making laws about bicycle lighting. Cars needed lights earlier,  because of the higher speeds. The cyclist is vulnarable, certainly when he is not visible in the dark. Reflective materials can prevent accidents; taillights can be enhanced by reflectors, white fenders, spoke reflectors and tire reflection. They all help the cyclist to survive at night.

     The hub dynamo FIG.7a performed  much better; it  was often combined with a battery holder (sometimes in the headlight). The hub dynamo was certainly more expensive. On top of the S.A. headlight of FIG.7b, there was a button to switch on the lighting. If a battery holder was in place, the light would be taken over automatic below a certain voltage. This avoided flickering  lights at low speeds. It was a well-functioning system, which actually covered all the weak points of bicycle lighting. At S.A. they thought in the 80s that the turnover was too low and they stopped production. With new magnets developed around 1990, others saw the gap in the market. Shimano makes affordable dynamo hubs with Neodynium magnets (see FIG.7c).  Schmidt Original Nabendynamo (SON) uses the more expensive Samarium-Cobalt for the high-end market.

       At speed a dynamo can overpower the lamp, causing it to fail. Historically this was a nuisance, but modern lamps and dynamos often incorporate Zener diodes to prevent overvoltage,
To produce light when the bike is stationary, some dynamo lights have a stand light facility, usually a single blue-white (front light) or red (rear light) LED powered by a capacitor, which runs for around five minutes. 

Developments since the seventies....(Wikipedia summary)

In the UK, the regulations governing bicycle lights are set out in the Road Vehicles Lighting Regulations 1989[3] and their subsequent amendments, summarized in the Highway Code.[4] The regulations require a
white front light, a red rear light, a red rear reflector, and amber/yellow pedal reflectors on the front and rear of both pedals. There are no EU rules on bike lights; every country has its own.

Reflectors must conform to BS 6102/2 or an equivalent European standard. The situation for lights is more complicated: a light with a steady mode is considered approved only if it conforms to BS 6102/3 or an equivalent European standard. A  light without a steady mode is considered approved only if it flashes at a constant rate of between 60 and 240 flashes per minute and has a luminous intensity of at least 4 candela;
The majority of LED lights available are not approved for UK use since they have steady modes that donot conform to BS 6102/3. National cyclists' organisations such as LAB (US) or CTC (UK)are a source of info. 
LED systems often include an option to dim the LEDs, or to provide a flashing light. Low-cost battery lights can be a good choice for occasional use, and usually meet legislative requirements.

However, regular bicycle commuters would probably find it cost-effective to use rechargeable cells. Some headlamps use lithium-ion rechargeablebattery packs, which may be recharged from a USB port.

The introduction of the Low self-discharge NiMH battery (LSDNiMH) in 2005 made rechargeable AA and AAA sized batteries more viable for powering LED bicycle lights. Previously, the selfdischarge effect of NiCAD and NiMH batteries caused the battery to run down over a period of weeks or a few months, even when not in use.

Alternatively, Li-ion and LiPo batteries have been becoming more popular with bicyclists due to their higher capacity and lighter weight compared with conventional batteries.