More bike and light articles
old fashioned bottle dynamo and rear light with normal bulb
Dynamos (aka generators) are really looked down on by almost all Americans, and many Brits as well. To the uninitiated, a dynamo sounds unappealing because it slows you down, makes alot of noise, slips in the wet, wears out your tires, and the lights aren't very bright and they go out as soon as you stop moving. In fact, almost all these things are true of older dynamo systems (where you have a dynamo that rubs against the side of your tire and powers incandescent lights both front and rear).
The one thing that is not true, even of older systems, is that the lights are dim. In fact, even a cheap dynamo front light is brighter than any small handlebar-mounted battery-powered front light. Just about the brightest of thse battery-powered lights is the Cateye HL-MC200 Micro-halogen, which is supposedly a 2.4W light. Dynamo headlights are also 2.4W when the dynamo is also used to power a .6W rear light. However dynamo lights are a bit more visible and are a bit better at lighting up the road than even this light. I think this is party because the optics used in dynamo lights concentrates the light where you need it (on a patch of ground right in front of you, and off to the sides for better side visibility, with very little going up). It is also partly because alkaline batteries really aren't that good at supplying the currents demanded of them by bright lights, while dynamos are very good at providing higher, steady currents. And you don't need to use the dynamo to power a rear light as well: if you use a standard battery-powered rear light your light will be 3W which is quite noticeably brighter than the Cateye Micro-halogen.
When you consider newer systems, you'll find that almost everything you learned about dynamo lighting systems is wrong. First, that they slow you down. Well, cheap dynamos that rub your tires still do slow you down a fair bit, but there are some tire-mounted dynamos (more properly called bottle dynamos because they are bottle-shaped) that are very efficient, and so have only minimal drag when powering your lights. Examples of these are the Lightspin and Busch & Mueller S6 dynamos. These can still slip in the wet and cause some wear to your tires, but hub dynamos don't.
Hub dynamos take the place of your front hub, and as they spin they silently generate electricity. They do this very efficiently indeed, so although they do slow you down a tiny bit, at least theoretically, you can't even feel the drag. There are two major ones, made by Shimano and Schmidt. The major difference between them is the amount of drag and the cost. The Schmidt and the Shimano both generate about the same amount of drag when the lights are on, but the Schmidt has almost no drag when the lights are off, while the Shimano still has significant drag when off. The old Shimano hub dynamos used to have the same drag on or off, but evidently the newer ones have less drag when off. The Schmidt is obviously better but it costs more than twice the price! The downsides of hub dynamos are obvious: they weigh a bit more than bottle dynamos and require you to rebuild your front wheel. You also need to get a switch to turn on and off your lgihts. These can be a separate item (Shimano sells a switch that can be used with its hub dynamo) or can be built into the front lamp (Busch & Mueller make these, and they work with either Schmidt or Shimano hub dynamos).
B&M dynamo lights with standlights Seculite Plus Lumotec Oval Plus
The next misconception is that your lights go off when you stop going forward. Nowadays you can get dynamo-powered lights with "standlights". These are LEDs that are powered by a capacitor which charges up as you ride along, and they stay lit for about 5 mins after you stop. For rear lights with standlights, usually the light is itself an LED, so you get the full brightness of the light when you stop. For front lights, which use halogen bulbs, the standlight is a separate LED, yellow or white, that comes on when you stop. Thus for front lights the standlight is quite a bit dimmer than the light you get while you're moving, but it's still bright enough to help you get noticed while you are stopped. These lights with standlights are made by Busch & Mueller (B&M).
my husband's commuter bike with Shimano hub dynamo and dynamo-powered B&M lamps
The ideal dynamo-powered setup is a Schmidt hub dynamo powering a B&M lamp setup such as the Lumotec Oval Senso Plus front light and a rear light with standlight such as Seculite Plus, 4DLite Plus, or DToplight Plus. If you don't have as much money you'd save a fair bit by getting a Shimano hub dynamo, but you should still pay the extra to get front and rear lights with standlights.
battery-powered DToplight on my commuter bike
One alternative to using the dynamo to power both front and rear lights, is to use the full power of the dynamo to power your front light, and to have a separate battery-powered rear light. Ideally the rear light should be a British standard or European standard LED light for good brightness both straight behind and from the sides, and for good battery run-time. This has two main benefits: it allows you to have a slightly brighter front light, and it makes installing the system somewhat easier (you don't have to wire up the rear light). The main disadvantage is that when you turn on your lights you must turn on both front and rear light separately. This may seem like a minor issue, but for a commuting bike when you're often riding home at dusk, it's a good idea to have lights that turn on a quickly and easily as possible. The ideal setup for this is to have a hub dynamo and an automatic sensor that turns on the lights when it's dark enough. This automatic sensor comes with the Shimano hub dynamo switch and with the Lumotec Oval Senso Plus front light. In both these cases, the front and rear lights will come on automatically when it gets a bit dark. My husband has this setup and just leaves the switch setting on automatic (you can also put it on ON or OFF settings as well), and I am jealous of his automatic setup.