Lamps are a contentious matter, and reliable opinion, firm facts, even independent tests are very hard to come by. For instance, the oft-quoted tests by one German magazine are straightforward selling tools for one manufacturer of dynamos and lamps, Schmidt Maschinenbau, the makers of the SON dynamo and Edelux lamps, the test being conducted by an employee of Schmidt.
The next problem you'll run into is relativity and common assumptions. People, including I'm sorry to say me, shorthand facts and opinions that are held in common by most of the likely readers of posts on, to take only one instance, a forum like this. You'll for instance find me saying, "The BUMM first series Cyo is the best bicycle lamp in the world." It is absolutely untrue, but nobody pulls me up on it because they all understand all the qualifications, those qualifications including your particular interest, that the Cyo has a designed and well-implemented horizontally flat upper cutoff. But, beyond that, the Cyo's beam is too narrow, and the cutoff is too low, for it even to approach the "best lamp in the world".
German laws, made decades ago for reasons that were even then passing into the dustbin of history, and now totally irrelevant, have misshapen bicycle lamp development. People who lack the wherewithal to put their brains in gear conclude from the fact that a lamp is "legal" that is therefore necessarily good. It doesn't follow. The first series Cyo, for instance, is just the best of the road-legal lamps; how far it is from sufficient may be judged from the fact that it was the first lamp, despite the claims of the BUMM Faithful for previous BUMM lamps (which objectively were dangerous), to be adequate.
A new Cyo is coming out which is claimed to improve on the intervening Cyos, which were steps backwards.
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What would a sufficient lamp be? Simple. It would be a car-strength lamp operating off a hub dynamo, with a dimswitch. How much cyclists are still regarded as second-class citizens may be judged from the fact that when BUMM recently provided a manner dim/bright function in the Luxos, the rider had no control over its operation. That is not good enough.
As you can see, you can't discuss lamps without touching on the larger metaphysical issues of cycling, in particular whether the cycle is another vehicle or the cyclist is a second class citizen to the motorist.
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Actually, my private opinion is that the best compromise of light where required and considerate road use is achieved by a pair of switched MR16 LED lamps, a high powered one arranged like a car's high beams, a lower-powered one pointed downwards and to the left (where one drives on the left).
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