Many front lights look rotational symmetric, also pretty powerful ones (>500 lumen). Is that usually the case? Or are there many that only look like that, but are still constructed to produce a good beam (powerful, illuminate the road well, not blind oncoming traffic)
If there are many with a good beam, are there giveaways that allow to weed out the bad ones?
Background / explanation
This question is probably not easy to understand for many people.
The construction of bike front lights broadly falls into three categories (from what I've seen):
- light source pointing down, mirror to front.
- light source and reflector rotational symmetric with asymmetric lens (right half)
The segments in the upper half probably reflect much of the light that goes through the upper half down onto the road
- complete rotational symmetric (left half)
I'm talking about the light source (LED) the reflector and the lens here, not the further assembly (battery, mount etc).
Rotational symmetric means that that you can turn it and it still looks the same.
For a light, having rotational symmetry means that the beam is rotationally symmetric - if you point it at a (vertical) wall, it will make a circle.
This is not what you want for vehicle front lights ("to be seen"-lights and off-road use somewhat excluded):
- if you point it at the horizon, half of the light will be above the horizon
- there's usually not so much to be seen, and what needs to be seen either retroreflects or has its own light (hopefully).
- If you go for powerful lights, you'll blind oncoming people which you usually want to avoid.
- if you point it below the horizon, the emphasis of the beam is a few meters in front of you, quickly diminishing forward
Instead, you want much of the light illuminating the (mostly) horizontal road.