Do rotational-symmetric-looking front lights usually throw half their light into oncoming traffic? How To dinstiguish?

Question

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):

1. light source pointing down, mirror to front.
2. 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
3. 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.

Answer 1

Unfortunately, there are a lot of head lights for bikes out there which do not have a sharp cutoff at the top of the light cone to avoid blinding oncoming traffic. You have to check each individual brand.

The best give-away is when the light is legal for use in Germany. Because German law prescribes that cutoff. So, best pretend you are German when you shop for a front light, and check that the light abides by the StVZO (StVZO = Straßenverkehrszulassungsordnung = the directive that defines which lights are legal and which are not).

Btw, bike shops may have images for how the light cone is supposed to look like on a dark road, allowing you to directly compare what parts of the road are illuminated by how much. Use these images if you can. They will show you which lights actually allow you to look far, and which only blind yourself by over-illuminating the patch right in front of you.

Answer 2

Summary of what I found when doing my DIY light:

If the light has only one symmetrical lens, then it won't be any good. If you want to see stuff near the front wheel to dodge potholes, and to the left and right to know where the curbs are, then the beam needs to be a bit wide, and the lens has to be aimed down. But in this case, it won't throw light in the distance, because the center of the beam will land about 5m in front of the bike. So if there is a pedestrian on the bike path, you'll only see them at the last second. That's no good.

With one narrow beam optic angled slightly down, with the center landing 25m in front of the bike, you'll see in the distance, but not up close and to the sides. That's also no good.

The ideal result with symmetrical optics turned out to be a wide lens angled down (about 15° beam, blue) and a narrow lens angled slightly down (about 6° beam, red). In fact, I don't remember, but the 15° might have been a non-symmetrical 15x30° lens. Anyway, it was quite good, not blinding incoming drivers, a good compromise using available off the shelf optics.

It is more difficult to build, because the two LEDs and lenses do not aim at the same angle. The LED has to sit on the heat sink, so this means one small heat sink per LED. I'd recommend this combo to anyone who wants to DIY. Get 2-3 small heat sinks, drill a hole through the fins, and put them on a skewer. Then you can mix and match optics and play with the angle of each LED until you find something that works. Note the narrow beam optics tend to be larger, so make sure it fits. With a dynamo, both available power and cooling airspeed are proportional to bicycle speed, so there is no risk of overheating when stopped, so the heat sink can be quite small as long as it gets good airflow.

It was not as good as a proper StVZO light, like the Saferide I got later. These are expensive because the optical design is quite difficult to do, requires custom molds, etc. At some point I got involved into designing one, but the optics company wanted $100k for the optics R&D, so... no go. That explains why there are so few on the market. But they are much MUCH better. Seriously if you have never tried one, you're missing out. The only drawback is they require a very accurate angle, because the brightest center spot, which throws light in the distance, is tiny with an abrupt cutoff. So they tend to cause problems at the beginning of an ascent, where the road ahead is above the cutoff, and at the top of a hill, where the beam will hit whoever's coming the other way in the face.

Wide symmetric lenses aimed horizontally are only suitable for mountain biking. In this case, you want to throw half the light up, to see the branch that will otherwise slap you in the face. So if you see a light with only one symmetric lens, or several but they're all at the same angle, there's no need to even try it for road cycling.

A helmet light with a narrow beam is a nice addition, because you can aim it, which is very useful when turning. Also to check stuff around that your main light doesn't illuminate. It needs to be a narrow beam, because it's about seeing stuff far away that's outside the main light beam, and to both point it away from people coming in the other direction to not blind them without having to turn your neck too much, and to briefly point towards drivers who come from the side at an intersection to make sure they see you. It also provides redundancy: if you're going downhill fast on a dark road, it's better to have two lights in case one dies.

Answer 3

There are two conflicting requirements for the light: do not dazzle the incoming traffic and see humans and other objects on the road so that you can stop in time. Riding a more remote road with no street lights is very different from riding a street where it is only important to ensure the good visibility of the cyclist instead.

If you only evade hitting the pedestrian because of the moonlight while your certified light seems not illuminating him at all, this is also unsafe. If you turn into the darkness because your light is aggressively shaped not to dazzle traffic on the side, this is unsafe. Same if your down-bent light is so bright that you do not see anything outside its highly restricted path.

The solution of this problem is known very well and is implemented on every car: if your road conditions change dramatically during the ride, you need both low and high beam. High end bicycle lights now feature a separate switch for a high beam. Unfortunately there are not many of them. Maybe a separate wide angle light for the usage outside the street/road may be a solution.

Very bright and "properly adjusted" light with expensive optics dangerously adapts the eyes of the cyclist for very poor visibility where this aggressively shaped beam does not reach.