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My "be seen" bicycle light is bar mounted and has to be reattached every time I jump on the bicycle, due to theft risk. How do I determine when it is correctly aligned?

In particular how are "be seen" lights correctly aligned so they don't blind on-coming traffic, but so that they're best visible from a distance for on-coming traffic?

This question is primarily about how to adjust beam position for courtesy and safety, rather than the mechanics of mounting any particular light.

related question: How to correctly align "to see" bicycle lights?

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With "be seen" lights good ones have a very wide beam pattern so they can be seen from all angles, which makes precise orientation less relevant. If you look at Nathan's bike light database most of those lights are about 180° horizontal beam spread and 90° or more vertically. Again, the better ones have a more even beam spread so there's really no "hot axis" that gets more light.

So I mount them for convenience. I look for somewhere I can attach them where they won't get bumped and point more or less along the axis of the bike, and more or less horizontally. Clipping them into the back pocket of my jeans rather than my belt, for example :)

If you have a poorly made "be seen" light that has a real beam and poor spread, the only real solution is to buy a better one. If you're using an old "see by" LED light in flash mode for this, I suggest setting it up as a "see by" light (per answers to your other question), and using it that way.

If it's particularly bright, the polite thing to do is turn it off or use a lower brightness setting when you're on the bike path. On roads the oncoming traffic should be some way out of your path so blinding shouldn't be an issue. If it is (really narrow roads, for example), I point mine down when I see oncoming traffic. Or turn them off. I have dyno lights that are always on, so turning the blinkie off doesn't break the law or make me invisible.

With my "see sense" lights that are designed to mount to a vertical tube and are annoyingly bright, I mount the front one to my head tube right under the stem so that I am shaded by the stem. That also helps a little with the upward part of the beam, but the real solution is to turn them down at night. It seems a bit odd, but a "daylight visible" flashing light is scary bright at night. So I run them on the lowest power setting at night (maybe ~30 lumens rather than the ~200 lumen peak). I can still see it lighting up reflective signs at 100m, so it's visible enough.

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Be seen lights can be rougly divided into two categories: focused and diffuse beams. Focused lights are much like to-see lights, they're just not actually bright enough to see with. They focus the light into a rougly circular shape that points mostly towards the front (or back) of the bike. Diffuse beams spread light much more evenly, and while they might have a hot spot, they're somewhat bright from every direction. I'm mostly going to address the latter, because focused be-seen lights should be treated effectively like see-with lights.

Before we get to positioning, we need to think about flash pattern. Generally speaking, even your be-seen lights don't need to flash at night. Check out the answers on which is safer, lights that blink or emit a steady beam for the justification behind this. Essentially, if your be-seen lights are bright enough, they'll be noticeable at night without flashing. A strobing light can be very disorienting at night, and could ultimately reduce your overall safety. More manufacturers are starting to offer lights that pulse instead of strobing, which is an acceptable compromise at night – the changing brightness is more attention grabbing without being distracting like a strobe. If you have a steady-burn see-with light already, a pulsing be-seen light can be a useful addition.

During the day, I would suggest using flashing lights. As the paper cited in the aforementioned flashing vs. steady post points out, the human eye perceives a flashing light as being brighter than a steady light with the same output. During the day your lights have a lot more to compete with because of the direct sunlight, so a flashing light helps you to be better seen. At the same time, it's not too distracting to drivers or other cyclists again because there's lots of other light. If you get into flashing lights that are above 500ish lumens, the flashing could still be distracting, but we're talking about be-seen lights here so that shouldn't be an issue.

Now, positioning. On the front, if you have a focused beam, defer to the advice for see-with lights. If it's a diffuse beam and you follow the flashing advice above, it shouldn't matter a whole lot where you aim it because the light output should be fairly uniform. I would suggest mounting the light such that the illuminated surface is perpendicular to the ground. This will ensure that your light is seen from the greatest distance.

For the rear, focused beams are sadly all too common. In order to be seen from a good distance, you should angle your light almost perfectly perpendicular to the ground, or tilted just a couple degrees down. What's most important is that you ensure the light isn't pointing too far up or too far down, else it won't actually be visible from a distance. Unless your light is both absurdly bright and very focused, you're unlikely to cause anybody serious discomfort (again, if following the flashing advice above).

With a diffuse beam on the back, figure out which axis most of the light is spread across. Good diffuse lights will be designed such that the beam is very wide in one direction and narrow in another. Point your light at a wall to figure out which way is which, then mount it so that the wide axis and the spread of the beam is parallel to the ground, and center it so that an even amount of light comes out each side. This will ensure optimal off-axis and distance visibility. Again, following the flashing advice, you shouldn't have any significant issues with blinding people.

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