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This question is related to this recent question: Best ways to avoid getting hit by cars?

I have a 750 lumens front head light - it's bright! Some of my riding buddies say it is too bright for the streets as it blinds drivers. I disagree that it is truly blinding drivers and think that means it is just doing a better job of making me visible. Is it possible for a light to be too bright?

Another consideration for this is bike paths at night. Obviously a light doesn't need to be as bright on a bike path as a busy city street, but I do feel bad for the oncoming pedestrians that seem to be wincing when I approach. What is the proper etiquette for approaching in these situations. Turning off my light doesn't seem like a good idea. Also, I'm not really interested in jimmy rigging a lens-guard on my light, or purchasing a new one (Bike lights with beam cut-off to avoid dazzling drivers?).

As a side note, on a couple occasions, drivers at intersections have complimented my lights noting how easy it was to see me! I assume they are referring to my tail light, though.

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    On the etiquette side, A light that is overly bright or is aimed at other road / path users eyes tends to generate negative feeling against cyclists. – Jahaziel Jun 27 '15 at 0:15
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    You can put your hand over the light when pedestrians or cyclists are approaching. On a path, you risk a head-on collision if oncoming cyclists cannot see. – andy256 Jun 27 '15 at 6:32
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    Ride towards a wall or parked cars and see if there is considerable light above ~1m. A proper, street-legal bicycle headlight has a horizontal cutoff to avoid blinding. Helmet lights, trail lights and flashlights are not made for the road and will blind other people, unless you aim them really low. I recommend a Phillips SafeRide 80 if you need a battery powered light. – Michael Jun 27 '15 at 9:01
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    This reminds me of a brief conversation I once had with a cyclist coming the other way. I yelled at him, "Your light is blinding me!" He yelled back, "Good!" WTF?!? – David Richerby Jun 27 '15 at 11:26
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    I have found from personal experience cycling home over bike lanes that, whenever someone with extremely bright lights approaches, it blinds me so much that I have troubles seeing anything else, which paradoxically enough includes the bicycle that is using those lights, but also anyone near them. It also screws with adjusting to the dark again once the lightbearer has passed by. I have had times where I almost went off the bike lane because I was adjusting to the darkness again. – Nzall Jun 27 '15 at 13:19
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Blinding road users will be the result of the following factors:

  1. Total light output
  2. Mirror design (how is the light shaped)
  3. How you aimed your light

Most trail lights (and high output battery powered lights) use a mirror that casts the light in a symmetrical shape. This means light is cast up, down, left and right. Light cast above the horizon is what binds other road users. Light above the horizon is however great for trail riding as it lets you see overhanging branches and other impediments.

Car running lights (i.e. not high beams) are designed with a horizontal cut-off, where light does not shine above the horizon (actually a low percentage does, this called spill light). If the car light is aimed correctly, the the horizontal cut-off falls below oncoming road users line of vision. This is why you are typically not horribly blinded by oncoming cars, if they are a) using their running lights and b) have their lights aimed correctly. High beams do not have a horizontal cut-off, which is the primary reason you get blinded so badly.

Some bike lights (those that are approved for German roads) will have a mirror that has this sharp horizontal cut-off. They usually will usually be indicated by the some statement on StVZO compliance. Most dynamo lights (e.g., B & M, Schmidt, etc) will have this mirror design. There are also a few battery powered lights (e.g., some B & M models), but they are few and far between.

If you have a light with a symmetrical beam, you can lessen the blinding effect by pointing the light down (as suggested in @Batman's answer), however due to the symmetrical nature of the beam you will still blind road users more than if you had a light with a horizontal cutoff and was aimed correctly as there will still be more spill light above the horizon. This effect is also confounded by how narrowly or wide the symmetrical beam is focused. Wide flood lights will almost always blind, despite how much you aim them down, spot lights will blind less when pointed down.

Finally, total output does have some impact too, but it is also confounded with mirror design (e.g., high output with a horizontal cut-off may blind less than a low output symmetrical beam). What really determines whether or not you blind other users is the total amount of light shining above the horizon at their eyes.

In your case your 750 lumen lights are likely trail lights with a wide symmetrical beam. If you run them at full output you will likely blind others unless you severely aim it downwards.

  • Properly engineered lights are usually not cut-off in the sense that parts of the beam are discarded but rather mirrors and lenses are used to produce a beam from most available light - this light illuminates the road. Another fraction is used to shine "everywhere" - this is the part that makes you visible for others, and is probably what @Rider_X means with "spill light" – chichak Jun 29 '15 at 23:50
  • @chichak - You are correct, lights with a horizontal cut-off create this cut-off by shaping the light with mirrors, not by discarding light. This creates a brighter patch on the ground in front of you for the same total output as a symmetrical beam light. Light shining "off-target" is sometimes referred to as "spill light." Some of it is by design, some of it is due to the limitations in how well light can be shaped of different light emitters. – Rider_X Jun 30 '15 at 16:15
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Yeah, it's like overkill if you're riding on the street. I use a Busch & Muller lamp powered by a dynamo hub and it puts out plenty of light even when riding downhill on potholed roads. Also keep in mind lumens is only one factor in determining light quality. It's sort of like having a computer with a fast processor but not enough memory.

More important are the actual optics which disburse the light. A lot of lights - even really expensive ones - come with nothing more than a plain lens on them which means a lot of the light they put out isn't lighting up the ground in front of you. No point in having 750 lumens if most of it doesn't illuminate the ground in front of you. Peter White has some good examples of how important beam pattern is in determining how good a light is in the real world: http://www.peterwhitecycles.com/headlights.asp

Another problem is where the light is mounted. A powerful light mounted up high (helmet or handlebars) can wash out the terrain making it tricky to spot a pothole. There's a reason why randonneurs - who often ride throughout the night - prefer to mount their lights lower (usually on the fork)

If people are wincing when you approach then your light is almost certainly aimed too high.

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    +1 for "If people are wincing when you approach then your light is almost certainly aimed too high." Don't do it, please. Yes, cars and other cyclists will see "something" and probably won't hit you. But instead they hit who/whatever is next to or behind you. If you blind them, everything in your vicinity is virtually invisible. Seen it happen more than once. – linac Jun 26 '15 at 22:59
  • A powerful light on a helmet also means that you're aiming it directly at drivers every time you look at them, and likely to continue aiming it at them for some time while your head tracks their motion. – David Richerby Jun 27 '15 at 11:31
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Yes. Blinding oncoming traffic is certainly not in your best interest.

Operating multiple running lights like you see on a semi truck would be better than purchasing a high power front or rear facing light. A brighter rear red light, with a touch of blue for colorblindness like stoplights have, is good to see you from behind. A more powerful front facing light will generally not increase reaction time of oncoming traffic. Drivers will have to make out your shape to influence reactions. While the oncoming driver is blinded, you are reducing the time they have to ensure your safety.

To explain the point people are making here. Imagine a house at the end of a dark street. The home owners point the super bright lights towards oncoming traffic. Drivers are unlikely to make out the fact there is a house because of the high power lights and the property is suffering traffic damage. When the owner finally turns the lights around towards the home, blinding drivers was then stopped and the shape of home was made out at a much further distance allowing safe traffic to resume.

So definitely have your headlight. Just keep it dim enough where it is comfortable for you and everyone else to look directly into and aim it properly. If it blinds you, you bet it's going to blind traffic. Increasing lumens of an already good light will not increase your safety. It might, however, make you a target if they are airplane runway bright.

If your friends are saying it is too bright, you should pay attention to them.

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It's not just the number of lumens you have (*) -- its the distribution. You need to aim your light at the ground at an appropriate angle, not into the eyes of motorists. If you have something like a flood light like the coast guard uses, thats no good to aim -- bicycle-specific lights will be easier to aim in a proper manner.

If your light is aimed properly, pedestrians shouldn't really have a problem as well and you shouldn't be blinding cars. Look at the beamshots on @nhinkle's bikelightdb for seeing what appropriate aim looks like.

(*) You didn't mention the model of light. Just because it says x lumens doesn't mean you're getting x lumens. Also, in regards to distribution, there were some cheap Chinese lights (~10-20 USD) which had very high light output and were very cheap a few years ago (used big batteries and stuff). These were somewhat problematic as you couldn't aim the beam.

An aside: As a driver, I find these new blue-white LED lights on Audis and similar a bit distracting. So the color can be a bit distracting, but no bike light is nearly that intense anyway so the point is somewhat moot.

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You can easily test if your light is too bright. Leave it on your bike and see how it looks.

Nowadays there are lots of too bright LED flashlights and headlamps the market that were not designed for cycling. Problem is they flood the light to very wide angle.

I would recommend a light made for cycling that has different modes for different use cases.

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    Or ride towards a wall or parked cars and see if there is considerable light above ~1m. A proper, street-legal bicycle headlight has a horizontal cutoff to avoid blinding. – Michael Jun 27 '15 at 8:54
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If your main worry is being seen by cars, then a superbright light isn't really the way forward. Besides being dazzling/blinding, a front light does very little for visibility from the sides and from behind, from which many (most?) collisions occur. A front light helps your visibility mainly when a car pulls out of or turns into a side road across your path while you approach. In my experience, in most of these cases the problem isn't that the driver didn't see me, but that they had difficulties judging my speed. Also, these are cases where I can mitigate some of the risk by reading the situation, observing the car (are they starting to move?) and being ready to brake.

A front light doesn't protect you against close overtakes, right hooks (UK: left hooks) and all the other frequent types of collisions.

If you are worried about visibility, then it's better to wear hi-vis and add reflectors or possibly several smaller, cheap lights on different parts of the bike. For the driver, one superbright light is just one point, whereas hi-vis, reflectors and several smaller lights will outline the shape and size of your bicycle and help drivers to judge distance and speed. I've also seen the suggestion to have a small light on the top tube that points at yourself to light up your own shape.

The only real use for a very bright light is a dark unlit path (without pedestrians) where you want to see where you're going. If you go along unlit paths a lot, I would invest in a light with a proper shaped beam, like the B&M models, that light up the path properly but don't dazzle other users.

I agree with everybody else that dazzling other users is a very bad idea, for many reasons (your own safety as well as etiquette).

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How to put this politely.... Disagree with you buddies all you like, but stop being a Selfish Cyclist who cares for no one but himself. Consider that your actions in running a light that bright on the road are the ones that make otherwise normal drivers cyclist hating psychopaths.

The law will not be on your side if your light is bright enough to be blinding or distracting to drivers. Unless it has proper optics behind it, 750 lumen on the road is too bright, your friends are right.

The end game of too many people running these lights at full brightness are laws banning anything other than standards approved lights, These approved lights won't cost $10 or $20 or $50 dollars, they will cost $500 or $1000.

If you want to be safe, add more lights and reflectors to your Helmet, arms, body, wheels....and High vis (white) clothing. Light from one point cannot be used to judge distance. The more points, the further apart, the more accurate and quickly the driver can determine the distance between you and him. Use flashing to attract attention but steady lights are needed to judge distance accurately and quickly.

I run 2x 1000 lumen lights off road (usually on mid setting), as soon as I hit the road, I turn them down to low (about 250 lumen). Its clear cars have no problem seeing me at those levels.

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    The problem is where you aim the light, not the power. Trail lights are really bad and dangerous on the road. If you dim them far enough you won’t see the road anymore. – Michael Jun 27 '15 at 8:59
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    Most of this answer is good but -1 for the ludicrous claim that regulation of bicycle lights will make them cost hundreds of dollars. For example, bicycle lights sold in the UK must comply with British Standard BS6102/3 or equivalent European Union regulations. Guess what? The cheapest lights still cost next to nothing. – David Richerby Jun 27 '15 at 11:25
  • I think the question is evidence that I care about the opinions of others and the safety of drivers too. But thanks for your other thoughts. – Trevor Jun 27 '15 at 19:13

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