I am just curious, I use handlebar mounted lights, and since I have multiple bikes to switch, it's a lot of trouble changing the light from one to another. So, I am thinking of switching to helmet mounted lights.
Give me your point of view. Why or why not? Which is better suitable for which riding type?
The answer, as others have said, is "both." However, if you are only going to use one light, there is a disadvantage to that one light being helmet-mounted. When the light is mounted close to the eye, everything that is illuminated is "flattened" since, from the eye's perspective, there are no shadows to provide information about depth. In particular, it's harder to see potholes or other road imperfections. That's why randonneurs often mount a light on the front fork near the hub -- the distance between the light source and the eye helps highlight the road surface. A second issue arises in fog or rain: in that case, with a helmet light you'll get reflections directly back into your eye. That's why you don't use "high" beams when driving your car in foggy conditions, and why fog lights are mounted low.
Given these reasons, a helmet-mounted light can be an excellent supplement but shouldn't be used as one's only light unless you are riding only in urban areas where you can rely on additional street lights to keep the road surface well-illuminated.
There's no single answer to this other than "do what works for you". While the optimal answer to this question is to have both a helmet light and one mounted on the bike, not all cyclists have the money to do that.
That said, there are a few things that can help you decide, Mac or PC helmet- or handlebar-mounted light:
Will you be using more than one bike? In this case, the answer's clearly "yes". If you bring the light with you, you can get one great light setup as opposed to several less-expensive lights. (You can do this with rear blinkies as well as headlights, but a blinky on your helmet is best as one that's supplementing a properly aimed blinky mounted on the bike.)
Do you commonly need to direct light in places other than right in front of you? If you ride on a lot of twisty roads or paths, often need to read street signs, need to watch closely for potholes, etc., then a helmet-mounted headlight would be a handy thing. If you usually ride roads you know well in good repair, you may find a helmet light less useful than one mounted on the bike.
Is the weight on your head a problem? This is something you can really only decide by trying it out, or by attaching the equivalent weight to your helmet and riding for a bit. Are you prone to neck problems? Does the weight make your helmet chafe? (It does with mine.) This problem will be more noticeable if you often ride offroad or on bumpy roads.
In the end, you'll have to try this both ways and see what works for you. You can give a helmet mount a test-ride by attaching the light to your helmet with a nest of zip ties, duct tape, and velcro. If you like having the light on your head, you can look for a more elegant solution.
You haven't said what country you're riding in or whether it's on or off-road, and this makes a big legal difference. In the UK at least, if you're riding on the road you must have a white front light on your bike. In addition, the light must be on the centre-line on your bike, or to the off-side of that (i.e. towards the centre of the road). It must not be on the near-side (the half nearer the kerb) because that might give other road users a false idea of your position.
In this country, only lights attached to your bike count: any lights or reflectives on your person (clothes, helmet, or anything else) are irrelevant. If you're riding with only helmet light(s), you are not legally lit. At first glance, this might not seem to matter, as it's unlikely the police would stop you, even when they're specifically targeting unlit bikes, but in the event of an accident, being legally unlit would make it much harder for you to make a claim against the other party, even if the accident is their fault.
In an ideal world, use both. A wide beam light on the handlebar keeps the light on the road in front of you, even when your viewpoint changes, which can save you if an obstacle comes up in front of you in a hurry.
However, if the trail curves sharply, and your light is fixed to the handlebar, then the light can be pointed in the wrong direction, which can have the opposite effect. Adding a helmet mounted light ensures that you have light where you are looking, i.e. down the trail.
This can be expensive, but it is the best choice.
Barring that option, use a bar mounted light with a bright, wide beam. It is more important to keep light on the ground in front of the bike, that off the trail where your viewpoint might be focused, and a helmet mounted light can be more annoying to other riders.
Just my 2 cents.
I will too answer that the best is to have both lights.
It's more failsafe but also give you a broader light spread. Different answers point out that lightsources being close to eye level make things look flat and reflection from mist, fog and suspended particles are more an issue, but also, handlebar mounted lights can cause long shadows behind objects not so tall, making it appear like a false hole. Some answer mentions it makes potholes looking bottomless.A low placed light is also more easily blocked by other riders if you ride in a group. Having both lights gives you the advantage of both and one is likely to overcome the disadvantages of the other at the expense of a few extra grams and an extra battery.
The helmet light is extremely handy if yo need to make roadside repairs, because you don't need an extra hand just to aim the light. For mountain biking I prefer the helmet mounted, because it is more stable (less shaky) than the handlebar. I've even had complaints from fellow riders that the shaky light is distracting. For night city riding the handlebar light is almost enough, but I like having also the helmet one to make me more visible to other road users, particularly when crossing streets, as you naturally look to the sides, you temporarily shine the light into drivers and pedestrians, which will draw their attention. Proper bike lights shine only a portion of their light to the sides, so they are less visible.
To help with eye level reflection, I recommend installing the light as high as possible on the helmet, this helps noticeably.
Here are some (subjective) Pros of helmet mounted light:
But really, the first point is all that matters to me. Not sure why all other answers in this thread condemn it. I have owned powerful handlebar light. I have owned a weak helmet mounted light. The latter is much more comfortable for mild trail downhill.
I'll just add some points no-one's mentioned yet: if you're riding in urban areas your lights are mostly not for seeing with but to be seen by cars and other traffic. So a light that makes you visible is what you need. Generally you want this light to be pointed straight out, parallel to the ground so that it can be seen for as far as possible.
Another disadvantage of helmet mounted lights generally is that they can get snagged on stuff. Many of them are connected quite strongly to the helmet, and if you hit a low branch they're going to jerk your head back, which puts you at risk of neck injuries at least. In an urban setting they increase the risk of injury if you come off, not only because they get caught on things, but because they could also end up being mashed into the helmet and your head. I've had helmet lights before that I've made myself, and they all were mounted on with velcro, which is enough to keep them in place, but will separate with minimal force.
As a 6-days/week day/night commuter for more than 10 years, I have tried several combinations of lighting described here. My trip is 25% city, 25% bush and 50% 4-lane national road. The dangerous part is the 4-lane where I have been side-swept and rear-bumped. I am now using only a helmet-mounted "quality" front white and rear red light. I have found that being able to aim my front light at whomever I suspect is not paying attention to me, be it human or animal, extremely valuable. On the 4-lane, when I hear or see the casted lights of a vehicle coming from behind, I shake my head from side to side to "sweep" the road behind me with my red light to make me even more noticeable. I have developped this strategy over time and after a couple of accidents and a few close calls. My most efficient safety device that is part of the overall "safety package" is not a light but a 1-m flag pole extending from my handlebar that I tilt from a vertical to a horizontal position. I use it on the 4-lane and have reduced the number of close calls to about 1 a year instead of 10 before. I am thinking of a way to light up this pole either through leds or indirectly from narrow beamed lights attached to the bike frame. I was once thrown violently to the ground and my helmet light got simply unclamped from its mount. When I cross with other cyclists or pedestrians, after ensuring that they have noticed me, I simply lower my head beam to a position a few meters in front of my front wheel so as not to blind them. Even though it is not allowed (in France), I mostly use the forward-looking flashing mode, simply as a power management strategy - I do not want to risk running out of light on the portion of road I have to share with speedy drivers. I do have to suffer reflection in rare foggy situations but overall, I think I have found the "compromise" that suits me best.
I am extremely aware of the importance of being seen and I wear all sorts of reflective materials and other lights. My major accident (side swept by a vehicle coming from behind) took place in full daylight and at the time, on top of all the equipment already described, I was wearing a flashing harness making me look like a Christmas tree... The driver said he had not seen me - his mobile phone still switched on in his hand.
My light is an 6 year old Exposure Joystick with a Red Eye Just wanted to share my experience.
Stephan
In addition to my bike-mounted dynamo-powered light, I have this helmet mounted headlight:
The reason I got it (and that I haven't seen mentioned in other responses) is that I also really wanted the helmet mounted REAR LIGHT. I'm just as afraid of being rear-ended on my commute home (especially in the winter) as I am of being side-swiped. The rear helmet light (set of three LEDs on the battery module) makes sure that there's one more set of warning lights from the rear -- and the distance between my head rear lights and the taillight also helps drivers gauge distance.
I keep the front helmet mounted light on low (in conjunction with my bike-mounted light). Putting the helmet-mount on high is usually too powerful (makes pothole shadows disappear, which is a bad thing) and also blinds and annoys drivers too much. Low is just right on this model. I can point my head at cars approaching from side streets and make sure they won't side swipe me.
Note that if I'm biking with a friend in the evening, I have to turn the helmet light off as it's extremely to aggressively annoying.
tl;dr: Get both, and a helmet mounted light that has a rear safety light.
Do not use a bright helmet light on paths with oncoming other cyclists or pedestrians, as you will easily dazzle them. As we naturally tend to look at other peoples' faces, you will very often shine your light right into their eyes. This can actually be more dangerous (I once almost collided with a cyclist who blinded me so that I lost control). On the other hand, a helmet light is useful when you are on a lonely dark path because you may want to look to the side when you e.g. hear something. So please switch a helmet light off or reduce the brightness when there is a chance you can blind others.
On a road with motor traffic, the main issue is to be seen (not so much the light helps you see). I would have at least one light fixed on the bike, as the moving light on the helmet can make it difficult for drivers to judge your speed, directions and intentions.
