How Bright Should a Bike Light Be?
Whether it is the best night riding light for your road, gravel, or mountain bike, or if you just need a light for your daily commute, we dive into the technical aspects of how many lumens you need, helmet vs. handlebar mounting, and how to get started.
What are lumens? How many lumens do you need?
When you replace a lightbulb in your home, you refer to a bulb’s wattage. But most bike lights are measured in lumens, not watts. What gives?
Nearly all modern bike lights use LEDs. Those light-emitting diode bulbs are becoming increasingly efficient, meaning they use the same amount of wattage but produce more light. Wattage simply measures power to the bulb.
“Lumens were developed specifically for the human eye,” says Emerson CEO of Light & Motion. “We humans can only see certain wavelengths of light. Lumens are only wavelengths of light useful specifically for humans.”
Emerson recommends a minimum of a 1,000-lumens (this can be achieved with a single light or a pair of lights) for most mountain bike trail riding. This will provide enough light to easily see trail features and obstacles in total darkness.
You can get away with less though. For commuting on the road, a 500 lumen light on the handlebars works very well.
Does bike light shape matter?
Lumens are an easy apples-to-apples comparison. However, a light’s beam shape can also greatly impact how your eyes perceive what’s ahead — or in your periphery.
“As we start to bring in more light, we tend to spread it,” Emerson says. “With 1,000 lumens, you can see far ahead, but you won’t be able to see much to either side.
“Our top-of-line Seca has four LEDs — one in the center is literally plowing light down the center, and the two to the side are angled off-center. Instead of a round spot down the trail, you’re getting an oval shape. The bottom LED spreads light in a nearby field, illuminating the trail near to you to see where you want to steer, like on a slow corner.”
Can you have a bike light that is too bright?
If money is no object, it’s easy to assume that more is always better in terms of lumens. However, Emerson cautions that too much light can lead to what he calls the “Snowball Effect.” Essentially, a too-bright light can interfere with your eye’s natural ability to compensate for low light, temporarily blinding you.
“If you project a really bright light down the trail, and your eye adjusts for that bright light, the minute you look aside you’re blind, it destroys your night vision,” he says. “With our beam, we have a very even fall-off. It is bright enough in the center to see well, but when you look to the side, your pupil hasn’t completely dilated. It’s a much more comfortable light to ride with, and you can see much more broadly.”
Light & Motion doesn’t go beyond 2,500 lumens for its bike lights. In part that’s because Emerson and his team feel the beam shape is more important. Also, he notes that lights with more lumens usually require heavier batteries.
“I like the Seca Race,” he says, which at 2,000 lumens is 500 lumens less than the brightest option, the Seca 2500. “While more lumens are nice, I like light weight and I can always add a second battery for back-up. The heads are the same weight, it is the battery that adds the heft.”
Handlebar mounted vs. helmet mounted bike lights
You generally have two options to mount bike lights: your handlebars or your helmet. There are tradeoffs and benefits to either placement, so it boils down to personal preference.
A helmet light illuminates exactly where you are looking. Compared to a bar mount, a helmet light doesn’t bounce around quite as much on rough trails. Mounting a light on your bars takes the weight off of your head, and avoids annoying wires that go down your back to an external battery (depending on your light).
Serious night riders often pair a helmet and a bar light, and often one is more powerful than the other.
“[World champion] Rebecca Rusch likes the powerful light on the helmet and just a pointer light on the bar,” Emerson adds. “I prefer the big broad Seca on the bar because I don’t want to deal with wires coming down my back.”
Most lights can mount on either your handlebars or helmet, so you can try both options. Eventually, you might opt for a light on both helmet and bars, but for starters, just one will be fine. Some might prefer a helmet-mounted light because it casts light exactly where you want it. It’s also less bouncy than a bar-mount, but you do need to be careful not to snag it on low-hanging branches.