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Jun 17, 2023

Headlights Measured In Luminance vs Lux

Lux is a much more accurate—though rarely talked about—metric for how well they’ll illuminate the road. The lumen is one of the most common units for measuring the brightness of any given light

Lux is a much more accurate—though rarely talked about—metric for how well they’ll illuminate the road.

The lumen is one of the most common units for measuring the brightness of any given light source. Despite its ubiquity, it’s actually also one of the worst ways to gauge illumination. Here, we dispel some of the misconceptions behind luminance and shed light on, if you will, more accurate ways of determining headlight power. Regardless of whether you’re replacing a bulb to meet inspection or installing a light bar on your overland rig, the same rules apply.

To start, lumens simply measure the raw amount of light emitted from the individual bulbs that make up each headlamp. At one point in your life, you’ve likely heard the old saying that brighter lights throw light further. However, that isn’t exactly true.

Case in point: There are plenty of cheap light bars that you can buy on Amazon that promise ridiculous numbers like 50,000 lumens. As this number reflects just the light coming out of each headlamp, it’s a somewhat deceptive metric; manufacturers can simply pack in more bulbs and advertise an eye-popping lumen count. What this doesn’t take into account is the housing of the light itself, which can make a substantial difference.

According to KC HiLites—who make high-quality lights for overlanding vehicles—the size and geometry of the reflector, as well as the clarity of the lens, can greatly affect a headlight’s raw output. There are virtually hundreds of other factors that can affect how well that headlight actually helps you see what’s in front of you. For this fact alone, one set of headlights isn’t necessarily better than another just because the bulbs offer more lumens.

Lux refers to the amount of usable light that lands on a certain surface—and is measured as how many lumens hit a single square meter of that surface. So a light that has a focused beam will put more lumens on the object, making it brighter, while a less focused beam will spread its lumens out and not show the object as brightly. This makes lux better for comparing how much illumination lights provide in the real world. Overlanding experts will know that manufacturers of off-road light bars also measure beam distance as well as pattern, which have quite a profound effect on the lux reading. When light is focused into a narrower beam, it theoretically becomes brighter. However, these would blind motorists and would leave your peripheral vision pitch dark.

Jon Tuico, a YouTuber and overland enthusiast, mentioned that his lights are actually measured by candela. Compared to lux, candela measures the total amount of light traveling in a certain beam angle and direction. It’s a great metric to evaluate the strength, volume, and intensity of any given light source.

To illustrate lux’s usefulness over luminance, we broke out our light meter and compared the headlights of two different vehicles at various distances. We parked each at the same point and measured the lux readings at 0, 10, 20, 30, and 40 meters to evaluate the amount of usable light from both vehicles. One was a co-worker’s 2003 Jeep Grand Cherokee with faded headlamps, while the other was my personal 2015 Volkswagen Golf GTI. While the old lights are nearly just as bright from close up, their lux drops off considerably as you get farther away. The GTI’s lights perform similarly in close proximity, but it’s clear to see that the lux output remains much greater at farther distances.

2015 Volkswagen Golf GTI

2003 Jeep Grand Cherokee

Regardless of whether you look at lux or luminance, let this be a reminder that faded headlights are one of the most dangerous things to leave untouched. We’ve previously written about headlight restoration kits, which are an easy and inexpensive way to keep yourself safe during nighttime motoring. Another easy fix involves replacing the bulbs in your headlights.

Unfortunately, it’s quite rare for headlight manufacturers to list lux on the packaging. If you’re looking to replace a bulb, keep an eye out for name brands like Sylvania or Philips, as we’ve found theirs to be reliable and usefully bright.

Matt Crisara is a native Austinite who has an unbridled passion for cars and motorsports, both foreign and domestic, and as the Autos Editor for Popular Mechanics, he writes the majority of automotive coverage across digital and print. He was previously a contributing writer for Motor1 following internships at Circuit Of The Americas F1 Track and Speed City, an Austin radio broadcaster focused on the world of motor racing. He earned a bachelor’s degree from the University of Arizona School of Journalism, where he raced mountain bikes with the University Club Team. When he isn’t working, he enjoys sim-racing, FPV drones, and the great outdoors.

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