Shining a Light on Headlight Beam Patterns: What You Need to Know

Every fall, car stereo and automotive accessory shops install thousands of lighting upgrades. These days, the most common upgrade is a set of LED headlight bulbs that fit into the stock light locations. While seemingly simple, the resulting beam pattern can be negatively affected if the chosen bulbs are not designed specifically for the lens or projector application. Let’s take a close look at understanding headlight beam patterns.

What Is a Headlight Beam Pattern?

Let’s kick this off with a bit of a definition. A beam pattern describes how the light from the headlight assembly illuminates the road. A lot of science goes into designing the reflector and lens in a headlight to focus the light energy where it does the most good. Of equal importance, headlights are designed to minimize blinding oncoming drivers. If an approaching driver can’t see correctly because your headlights aren’t aimed or functioning properly, everyone on the road is at risk. Crazy, bright headlights that blind everyone aren’t just dangerous for other drivers; they likely don’t work well at long distances. This reduces your safety.

Off-Road Versus On-Road Lighting

In an off-road application, the driver wants as much light coverage as possible. If the trees are lit up, that isn’t a significant problem. However, on the road, we need to control where all the light from the vehicle goes. The issue with this light is that the beam is still directed at the oncoming driver’s eyes. This is incredibly dangerous as it prevents them from seeing any obstacles in front of them, including your vehicle, the road, or animals.

The solution might be to angle the lights down so the top of the beam is below horizontal. The problem with this method is that the hot spot in the middle of the light is no longer way out in front of the vehicle. Most of the light will illuminate an area about 50 feet away. We don’t need to see this part of the road as much as we need to see 100 feet and beyond. Further, with the foreground so bright, the iris in our eyes will close to reduce the total light we see. This reduces how much we can see objects in the shadows or long distances.

Proper Automotive Headlight Systems

Many new cars and trucks use LED projector headlights; although some models may use reflector-type headlights, HID lights, or halogen bulbs. In the case of projector light assemblies, a cut-off feature assists in creating a good beam pattern. The goal of the lighting system is to illuminate the road as far as possible without making the foreground too bright. This requires a well-focused hot spot in the middle of the light and a way to prevent oncoming drivers from being blinded.

Looking at the image closely, you’ll see a metal plate in the middle of the light assembly. The plate has a notch in it. That plate is oriented so it bisects the hot spot of the light beam. If the vehicle is pointed at a wall, you will see the step down on the left side of the light and a hot spot in the middle of that step.

Asymmetric Low Beam Pattern

The step in the shutter reduces the light that shines on the left side of the road. This allows the lane in front of you to be well-illuminated while reducing glare for oncoming drivers. This is called an Asymmetric Low Beam Pattern. Here’s an aerial image of the beam pattern from the SUV to show the effect on the ground.

If you look closely at the pattern produced by the headlights, you can see a beam of light extending almost off the image’s right side. This light is in line with the vehicle’s direction of travel and will light the road far off into the distance.

What to Know When Upgrading Headlights

If you plan on upgrading the lighting on your car, truck, or SUV, it’s paramount that the changes you make don’t negatively affect the beam pattern. First, the hot spot should remain in the center and not dramatically brighten the foreground. The cutoff must remain sharp so as not to blind oncoming drivers. Technically, any bulbs you install should comply with FMVSS and be marked accordingly to be legal for on-road use.

Something is wrong if the lights don’t seem as bright or there are streaks of light pointing off to the side or upward. Have the shop reinstall the original bulbs until you find a perfect solution.

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