Car Advice

How do dusk-sensing headlights work?

More and more vehicles are being fitted with dusk-sensing headlights and you can even buy kits to retro-fit them to older vehicles. But what are dusk sensing headlights and how do they work?

Last week we explained rain-sensing windscreen wipers and you can read about that by clicking the link, and this week we’re looking at their partner, dusk-sensing headlights. We’ll keep this one brief.

Most vehicles on the market are offered with both rain-sensing wipers and dusk-sensing headlights, indeed, I think it would be impossible to find a vehicle offered with one and not the other. And that’s because they’re linked.

And, by that I mean they piggyback off the same light-sensor. And, in many cases, when the rain-sensing wipers activate the headlights are set to also automatically switch on, because of the low-light you encounter when it’s raining. Of course, for all of this to work, you need to roll your light switch to A for Automatic. This tells the system you want it to determine when and if the headlights should activate; usually the daytime running lights (if fitted) are on all the time.

More and more vehicles are being fitted with dusk-sensing headlights and you can even buy kits to retro-fit them to older vehicles. But what are dusk sensing headlights and how do they work?

What’s the history of headlights?

Very, very briefly headlights were, for obvious reasons, one of the first things to be made electric on vehicles with 6V electric headlights arriving in around 1906 and carrying on, more or less unchanged until around 1962. Halogen headlights took over, and then Xenon lights took over from them in 1991 and now LED headlights are becoming more and more common and this is because they afford a more natural-looking light, are more energy efficient and can be linked to sensors to allow for very clever lighting systems, like Audi’s Matrix system.

What’s the difference between, sunset, dusk and twilight?

We call them dusk-sensing headlights, but what does that mean? Well, let’s look at ‘sunset’ first because some of these lighting systems are set to activate at sunset rather than later in the day. Sunset refers to that moment when the sun hits the horizon. Dusk is the moment just after the sun has slipped below the horizon. Now, you might think that Twilight occurs after dusk but it doesn’t, rather it refers to the period between sunset and dusk.

What does a dusk-sensing headlight do?

Unlike it says on the box, a dusk-sensing headlight doesn’t just sense dusk and then activate, it is designed to detect low-light and then switch on the main-beam headlights. Usually the systems are designed in a way that they won’t activate when you travel under a bridge, although I’ve driven vehicles where the headlights do activate in that situation and then they’ll immediately switch off again when you pass out of the bridge’s shadow. You end up with an almost constant, and annoying, on-off situation.

So, most dusk-sensing systems are tuned to determine the difference between the shade of a tree, or bridge, and, say, a tunnel or indeed dusk. While most of these systems are provided to car makers by one or two external partners, some car makers set different parameters, like a more sensitive setting. For instance, Nissan has what it calls intelligent auto headlights and this system is simply designed to turn on a little earlier in the afternoon (around sunset) than some other system and this is based on research into pedestrian-based collisions where, according to Nissan, there’s a spike during sunset and twilight.

How does a dusk-sensing headlight work?

It’s quite simple, like the rain-sensing windscreen wiper, the sensor is the same unit for both applications but uses different sensors to determine the difference between rain drops on the windscreen and low-light but no rain drops on the windscreen. Of course, it only ‘activates’ when the headlights (or windscreen wipers) are set to Auto.

For detecting low light there are two sensors, in general, there’s a light sensor looking for ambient light and a forward-facing light sensor which is used to measure the light level directly in front of a vehicle. In combination, these two sensors are tuned to tell the difference between natural ambient light (or a lack of it) or artificial light from, say, street lights. Once the system determines the level of natural light has dropped below a certain level the headlights are switched on.

Sometimes, the sensor, as in the picture below is placed at the top of the windscreen while others are placed at the base of the windscreen. You can usually tell the location because of what looks like a little ice cube – this is the sensor.

how a rain-sensing windscreen wiper works

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Isaac Bober

Isaac Bober