What does that sensor on my car windscreen do?

Modern cars are packed with technology, including sensors fitted behind the interior rear-view mirror. This is how they work.

Windscreen sensors explained

The latest cars are crammed with new features designed to make life easier and safer for drivers.

Many of a car’s sensors are mounted behind the windscreen, between the rear-view mirror and the glass. This gives them an optimal vantage point to keep an eye on the road and the conditions outside. 

If you have ever wondered what all those sensors are doing, read on…

An enlightening history lesson

Windscreen sensors explained

Although they may seem like modern ideas, sensors that react to weather or lighting conditions are not new. 

General Motors pioneered automatically dipping main beam headlights as far back as 1952. The ominously named Autronic Eye sensor was made available for Oldsmobile and Cadillac models as an option.

Buick continued the General Motors innovations, later introducing the Twilight Sentinel. This could detect darkness, then automatically turn on the exterior lights. 

Rain sensors are also not a contemporary innovation. Cadillac created a version of the opulent 1958 Eldorado convertible that was able to sense rain and close its roof automatically.

Lights, Camera, Action

Windscreen sensors explained

Although there are variations between individual manufacturers, the sensor setups used in most modern cars have a similar design. 

For example, Skoda uses a sensor package officially known as the RLFS. This initialism comes from the German wording Regen-Licht-Feuchte-Sonne

This translates into English as ‘Rain-Light-Moisture-Sun’, giving a clue as to what the most common sensor modules can do.

Sensing the rain

Windscreen sensors explained

One of the most useful features offered by an RLFS module is the control of rain-sensing automatic windscreen wipers. 

To do this, the sensors measure the amount of moisture on the windscreen. Too many raindrops results in a signal being sent to the wipers to start. 

All the driver needs to do is leave the control stalk in the ‘Auto’ position, and adjust the level of sensitivity to rainfall that the sensor allows.

All about the humidity

Windscreen sensors explained

Another important part of the RLFS package is the ability to monitor the level of humidity inside the car. 

It does this along by monitoring the level of sunlight and the interior temperature of the car, to let the climate control system function as efficiently as possible.

The latest sensors can even detect what side of the car sunlight is strongest on, and adjust the level of cooling between left and right sides of the cabin.

Lighting up the dark

Windscreen sensors explained

From their origins in the 1950s, light sensors have progressed to become standard equipment in many modern cars. 

Monitoring the level of daylight, and turning the headlights on when it gets dark, can have a major effect on road safety. The latest RLFS sensors can differentiate between entering a tunnel and simply driving along a tree-lined road, when deciding whether to turn on the headlights. 

Emulating the Autronic Eye created by General Motors, such sensors can also control main-beam headlights. Cameras can spot oncoming traffic, then automatically dip the headlights to avoid dazzling others.

What else is found behind the rear-view mirror?

Windscreen sensors explained

The pace of innovation means manufacturers are incorporating more technology into the prime real estate behind the interior mirror. 

Forward-facing cameras are now frequently added to complement sensor packages, extending the range of functions. This includes using a camera to read road signs as you pass. 

This can be used to update a speed limit setting for the cruise control, or create a warning for pedestrian crossings.

Keeping an eye on safety

Windscreen sensors explained

On certain cars, cameras are used to scan for potential collision hazards, and take action accordingly. This includes autonomous emergency braking systems, like the ‘stereo’ camera setup used by Jaguar Land Rover. 

Subaru introduced a camera-based adaptive cruise control system in 1999. The company now incorporates emergency braking functionality as part of its EyeSight range. 

Mercedes-Benz also offers a camera technology to scan the road ahead for bumps and dips. On the GLE SUV it can preemptively adjust the air suspension to reduce body movement as much as possible.

What if my windscreen needs replacing?

Windscreen sensors explained

Having so much complex technology crammed behind the windscreen means extra care needs to be taken should the glass need replacing. 

Before replacing or refitting a windscreen, companies should ask whether the car has any special sensors installed. 

A layer of gel separates the sensor components from the actual glass, and needs to be replaced with the windscreen. 

More importantly, cars with features such as automatic braking need to have the sensors and cameras recalibrated after the new glass is fitted. This ensures they operate as intended, keeping drivers and others safe.

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John Redfern
U.S. Editor with a love of all things Americana. Woodgrain-clad station wagons and ridiculous muscle cars a speciality.

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