From The Editor | September 26, 2023

Clear Vision: Optical Coatings, Rain Sensors, And Safer Roads

John Headshot cropped  500 px wide

By John Oncea, Editor


Rain sensors play a critical role in the automotive industry, enhancing safety while driving. Optical coatings, thin layers applied to optical components to control light reflection, transmission, and absorption, help rain sensors do their thing.

Optical coatings use the optical properties of materials to guide and manipulate light. They are omnipresent in the automotive industry, used in headlight lens systems, rearview mirrors, motion detectors, illumination reflectors, and camera systems, as well as in rain sensors to help reduce ghosting and maximize reflectance in a broad range of wavelengths.

Let’s take a look at optical coatings and rain sensors, two distinct technologies serving different purposes with a common goal: safer driving.

Optical Coatings 101

Optical coatings are thin layers of materials applied to the surface of optical components, such as lenses, windows, or mirrors, to enhance their optical performance, according to Edmund Optics. These coatings are designed to control the reflection, transmission, and absorption of light at specific wavelengths or angles. Common types of optical coatings and their purposes include:

  • Anti-reflective coatings: These coatings reduce unwanted reflections from the surface of optical components, allowing more light to pass through and improving visibility.
  • Anti-scratch coatings: These coatings provide a protective layer that helps prevent scratches and damage to the optical surfaces.
  • Anti-fog coatings: Anti-fog coatings reduce condensation and fogging on optical surfaces, ensuring clear visibility, particularly in humid or cold conditions.
  • UV-protective coatings: These coatings block harmful ultraviolet (UV) radiation from the sun, protecting both the optics and occupants of a vehicle.
  • Hydrophobic coatings: These coatings repel water, making it easier for raindrops to bead up and roll off the surface.

Rain, Rain, Go Away

Rain sensors, according to Safelite AutoGlass, are electronic devices integrated into vehicles to detect the presence and intensity of rain on the windshield. They are typically used in conjunction with windshield wipers to automatically adjust their speed based on the amount of rain or moisture detected. The main purposes of rain sensors are:

  • Automatic Wiper Control: Rain sensors enable the automatic activation and adjustment of windshield wipers in response to changing weather conditions. When rain is detected, the wipers can be set to operate at an appropriate speed to maintain clear visibility.
  • Enhanced Safety: By providing optimal wiper control, rain sensors improve driver visibility during inclement weather, reducing the likelihood of accidents caused by reduced visibility.
  • Convenience: Rain sensors eliminate the need for the driver to manually adjust the wiper speed, making driving in the rain more convenient.
  • Energy Efficiency: Rain sensors can help conserve energy by operating the wipers only when necessary, reducing wear and tear on wiper blades and the wiper motor.

“Your car’s sensors can tell it is raining by measuring how many raindrops are on the windshield,” Safelite writes. “The sensor detects the light reflected internally by the windshield glass, so if there were more raindrops on the windshield, the less light would be reflected to the sensor.”

The sensors detect the amount of rain on the windshield and adjust the speed of the wipers accordingly. These sensors are mounted on a special bracket behind the rearview mirror and wired through the roof of the vehicle.

Coatings + Sensors = Safety

Automakers have tried various methods over the years to either eliminate wipers or automatically control the speed, writes HowStuffWorks. “Some of the schemes involved detecting the vibrations caused by individual raindrops hitting the windshield, applying special coatings that did not allow drops to form, or even ultrasonically vibrating the windshield to break up the droplets so they don't need to be wiped at all. But these systems were plagued by problems and either never made it to production or were quickly axed because they annoyed more drivers than they pleased.”

Enter rain sensors, which were first fitted in a vehicle’s mirror base of the rear-view mirror in the mid-1990s, according to Hella. Today they are fitted “inside of the front windshield outside the driver's field of vision.

“Within the sensor area, the sensor detects when it is raining and transmits its information to the windshield wiper’s control electronics. The wiper frequency in the intermittent wiper setting can therefore be automatically adapted to the intensity of the rain. Thus, manual intervention by the driver is now more or less unnecessary.”

The optoelectronic method of measurement used by the rain sensor detects precipitation on the windshield. The sensor element includes transmitters in the form of light-emitting diodes, a prism, and a receiver in the form of a photodiode.

The light beam produced by the diode passes through the prism to the windshield, reflects several times off the outer surface of the windshield, and is then directed toward the photodiode. Maximum reflection of the light beam is achieved when the sensor is in the right position and the windshield is dry. Raindrops on the windshield alter the reflection properties, causing the light beams to be deflected by the water droplets, thereby no longer reaching their target.

“The greater the intensity of the rain, the less light reaches the photodiode. Evaluation electronics use irradiance to calculate the amount of rain currently on the windshield and transmit the information needed from the windshield electronics to control the wiper speed,” writes Hella. “The sensor continually takes measurements so that wiping can be individually adapted to the amount of precipitation. If heavy rain or spray from a vehicle driving in front is detected, the system automatically switches from the intermittent setting to the highest wiper setting.”

All of this is aided by an optical coating that allows the sensor to project infrared light into the windshield at a 45-degree angle. “If the glass is dry, most of this light is reflected into the sensor by the front of the windshield. If water droplets are on the glass, they reflect the light in different directions – the wetter the glass, the less light makes it back into the sensor,” HowStuffWorks writes.

When the amount of light reflected onto the sensor decreases to a certain level, the electronics and software in the sensor activate the wipers. The software then determines the speed of the wipers based on the rate of moisture build-up between each wipe. The wipers can operate at any speed and the system adjusts the speed as often as needed to match the rate of moisture accumulation.

Even though modern cars are designed to be easier to drive and more dependable, drivers are now confronted with more distractions than ever before. Rain sensors, however, can eliminate one of them, allowing drivers to focus on safety.