of rain. The first such device appears to have been used for that same purpose in a concept vehicle designated Le Sabre and built around 1950–51. General Motors' automatic rain sensor for convertible tops was available as a dealer-installed option during the 1950s for vehicles such as the Chevrolet Bel Air. For the 1996 Model Year, Cadillac once again equipped cars with an automatic rain sensor; this time to automatically trigger the windshield wipers and adjust their speed to conditions as necessary. In December 2017 Tesla started rolling out an OTA update (2017.52.3) enabling their AP2.x cars to utilize the onboard cameras to passively detect rain without the use of a dedicated sensor. Most vehicles with this feature have an "AUTO" position on the control column. Physics of rain sensor The most common modern rain sensors are based on the principle of total internal reflection. At all times, an infrared light is beamed at a 45-degree angle into the windshield from the interior. If the glass is dry, the critical angle for total internal refraction is around 42°. This value is obtained with the total internal refraction formula where is the approximate value on air's refraction index for infrared and is the approximate value of the glass refraction index, also for infrared. In that case, since the incident angle of light is 45°, all the light is reflected and the detector receives maximum intensity. If the glass is wet, the critical angle changes to around 60° because the refraction index of water is higher than air (). In that case, because the incident angle is 45°, total internal reflection is not obtained. Part of the light beam is transmitted through the glass and the intensity measured for reflection is lower : the system detects water and the wipers turn on. See also List of sensors Rain gauge References Irrigation Sensors Meteorological instrumentation and equipment Windscreen wiper