Automotive cameras, radar and lidar are the three kinds of sensor that a car uses to see the road. A car often uses all three together, because each detects something the others cannot. Most cars sold in India that have driver assistance use a camera and radar. Lidar is costlier and is still rare in them.
Cameras are cheap. They read colours, road signs and lane markings. The picture is then understood with computer vision. But cameras struggle in darkness and in heavy rain.
Radar sends out radio waves and measures how long they take to return. This gives the distance and speed of an object, in any weather. But it shows little detail. It can tell that something is 40 m ahead, but not easily what it is.
Lidar sends out laser pulses and builds a precise three-dimensional picture of everything around the car. It shows the shape of a scooter or a person very well. It costs more than the other two. Some lidar also measures the speed of everything it sees, as well as its distance.
Cars also carry short-range ultrasonic sensors in the bumpers. These produce the beeps while parking.
Combining the readings of several sensors is called sensor fusion. It matters because each sensor can be wrong in its own way, and the others can correct it.
Take, for example, a car on a highway at night in light rain. The camera sees poorly in the dark. The radar detects a stopped truck 80 m ahead and measures that the car is approaching it at 20 m every second. That leaves about 4 seconds. Software combines both readings, decides that a crash is coming, and asks the brakes to act.
The sensors do not decide anything. They send their readings to driver assistance and to autonomous driving software, which interpret them. The readings travel over the wiring and computers of the vehicle E/E architecture. Sensors for pressure and position, used in braking and steering, are described under industrial sensors. Cars with higher levels of autonomy carry more sensors. The carmaker pays for each sensor in every car it builds, so the choice of sensors decides both the cost and the safety features.