Some elevators use multiple infrared transmitter/receiver pair of sensors. Each receiver can detect when the corresponding transmitter has been obstructed by an object, and automatically trigger the opening of the Selcom Doors.

Also, some elevator doors are built with mechanical safety edges to detect physical contact and open the door.

The doors have an infra sensor

Another type of sensor is the infrared sensor. Infrared sensors use infrared technology to open automatic doors. The infrared sensors are able to detect a change in temperature. When the sensors detect a temperature that is warmer, representing a human, it will activate the opening and closing mechanism of the door. Infrared sensors are usually used as in primary sensor in automatic door systems.

Another type of motion detector that doesn’t use infrared technology uses ultrasonic waves or microwaves instead. Like bats, the sensors generate pulses of ultrasonic waves or microwave energy to judge at detect if there is an object is in the way. If there is an object in the way, the ultrasonic waves and microwaves will bounce off the object. The motion detectors use the time taken for the ultrasonic wave or microwave to arrive back to determine if there is an object in the way.

As well as the two previous answers earlier elevators used a light beam across the doors which when broken triggers the door opener. The other type used was in the 

opening mechanism which Elevator Sensor the extra resistance during door travel and opened the doors. These have been superseded by the infra red type.