According to the action form, it can be divided into active and passive sensors.

Active sensor has action type and reaction type. This kind of sensor can send a certain detection signal to the measured object, detect the change of the detection signal in the measured object, or form a signal by the detection signal producing some effect in the measured object. Detecting the change mode of detection signal is called action type, and detecting the response to form the signal mode is called reaction type. Radar and radio frequency range detector are examples of action type, while photoacoustic effect analysis device and laser analyzer are examples of reaction type.

The passive sensor only receives the signal generated by the measured object itself, such as infrared radiation thermometer, infrared camera device, etc