It would be determined by follows;
1)Sensitivity of Pyroelectric infrared sensor
2)Amplification factor of amplifier circuit.
3)Type of Fresnel lens. 4)Frequency Characteristic of amplifier circuit.
5)Threshold of a comparator. Fresnel lens is mostly affected.
The ordinary sensor can detect the area between 20m to 30m with lens which has large collection efficiency. Conversely sensor can detect for 1m only without lens.