How PIR Fresnel lenses work?
Focusing of infrared radiation: the PIR Fresnel lens focuses the infrared radiation emitted by the human body or an object into the sensitive area of the PIR sensor.
Signal enhancement: by focusing the infrared radiation, the signal strength and detection range of the sensor is increased.
Multi-area detection: the lens is usually designed with multiple Fresnel rings, each ring corresponds to a detection area, realizing wide angle or multi-area detection.

Highly Recommended Optical Fresnel Lenses from Yutai Optics:
Optical Fresnel Lenses
Rectangular Fresnel Lenses
Round Fresnel Lenses
Liner Fresnel Lenses