There are two main ways for sensors to detect when an area is occupied:

Microwave sensors :

transmit an inaudible microwave and monitor reflections from walls or objects in the room irrespective of heat or light. Line of sight issues do not affect microwave sensors as any movement of solid objects changes the pattern of the reflections and activates the sensor.

Key points:
1)  Activated by motion – speed and size not heat and light
2)  Unaffected by background temperature
3) Completely enclosed within the lighting fixture as microwave operation can safely penetrate non-metallic objects such as glass and plastic
4) Very stable performance which is suitable for any climate
5) In line with the fittings IP rating, the units are dust and smoke proof as they are inside the fitting
6) Very long life span around 10 Years

Passive infrared (PIR) sensors:

relate to the movement of objects by detecting their heat and light, but only in their field of view. As these products rely on line of sight their performance is affected when inanimate objects obstruct their field of view

Key points:
1) Activated by infrared – heat and light
2)  Do not function well in temperatures >35 degrees Celsius
3)  Cannot penetrate plastic or glass, so the detector has to be positioned externally to a light fitting, therefore they can become vulnerable to smoke and dust
4)  Lenses can age due to exposure to the atmosphere, which results in reduced performance over time
5) Short life span of around 3 Years 

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