hi Shahzad Aslam

pyro sensors are the sensors used to detect pyroelectric radiations emmited by a body, for example human body. The wavelenth of these radiations is near to infrared. The sensors are mostly used in security systems.
human body emmits pyro electric radiations and hence these sensors can detect these radiations hence security is the main applications of these sensors. These sensors require defferent lenses depending on the field of applications to improve their perfprmance. These lenses are not the glass lenses that we use in the daily life. To make these lenses they make use of a plastic like material which is milky in colour.(i know only this much about the lenses) The sensor consists of a pyro electric filter on top (the glass window)and a sensor element allong with a FET(i think so).

if you find any more information about pyro sensors please mail me.

All the best.

Have a nice day.

Mayur......