Ther fresnal lens is what focus's the IR heat on the sensor. It acts collector and lens. without it you will only get 5 feet max and with glass or plexiglass you'll get 0 feet. the IR wont pass through most materials. You just have to deal with it.
Welcome to the forum. The fresnel lens focuses the infared radiation on the PIR detector. Material in the fresnel lens is selected very carefully for it's ability to pass IR radiation. Not all plastics have this property. If I remember correctly plain glass does not pass IR very well so the range would be very limited.
You guys are great. When I found this page it was like I found a whole new world of Kindred spirits. I am a pastor in mississippi and have limited electronics knowledge. However, I deployed my first camera this evening. A canon PF. I mounted it in a small cash box. Another Question. Does the Fresnal lens need to stay curved like it is in the sensor housing, or can you mount it flat on to the outer cam. housing?