Be careful when mounting and aiming your cameras not to have the mounting surface included in the view of the camera. In total darkness the IR illuminators will bathe the surface close to the camera with intense IR light, causing overexposure of the foreground surface closest to the camera. Since all cameras employ light averaging, the overexposed bright foreground surface causes the area beyond to darken to the point that an intruder can’t be seen!
In this example, the camera needs to be aimed away from the mounting surface so none of that surface will be seen by the camera.
Remember too that the effective range of the IR illumination in total darkness is about 35 feet and the maximum trigger distance of the PIR for a single human can be up to 50 feet if mounted looking parallel with the ground so it is possible under these conditions to trigger the camera but not be able to see the intruder!
A good rule of thumb on sites without artificial light is to mount the cameras with a slight (10 degree) downward tilt so the PIR will trigger within the effective IR illumination range, allowing our dispatchers to see the intruder in total darkness and dispatch the police to a video verified crime in progress.
If your site does have artificial lighting at night, you can mount your cameras parallel with the ground to achieve the maximum detection range.