Our Guide to Passive Infrared Detection

Understanding the basics of Infrared Detection will help to get better results from any Trail Camera whatever the manufacturer & explain some of the abnormalities of "Trigger times" & "Missed subjects"

The Basics of IR Detection:

All objects have a temperature which generates variable infrared radiation at a wavelength that is beyond visible light.  Although IR is unseen it can be detected. A basic Passive Infrared Detector  (PIR) consists of a Sensor, a Freznel lens to focus the infrared radiation onto the surface of the sensor & a miniature circuit board & relay to activate the camera. If an object has a different IR radiation (Temperature) to it's surroundings it is detected as it passes through the detection zones created by the freznel lens & the camera is triggered.  Stationary, slow moving & objects moving directly towards the PID are harder to detect as they may not move through the zones created by the lens.  The insulation properties of fur & clothing can also limit the detection rate if IR radiation is similar that of the surroundings.

The Fresnel Lens:				Each Faucet of the lens collects the IR information from a different area & focuses it onto the IR Sensor.
The  IR Sensor:				To help avoid false triggers the IR sensor has two or more sensing areas.  As the target object passes through each sector the IR radiation compares the two levels & sensing a IR radiation difference the camera is triggered. If there was only one single sensing element on the IR sensor the continual fluctuations in the IR received would trigger the camera almost continually.

A common way of depicting PIR Detectors sensing fields is with a "Spider chart"  The top depiction shows the the sensed areas of each facet of the fresnel lens as viewed from above, whereas the lower depicts the sensing areas of the fresnel lens from the side.  The depicted gaps are for illustration puposes only as detection covers the full arc.
Camera Height & Detection: There is no golden rule to the height a Trail Camera should be set at. Most manufacturers recommend a height of one to two metres but you can go higher if you tilt your camera downwards to cover a given area. As can be seen from the diagram the height you set your trail camera is relevent to it's detection range. Small creatures may not be detected at the greater limits of the detection range when the camera is placed higher. Larger animals would get considerably closer before detection if the camera is placed low.

As can be seen from the imformation above the ideal position for the IR Sensor to be set is so that the expected entry of the target into the detection zone is at right angles to it. The problem then is that the target will continue through the picture!  Although a picture will be recorded there will be no indication of what triggered the camera as it has already passed out of the frame.  Alternately if the camera is placed so that the target enters the detection zone face on it may not cross from one zone to the next for the sensor to sample the two IR radiations it is receiving & therefore may not be detected till the last minute. The best placement of the camera is at a forty five degree angle to the expected path of your target. Alternately a little bribe to keep the animal in range should pay dividends. (Food, we mean, not money)

Note:- To reduce the size of Trail Cameras the PIR, although efficient for it's size, it is only about a quarter of the size of a standard unit as fitted to CCTV & static intruder detectors. Correct camera placement & alignment is important to reduce any missed triggers.

Like the Camera Lens a mark or a finger print on the Fresnel Lens could seriously affects its efficiency