Cameratraps only record when there is actual activity in front of the camera. For this purpose, they depend on a so-called passive infrared (PIR) sensor. This advanced sensor activates the camera when observing a moving heat source. Cameratraps do not react to just movement (i.e. not to a moving branch or a falling leaf), but only if the moving object is warmer or colder than its immediate surroundings. This means that the PIR sensor actually determines the preformance of a cameratrap and is perhaps the most important part. The PIR sensor is kept under wraps, so most manufacturers are not transparent about the precise operation of the PIR sensor they use. By looking closely at the performance of a sensor, such as the response time (the time between detecting the moving object and actually starting the recording), the recovery time (the time between two consecutive triggers) and the detection distance and angle (the distance and maximum angle at which the moving object is detected), you get a good idea of how it works. A fast response- and recovery time increases the chance in which a fast moving animal is captured in its entirety (and therefore you do not only just see a tail, or even worse, an empty image without an animal).
PIR sensors consist of two components, a pyroelectric sensor (A) and one or more Fresnel lenses (B). The pyroelectric sensor consists of two elements that absorb the infrared radiation and thus perceive the difference in heat. The sensor does not penetrate through objects, so an open landscape creates a larger detection zone than a forest area. As soon as the two elements detect a difference in heat radiation, the pyroelectric sensor sends a signal to the camera to take a picture. The Fresnel lenses ensure an accurate recording of the infrared radiation by the pyroelectric sensors. The quality of the sensor and the design of the Fresnel lens varies between cameratraps, making one cameratrap more sensitive than the other.
The image below shows the interaction between the detection zones of a PIR (red) and motion sensor (numbers 1 to 6) on a Reconyx HyperFire 1 cameratrap. The PIR sensor only sends a signal to the camera once an animal is in the PIR detection zone and a difference in temperature is detected, and the animal enters or leaves one of the six areas. The deer (top left) is therefore not detected by the camera (image from Reconyx).