Infrared motion detectors are especially effective for detecting intruders moving where?

• A. Toward the detector
• B. Across its field
• C. Behind walls
• D. In corners

Answer: (B)

Infrared motion detectors work by sensing changes in heat in the scene they cover. The Fresnel lens splits that area into many small zones, and the detector looks for changes between neighboring zones as a warm body moves. When someone crosses the field of view, their body heats up and cools the zones in sequence, creating a pattern the sensor easily detects. This makes movement across the field of view the most effective for triggering.

Moving directly toward or away from the sensor doesn’t create as strong a differential change across the adjacent zones, so detection is less reliable in that orientation. Also, these detectors don’t see through walls, so movement behind walls isn’t detected. In corners, detection depends on whether the path crosses the active zones, but the key strength is lateral (across the field) motion.