PIR sensors respond to sudden changes in background heat energy by detecting the presence of heat energy at a wavelength emitted by humans. They must be able to have a direct ?????line of sight????? to an occupant to detect his presence. A curved faceted lens defines the field of view as a fan-shaped series of vertical and horizontal ?????cones????? of detection projected from the sensor. The further an occupant is from the sensor, the wider the gaps between these cones. Therefore, the sensor?????s sensitivity to motion decreases with distance away from the sensor.