Most motion-sensitive security lights detect heat from
your body and are triggered if the infrared radiation received
at the sensor changes sufficiently quickly. The lamps ignore
gradual variations – if they didn’t, the temperature changes of
the surroundings between day and night would set them off. It
depends on the lamp, but my own experiments reveal that
you have to move slower than 0.1 metres per second, or
0.4km/h, to avoid being rumbled.
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