Schematic diagram of the reference structure. (a) Type I mask for PIR1, PIR2, PIR3, and PIR4. (b) Type II mask for PIR5, PIR6, and PIR7. (c) The multiplexing of PIR sensors with masks form sampling cells. (d) The measure space, object space and the thermal target. The human target can be modeled by a vertical cylinder thermal source approximately. During the process of fall, the thermal target will cross the boundaries of the sampling cells and PIR sensors will generate output correspondingly.