Wireless Sensor and Actuator Networks for Lighting Energy ...

(a)
Figure 8-6 Sensor placement of subject No. 2.
(b)
In six out of the ten cases, the fused values were constantly lower than the meter
readings by 11.37-25.64%. In two of these cases, subjects No. 3 and 4, one sensor
measurement was close to the meter measurement and the other two were constantly
lower. Figure 8-7 (a) illustrates the readings from one such case with the corresponding
sensor locations in Figure 8-7 (b). Even though the measurement of one sensor was
close to the meter readings, they were weighted less by the fusion algorithm as the
readings were significantly higher than the other two. As a result, the fused values were
lower than the real illuminances. In the other four cases of subjects No. 5-8, all three
sensor measurements were constantly lower than the meter readings. Figure 8-8
demonstrates one of the cases along with the sensor locations. These results show that
the sensors are likely to be placed at spots where the illuminances are lower than the
work area of the occupants. Those places with lower illuminance may be under subtle
shadows of other objects on the desktop, or simply farther away from the overhead
luminaire since the illuminance distribution is not perfectly even as Figure 6-2 implies.
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