This reference coupon was stored in a dry and clean environment to prevent the accumulation of soiling
on its surface. There were some differences between transmittance measurements conducted at the
calibration stage and the ones conducted during the validation stage. First, during the calibration stage,
the screening masks were fixed to the surface of the reference coupon; whereas during the validation
stage only the transmittance of the naturally soiled coupons was measured. Second, to take into
account the presence of non-uniform soiling over naturally soiled coupons, four transmittance
measurements were performed in different points of their surface during the validation stage. The
average of these measurements was calculated. Also the standard deviation was calculated as a way to
quantify the level of nonuniformity. In this work, the DUSST measurements was conducted by recording
the PV cell current and LED temperature during a 21-minute period with a 2-seconds sampling interval.
Then, the data were processed to obtain a corrected value of current, independently of the LED
temperature and external conditions. This methodology is shown in Fig. 6. It should be noted that, in
order to minimize the impact of outliers, when the difference between two consecutive cell current
measurements was higher than 0.2 mA, the measurements was replaced with the average of the 5
previous and 5 posterior measurements.