For the 10 test sites, the linear curve fits (R2) were evaluated to determine the extent to which the soiling effect is linear. An R2 of 0.7 or higher was considered sufficient to support the use of a linear approximation. The actual average of all curve fits for the 10 sample sites was 0.835. Table 1 details the R2 values for all four performance metrics for each of the 10 sample sites. The best fits for each system are highlighted.