Int. J. Environ. Res. Public Health 2010, 7 2445 Figure 6. Linear models with reflectance ratios of characteristic bands. (a) R705.2/R572.4 (b)R705.2 / R682.1 As shown in Figure 6 the Chl-a concentration estimation does not produce good results on reflectance ratios of characteristic bands, with R2 of 0.0439 and 0.0037 respectively in Tangxun Lake. The fitting line will cause a huge error in Chl-a concentration estimation, which could not provide the accuracy needed for practical Chl-a concentration monitoring. Therefore, it is concluded that reflectance ratios of characteristic bands are not universally applicable. To find the optimal bands suitable for the method for estimating reflectance ratio in Chl-a concentration in Tangxun Lake, correlation analysis between the reflectance ratios of different bands and Chl-a concentration in field measurements was applied. Reflectance ratios between 384 pairs of bands from 380 nm to 900 nm were calculated for correlation analysis. The distribution of correlation coefficients between reflectance ratios and Chl-a are shown in Figure 7. The absolute values of correlation coefficients varied from 0 to 0.928 with different band combinations, and the maximum absolute value of 0.928 was achieved for the R861.1/R865.7 ratio combination. Based on the correlation analysis, the optimal band combination is easy to obtain, and can be used for Chl-a concentration estimation in Tangxun Lake. Figure 7. The correlation coefficients between Chl-a and reflectance ratios.