The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B7. Beijing 2008 
400-800nm region. However, the greatest correlation between 
8R and chlorophyll content was near 540-550rmK 675-685nm 
and 720-735nm. 
2.3 optimal bands range 
The criterions to determine the optimal wavelength regions for 
estimating the mixed canopy chlorophyll content are: (1) the 
reflectance or the first derivative of reflectance in these 
wavelength regions should show the higher correlation with 
chlorophyll content; (2) the optimal wavelength regions should 
be those regions in which the reflectance or the first derivative 
of reflectance shows the highest correlation both linear and 
nonlinear with the mixed leaves chlorophyll content. Due to the 
nonlinear correlation involving the saturation problem, the 
stronger linear correlation is dominant criterion. The criterions 
not only ensure high correlation between reflectance of optimal 
wavelength regions and chlorophyll content, but also avoid the 
information saturation problem in relevant bands. 
In order to develop better algorithms for estimating chlorophyll 
content, the wavelength bands with maximum and minimum 
sensitivities to chlorophyll content were identified from 
correlogram plots showing the correlation coefficients between 
reflectance and chlorophyll content at all wavelength. The 
determination of optimal bands to indicate the mixed canopy 
chlorophyll content depended on the integration of linear and 
nonlinear correlation analysis. The overlap regions with higher 
linear and nonlinear correlation were determined as optimal 
bands. Figurel, Figure2, Figure3, and Figure4 provided the 
information for locating the overlap regions of reflectance or the 
first derivative of reflectance. In addition, the overlap 
wavelength regions, in which the reflectance was insensitive to 
chlorophyll content, can serve as the reference bands. 
Band ratios were calculated by dividing the reflectance in a 
band insensitive to pigment content to the reflectance in bands 
highly sensitive to pigment content. The 698-7 lOnm band was 
found to be the best sensitive band for constructing reflectance 
ratios. Pigment do not absorb NIR radiation, so the NIR bands 
as the insensitive terms for developing reflectance-based 
algorithms for estimating leaf pigment content. 
Correlation analysis 
Significant correlated 
spectra region(nm) 
Insensitive 
bands(nm) 
Sensitive 
bands(nm) 
Extend 
bands(nm) 
Reference 
bands(nm) 
reflectance spectra 
Linear model (R) 
698-715|R|>0.4 
530-570|R|>0.42 
740-900 
698-710 
698-715 
740-900 
Nonlinear model (R 2 ) 
581-712 R 2 >0.5 
and: 621-706 R 2 >0.6 
740-900 
the first derivative 
of reflectance 
Linear model (R) 
723-742 R>0.6, 
and: 727-736 R>0.65 
531-545 R>0.57 
557-563 
670-674 
703-708 
723-735 
539-545 
720-745 
535-550 
703-708 
670-674 
Nonlinear model (R 2 ) 
719-735 R 2 >0.54, 
and: 720-728 R 2 >0.6 
675-685 R 2 >0.5, 
and: 678-682 R 2 >0.61 
537-552 R 2 >0.5, 
and: 539-550 R 2 >0.61 
564-587 
694-705 
656-674 
Table 1. Result of correlation analysis between chlorophyll content and reflectance spectra, 
the first derivatives of reflectance spectra, respectively 
Reflectance in the main chlorophyll absorption region near 
680nm has been used for a long time as an indicator of 
chlorophyll content of leaves. However, the relationship 
between reflectance near 680nm and chlorophyll content has 
been shown to become saturated at medium to high chlorophyll 
contents, and is therefore the least sensitive to variation in 
chlorophyll content. The reflectance near 680nm was found to 
be the better reference band than the NIR bands (Datt et al., 
1998). In this study, correlation analysis results showed that 
740-900nm region is the better reference bands, besides the 
region near 680nm. 
Table 1 showed that the sensitive reflectance regions to 
chlorophyll content are not the same regions of the sensitive 
derivative reflectance. 
3. RESULTS 
The chlorophyll index (Cl = (R 75 o-/?705)/( Riio+Rios)), which was 
proved to be the better indictor of mixed leaves chlorophyll 
content (Figure 5) using LOPEX93 database. However, Cl is 
narrow band index. According to analysis on the optimal 
wavelength regions for estimation of mixed canopy chlorophyll 
content, it is possible to extend Cl to be wide band index. To 
validate the optimal bands in table 1, we defined the CI widc as 
_ 1*760 <*710 /*760 1*710 ... 
CT ~ + L*A)< 2 > 
Where CI wide is the extended chlorophyll index, R x is the 
reflectance at wavelength X. The index CI wide described by 
equation (2) was tested. The strong relationship between 
chlorophyll content and CI wide is indicated by the scatterplot in 
Figure 6. 
In addition, the reflectance integration of random 1 Onm interval 
between 740nm and 900nm can substitute the reflectance 
integration between 750nm and 760nm in equation (2). In 
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