303 
23 Development of the chlorophyll absorption ratio index (CARI) 
Although CAR appears to be a suitable method for estimating absorption from leaf level reflectance spectra, it 
was found to be an inadequate model for estimating A par at the canopy level. Canopy level CAR is a function of 
absorption due to green biomass and reflectance characteristics of background materials. Thus, the behavior of 
CAR is such that changes in the 550-700 nm slope in the canopy reflectance results changes the value of CAR. 
The canopy level CAR 
was modified to compensate for 
the effects of background 
reflectance. A multiplication of 
the reflectance band ratio of 700 
nm and 670 nm with CAR, was 
defined as the chlorophyll 
absorption ratio index (CARI = 
CAR Canopy* (^70o/^67o))' 
We believe that the ratio 
of the 700 nm and 670 nm bands 
counteracts the effects of 
background reflectance. This 
ratio is the slope of background 
materials when the canopy 
contains no green biomass. At 
670 nm, chlorophyll a in 
vegetation has an absorption 
maximum absorption which 
minimizes the background 
reflectance. The 700 nm band 
where chlorophyll a absorption 
becomes minimum is a band 
where the background reflectance 
becomes relatively significant. In 
the case of partial canopy 
closure, this ratio would depict 
the combined effects of the 
background reflectance as well as the fraction of nonphotosynthetic to photosynthetic materials. 
Figure 5 
The SAIL model simulated soybean canopy reflectance (LAI 
= 1.0). The ratio of 700/550 nm differences are the effect 
of reflectance characteristics of the background materials. 
3. ASSESSMENT OF CHLOROPHYLL ABSORPTION RATIO INDEX 
Soybean canopy reflectance at 10 nm spectral resolution at 550, 670, 700 nm, and 600-690 nm and 750-850 nm 
bands were simulated using the SAIL model. Table 2 lists the optical properties of soybean leaf and parameters 
used for the simulation. Reflectance of soils obtained from National Soil Erosion Research Laboratory described 
TABLE 2. Soybean leaf reflectance and transmittance plus other input parameters used for the SAIL model 
simulation of soybean canopy reflectance. 
Waveband 
% Refi. 
% Trans. 
TIME 
11:00 
550 ± 5 nm 
12.88 
18.03 
SOLAR DECLINATION ANGLE 
23.5CP 
670 ± 5 nm 
5.83 
3.18 
SOLAR ZENITH ANGLE 
20.79’ 
700 ± 5 nm 
14.80 
20.23 
LATITUDE 
40.00’ 
RED (600-690 nm) 
6.87 
6.98 
VIEW ZENITH ANGLE 
0.0Cf 
NIR (750-850 nm) 
45.85 
52.47 
VIEW AZIMUTH ANGLE 
O.OCf