Agriculture Reference
In-Depth Information
0.8
diff. of inversions
reflect. ratio minus 1
(R
l
)
-1
- (R
NIR
)
-1
(R
NIR
/R
l
)
-1
0.6
0.4
inverted refl.
(
R
l
)
-1
explanation of symbols in indices:
= reflectance for respective wavelength
= reflectance in near-infrared range
0.2
R
l
R
NIR
red edge
0.0
400
450
500
550
600
650
700
750
wavelength
l
in nm
blue
green
yellow
red
near-infr.
Fig. 6.8
Coefficient of determination (r
2
) of three reflectance indices versus chlorophyll content
of leaves (From Gitelson et al.
2003
, altered)
laboratories. Along the whole range of visible wavelengths plus the red edge, the
prediction is slightly improved in the order of firstly the simple inverted reflectance
(R
λ
)
−
1
, secondly the difference of inverted reflectances
(R
λ
)
−
1
−
(R
NIR
)
−
1
and thirdly
the reflectance ratio minus one
(R
NIR
/R
λ
)
−
1
. So the logical expectations regarding
the corrections or enhancing the predictions have been confirmed.
Yet within the visible radiation, the differences between these three indices on
the reliability of the estimates are very small compared to the effect of the wave-
lengths λ. For the best index - the
reflectance ratio minus one
- the r
2
for the
wavelengths λ involved goes from 0.20 to about 0.95 (Fig.
6.8
). Hence getting reli-
able predictions about the chlorophyll in the leaves is primarily a matter of selecting
the most suitable wavelength bands. The bands should be located either in the green
range or in the red edge range and perhaps also in both of them.
Based on the reflectance ratio minus one, the recommended indices and wavelengths
for sensing of chlorophyll in leaves are either (Gitelson et al.
2003
)
• the green chlorophyll index with the formula
(R
NIR
/R
520-585
)
−
1
,
or
• the red edge chlorophyll index with the formula
(R
NIR
/R
695-740
)
−
1
.
The rather wide wavelength ranges in the green region (520-585 nm) and in the
red edge region (695-740 nm) leave some tolerances for narrower bands within and
hence for adapting radiometers.
