SRS Sensors
3
THEORY
diometer. See Royo and Dolors (2011) for an extensive introduction
to using spectral indices for plant canopy measurements.
Figure 3: Reflectance spectra for bare soil (Soil) and a healthy
wheat crop at various stages of development: heading (H), anthesis
(A), milk-grain stage (M), and post maturity (PM). Consider two
things about this figure: First, the considerable difference between
reflectance spectra from the soil and all stages of plant
development. Second, the changes in the visible spectra as the
canopy matures and senesces. Figure reproduced with permission
from Royo and Dolors (2011).
Calculate NDVI as:
N DV I
=
ρ
N IR
−
ρ
red
ρ
N IR
+
ρ
red
(1)
where,
ρ
red
and
ρ
N IR
are percent reflectances in the red and near
infrared (NIR). We assume percent reflectance to be the ratio of
reflected to incident radiation in the specified waveband. A detailed
description of how to calculate reflectances from measured radiation
values is provided in equation number 4.
7
