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SRS Sensors
temporal variability in radiation conditions. In cases where multiple
down looking sensors have been deployed within close proximity to
each other, it is only necessary to have one up looking sensor. The
measurements from the single up looking sensor can be combined
with the measurements from each of the down looking sensors to cal-
culate reflectances
In the event that up looking measurements are not available, re-
arrangement of the vegetation index equations allows for a rough
approximation of the measurements. The following derivation is for
NDVI, but similar equations apply to the PRI. If R
NIR radiation from the canopy, R
is the incident NIR, and I
R
N DV I =
R
Where α = I
/I
r
just the down facing measurements if you know the ratio of red to
NIR spectral irradiance, α. Although not extensively tested, we have
found that this ratio (α = 1.86 for NDVI bands) can be used as a
rough approximation during midday under relatively clear sky condi-
tions. However, we caution that direct measurements of downwelling
radiation is more accurate by accounting for any fluctuations in α
that occur with changes in atmospheric conditions or across large
variations in sun elevation angle.
In the event that you do not want to use the default α value or
measurements from an up facing sensor are not available, it is is pos-
sible to use a spectralon panel or similar reflectance standard with a
field stop SRS to measure incident irradiance. To measure incident
irradiance with a down facing sensor, place a reflectance standard
within the field of view of the field stop sensor, making sure that the
reflectance panel is level, uniformly illuminated and that the field of
view of the sensor is fully within the area of the reflectance panel.
Measurements obtained from field stop sensors pointed at the re-
flectance panel must be multiplied by π to convert radiance values
to irradiance values. Irradiance values can then be used in Equation
4 or to calculate α directly.
is the incident red, then
r
/I
R
/I
n
n
r
r
=
/I
+ R
/I
n
n
r
r
, equation 4 allows the computation of NDVI from
n
is the reflected red radiation, I
r
(I
/I
)R
R
r
n
n
r
(I
/I
)R
+ R
r
n
n
r
15
3 THEORY
is the reflected
n
αR
R
n
r
=
αR
+ R
n
r
n
(4)
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