SRS Sensors
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ABOUT SRS
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About SRS
2.1
Overview
The SRS are two-band radiometers we designed to measure either in-
cident or reflected radiation in wavelengths appropriate for calculat-
ing the Normalized Difference Vegetation Index (NDVI) or the Pho-
tochemical Reflectance Index (PRI). They are designed to be an al-
ternative to more complex and costly spectrometers. The SRS sensor
comes in four different versions: NDVI-hemispherical (Ni), NDVI-
field stop (Nr), PRI-hemispherical (Pi) and PRI-field stop (Pr). The
hemispherical versions (Figure 1) are built with Teflon diffusers for
making cosine-corrected measurements, with a hemispherical 180
◦
FOV, and are primarily designed for up looking measurements of in-
cident radiation. The field stop versions (Figure 2) have a field of
view restricted to 36
◦
(18
◦
half angle) and are designed for pointing
downward to measure canopy reflected radiation.
Calculating NDVI or PRI requires knowing both incoming and re-
flected radiation. Unlike the reflected radiation, the incoming radia-
tion is spatially uniform above the canopy. So, you only need one up
facing radiometer to compute the vegetation indices for many down
facing radiometers that are within the same general area. The up
looking radiometer must be leveled and have a hemispherical field of
view.
The SRS is a digital sensor. Its outputs follow the SDI-12 standard.
The SRS is best suited for use with METER’s data loggers. How-
ever, customers can use the SRS with other loggers, such as those
from Campbell Scientific.
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