3
THEORY
SRS Sensors
3
Theory
METER designed the SRS to measure NDVI and PRI vegetation in-
dices from plant canopies. We caution users that NDVI and PRI are
derived from measurements of radiation reflected from canopy sur-
faces, and therefore provide only indirect or correlative associations
with several canopy variables of interest and should not be treated
as direct measurements of these variables.
NDVI has a well-established and long history of use in remote sens-
ing research and ecological applications related to canopy structure.
PRI, while showing great promise for quantifying canopy physiolog-
ical function, is far more experimental with new uses and caveats
continually being discovered. While NDVI and PRI can be power-
ful tools for inferring structure and function of plant canopies, you
must take into account their limitations when interpreting the data.
Section 3 provides an overview of the theory and discusses some of
the uses and limitations of each vegetation index.
3.1
Normalized Difference Vegetation Index (NDVI)
A number of nondestructive methods exist for remotely monitoring
and quantifying certain canopy characteristics. Some of those char-
acteristics are: foliar biochemistry and pigment content, leaf area
index (LAI, Nguy-Robinson et al., 2012), phenology, and canopy
photosynthesis (Ryu et al., 2010). One nondestructive method in-
volves measuring NDVI. The underlying principle of NDVI derives
from a well known concept that vegetation reflects light differently in
the visible spectrum (400 to 700 nm) compared to the near infrared
(
>
700 nm).
Green leaves absorb light most strongly in the visible spectrum, es-
pecially at red wavelengths, but are highly reflective in the near in-
frared region (Figure 3). Because bare soil, detritus, stems, trunks,
branches, and other non-photosynthetic elements show relatively lit-
tle difference in reflectance between the visible and near infrared,
measuring the difference between reflectance in these two bands can
be related to the amount green vegetation in the field of view of a ra-
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