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INTRODUCTION
Radiation reflected from surfaces (e.g., plant canopies, soil) provides information about the state of the surface.
Reflectance is the ratio of radiation reflected by the surface to radiation incident on the surface.
A common index calculated from reflectance measurements is normalized difference vegetation index (NDVI).
NDVI is calculated from red and near infrared (NIR) reflectance and provides a measure of surface greenness.
The typical application of NDVI sensors is monitoring plant canopies. NDVI is related to leaf area of the plant
canopy and canopy chlorophyll content of leaves and is often used to monitor green up in the spring and
senescence in the fall.
Apogee Instruments S2 series two-band sensors consist of a cast acrylic diffuser (upward-looking sensor) or acrylic
window (downward-looking sensor), pair of photodiodes that measure specific wavelength ranges, and signal
processing circuitry mounted in an anodized aluminum housing. A cable to connect the sensor to a measurement
device is also included. S2 series sensors are designed for continuous irradiance (upward-looking sensor) or
radiance (downward-looking sensor) measurements in indoor and outdoor environments. Reflectance derived
from paired upward-looking and downward-looking sensors can be used to calculate NDVI. Apogee NDVI sensors
are offered with digital SDI-12 output or with analog voltage output.