Apogee Instruments S2-411 Manuel du propriétaire - Page 12
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OPERATION AND MEASUREMENT
The S2-400 series two-band sensors have an SDI-12 output, where irradiance or radiance is returned in digital
format. Measurement of S2-400 series two-band sensors requires a measurement device with SDI-12 functionality
that includes the M or C command.
Wiring for S2-400 series
Reflectance and Reflectance Indices (NDVI)
Reflectance (ρ) is the ratio of radiance, the calibrated signal returned by the downward-looking sensor, to
irradiance, the calibrated signal returned by the upward-looking sensor, for a specific wavelength range:
= radiance reflected from the surface / irradiance incident on the surface
Typically, a measurement of radiance is used in the numerator of the equation above to determine reflectance.
Theoretically, when ρ is determined from radiance reflected from the surface it is called directional reflectance,
which approximates the bidirectional reflectance factor. When ρ is determined from irradiance reflected from the
surface it is called hemispherical reflectance. Both terms are referred to as reflectance herein.
NDVI is calculated as the difference between near infrared (NIR) and red reflectance divided by the sum of NIR and
red reflectance:
It is important to ensure that paired upward-looing and downward-looking sensors make measurements at the
same time, otherwise temporal changes in sky conditions can result in errors in reflectance and calculated NDVI
values.
A single upward-looking sensor can be deployed with multiple downward-looking sensors, and irradiance from the
single upward-looking sensor can be used to calculate NDVI at the location of all the downward-looking sensors, if
the downward-looking sensors are in close proximity to each other (e.g., within a research plot).
−
=
+
White: Output Signal
Red: Power in (4.5-24 V DC)
Black: Ground
Clear: Shield