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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-looking 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).
While paired upward-looking and downward-looking sensors provide the most accurate measurements of NDVI,
radiance measurements from downward-looking sensors can also be used to approximate NDVI without
calculating reflectance from paired upward-looking and downward-looking sensors. The equation has a similar
form to the NDVI equation above, but uses radiances (R) in the calculation:
where (alpha) is the ratio of red irradiance to NIR irradiance (the values measured by an upward-looking sensor).
Data from multiple solar spectra indicate ranges from about 1.1 to 1.4, with lower values occurring at high solar
zenith angles or under overcast sky and higher values occurring under clear sky at low solar zenith angles.
Approximations of NDVI from the equation above are least sensitive to the value of when the difference
between NIR and Red radiances are relatively large (e.g., measurements over green vegetation) and most sensitive
to the value of when the difference between NIR and Red radiances are relatively small (e.g., measurements
over senesced vegetation or soil).
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