Light Scattering Correction
One of the traditional underlying assumptions of the LAI‑2000 and LAI‑2200 has been that foliage absorbs all the radiation in the blue waveband seen by the sensor (320-490 nm). This is usually a good assumption under diffuse light conditions such as uniform overcast, just before sunrise, or just after sunset. In direct sunlight, however, reflectance off foliage causes a much greater overestimation of the gap fraction and underestimation of leaf area index.
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The software package FV2200 version 2.0 allows this assumption to be set aside (following the model presented in Kobayashi et al., 2013). It provides a mechanism for correcting measurements for the radiation reflected and transmitted by the foliage. You should apply this correction for data taken in direct sun, since that’s when the scattering errors are the highest, but you can also correct data taken when the sun is obscured, adjusting for the actual foliage scattering properties in your plots rather that assuming reflectance and transmittance are both zero.
Scattering correction is a significant improvement, especially where many below-canopy readings are needed, such as heterogeneous forest canopies. Measurements can now be done throughout the daylight hours, even under clear skies. Partly cloudy skies are still challenging, especially with fast-moving clouds, but can be accommodated with the proper sampling technique.
Inputs for light scattering corrections:
- Solar position. Provided automatically in the LAI‑2200C (or the LAI‑2200 equipped with the 2200CLEAR upgrade kit). The FV2200 PC software uses latitude, longitude, and UTC to compute solar zenith and azimuth angles.
- Sky radiation properties (measured with the optical sensor)
- Fraction beam. Fraction of the total incident radiation (in the blue waveband) from direct beam. Derived from two readings with the optical sensor covered by a diffuser cap. The sensor is shaded for one of the readings.
- Sky brightness distribution. Two types are needed: One for the whole sky, and one for the region you are using as a reference (“above” readings).
- Scattering properties. Foliage reflectance and transmittance as well as ground surface reflectance, all for the blue waveband.