Beam Fraction Calculation in the LP80 (13469-01 AN)
The radiation reaching the probe of the LP80 can come directly from the solar beam, or be scattered from the sky or clouds. These two sources are affected differently by canopy architecture, and must therefore be treated separately in the computation of leaf area index from canopy transmission measurements. The information needed to make the computation is the beam fraction, or ratio of radiation that comes directly from the solar beam to the total radiation (beam plus scattered or diffuse PAR) incident on the probe. The previous version of AccuPAR required the user to measure beam fraction by shading the probe. The LP80 computes it using measurements it has available. The method used is modified from one published by Spitters et al. (1986) to find beam fraction for total radiation. They correlated beam fraction with the ratio of measured total global radiation to potential radiation on a horizontal surface outside the earth’s atmosphere.
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