TRAC is a new optical instrument for measuring the Leaf Area Index (LAI) and the Fraction of Photosynthetically Active Radiation absorbed by plant canopies (FPAR). TRAC measures canopy 'gap size' distribution in addition to canopy 'gap fraction'. Gap fraction is the percentage of gaps in the canopy at a given solar zenith angle. It is usually obtained from radiation transmittance. Gap size is the physical dimension of a gap in the canopy. For the same gap fraction, gap size distributions can be quite different.

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