FIGURE 4-19 Anisotropy factors for three wavelengths measured in perennial ryegrass with a field goniometer. Red displays the strongest BRDF, green is intermediate, and near-infrared is weakest. Adapted from Sandmeier (2004, fig. 3-3).

shadowed zones. Thus, CIR images that contain active vegetation, water bodies, and shadowing are often high contrast—quite bright and dark portions with little mid-range of brightness. Furthermore, darkening of water and shadows tends to exaggerate the appearance of sun glint and the hot spot (Fig. 4-18).

Numerous laboratory and field studies have demonstrated that the BRDF effect displays strong spectral variation for vegetated surfaces (Sandmeier, 2004). This is a consequence of multiple scattering effects within vegetation canopy and selective absorption of certain wavelengths. In general, high-absorbing red light shows the strongest response, whereas high-reflecting near-infrared is the weakest, and green is intermediate (Fig. 4-19).

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