18 No comparisons of ground measurements with data from remote-sensing instruments that observe in retroreflection (such as POLDER: polarization and directionality of the Earth’s reflectances 5 ) have been available for systematic study.
10,16 Few laboratory measurements have been available for reference in the study of land surfaces, except in a few special cases designed for studying the physical mechanisms that cause hot spots and the opposition effect of ices. Thus far, polarization has received more attention in studies of optics and biological media, 17 but it has found a few applications in planetary and space studies as well. 4,16 Changes in the polarization ratio are also common near backscatter, and the polarization features seem to be even more distinctive than intensity effects. Some complicated physical mechanisms also make BRDF near backscattering a special case: The current physical phenomena related to brightening are shadow hiding and coherent backscattering, but their contributions to the intensity peak are some- what difficult to separate in experimental data. 14 Backscattering is involved in laser scanning altim- etry, 15 but the intensity data have not been used thus far. 13 Ra- dar backscattering signatures from land surfaces have also been shown to be distinguishable such that they can be utilized in radar scatterometers for, e.g., identification of snow-free and snow-covered areas.
11,12 Applications include medicine, in which backscattering instruments facilitate use of noncontact diagnostic methods. The effect is called weak localization (of photons), and it has been the subject of various studies, mostly of small particles. 8 –10 In condensed-matter physics, narrow coherent backscattering peaks are observed on a milli- radian scale. This phe- nomenon is known in planetary science as the opposition effect and has been studied for decades for solar system regolith surfaces and ices (e.g., Saturn’s rings). The BRDFs for field targets (such as lichen 6,7 ), measured with ground-based goniometers, often show enhancement in the backward direction, although zero phase has not been reached. A surge in brightness near zero phase (often referred to as exact backscattering to distinguish it from backward scattering, i.e., reflection to the same hemisphere as the light source) is a common feature of most remote-sensing targets, such as forest and vegetation, 4,5 where it is seen as a bright spot (called a hot spot) in satelliteborne or airborne images. 1) and because it is utilized in several applications described below. The BRDF at small phase (light source–target– detector) angles is often studied as a special case, both because it requires special instrumentation to prevent the detector from obscuring the light source (see Fig. 1–3 Laboratory and field data are especially useful in developing methods for investigating spatially complex targets, such as forests and vegetation canopies. These features are characteristic of surface structure and have led to applications in both terrestrial and planetary remote sensing for characterizing targets and retrieving the textural properties of surfaces. bidirectional reflectance distribution function (BRDF) describes the directional features of light reflected from a surface (see Fig.
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