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neutral point1. In the clear sky, one of several points in the principal plane (or very near it) where the degree of linear polarization P = 0 (i.e., polarization is neutral). Stated another way, a neutral point occurs where Stokes parameter Q = U = 0. In fact, P often differs indistinguishably from zero within a small area (∼3° radius) around the nominal principal point, and thus neutral polarization routinely occurs slightly outside the principal plane. For a single-scattering molecular atmosphere, the neutral points would coincide with the solar and antisolar points. In the real atmosphere, scattering by large particles (e.g., haze) and multiple scattering displace the observed neutral points. The Arago neutral point occurs ∼10°–30° above the antisolar point. The Brewster neutral point occurs below the sun and the Brewster point occurs above it. When the sun is low in the sky, the Brewster and Babinet points can be as much as 25°–35° from it, although their angular distances need not be equal. In principle, the Brewster and Babinet points coincide with the zenith sun. 2. Same as hyperbolic point.
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