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Heiligenschein, or hotspot, around the shadow of a hot-air balloon cast on a field of standing crops (Oxfordshire, England)

Heiligenschein (German for "halo" or "aureola", pronounced [ˈhaɪlɪɡənˌʃaɪn]) is an optical phenomenon in which a bright spot appears around the shadow of the viewer's head. In photogrammetry and remote sensing, it is more commonly known as the hotspot. It is also occasionally known as "Cellini's halo" after the Italian artist and writer Benvenuto Cellini (1500-71), who described the phenomenon in his memoirs in 1562.[1]

This diffuse reflection is due to the opposition surge, the reduction in the proportion of shadows viewed at angles close to the backscatter direction. It may also be created when the surface on which the shadow falls has retroreflective optical properties. Both dry regolith and dewy grass are known to exhibit these characteristics. Nearly spherical dew droplets act as lenses to focus the light onto the surface behind them. When this light scatters or reflects off that surface, the same lens re-focuses that light into the direction from which it came. This configuration is sometimes called a cat's eye retroreflector. Any retroreflective surface is brightest around the antisolar point.

Heiligenschein appears around head of Buzz Aldrin's shadow due to the retroreflective properties of lunar regolith.[2] This is a close up of the reflection in Aldrin's visor, cropped from the famous image.[3] The white figure to the right is the photographer, Neil Armstrong.

The glory is a similar halo effect caused by a different mechanism.

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  1. ^ "Heiligenschein". The American Meteorological Society. Retrieved 8 March 2019.
  2. ^ Hapke, Bruce; Nelson, Robert; Smythe, William (1998). "The Opposition Effect of the Moon: Coherent Backscatter and Shadow Hiding". Icarus. 133: 89–97. Bibcode:1998Icar..133...89H. doi:10.1006/icar.1998.5907.
  3. ^ Catalog number AS11-40-5903

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