Willkommen bei Paraselene.de


0 Pages
Graphic: Symbol MemoPad.


Opposition Effect

The opposition effect has been called a 'dry heiligenschein' (Minnaert). In fact it produces the same diffuse brightening around the observer’s shadow at the antisolar point although water is completely absent. The opposition effect is partly caused by shadow hiding. Along the line of sight of the observer each small object hides its own shadow and sunlight is reflected more or less completely to the observer’s eye. Farther away from the antisolar point this is no longer the case, shadows appear to the side of objects and on average the ground therefore looks darker. The opposition effect is most easily seen when the Sun is very low. Winter is the right season to look for it. There is an opposition effect on grass, the stubbles of a harvested field produce it, and it can even be observed on rough surfaces like asphalt or bare earth. If the objects are cylindrical and upright, a vertical streak is formed.

The opposition effect is slightly more yellow than its surroundings because of the absence of blue always present in shadows (because the shadows are illuminated by blue sky).

There are other processes which strongly contribute to the opposition effect. One of them is scattering and retro-reflection. There is a process called coherent backscattering which was first discovered by astronomers having to explain the unexpected brightness of planets in opposition to Earth and the Moon. You are much more likely to observe an opposition effect on a sandy road than on fresh and clean asphalt. The same effect works in licence plates of cars and road signs. There is a phenomenon half-way between the heiligenschein and the opposition effect which is called ‘sylvanshine’ (Lynch and Livingston). It is formed by the impermeable coatings of leaves and droplets of sap of certain shrubs and trees.

Opposition effect on asphalt near a house under construction. Olympus C-2000 Zoom digital camera. Processed with Adobe Photoshop 5.0 LE and Adobe Photoshop Elements.

Grafik: URL dieser Seite.
Permanent link:

nach oben