Diffuse Sky Radiation - Neutral Points

Neutral Points

There are three commonly detectable points of zero polarization of diffuse sky radiation (known as neutral points) lying along the vertical circle through the sun.

• The Arago point, named after its discoverer, is customarily located at about 20° above the antisolar point; but it lies at higher altitudes in turbid air. The latter property makes the Arago distance a useful measure of atmospheric turbidity.
• The Babinet point, discovered by Babinet in 1840, is located about 15° to 20° above the sun, hence it is difficult to observe because of solar glare.
• The Brewster point, discovered by Brewster in 1840, is located about 15° to 20° below the sun; hence it is difficult to observe because of solar glare.