Journal Contributor Gary Swearingen suggested the addition of a brief explanation of coherent backscattering. The following was found in a Wikipedia article on the general topic of scattering.
"(A) special type of EM (electromagnetic) scattering is coherent backscattering. This is a relatively obscure phenomenon that occurs when coherent radiation (such as a laser beam) propagates through a medium which has a large number of scattering centers, so that the waves are scattered many times while traveling through it. A thick cloud is a typical example of this sort of multiple-scattering medium. The effect produces a very large peak in the scattering intensity in the direction from the which the wave travels—effectively, the light scatters preferentially back the way it came. For incoherent radiation, the scattering typically reaches a local maximum in the backward direction, but the coherent backscatter peak is two times higher than the level would have been if the light were incoherent. It is very difficult to detect and measure for two reasons. The first is fairly obvious, that it is difficult to measure the direct backscatter without blocking the beam, but there are methods for overcoming this problem. The second is that the peak is usually extremely sharp around the backward direction, so that a very high level of angular resolution is needed for the detector to see the peak without averaging its intensity out over the surrounding angles where the intensity can undergo large dips. At angles other than the backscatter direction, the light intensity is subject to numerous essentially random fluctuations called speckles."
The lunar surface is composed of numerous small grains of various sizes, shapes, and orientations and each of these serves as a 'scattering center' of direct sunlight.
Ron Wells provided the following references to the subject, with some annotation. References No. 8 and 9 are the current standard works.
- 1. Hapke, B. Bidirectional reflectance spectroscopy. 1. Theory.
J. Geophys. Res. 86: 3039-3054 (1981).
- 2. Hapke, B. Bidirectional reflectance spectroscopy. 3. Correction for macroscopic roughness. Icarus 59: 41-59 (1984).
- 3. Hapke, B. Bidirectional reflectance spectroscopy. 4. The extinction coefficient and the opposition effect. Icarus 67: 264-280 (1986).
- 4. Helfenstein, P., and Veverka, J. Photometric properties of lunar terrains derived from Hapke's equation. Icarus 72: 342-357 (1987).
- 5. Hapke, Bruce, Theory of Reflectance and Emittance Spectroscopy, (New York, NY: Cambridge University Press, 1993).
- 6. Hapke, B. W., Nelson, R. M., Smythe, W. D., The opposition effect of the moon - the contribution of coherent backscatter. Science 260: 509-511 (1993).
- 7. Hapke, B., Are planetary regolith particles back scattering - response.
J. Quan. Spectr. Rad. Trans. 55: 837-848 (1996).
- 8. Helfenstein, P., Veverka, J. and Hillier, J. The lunar opposition effect:
A test of alternative models. Icarus 128: 2-14 (1997). | Derivation of a composite coherent backscatter and shadow hiding surface reflectance model of the lunar surface.
- 9. Hapke, B., Nelson, R., Smythe, W., The opposition effect of the moon: coherent backscatter and shadow hiding. Icarus 133: 89-97 (1998). | Laboratory measurements of lunar soils verifying the composite model derived in ref. .
- 10. Hapke, B., Scattering and diffraction of light by particles in
planetary regoliths. J. Quan. Spectr. Rad. Trans. 61: 565-581 (1999).
- 11. Helfenstein, P., and Shepard, Michael K., Submillimeter-Scale Topography of the Lunar Regolith. Icarus 141: 107-131 (1999). | A comparison of Gaussian, fractal, and photometric statistics on the stereo photogrammetric representation of Apollos 11, 12, and 14 soils. The first part of the paper describes iterated procedures of measuring parallax offsets of individual features in both the left and right stereo frames.
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