Editing Diffuse reflection (section)

==Mechanism==
[[Image:Diffuse reflection.gif|thumb|right|Figure&nbsp;1&nbsp;– General mechanism of diffuse reflection by a solid surface ([[refraction]] phenomena not represented)|250px]]
[[File:Diffuse reflection.svg|thumb|Figure&nbsp;2&nbsp;– Diffuse reflection from an irregular surface|250px]]

Diffuse reflection from solids is generally not due to surface roughness. A flat surface is indeed required to give specular reflection, but it does not prevent diffuse reflection. A piece of highly polished white marble remains white; no amount of polishing will turn it into a mirror. Polishing produces some specular reflection, but the remaining light continues to be diffusely reflected.

The most general mechanism by which a surface gives diffuse reflection does not involve ''exactly'' the surface: most of the light is contributed by [[Subsurface scattering|scattering centers beneath the surface]],<ref>P.Hanrahan and W.Krueger (1993), ''Reflection from layered surfaces due to subsurface scattering'', in [http://www.cs.berkeley.edu/~ravir/6998/papers/p165-hanrahan.pdf SIGGRAPH ’93 Proceedings, J. T. Kajiya, Ed., vol.&nbsp;27, pp.&nbsp;165–174] {{webarchive|url=https://web.archive.org/web/20100727005751/http://www.cs.berkeley.edu/~ravir/6998/papers/p165-hanrahan.pdf |date=2010-07-27 }}.</ref><ref>H.W.Jensen et al. (2001), ''A practical model for subsurface light transport'', in '[http://www.cs.berkeley.edu/~ravir/6998/papers/p511-jensen.pdf Proceedings of ACM SIGGRAPH 2001', pp.&nbsp;511–518] {{webarchive|url=https://web.archive.org/web/20100727005456/http://www.cs.berkeley.edu/~ravir/6998/papers/p511-jensen.pdf |date=2010-07-27 }}</ref> as illustrated in Figure&nbsp;1.
If one were to imagine that the figure represents snow, and that the polygons are its (transparent) ice crystallites, an impinging ray is partially reflected (a few percent) by the first particle, enters in it, is again reflected by the interface with the second particle, enters in it, impinges on the third, and so on, generating a series of "primary" scattered rays in random directions, which, in turn, through the same mechanism, generate a large number of "secondary" scattered rays, which generate "tertiary" rays, and so forth.<ref>Only primary and secondary rays are represented in the figure.</ref> All these rays walk through the snow crystallites, which do not absorb light, until they arrive at the surface and exit in random directions.<ref>Or, if the object is thin, it can exit from the opposite surface, giving diffuse transmitted light.</ref> The result is that the light that was sent out is returned in all directions, so that snow is white despite being made of transparent material (ice crystals).

For simplicity, "reflections" are spoken of here, but more generally the interface between the small particles that constitute many materials is irregular on a scale comparable with light wavelength, so diffuse light is generated at each interface, rather than a single reflected ray, but the story can be told the same way.

This mechanism is very general, because almost all common materials are made of "small things" held together. Mineral materials are generally [[polycrystalline]]: one can describe them as made of a 3D mosaic of small, irregularly shaped defective crystals. Organic materials are usually composed of fibers or cells, with their membranes and their complex internal structure. And each interface, inhomogeneity or imperfection can deviate, reflect or scatter light, reproducing the above mechanism.

Few materials do not cause diffuse reflection: among these are metals, which do not allow light to enter; gases, liquids, glass, and transparent plastics (which have a liquid-like [[amorphous]] microscopic structure); [[Monocrystal|single crystals]], such as some gems or a salt crystal; and some very special materials, such as the tissues which make the [[cornea]] and the [[Lens (anatomy)|lens]] of an eye. These materials can reflect diffusely, however, if their surface is microscopically rough, like in a [[frost glass]] (Figure&nbsp;2), or, of course, if their homogeneous structure deteriorates, as in [[cataract]]s of the eye lens.

A surface may also exhibit both specular and diffuse reflection, as is the case, for example, of [[glossy]] [[paint]]s as used in home painting, which give also a fraction of specular reflection, while [[matte (surface)|matte]] paints give almost exclusively diffuse reflection.

Most materials can give some specular reflection, provided that their surface can be polished to eliminate irregularities comparable with the light wavelength (a fraction of a micrometer). Depending on the material and surface roughness, reflection may be mostly specular, mostly diffuse, or anywhere in between. A few materials, like liquids and glasses, lack the internal subdivisions which produce the subsurface scattering mechanism described above, and so give ''only'' specular reflection. Among common materials, only polished metals can reflect light specularly with high efficiency, as in aluminum or silver usually used in mirrors. All other common materials, even when perfectly polished, usually give not more than a few percent specular reflection, except in particular cases, such as [[Angle of incidence (optics)#Grazing angle|grazing angle]] reflection by a lake, or the ''[[total reflection]]'' of a glass prism, or when structured in certain complex configurations such as the silvery skin of many fish species or the reflective surface of a [[dielectric mirror]]. Diffuse reflection can be highly efficient, as in white materials, due to the summing up of the many subsurface reflections.