Diffuse reflection: Difference between revisions

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{{short description|Reflection with light scattered at random angles}}

{{For|reflection of charged particles|Scattering from rough surfaces}}

{{short description|Reflection with light scattered at random angles}}

[[File:Lambert2.gif|thumb| Diffuse and specular reflection from a glossy surface.<ref>

{{cite book

|title = Photoelectric sensors and controls: selection and application

|edition =

|author = Scott M. Juds

|publisher = CRC Press

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|isbn = 978-0-8247-7886-6

|page = 29

|url = https://books.google.com/books?id=BkdBo1n_oO4C&pg=PA29&dqq=%22diffuse+reflection%22+lambertian#v=onepage&qpg=%22diffuse%20reflection%22%20lambertian&f=falsePA29

|deadurlurl-status = nolive

|archiveurlarchive-url = https://web.archive.org/web/20180114234340/https://books.google.com/books?id=BkdBo1n_oO4C&pg=PA29&dq=%22diffuse+reflection%22+lambertian#v=onepage&q=%22diffuse%20reflection%22%20lambertian&f=false

|archivedatearchive-date = 2018-01-14

|df =

}}</ref> The rays represent [[luminous intensity]], which varies according to [[Lambert's cosine law]] for an ideal diffuse reflector.

]]

'''Diffuse reflection''' is the [[reflection (physics)|reflection]] of [[light]] or other [[radiationWave|waves]] or [[particles]] from a surface such that a [[ray (optics)|ray]] incident on the surface is [[scattering|scattered]] at many [[angle]]s rather than at just one angle as in the case of [[specular reflection]]. An ''ideal'' diffuse reflecting surface is said to exhibit [[Lambertian reflection]], meaning that there is equal [[luminance]] when viewed from all directions lying in the [[half-space (geometry)|half-space]] adjacent to the surface.

A surface built from a non-absorbing powder such as [[plaster]], or from fibers such as paper, or from a [[polycrystalline]] material such as white [[marble]], reflects light diffusely with great efficiency. Many common materials exhibit a mixture of specular and diffuse reflection.

The visibility of objects, excluding light-emitting ones, is primarily caused by diffuse reflection of light: it is diffusely-scattered light that forms the image of the object in thean observer's eye over a wide range of angles of the observer with respect to the object.

==Mechanism==

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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).

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==Colored objects==

Up to this point white objects have been discussed, which do not absorb light. But the above scheme continues to be valid in the case that the material is absorbent. In this case, diffused rays will lose some wavelengths during their walk in the material, and will emerge colored.

Diffusion affects the color of objects in a substantial manner because it determines the average path of light in the material, and hence to which extent the various wavelengths are absorbed.<ref>Paul Kubelka, Franz Munk (1931), ''Ein Beitrag zur Optik der Farbanstriche'', Zeits. f. Techn. Physik, '''12''', 593–601, see [https://web.archive.org/web/20110717155703/http://web.eng.fiu.edu/~godavart/BME-Optics/Kubelka-Munk-Theory.pdf ''The Kubelka-Munk Theory of Reflectance''] {{webarchive|url=https://web.archive.org/web/20110717155703/http://web.eng.fiu.edu/~godavart/BME-Optics/Kubelka-Munk-Theory.pdf |date=2011-07-17 }}</ref> Red ink looks black when it stays in its bottle. Its vivid color is only perceived when it is placed on a scattering material (e.g. paper). This is so because light's path through the paper fibers (and through the ink) is only a fraction of millimeter long. However, light from the bottle has crossed several centimeters of ink and has been heavily absorbed, even in its red wavelengths.

And, when a colored object has both diffuse and specular reflection, usually only the diffuse component is colored. A cherry reflects diffusely red light, absorbs all other colors and has a specular reflection which is essentially white (if the incident light is white light). This is quite general, because, except for metals, the reflectivity of most materials depends on their [[refractive index]], which varies little with the wavelength (though it is this variation that causes the [[chromatic dispersion]] in a [[Prism (optics)|prism]]), so that all colors are reflected nearly with the same intensity. Reflections from different origin, instead, may be colored: metallic reflections, such as in gold or copper, or [[Interference (wave propagation)|interferential]] reflections: [[iridescence]]s, peacock feathers, butterfly wings, beetle [[elytra]], or the [[antireflection coating]] of a lens.

==Importance for vision==

The vast majority of visible objects are seen primarily by diffuse reflection from their surface.<ref name="z">{{cite book

Looking at one's surrounding environment, the vast majority of visible objects are seen primarily by diffuse reflection from their surface. This holds with few exceptions, such as glass, reflective liquids, polished or smooth metals, glossy objects, and objects that themselves emit light: the Sun, lamps, and computer screens (which, however, emit ''diffuse'' light). Outdoors it is the same, with perhaps the exception of a transparent water stream or of the iridescent colors of a beetle. Additionally, [[Rayleigh scattering]] is responsible for the blue color of the sky, and [[Mie scattering]] for the white color of the water droplets of clouds.

Light scattered from the surfaces of objects is by far the primary light which humans visually observe.<ref name="z">{{cite journal

|author=Kerker, M.

|title=The Scattering of Light

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|page=381

|year=1926

|author-link=Milton Kerker

}}</ref>

Exceptions include objects with polished (specularly reflecting) surfaces, and objects that themselves emit light. [[Rayleigh scattering]] is responsible for the blue color of the sky, and [[Mie scattering]] for the white color of the water droplets in clouds.

==Interreflection==

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In [[3D computer graphics]], diffuse interreflection is an important component of [[global illumination]]. There are a number of ways to model diffuse interreflection when rendering a scene. [[Radiosity (3D computer graphics)|Radiosity]] and [[photon mapping]] are two commonly used methods.

==Spectroscopy==

[[Diffuse reflectance spectroscopy]] can be used to determine the absorption spectra of powdered samples in cases where transmission spectroscopy is not feasible. This applies to [[Ultraviolet–visible spectroscopy|UV-Vis-NIR]] spectroscopy or [[Diffuse reflectance infrared fourier transform spectroscopy|mid-infrared spectroscopy]].<ref name="Griffiths">{{Cite journal|last1=Fuller|first1=Michael P.|last2=Griffiths|first2=Peter R.|date=1978|title=Diffuse reflectance measurements by infrared Fourier transform spectrometry|journal=Analytical Chemistry|language=en|volume=50|issue=13|pages=1906–1910|doi=10.1021/ac50035a045|issn=0003-2700}}</ref><ref name="Kortuem">{{Cite book|title=Reflectance spectroscopy Principles, methods, applications.|last=Kortüm|first= Gustav|date=1969|publisher=Springer|isbn=9783642880711|___location=Berlin|oclc=714802320}}</ref>

==See also==

{{Portal|Computer graphics}}

* [[Diffuser (optics)|Diffuser]]

* [[List of reflected light sources]]

* [[Reflectivity]]

* [[Oren–Nayar reflectance model]]

* [[Reflectivity]]

* [[Remission (spectroscopy)|Remission]]

==References==

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{{DEFAULTSORT:Diffuse Reflection}}

[[Category:OpticsOptical phenomena]]

[[Category:Shading]]