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reflectivity
Snippet from Wikipedia: Reflectance

Reflectance of the surface of a material is its effectiveness in reflecting radiant energy. It is the fraction of incident electromagnetic power that is reflected at an interface. The reflectance spectrum or spectral reflectance curve is the plot of the reflectance as a function of wavelength.

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Reflectivity or reflectance is the fraction of incident electromagnetic power that is reflected at an interface, in contrast to the reflection coefficient, which is the ratio of the reflected to incident electric field.<ref name=“Klein”>Klein and Furtak, ''Optics''</ref>

Reflectance

File:Fresnel reflection coefficients.svg

The reflectance or reflectivity is the square of the magnitude of the reflection coefficient.<ref>E. Hecht (2001). Optics (4th ed.). Pearson Education. ISBN 0-8053-8566-5.</ref> The reflection coefficient can be expressed as a complex number as determined by the Fresnel equations for a single layer, whereas the reflectance (or reflectivity) is always a positive real number.

According to the CIE (the International Commission on Illumination),<ref name=“CIE”>CIE (the International Commission on Illumination)</ref> reflectivity is distinguished from reflectance by the fact that reflectivity is a value that applies to thick reflecting objects.<ref name=“ILV”>CIE International Lighting Vocabulary</ref> When reflection occurs from thin layers of material, internal reflection effects can cause the reflectance to vary with surface thickness. Reflectivity is the limit value of reflectance as the sample becomes thick; it is the intrinsic reflectance of the surface, hence irrespective of other parameters such as the reflectance of the rear surface. Another way to interpret this is that the reflectance is the fraction of electromagnetic power reflected from a specific sample, while reflectivity is a property of the material itself, which would be measured on a perfect machine if the material filled half of all space.<ref name=“Grant”>Palmer and Grant, ''The Art of Radiometry''</ref>

The reflectance spectrum or spectral reflectance curve is the plot of the reflectance as a function of wavelength.

Surface type

Going back to the fact that reflectivity is a directional property, most surfaces can be divided into those that give specular reflection and those that give diffuse reflection.

  • For specular surfaces, such as glass or polished metal, reflectivity will be nearly zero at all angles except at the appropriate reflected angle - that is, reflected radiation will follow a different path from incident radiation for all cases other than radiation normal to the surface.
  • For diffuse surfaces, such as matte white paint, reflectivity is uniform; radiation is reflected in all angles equally or near-equally. Such surfaces are said to be Lambertian.

Most real objects have some mixture of diffuse and specular reflective properties.

Water reflectivity

Reflection occurs when light moves from a medium with one index of refraction into a second medium with a different index of refraction.

Specular reflection from a body of water is calculated by the Fresnel equations.<ref name=“Ottav”>Ottaviani, M. and Stamnes, K. and Koskulics, J. and Eide, H. and Long, S.R. and Su, W. and Wiscombe, W., 2008: 'Light Reflection from Water Waves: Suitable Setup for a Polarimetric Investigation under Controlled Laboratory Conditions. Journal of Atmospheric and Oceanic Technology, 25 (5), 715–728.</ref> Fresnel reflection is directional and therefore does not contribute significantly to albedo which is primarily diffuse reflection.

A real water surface may be wavy. Reflectivity assuming a flat surface as given by the Fresnel equations can be adjusted to account for waviness.

Grating efficiency

The generalization of reflectance to a diffraction grating, which disperses light by wavelength, is called diffraction efficiency.

Applications

Reflectivity is an important concept in the fields of optics, solar thermal energy, telecommunication and radar.

See also

References

Fair Use References are embedded in the above article as footnotes.

reflectivity.txt · Last modified: 2020/03/12 18:37 (external edit)