Editing Reflectance (section)

==Reflectivity==
[[File:Fresnel equations - reflectance.svg|thumb|400px|Fresnel reflection coefficients for a boundary surface between air and a variable material in dependence of the complex refractive index and the angle of incidence]]
{{redirect|Reflectivity|the EM formulation|Fresnel power reflection}}
For homogeneous and semi-infinite (see [[Half-space (geometry)|halfspace]]) materials, reflectivity is the same as reflectance. 
Reflectivity is the square of the magnitude of the [[Fresnel reflection coefficient]],<ref>E. Hecht (2001). Optics (4th ed.). Pearson Education. {{ISBN|0-8053-8566-5}}.</ref>
which is the ratio of the reflected to incident [[electric field]];<ref name=GoldBook>{{GoldBookRef|title=Reflectance|file=R05235|accessdate=2015-03-15}}</ref> 
as such the reflection coefficient can be expressed as a [[complex number]] as determined by the [[Fresnel equation]]s for a single layer, whereas the reflectance is always a positive [[real number]].

For layered and finite media, according to the [[International Commission on Illumination|CIE]],{{citation needed|date=May 2015}} reflectivity is distinguished from ''reflectance'' by the fact that reflectivity is a value that applies to ''thick'' reflecting objects.<ref name="ILV">{{Cite web |url=http://www.cie.co.at/index.php/index.php?i_ca_id=306 |title=CIE International Lighting Vocabulary |access-date=2010-12-04 |archive-date=2016-06-16 |archive-url=https://web.archive.org/web/20160616060734/http://www.cie.co.at/index.php/index.php?i_ca_id=306 |url-status=dead }}</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">[https://www.amazon.com/dp/081947245X Palmer and Grant, ''The Art of Radiometry'']</ref>