reflectivity (original) (raw)

Author: the photonics expert (RP)

Definition: the ratio of reflected optical power to the incident optical power at a surface

Category: general optics

Related: reflectionFresnel reflectionsFresnel equationsspecular reflectionreflectanceabsorbancetransmissivityreflectometersoptical time-domain reflectometers

Formula symbol: ($R$)

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DOI: 10.61835/l0j Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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Contents

What is a Reflectivity?

The reflectivity of a surface (e.g. an interface between two optical media) is defined as the ratio of reflected optical power to the incident optical power. Usually, it is applied to cases with specular reflections on exactly flat unstructured surfaces, i.e., not to extended objects and diffuse reflections.

Strictly speaking, according to the rules of radiometry, the term reflectivity should be used only for reflections occurring at one particular pure surface — not for example for reflections at extended objects, such as glass plates or optical resonators, or on rough surfaces; in such cases, the term reflectance is appropriate. However, it is common, for example, to specify reflectivities of laser mirrors or anti-reflection coatings, although these are usually based on dielectric coatings, involving interferences between reflections from many different optical interfaces. One then actually considers a whole thin-film structure, usually having an overall thickness far below 1 mm, as one surface. A more questionable case is a fiber Bragg grating, which can be far more extended.

Relation to Reflection Coefficients

The reflection at an optical surface is also often described with a complex reflection coefficient. Its squared modulus is the reflectivity, and it also carries a complex phase according to the optical phase change upon reflection.

Fresnel Equations

Complex reflection coefficients and reflectivities at optical interfaces can be calculated with Fresnel equations. They depend only on the refractive indexes of both optical materials.

Frequently Asked Questions

This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).

What is reflectivity?

Reflectivity is the ratio of reflected optical power to the incident optical power at a surface. The term is typically used for specular reflections from flat, unstructured surfaces.

What is the difference between reflectivity and reflectance?

Strictly, reflectivity applies to reflection at a single pure surface. Reflectance is a more general term, used for extended objects like coated mirrors or for rough surfaces causing diffuse reflections.

How can the reflectivity of a surface be calculated?

The reflectivity of an optical interface can be calculated using the Fresnel equations, which require the refractive indexes of the two media involved.

What is the relation between reflectivity and the reflection coefficient?

Reflectivity is the squared modulus of the complex reflection coefficient. The coefficient also contains information on the optical phase change which occurs upon reflection.

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