Thereflectance of the surface of amaterial is its effectiveness inreflectingradiant energy. It is the fraction of incident electromagnetic power that is reflected at the boundary. Reflectance is a component of the response of theelectronic structure of the material to the electromagnetic field of light, and is in general a function of the frequency, orwavelength, of the light, its polarization, and theangle of incidence. The dependence of reflectance on the wavelength is called areflectance spectrum orspectral reflectance curve.

Thehemispherical reflectance of a surface, denotedR, is defined as^[1]$${\displaystyle R=\frac{{\mathrm{\Phi }}_{\mathrm{e}}^{\mathrm{r}}}{{\mathrm{\Phi }}_{\mathrm{e}}^{\mathrm{i}}},}$$whereΦ_e^r is theradiant fluxreflected by that surface andΦ_eⁱ is the radiant fluxreceived by that surface.

Thespectral hemispherical reflectance in frequency andspectral hemispherical reflectance in wavelength of a surface, denotedR_ν andR_λ respectively, are defined as^[1]$${\displaystyle R_{\nu }=\frac{{\mathrm{\Phi }}_{\mathrm{e},\nu }^{\mathrm{r}}}{{\mathrm{\Phi }}_{\mathrm{e},\nu }^{\mathrm{i}}},}$$$${\displaystyle R_{\lambda }=\frac{{\mathrm{\Phi }}_{\mathrm{e},\lambda }^{\mathrm{r}}}{{\mathrm{\Phi }}_{\mathrm{e},\lambda }^{\mathrm{i}}},}$$where

• Φ_e,ν^r is thespectral radiant flux in frequencyreflected by that surface;
• Φ_e,νⁱ is the spectral radiant flux in frequency received by that surface;
• Φ_e,λ^r is thespectral radiant flux in wavelengthreflected by that surface;
• Φ_e,λⁱ is the spectral radiant flux in wavelength received by that surface.

Thedirectional reflectance of a surface, denotedR_Ω, is defined as^[1]$${\displaystyle R_{\mathrm{\Omega }}=\frac{L_{\mathrm{e},\mathrm{\Omega }}^{\mathrm{r}}}{L_{\mathrm{e},\mathrm{\Omega }}^{\mathrm{i}}},}$$where

• L_e,Ω^r is theradiancereflected by that surface;
• L_e,Ωⁱ is the radiance received by that surface.

This depends on both the reflected direction and the incoming direction. In other words, it has a value for every combination of incoming and outgoing directions. It is related to thebidirectional reflectance distribution function and its upper limit is 1. Another measure of reflectance, depending only on the outgoing direction, isI/F, whereI is the radiance reflected in a given direction andF is the incoming radiance averaged over all directions, in other words, the total flux of radiation hitting the surface per unit area, divided by π.^[2] This can be greater than 1 for a glossy surface illuminated by a source such as the sun, with the reflectance measured in the direction of maximum radiance (see alsoSeeliger effect).

Thespectral directional reflectance in frequency andspectral directional reflectance in wavelength of a surface, denotedR_Ω,ν andR_Ω,λ respectively, are defined as^[1]$${\displaystyle R_{\mathrm{\Omega },\nu }=\frac{L_{\mathrm{e},\mathrm{\Omega },\nu }^{\mathrm{r}}}{L_{\mathrm{e},\mathrm{\Omega },\nu }^{\mathrm{i}}},}$$$${\displaystyle R_{\mathrm{\Omega },\lambda }=\frac{L_{\mathrm{e},\mathrm{\Omega },\lambda }^{\mathrm{r}}}{L_{\mathrm{e},\mathrm{\Omega },\lambda }^{\mathrm{i}}},}$$where

• L_e,Ω,ν^r is thespectral radiance in frequencyreflected by that surface;
• L_e,Ω,νⁱ is the spectral radiance received by that surface;
• L_e,Ω,λ^r is thespectral radiance in wavelengthreflected by that surface;
• L_e,Ω,λⁱ is the spectral radiance in wavelength received by that surface.

Again, one can also define a value ofI/F (see above) for a given wavelength.^[3]

For homogeneous and semi-infinite (seehalfspace) materials, reflectivity is the same as reflectance. Reflectivity is the square of the magnitude of theFresnel reflection coefficient,^[4]which is the ratio of the reflected to incidentelectric field;^[5] as such the reflection coefficient can be expressed as acomplex number as determined by theFresnel equations for a single layer, whereas the reflectance is always a positivereal number.

For layered and finite media, according to theCIE,^{[citation needed]} reflectivity is distinguished fromreflectance by the fact that reflectivity is a value that applies tothick reflecting objects.^[6] 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.^[7]

Given that reflectance is a directional property, most surfaces can be divided into those that givespecular reflection and those that givediffuse reflection.

For specular surfaces, such as glass or polished metal, reflectance is nearly zero at all angles except at the appropriate reflected angle; that is the same angle with respect to the surface normal in theplane of incidence, but on the opposing side. When the radiation is incident normal to the surface, it is reflected back into the same direction.

For diffuse surfaces, such as matte white paint, reflectance is uniform; radiation is reflected in all angles equally or near-equally. Such surfaces are said to beLambertian.

Most practical objects exhibit a combination of diffuse and specular reflective properties.

Reflection of light occurs at a boundary at which theindex of refraction changes. Specular reflection is calculated by theFresnel equations.^[8] Fresnel reflection is directional and therefore does not contribute significantly toalbedo which primarily diffuses reflection.

A liquid surface may be wavy. Reflectance may be adjusted to account forwaviness.

The generalization of reflectance to adiffraction grating, which disperses light bywavelength, is calleddiffraction efficiency.

• Bidirectional reflectance distribution function
• Colorimetry
• Emissivity
• Lambert's cosine law
• Transmittance
• Sun path
• Light Reflectance Value
• Albedo
• Reststrahlen effect
• Lyddane–Sachs–Teller relation

Look upreflectance in Wiktionary, the free dictionary.

• Reflectivity of metalsArchived 2016-03-04 at theWayback Machine.
• Reflectance Data.

• Physical quantities
• Radiometry
• Dimensionless numbers of physics

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