# Reflectivity

 related topics {math, energy, light}

In optics and photometry, reflectivity is the fraction of incident radiation reflected by a surface. In general it must be treated as a directional property that is a function of the reflected direction, the incident direction, and the incident wavelength. However it is also commonly averaged over the reflected hemisphere to give the hemispherical spectral reflectivity:

$\rho(\lambda) = \frac{G_{\mathrm{refl}}(\lambda)}{G_{\mathrm{incid}}(\lambda)}$

where Grefl(λ) and Gincid(λ) are the reflected and incident spectral (per wavelength) intensity, respectively.

This can be further averaged over all wavelengths (in a given band of interest—say, optical) to give the total hemispherical reflectivity,

$\rho = \frac{G_{\mathrm{refl}}}{G_{\mathrm{incid}}}$

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

## Contents

### Reflectance

Reflectivity and reflectance generally refer to the fraction of incident electromagnetic power that is reflected at an interface, while the term "reflection coefficient" is used for the fraction of electric field reflected.[1][2] The reflectance or reflectivity is thus the square of the magnitude of the reflection coefficient. 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)[3], reflectivity is distinguished from reflectance by the fact that reflectivity is a value that applies to thick reflecting objects.[4] 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 surface 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.[5]

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