Reflectivity is an optical property of material, which describes how much light is reflected from the material in relation to an amount of light incident on the material. The reflection occurs always on the surface of the material, for the light-diffusing (translucent) materials also in the volume of the material.
Reflectivity depends on the wavelength of light, direction of the incident and reflected light, polarization of light, type of the material (metal, plastic, etc.), chemical composition and structure of the material, and state of the material and its surface (temperature, surface roughness, degree of oxidation and contamination).
On a smooth / polished surface the reflection is specular (direct). On a rough surface / scattering volume the reflection is diffuse (scattered). Specular reflection is governed by the law of reflection (angle of reflection = angle of incidence, both from the surface normal). Diffuse reflection is the reflection in different directions, when most of the reflected light can be scattered in directions close to the specular reflection or reflected light is scattered uniformly in all directions (even back to the source).
For a reflection at a large angle of incidence the reflected light is partially polarized. Two main directions of polarization are defined – S and P. S is the direction perpendicular to the plane of reflection and incidence (from German Senkrecht = perpendicular). P is the direction parallel to the plane of reflection and incidence.
Reflectivity depends material, chemical composition and structure of the material, state of the material and its surface, wavelength, direction of the incident and reflected light and polarization of light.
Mathematically[]
Reflectivity ρ is the ratio of the radiation flux Φr reflected by a sample surface to the incident radiation flux Φi:
Sometimes the term "reflectivity" is understood as the ratio of the mentioned fluxes when the sample reflects volumetrically including its interior if it is semitransparent to thermal radiation. In this case the reflection depends on sample thickness and instead of "reflectivity" we may recommend the use of the term "reflection coefficient". The methods for calculation and measurement of reflectivity are well developed.
For optically smooth sample surface (if its mean square microroughness is at least 100 times less than the wavelength of the radiation) the reflection is specular and the angle of reflection is equal to the angle of incidence. The value of reflectivity as a function of an incident angle may be calculated using Fresnel's formulas if refractive index n and absorption index χ of sample material are known.
In spectral range of transparency of two adjacent media (1 and 2), when the absorption indexes x1and x2 are small in comparison with refractive indexes n1 and n2, the following expressions are applied for the perpendicular and parallel (||) polarized components of incident radiation:
where n21 = n2/nh and θ is an incident angle of radiation from first medium onto the second relative to the surface normal.
For incident natural unpolarized radiation the parallel and perpendicular components have an equal intensity and the mean arithmetic value for this radiation may be taken as reflectivity:
References[]
- Hsia, J. J., Richmond, J. C. (1976) Bidirectional reflectometry, 1. A high resolution laser bidirectional reflectometer with results of several optical coatings, J. Res. Nat. Bur. Stand. A., 80–2, 189–205, 1976.
- Toulukian, Y. S., De Witt, D. P. (1972) Thermophysical properties of matter, Thermal Radiation Properties, Vol. 7, Metallic Elements and Alloys, 1970, Vol. 8, Nonmetallic solids, 1972, New York—Washington, 1FI—Plenum.