Reflectivity

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 on:

  • material (metal, plastic, etc.)
  • chemical composition and structure of the material
  • state of the material and its surface (temperature, surface roughness, degree of oxidation and contamination)
  • wavelength
  • direction of the incident and reflected light
  • polarization of light

Specification

Specification of reflectivity according to a region of the spectrum

Spectral
Reflectivity values for particular wavelengths.
Band
Average of reflectivity values of a certain wavelength band.
Solar
Average of reflectivity values of all wavelengths with the weight of the intensity of solar radiation at a given wavelength. It is one value valid for solar radiation.
Integrated
Average of reflectivity values of all wavelengths with the weight of the intensity of black body radiation at a given wavelength and a given temperature of the surroundings. It is one value valid for the thermal radiation of surroundings.

Specification of reflectivity according to a geometry

Normal
Specular reflectivity with angle of incidence and reflection close to the surface normal (<15°).
Angular
Specular reflectivity with the same angle of incidence and reflection.
Angular angular
Diffuse reflectivity measured at different angles of incidence and reflection, but the reflected light is collected in one particular direction.
Normal / angular hemispherical
Diffuse reflectivity with one angle of incidence, the reflection is collected from all directions of a hemisphere together.
Hemispherical normal / angular
Diffuse reflectivity with all incident angles together (sample is irradiated from all directions), the reflection is collected in one direction only.

Relative / absolute reflectivity

Relative
Reflectivity measured against a reference with unknown reflectivity value.
Absolute
Reflectivity measured against calibrated standard and recalculated by its absolute reflectivity.

Examples of use

  • Optical elements - mirrors (important is high reflectivity), windows, lenses (important is low reflectivity), beam splitters (precisely defined percentage of reflection), filters and selective mirrors (important is high reflectivity at certain wavelengths - specific wavelengths are reflected and other transmitted) - especially spectral normal / angular reflectivity.
  • Head lights, lampshades of light sources - important is high reflectivity to prevent light loss - especially spectral angular / angular hemispherical reflectivity.
  • Black coatings for walls of chambers or similar sites - important is low reflectivity to prevent further spreading of light - especially spectral angular hemispherical reflectivity.
  • Solar reflectance index - important is high reflectivity - especially solar reflectivity.

Measurement possibilities

  • Spectral normal / angular reflectivity
    • Accessory for specular reflection combined with the spectrometer
    • Different angles of incidence
    • Different polarization (especially for higher angles of incidence from the normal) - a combination of accessory for specular angular reflection with polarizer
  • Spectral normal hemispherical reflectivity
    • Accessory for diffuse reflection (integrating sphere) in combination with the spectrometer
  • Spectral proportion of specular and diffuse components of reflectivity
    • Accessory for diffuse reflection with a function to eliminate specular component of reflection in combination with the spectrometer
  • Angular angular reflectivity
    • Goniometers
  • Band, integrated or solar reflectivity is calculated from spectral reflectivity
  • Absolute reflectivity
    • Calculated from the measured relative reflectivity using a calibrated standard
    • Or measured using accessory for absolute reflection without standard

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