Definitions

Reflectivity

Reflectivity

[ri-flek-tiv]

In photometry and heat transfer, reflectivity is the fraction of incident radiation reflected by a surface. In full generality 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_{refl}(lambda)}{G_{incid}(lambda)}

where G_{refl}(lambda) and G_{incid}(lambda) are the reflected and incident spectral (per wavelength) intensity, respectively.

This can be further averaged over all wavelengths to give the total hemispherical reflectivity,

rho = frac{G_{refl}}{G_{incid}}

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

Reflectance

Reflectivity measures the fractional amplitude of the reflected electromagnetic field, while reflectance refers to the fraction of incident electromagnetic power that is reflected at an interface. The reflectance is thus the square of the magnitude of the reflectivity. The reflectivity can be expressed as a complex number as determined by the Fresnel Equations for a single layer, whereas the reflectance is always a positive real number.

In certain fields, reflectivity is distinguished from reflectance by the fact that reflectivity is a value that applies to thick reflecting objects. 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.

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

Surface type

Going back to the fact that reflectivity is a directional property, it should be noted that most surfaces can be divided into those that are specular and those that are diffuse.

  • For specular surfaces, such as glass or polished metal, reflectivity will be nearly zero at all angles except at the appropriate reflected angle.
  • For diffuse surfaces, such as matte white paint, reflectivity is uniform; radiation is reflected in all angles equally or near-equally. Such surfaces are said to be Lambertian.

Most real objects have some mixture of diffuse and specular reflective properties.

Water reflectivity

Reflection occurs when light moves from a medium with one index of refraction into a second medium with a different index of refraction.

That part of incident light that is reflected from a body of water is specular and is calculated by the Fresnel equations. Fresnel reflection is directional and therefore does not contribute significantly to albedo which is primarily diffuse reflection.

A real water surface may be wavy. Reflectivity assuming a flat surface as given by the Fresnel equations can be adjusted to account for waviness.

Grating efficiency

The generalization of reflectance to a diffraction grating, which disperses light by wavelength, is called diffraction efficiency.

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