REFLECTIVITY

In optics 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 common averaged over the reflected hemisphere as follows
$$
ho(lambda) = rac{G_{refl}(lambda)}{G_{incid}(lambda)}
where $G\_\{refl\}(lambda)$ and $G\_\{incid\}(lambda)$ are reflected and incident spectral intensities, respectively.
'Note: The above equation is actually a ratio rather than an average. This should be fixed.'
This can be further averaged over all wavelengths to give the 'total hemispherical reflectivity',
$$
ho = rac{G_{refl}}{G_{incid}}
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 matt 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.
See also Specular reflection and Diffuse reflection. In climatology, reflectivity is also called albedo.

Contents

In depth

Fresnel reflection

Water reflectivity

Metal reflectance

See also

External links

In depth

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.

Fresnel reflection

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

Water reflectivity

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. A formula and graph for correction for waviness for two different wave energy spectrum definitions exists. [1]

Metal reflectance

See also

★ Photometry (optics) Main Photometry/Radiometry article

★ Transmissivity

★ Absorptivity

★ Emissivity

★ Federal Standard 1037C (clarification required)

External links

★ Reflectance Data Painted surfaces etc.
Reflectivity is an important concept in the fields of

★ Solar thermal energy

★ Telecommunication and Radar