Dictionary Definition
reflectivity
Noun
1 the fraction of radiant energy that is
reflected from a surface [syn: coefficient
of reflection, reflection
factor, reflectance]
2 the ability to reflect beams or rays [syn:
reflection, reflexion]
User Contributed Dictionary
Extensive Definition
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
where G_(\lambda) and G_(\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
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.
See also
Reference
External links
- Reflectance Data Painted surfaces etc.
reflectivity in German: Reflexionsgrad
reflectivity in Estonian: Peegeldustegur
reflectivity in French: Réflectivité
reflectivity in Italian: Riflettanza
reflectivity in Polish: Współczynnik
odbicia