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In photometry
Photometry (optics)

Photometry is the science of measurement of light, in terms of its perceived brightness to the human eye. It is distinct from radiometry, which is the science of measurement of radiant energy in terms of absolute power; rather, in photometry, the radiant power at each wavelength is weighted by a luminosity function that models human b...
 and heat transfer
Heat transfer

Heat transfer is the transition of thermal energy or simply heat from a hotter object to a cooler object . When an object or fluid is at a different temperature than its thermodynamic system or another object, transfer of thermal energy, also known as heat transfer, or heat exchange, occurs in such a way that the body and the surround...
, reflectivity is the fraction of incident radiation reflected
Reflection (physics)

Reflection is the change in direction of a wavefront at an wiktionary:interface between two differentmedium so that the wavefront returns into the medium from which it originated....
 by a surface. In general 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:

where and are the reflected and incident spectral (per wavelength) intensity, respectively.

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

Reflectivity is an important concept in the fields of solar thermal energy
Solar thermal energy

Solar thermal energy is a technology for harnessing solar energy for thermal energy . Solar thermal collectors are defined by the USA Energy Information Administration as low-, medium-, or high-temperature collectors....
, telecommunication
Telecommunication

Telecommunication is the assisted Transmission of Signal over a distance for the purpose of communication. In earlier times, this may have involved the use of smoke signals, Drum , Semaphore line, flag signals or heliograph....
 and radar
Radar

Radar is a system that uses electromagnetic radiation waves to identify the range, altitude, direction, or speed of both moving and fixed objects such as aircraft, ships, motor vehicles, weather formations, and terrain....
.

ectivity 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.






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In photometry
Photometry (optics)

Photometry is the science of measurement of light, in terms of its perceived brightness to the human eye. It is distinct from radiometry, which is the science of measurement of radiant energy in terms of absolute power; rather, in photometry, the radiant power at each wavelength is weighted by a luminosity function that models human b...
 and heat transfer
Heat transfer

Heat transfer is the transition of thermal energy or simply heat from a hotter object to a cooler object . When an object or fluid is at a different temperature than its thermodynamic system or another object, transfer of thermal energy, also known as heat transfer, or heat exchange, occurs in such a way that the body and the surround...
, reflectivity is the fraction of incident radiation reflected
Reflection (physics)

Reflection is the change in direction of a wavefront at an wiktionary:interface between two differentmedium so that the wavefront returns into the medium from which it originated....
 by a surface. In general 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:

where and are the reflected and incident spectral (per wavelength) intensity, respectively.

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

Reflectivity is an important concept in the fields of solar thermal energy
Solar thermal energy

Solar thermal energy is a technology for harnessing solar energy for thermal energy . Solar thermal collectors are defined by the USA Energy Information Administration as low-, medium-, or high-temperature collectors....
, telecommunication
Telecommunication

Telecommunication is the assisted Transmission of Signal over a distance for the purpose of communication. In earlier times, this may have involved the use of smoke signals, Drum , Semaphore line, flag signals or heliograph....
 and radar
Radar

Radar is a system that uses electromagnetic radiation waves to identify the range, altitude, direction, or speed of both moving and fixed objects such as aircraft, ships, motor vehicles, weather formations, and terrain....
.

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
Complex number

In mathematics, the complex numbers are an extension of the real numbers obtained by adjoining an imaginary unit, denoted i, which satisfies:...
 as determined by the Fresnel Equations
Fresnel equations

The Fresnel equations, deduced by Augustin-Jean Fresnel , describe the behaviour of light when moving between medium of differing refractive index....
 for a single layer, whereas the reflectance is always a positive real number
Real number

In mathematics, the real numbers may be described informally in several different ways. The real numbers include both rational numbers, such as 42 and −23/129, and irrational numbers, such as pi and the square root of two; or, a real number can be given by an infinite decimal representation, such as 2.4871773339...., where the digits co...
.

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 reflection
Specular reflection

Specular reflection is the perfect, mirror-like reflection of light from a surface, in which light from a single incoming direction is reflected into a single outgoing direction....
 and those that are diffuse reflection
Diffuse reflection

Diffuse reflection is the reflection of light from an uneven or granular surface such that an incident ray is seemingly reflected at a number of angles....
.

  • 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
    Lambertian reflectance

    If a surface exhibits Lambertian reflectance, light falling on it is scattered such that the apparent brightness of the surface to an observer is the same regardless of the observer's angle of view....
    .


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
Fresnel equations

The Fresnel equations, deduced by Augustin-Jean Fresnel , describe the behaviour of light when moving between medium of differing refractive index....
 can be adjusted to account for waviness.

Grating efficiency


The generalization of reflectance to a diffraction grating
Diffraction grating

In optics, a diffraction grating is an optical component with a regular pattern, which splits light into several beams travelling in different directions....
, which disperses light by wavelength
Wavelength

In physics, wavelength is the distance between repeating units of a propagating wave of a given frequency. It is commonly designated by the Greek language letter lambda ....
, is called diffraction efficiency.

See also

  • Albedo
    Albedo

    The albedo of an object is the extent to which it diffusely reflects light from the Sun. It is therefore a more specific form of the term reflectivity....
  • Bidirectional reflectance distribution function
    Bidirectional reflectance distribution function

    The bidirectional reflectance distribution function is a 4-dimensional function that defines how light is reflected at an opaque surface. The function takes an incoming light direction, , and outgoing direction, , both defined with respect to the surface normal , and returns the ratio of reflected radiance exiting along to the irradiance...
  • Absorptivity, Emissivity
    Emissivity

    The emissivity of a material is the ratio of energy Radiation by a particular material to energy radiated by a black body at the same temperature....
    , Transmissivity
    Transmissivity

    Transmissivity may refer to:*Hydraulic conductivity#Transmissivity*Transmittance, in optics...
  • Diffuse reflection
    Diffuse reflection

    Diffuse reflection is the reflection of light from an uneven or granular surface such that an incident ray is seemingly reflected at a number of angles....
    , Specular reflection
    Specular reflection

    Specular reflection is the perfect, mirror-like reflection of light from a surface, in which light from a single incoming direction is reflected into a single outgoing direction....
  • Lambert's cosine law
    Lambert's cosine law

    Lambert's cosine law in optics says that the radiant intensity observed from a "Lambertian" surface is directly proportional to the cosine of the angle ? between the observer's line of sight and the surface normal....
  • Transmittance
    Transmittance

    In optics and spectroscopy, transmittance is the fraction of incident light at a specified wavelength that passes through a sample. Specifically, visible transmittance is this fraction for visible light....
  • Sun path
    Sun path

    File:Solar altitude.svgSun path refers to the apparent significant seasonal-and-hourly positional changes of the sun as the Earth rotates, and orbits around the sun....


External links

  • Painted surfaces etc.