Reflectance

It is the fraction of incident electromagnetic power that is reflected at the boundary.

Reflectance is a component of the response of the electronic structure of the material to the electromagnetic field of light, and is in general a function of the frequency, or wavelength, of the light, its polarization, and the angle of incidence.

The hemispherical reflectance of a surface, denoted R, is defined as[1]

where Φer is the radiant flux reflected by that surface and Φei is the radiant flux received by that surface.

The spectral hemispherical reflectance in frequency and spectral hemispherical reflectance in wavelength of a surface, denoted Rν and Rλ respectively, are defined as[1]

where The directional reflectance of a surface, denoted RΩ, is defined as[1]

In other words, it has a value for every combination of incoming and outgoing directions.

It is related to the bidirectional reflectance distribution function and its upper limit is 1.

Another measure of reflectance, depending only on the outgoing direction, is I/F, where I is the radiance reflected in a given direction and F is the incoming radiance averaged over all directions, in other words, the total flux of radiation hitting the surface per unit area, divided by π.

[2] This can be greater than 1 for a glossy surface illuminated by a source such as the sun, with the reflectance measured in the direction of maximum radiance (see also Seeliger effect).

The spectral directional reflectance in frequency and spectral directional reflectance in wavelength of a surface, denoted RΩ,ν and RΩ,λ respectively, are defined as[1]

Another way to interpret this is that the reflectance is the fraction of electromagnetic power reflected from a specific sample, while reflectivity is a property of the material itself, which would be measured on a perfect machine if the material filled half of all space.

When the radiation is incident normal to the surface, it is reflected back into the same direction.

For diffuse surfaces, such as matte white paint, reflectance is uniform; radiation is reflected in all angles equally or near-equally.

Most practical objects exhibit a combination of diffuse and specular reflective properties.

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

Specular reflection from a body of water is calculated by the Fresnel equations.

Reflectance, which assumes a flat surface as given by the Fresnel equations, can be adjusted to account for waviness.