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. Reflectivity is the limit value of reflectance as the sample becomes thick it is the intrinsic reflectance of the surface, hence irrespective of other parameters such as the reflectance of the rear surface. When reflection occurs from thin layers of material, internal reflection effects can cause the reflectance to vary with surface thickness. Which is the ratio of the reflected to incident electric field Īs such the reflection coefficient 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.įor layered and finite media, according to the CIE, reflectivity is distinguished from reflectance by the fact that reflectivity is a value that applies to thick reflecting objects. Reflectivity is the square of the magnitude of the Fresnel reflection coefficient, For the EM formulation, see Fresnel power reflection.įor homogeneous and semi-infinite (see halfspace) materials, reflectivity is the same as reflectance.

The spectral directional reflectance in frequency and spectral directional reflectance in wavelength of a surface, denoted R Ω, ν and R Ω, λ respectively, are defined as 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). 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. It is related to the bidirectional reflectance distribution function and its upper limit is 1. In other words, it has a value for every combination of incoming and outgoing directions. This depends on both the reflected direction and the incoming direction.

L e,Ω i is the radiance received by that surface.

L e,Ω r is the radiance reflected by that surface.

Intensity of light water equation