What is Reflectance (Reflectivity)

The ratio of the flux reflected to that incident on a surface. That varies according to the wavelength distribution of the incident radiation. 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. According to the CIE, 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 and reflectance generally refer to an electromagnetic power, while the term ‘reflection coefficient’ is used for the ratio of electric field reflected. The reflectance or reflectivity is thus the square of the magnitude of the reflection coefficient. The reflection coefficient can be expressed as a complex number as determined by the Fresnel equations for a single layer, whereas the reflectance (or reflectivity) is always a positive real number. Many common materials exhibit a mixture of specular and diffuse reflection.

Color Acquisition, Management, Rendering, and Assessment in 3D Reality-Based Models Construction

Marco Gaiani (Università di Bologna, Italy)